Brandywine Flood Study Report

Brandywine Flood Study 2025

Brandywine Flood Study

April 2025

PREPARED BY Brandywine Conservancy

Chester County Water Resources Authority University of Delaware Water Resources Center

A Letter from the Flood Study Partners

The Brandywine Flood Study was initiated in response to the impacts of Hurricane Ida (September 2021). While flooding is not a new issue in this watershed, that storm took a devastating toll on many communities, and ultimately galvanized a renewed interest in regional flood mitigation planning.

This work is especially important at this critical juncture in time, as climate change increases the frequency and intensity of storms. Beyond shifting precipitation patterns, development pressure in the region is increasing. Current planning forecasts predict an additional 150,000 residents in the Brandywine watershed by the end of this century. Both of these issues have the potential to exacerbate the impacts of flooding on local communities, unless a proactive approach is taken to mitigate those risks.

In order to better protect people and property from the impacts of future floods, this study provides a broader analysis of flood risk in the Brandywine watershed, including historic flooding, projections for future floods, potential impacts of future precipitation and storm events, and land use based on the Brandywine watershed’s projected population in the year 2100.

This report provides a summary of the study’s research and community outreach, along with an actionable suite of proposed flood mitigation recommendations throughout the watershed. Recommendations are presented in two distinct categories: structural and nonstructural. Structural mitigation measures are often the most visible, like the five major flood control dams and reservoirs that were built after extreme flood events during the early and mid 20th century and collectively provide over 6 billion gallons of flood storage capacity. Study partners analyzed numerous structural project opportunities, including upgrades to existing flood control infrastructure, bridge and culvert replacement, low-head dam removal, floodplain restoration, and stormwater basin retrofits. Ultimately, of the more than 300 individual sites evaluated, 16 were prioritized for their potential to reduce regional and localized flood risks, including:

• 10 bridge replacements

• 4 low-head dam removals

• 1 floodplain restoration project

• Upgrades to the existing Barneston Dam flood control structure.

The study also includes more than a dozen non-structural recommendations related to emergency planning, early warning systems, public education campaigns, and more robust road closure measures. For developed areas within the 100-year floodplain, it outlines strategies for enhanced floodplain management, flood insurance, structural elevations, floodproofing, and voluntary buyouts. The study also calls for increased land preservation to ensure the long-term functionality of natural floodplains and open space, which act as sponges during storm events.

In terms of the impact of existing infrastructure, the study concluded that during Hurricane Ida, all 5 of the flood control structures in the upper watershed performed as designed, which helped to prevent the catastrophic flooding from being truly cataclysmic. In terms of the watershed’s 1,200 analyzed stormwater basins, it was determined that though the basins do lessen stormwater runoff, those benefits are mostly limited to the local community. The study found that retrofits or enhancements to the stormwater basins would not provide significant additional benefits at the watershed scale during major storm/flood events. However, in some flood-prone areas, retrofits and restoration efforts could help address localized flooding issues during smaller storm events. In all cases, ongoing inspection and maintenance of these facilities is critical to ensuring they perform as designed.

Anywhere there is water, there is the potential for flooding. Even with unlimited financial and technological resources, it would be impossible to eliminate all flood risks. However, the Flood Study partners are confident that implementation of the recommendations laid out in this report can meaningfully reduce future flood risks to communities throughout the Brandywine watershed. The implementation recommendations include both collaborative action and individual stakeholder projects, and all potential implementers should evaluate and prioritize opportunities to work to improve flood mitigation in the watershed.

The Brandywine Flood Study partners are committed to supporting municipalities, stakeholders, and others in the implementation of these strategies, and to continually assess new opportunities to reduce localized and regional flooding in the future.

Brandywine Conservancy

Chester County Water Resources Authority

University of Delaware Water Resources Center

Acknowledgements

The Brandywine Conservancy, Chester County Water Resources Authority, and University of Delaware Water Resources Center would like to acknowledge the following organizations for their partnership in preparing the Brandywine Flood Study report:

• Gannett Fleming

• Geodesy, Inc.

• Meliora Design

• Stroud Water Research Center

• West Chester University

The report team extends gratitude to Chester County, PA; Delaware County, PA; Federal Emergency Management Agency (FEMA); and Pennsylvania Emergency Management Agency (PEMA) for funding support of the Brandywine Flood Study.

In addition to the partners listed above, the Flood Advisory Committee, which was active throughout the study process, was instrumental in developing and refining the results of this final report. Members of the Advisory Committee included representatives from the following organizations:

• 2nd Century Alliance

• Arcadis

• Brandywine Red Clay Creek Alliance

• Brandywine River Restoration Trust

• CDM Smith

• Center for Watershed Protection

• Chadds Ford Township

• Chester County Conservation District

• Chester County Department of Emergency Services

• Chester County Planning Commission

• Christina Watersheds Municipal Partnership

• City of Coatesville

• City of Wilmington

• Collaborate Northeast

• Delaware County Department of Emergency Services

• Delaware County Heritage Commission

• Delaware County Planning Commission

• Delaware Department of Natural Resources and Environmental Control

• Delaware Nature Society

• Delaware Sea Grant

• Downingtown Borough

• Downingtown Flood Advisory Committee

• Eleventh Street Bridge Community Long-Term Recovery Group, Inc.y

• Gaadt Perspectives, LLC

• Gannett Fleming

• Greater Wilmington Housing Providers

• Green Building United

• Hagley Museum and Library

• Kirkwood Community Center

• Natural Resources Conservation Service

• Partnership for the Delaware Estuary

• Pennsbury Township

• Pennsylvania Department of Conservation and Natural Resources

• The Nature Conservancy, Pennsylvania/Delaware Chapter

• United States Geological Survey

• University of Delaware Institute for Public Administration

• West Chester University Department of Geography and Planning

• Wilmington Area Planning Council

Table

Table

Lloyd Avenue bridge over Beaver Creek in Caln Township

List of Figures

Figure 1: Map of the Brandywine Watershed, along with the 17 Subwatersheds used in Flood Study Modeling ............................................................................................................ 6

Figure 2: Population Density in the Brandywine Watershed, 2020 8

Figure 3: Percentage of Major Land Cover Categories in the Brandywine Watershed, 2021 10

Figure 4: Land Cover in the Brandywine Watershed, 2021 ................................................. 11

Figure 5: Impervious Cover by Subwatershed in the Brandywine Watershed, 2021 12

Figure 6: Change in Developed land, 2011-2021 ................................................................ 13

Figure 7: Flood of Record and All Storms above Major Flood Stage (13.0”) at the Brandywine Creek at Chadds Ford, PA ................................................................................ 15

Figure 8: Precipitation Totals and Distribution throughout the Watershed during Hurricane Ida, September 2021 ........................................................................................... 19

Figure 9: Peak Flow Rate at the Stream Gage for the Brandywine Creek at Wilmington, 1947–2024 ............................................................................................................................. 19

Figure 10: Peak Flow Rates into and out of Marsh Creek Dam during Hurricane Ida 20

Figure 11: Flood Waters Detained by Beaver Creek Dam during Hurricane Ida ................ 21

Figure 12: Flood Hazard Sites in the Main Stem Brandywine Creek Watershed 22

Figure 13: Flood Hazard Sites in the East Branch Brandywine Creek Watershed .............. 23

Figure 14: Flood Hazard Sites in the West Branch Brandywine Creek Watershed 24

Figure

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Figure 20: Water Surface Elevation Model Output at U.S. Route 13/Northeast Boulevard Bridge .................................................................................................................. 47

Figure 21: Water Surface Elevation Model Output at Bancroft Dam No. 4 ....................... 47

Figure 22: Water Surface Elevation Model Output at DuPont Experimental Station Dam No. 6...................................................................................................................................... 47

Chapter 1

Flooding in the Brandywine –A Call to Action

The Brandywine Creek, which traverses through southeastern Pennsylvania and northern Delaware, has always had an incredible impact on the local landscapes and communities. Hundreds of years ago, industries established themselves along the banks of the Brandywine and its tributaries to harness its power. Townships and cities settled around those industrial hubs and continued to grow and expand even as the use of hydropower declined. Today, these streams provide natural character and numerous ecosystem services to their communities. Yet flooding along these waterways has the potential to endanger lives, disrupt economic activities, and cause extensive damage.

Communities along the Brandywine Creek and its tributaries are no strangers to the threat of rising waters. Many residents across the region can vividly recall hurricanes, tropical storms, and other major rain events that disrupted their lives in one way or another. The devastation caused by Hurricane Ida, an 800-year storm, brought renewed attention to flood mitigation efforts in the Brandywine watershed in 2021. That storm served as the main catalyst for this study to better understand the factors that contribute to and exacerbate flooding in the watershed, as well as identify actionable steps communities can take to reduce flood risks.

According to the National Oceanic and Atmospheric Administration (NOAA) Storm Events Database, since 1996, flood events have resulted in two deaths and more than $56 million in property damage in Chester County, PA. The remnants of Hurricane Ida alone, which occurred September 1 – 2, 2021, caused

nearly $45 million in damage to private property and public infrastructure in southeastern Pennsylvania and northern Delaware. In Chester County, the Department of Emergency Services reported that the 9-1-1 Communications Center processed more than 4,000 calls during Tropical Storm Ida, resulting in over 300 storm-related rescues within 10 hours. There was one storm-related fatality in Downingtown Borough, and four first responders were injured during rescue operations. As of early 2025, several communities within the watershed are still actively recovering from Hurricane Ida.

In addition to serving as a home for roughly 250,000 people, the Brandywine watershed is a critical piece of the regional economy. According to the 2018 Brandywine-Christina State of the Watershed Report, the watershed provides over 100,000 jobs and brings in $4.9 billion annually in economic activity. Floods have the potential to cause significant disruptions to the economy and transit in the region, on top of the risk to life and property. Closed roadways, impassable bridges, and inundated commercial areas create their own suite of problems in local communities, ranging from increased traffic congestion to major economic losses.

Planning for flooding along the Brandywine Creek and its tributaries has been an ongoing exercise for decades. Major floods in 1920, 1933, 1942, 1955, 1972, and 1973 were referenced in plans that ultimately resulted in the construction of five regional flood control facilities within the upper portions of the watershed.

Extensive flooding from Hurricane Ida occurred at Northeast Boulevard in Wilmington, DE. Photo courtesy of City of Wilmington Department of Public Works

Together, these structures provide 6 billion gallons of total flood storage capacity to protect thousands of lives and properties downstream. This amount of water could fill the entire Lincoln Financial Field stadium, home of the Philadelphia Eagles, seven times.

However, as storms become more frequent and intense and development continues throughout the watershed, the challenge of flooding continues.

To address these longstanding challenges, the Chester County Water Resources Authority (CCWRA), Brandywine Conservancy (BC), University of Delaware Water Resources Center (UDWRC), and Delaware County, PA, have conducted a flood study of the Brandywine Creek and its tributaries in Chester County and Delaware County, PA, and New Castle County, DE. This study builds upon the 2017 Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) updates and flood zone maps revisions for the region by incorporating updated land use data and climate model projections with hydrologic and hydraulic computer modeling.

The project analyzes the Brandywine Creek during intense storm and flooding events in order to produce an actionable suite of flood mitigation recommendations. This report provides a summary of the research and community outreach conducted, along with proposed implementation strategies to address future intense storm events and flooding throughout the watershed.

The Brandywine Flood Study included the following key elements:

1. Flood Working Group: Identified representatives from the public (focus on municipal governments), private, and nonprofit entities to serve on a Flood Working Group to inform and advise on the initiative. Conducted public outreach meetings.

2. Flood Identification: Identified and mapped chronic flooding areas through the review of literature from FEMA, U.S. Army Corps of Engineers (USACE), U.S. Geological Survey (USGS), Delaware Department of Natural Resources and Environmental Control (DNREC), Pennsylvania Department of Environmental Protection (PADEP), CCWRA, and the media. Conducted field reconnaissance to field survey and map flood areas.

3. Public Engagement: Solicited public input on flooding hot spots, areas of concern, and ideas for future improvements through multiple avenues, including live and pre-recorded presentations, web-based surveys, an interactive flood mapping tool, and community listening sessions in key areas throughout the watershed.

Route 30 Bypass in Chester County, PA during Tropical Storm Isaias in 2020.

Downingtown Borough is set at the confluence of multiple streams, including the East Branch Brandywine Creek and Beaver Creek.

Robert G. Struble, Sr. Dam and Lake in the upper East Branch Brandywine Creek watershed.

4. Municipal Outreach: Met with staff and officials from each municipality in the watershed to gather feedback on localized flooding challenges, as well as ongoing/planned efforts to address them.

5. Storm Event Analysis: Developed a series of model storm events using historical records at precipitation gages in the watershed to analyze hydrologic and hydraulic models. Used historical storm event analysis to develop storm events representing potential increases in intensity and duration of future events for the models.

6. Hydrologic Model: Utilized U.S. Department of Agriculture (USDA) Technical Release 55 (TR-55) hydrologic models and ArcView GIS to delineate watersheds/subwatersheds, incorporate USGS stream gages, stream/storage routing, and conduct existing/proposed (i.e., with flood solutions) conditions modeling for the 2-, 10-, 50-, 100-, 500-, 1000-year, and storm of record flood frequency scenarios. The scenarios incorporated projections related to climate change and the potential effects of future development throughout the watershed.

7. Hydraulic Model: Utilized existing USACE Hydrologic Engineering Center’s River Analysis System (HEC-RAS) hydraulic models and FEMA flood profiles for the mainstem, east and west branches, and Beaver Creek of the Brandywine Creek to evaluate existing flooding conditions and performed proposed future conditions modeling.

8. Flood Relief Analysis: For areas with chronic flooding or significant obstructions, used hydrologic/hydraulic models (where available) and FEMA Flood Insurance Study flood profiles to assess opportunities for structural and nonstructural mitigation projects.

This study was funded through grants from FEMA, PEMA, and Chester and Delaware Counties in Pennsylvania. The primary authors of this report are CCWRA, BC, and UDWRC. Multiple project partners have contributed significantly to the report by providing data, feedback, mapping support, written content, and technical review at all stages of the project.

In addition to the primary authors, the Brandywine Flood Study partners include the Stroud Water Research Center, West Chester University, and Meliora Design. The Brandywine Flood Study Technical Advisory Committee includes government officials, nonprofit organizations, and private entities, that provided continuous feedback and expertise throughout the project. Community members throughout the Brandywine provided the project team with meaningful input and have contributed significantly to inform and advise the project. This report was made possible by the robust support of this broad network of engaged stakeholders.

Houses in Birmingham Township were flooded in 2006. Photo courtesy of Chester County Planning Commission

Flood waters from Hurricane Ida in 2021 at Breaks Mill in New Castle County, DE.

Chapter 2 Current Watershed Conditions

The Brandywine watershed is one of the most historic small watersheds in the nation. It is part of the ancestral homelands of the Lenni Lenape, nestled within two of the original 13 U.S. colonies. The area boasts a rich agricultural heritage and is home to early mills, which helped to power the American Industrial Revolution. The watershed spans 325 square miles (sq. mi.), of which 303 sq. mi. (93%) are in Pennsylvania and 23 sq. mi. (7%) are in Delaware. It is currently home to more than 265,000 people (U.S. Census 2020).

The Brandywine Creek comprises three main branches: the main stem, running from the mouth at Wilmington, DE, north into Pennsylvania; the East Branch, from Pocopson through the Borough of Downingtown to the headwaters east of the Borough of Honey Brook in Chester County, PA; and the West Branch, running through the City of Coatesville to the headwaters west of Honey Brook. The watershed includes tributaries that define 17 distinct subwatersheds, which were modeled in the study (see Figure 1).

The Brandywine watershed begins near the Welsh Mountains along the northern border between Lancaster and Chester Counties at an elevation of over 1,000 feet. From its headwaters, the Brandywine Creek flows for more than 40 miles down to its confluence with the Christina River in Delaware.

For thousands of years, the watershed’s rolling hills and stream valleys have been part of the ancestral homelands of the Lenni Lenape. Later in its history, the watershed supported colonial populations, who took advantage of the fertile soils and ample waterpower for farms and mills. The area played an important role in the American Revolution as the location of the Battle of Brandywine. Industrialization in this region of the country began in the early 1800s with the development of textile and powder mills along the Brandywine Creek fallline, and later with steel mills in the central Great Valley at Coatesville. Today, a mix of residential, commercial, and industrial development forms the basis for growing communities and significant economic activity.

Right photo: Gateway Park in Coatesville City, PA sits along the West Branch Brandywine Creek next to Cleveland Cliffs Steel.

Left photo: Brandywine Creek in Wilmington from Brandywine Mills Park. Photo courtesy of Elena Hadley

Figure 1: Map of Brandywine Watershed, with the 17 Subwatersheds used in Flood Study Modeling

As of the 2020 U.S. Census, there were more than 265,000 residents in the Brandywine watershed, with 222,000 (84%) residing in Pennsylvania, and nearly 43,000 (16%) in Delaware (see Table 1). The population is concentrated in the urbanized central portion of the watershed, particularly in the areas surrounding the Borough of West Chester and along the Great Valley corridor, and in the City of Wilmington near the mouth of the Brandywine. The more rural areas in the headwaters near Honey Brook and the West Branch below Coatesville are less densely populated (see Figure 2).

The population of the watershed has been steadily growing in recent decades, reflecting trends in Chester, Delaware, and New Castle Counties. Between 2000 and 2020, the watershed saw a net increase of over 52,000 residents (24.6%), based on the U.S. Census Bureau Decennial Census. New Castle County, Delaware, in the lower portion of the watershed, increased by more than 2,700 residents (6.9%), while the Pennsylvania portion of the watershed increased by nearly 50,000 (28.7%).

Table 1: Population in the Brandywine Watershed by County from 2000 Census and 2020 Census

Uwchlan Township. Photo courtesy of Chester County Planning Commission

Figure 2: Population Density in the Brandywine Watershed, 2020

Housing construction in East Brandywine Township.

Uwchlan Township in the Shamona Creek subwatershed

East Branch Brandywine Creek in Downingtown Borough

Land cover has a large effect on the volume of stormwater runoff. Any conversion from forest to non-forest land cover will generate more runoff, which has the potential to result in increased flooding locally and downstream. The amount of additional runoff increases with the amount and intensity of development, with densely urbanized residential, commercial, and industrial areas contributing the most runoff during storm events.

Figure 3: Percentage of Major Land Cover Categories in the Brandywine Watershed, 2021

Land cover in the Brandywine watershed is roughly equally divided among developed, agricultural, and forested areas (see Figures 3 and 4). Agriculture dominates the landscape in the northern portion of the watershed, near the headwaters of the East and West Branches. Urban development is focused in the Great Valley along the Route 30 corridor and around the City of Wilmington, while less dense mixed residential and commercial development occurs in the suburban areas beyond those urbanized centers (see Figure 5).

Over the past 20 years (2001 to 2021) in the Brandywine watershed, development pressure has resulted in the decline of forested and agricultural land cover and an increase in developed impervious cover (see Figure 6). The impervious cover has increased by approximately 9.1% for the West Chester subbasin, 14% – 15% along the Route 30 corridor (Great Valley), and a relatively modest 1.3% in the already significantly built-out Wilmington subbasin, which is estimated at 45% impervious cover as of 2021.

West Caln Township in the Birch Run subwatrshed

Newlin Township in the West Branch Brandywine Creek watershed

Minch Park in Parkesburg Borough in the Buck Run subwatershed

Figure 4: Land Cover in the Brandywine Watershed, 2021

Figure 5: Impervious Cover by Subwatershed in the Brandywine Watershed, 2021

Figure 6: Change in Developed Land, 2001 – 2021, Brandywine Watershed

Chapter 3

Historic Flooding Challenges

According to a USACE report from 1962, records of flooding in the Brandywine Valley date back to January 1839. During that winter storm, the main stem of the Brandywine rose dramatically, and all but two of the bridges across the creek were swept away. Since then, dozens of flood events have impacted communities across the watershed. There have been 17 separate instances where flood waters peaked above the National Weather Service’s “Major Flood Stage” level since 1915, including 8 in the past 26 years. Tropical Storm Agnes produced over 7 inches of rain on the area between June 20 and 25, 1972, and resulted in a flood crest elevation of 167.0 feet (which is a stream height of 16.56’ at the gage) in the Brandywine Creek at Chadds Ford, PA. Prior to Agnes, the previous flood of record occurred on March 5, 1920, and

had a crest elevation of 165.5 feet (or 15’ at the gage). Agnes was considered the highest flood of record in the watershed until Hurricane Floyd in 1999, which was ultimately eclipsed by Hurricane Ida in 2021.

The graph in Figure 7 illustrates how the “Flood of Record” has changed over the past century at the Brandywine Creek at Chadds Ford. In 1915, flood waters reached a height of 14.7 feet measured at the gage, which is 1.7 feet above Major Flood Stage, and the elevation where U.S. Route 1 and PA Route 100 are closed. Since then, 18 separate storms have peaked above Major Flood Stage, with the Flood of Record being revised four times and the largest single increase due to Hurricane Ida in 2021.

= Major Flood Stage

= U. S. Route 1 and Brandywine River Museum take on water.

Figure 7: Flood of Record and All Storms above Major Flood Stage (13.0’) at the Brandywine Creek at Chadds Ford, PA

In response to severe storms (as well as growing water supply demands and drought concerns), in the 1950s, local, state, and federal partners active in the watershed collaborated on the development of the Brandywine Watershed Work Plan. The most significant outcomes of the plan’s implementation include the construction of five major flood control structures in the upper reaches of the watershed between 1971 and 1996:

• Robert G. Struble, Sr. Dam and Regional Flood Control Facility - built in 1971 on the East Branch Brandywine Creek in Honey Brook Township.

• Marsh Creek Reservoir and State Park - built in 1973 on Marsh Creek in the East Branch Brandywine watershed in Upper Uwchlan Township.

• Beaver Creek Regional Flood Control Facilitybuilt in 1975 on Beaver Creek in the East Branch Brandywine watershed in East Brandywine Township.

• Barneston Regional Flood Control Facility - built in 1983 on the East Branch Brandywine Creek in Wallace Township.

• Hibernia Regional Flood Control Facility - built in 1994 on Birch Run in the West Branch Brandywine Creek watershed in West Caln Township.

Combined, these structures provide 5.5 billion gallons of total flood storage capacity in the upper portions of the watershed. Though these structures provide significant protection for downstream communities during storms, they are not a cure-all, particularly as they only manage water from the drainage area above the structure itself.

Barneston Dam, which is maintained as a dry dam, impounded an estimated 277 million gallons during Hurricane Ida.

Hibernia Dam under construction in 1994.

Since the final flood control structure was built in the mid-1990s, numerous floods have negatively impacted communities across the watershed. These include, but are not limited to:

• Hurricane Floyd in September 1999

• Multiple severe storms in July 2003

• Tropical Storm Henri in September 2003

• Tropical Depression Ivan in September 2004

• Tropical Depression Frances in September 2004

• Hurricane Jeanne in September 2004

• Hurricane Katrina in September 2005

• Severe storms in June 2006

• Hurricane Irene in August 2011

• Tropical Storm Lee in September 2011

• Hurricane Sandy in October 2012

• Hurricane Ida in September 2021

What does the term "100-year flood" mean?

It doesn’t actually refer to a flood that can happen only once every 100 years. In fact, 100-year flood could happen 2 years in a row.

The term, 100-year flood, is used to describe the extreme nature of the flood and help us to compare how common some floods are annually. When we say that a recent flood was a 100-year flood, what we are really saying is that it is a statistically rare flood.

Some scientists prefer to say it is a 1-Percent Flood, to remove the confusion with how frequently it could occur.

For the Brandywine watershed, a 1-Percent Flood may be caused by approximately 5-inches of rain in 6 hours or 7.5 inches of rain in 24 hours.

Flooding along the Brandywine in Birmingham Township.

Photo Courtesy of Chester County Planning Commission.

Hurricane Ida’s Devastation

The results of Hurricane Ida were catastrophic for many communities in the central and lower portions of the watershed. In some cases, individual recovery efforts are still ongoing. These impacts were the catalyst for this study, to provide recommendations for communities to be better protected and prepared for future storms.

Hurricane Ida made landfall in Louisiana on Sunday, August 29, 2021, with winds of up to 150 miles per hour (mph). The storm’s remnants reached the Brandywine watershed three days later on Wednesday, September 1, 2021. Rainfall intensity varied throughout the watershed, as well as over the course of the day when Hurricane Ida passed over the watershed. Ida dropped 7.3 inches of rain at the City of Coatesville and 8.2 inches at Downingtown Borough over the duration of the storm (see Figure 8). However, most of the rainfall occurred in a 6-hour window. The maximum rainfall recorded in the 6-hour timeframe at the USGS gage in Modena Borough exceeded NOAA’s estimate for the 1,000-year storm (6.93 inches in 6 hours). Most other sites in the upper Brandywine Creek watershed exceeded the 200-year event.

While rainfall totals during Ida were less in the lower Brandywine watershed, flooding in the upper reaches was exacerbated by the inherently steep Piedmont topography, which creates a funnel-like shape in the watershed closer to the Pennsylvania/Delaware state line. Floodwaters overtopped the USGS gage at Chadds Ford in the early morning hours of September 2, 2021. USGS used high water marks and other data to determine that Ida’s peak discharge at that location was roughly 49,000 cubic feet per second (cfs). Based on that estimate, this would represent approximately an 800year flood event (Stuckey et.al., 2023) and the highest flood recorded at the site in two centuries. Though the wider, flatter floodplains in southern Chester County were able to attenuate some of the flood waters, the peak flow in the City of Wilmington reached 33,700 cfs on September 2, 2021 (see Figure 9). This is the highest flood discharge on record along the Brandywine Creek at Wilmington dating back to 1946, surpassing Hurricanes Agnes (29,000 cfs) in 1972 and Floyd (28,700 cfs) in 1999.

Severe flooding impacted Northeast Wilmington following Hurricane Ida. Photo courtesy of City of Wilmington Department of Public Works

Figure 9: Peak Flow Rate at the Stream Gage for the Brandywine Creek at Wilmington, 1947 – 2024

Since 1947, the Brandywine Creek at Wilmington, DE never exceeded 22,000 cfs until Hurricane Agnes in1972 which resulted in flood flows of 29,000 cfs. However, flood flows from Hurricane Ida set a new record and were estimated by the U. S. Geological Survey to be 33,700 cfs.

Figure 8: Precipitation Totals and Distribution throughout the Watershed during Hurricane Ida, September 2021

The University of Delaware's Delaware Environmental Observing System (UDELDEOS) has multiple gages within and surrounding the Brandywine Watershed. The UDEL-DEOS precipitation gages recorded as much as 8.2 inches of rain from Hurricane Ida in the upper portion of the watershed and a comparatively low 2.35 inches at the southern tip of the watershed in Wilmington.

Operation of Flood Control Dams

As part of the modeling evaluation of Hurricane Ida, the project team reviewed the performance of the five flood control dams in the upper portion of the Brandywine Creek watershed. The evaluation was used to assess whether the dams operated as designed, whether the dams reduced flood flows to downstream communities, and whether this report should recommend changes to the operation of the dams in the future.

Based on this evaluation, it was determined that all five dams operated as designed, detained floodwaters from their contributing watersheds, and reduced flood flows to downstream communities.

Marsh Creek Dam is the largest of the five flood control dams in the Brandywine watershed with a normal reservoir storage of 4.5 billion gallons and additional flood storage capacity of 1.15 billion gallons to the crest of the auxiliary spillway. There are two USGS stream gages on Marsh Creek, one above the dam and the second below the dam, as well as a stream gage on the

East Branch Brandywine Creek, downstream of the confluence of Marsh Creek. These gages, as well as plans from the design data from the construction of the dam, allowed the project team to develop a hydrologic model to estimate flood detention and compare the inflow of flood waters with the outflow from the dam during and after the storm.

Prior to Hurricane Ida reaching Pennsylvania, a water release was made from Marsh Creek Dam beginning at 11a.m. on August 30 and ending at 11a.m. on August 31 in anticipation of the storm. As documented by the USGS stream gages below Downingtown, at Chadds Ford, and at Wilmington, the water released from Marsh Creek Dam on August 30 and 31 completely flowed past the USGS gage at Wilmington by 5am on September 1, and the water levels returned to low levels before Hurricane Ida reached the watershed.

Figure 10: Peak Flow Rates into and out of Marsh Creek Dam during Hurricane Ida.

During Hurricane Ida, the peak flow rate into the reservoir was estimated at 4,130 cfs (or 110 million gallons per hour) based on the hydrologic model (see Figure 10). The model estimated a peak outflow rate from the dam at 370 cfs. This was verified by the USGS stream gage below the dam (USGS ID 01480685), which recorded a peak flow rate from the dam at 380 cfs (or 10 million gallons per hour). The flood storage provided by Marsh Creek Dam during Hurricane Ida significantly reduced flooding in downstream communities, including Downingtown, Chadds Ford, and Wilmington.

Figure 11 shows the Beaver Creek Dam in East Brandywine Township in the East Branch Brandywine Creek watershed following Hurricane Ida on September 2, 2021. The dam was completely dewatered prior to Hurricane Ida during construction of a rehabilitation project. During Hurricane Ida, the flood detention area upstream of the dam filled with over 163 million gallons of flood waters, which were detained and then released slowly for the following three days.

Figure 11: Flood Waters Detained by Beaver Creek Dam during Hurricane Ida. Photo courtesy of KC Construction Co.

Marsh Creek Dam is a multi-purpose dam which provides flood storage, water supply and recreation benefits. Photo courtesy of Chester County Planning Commisssion.

Areas of Recurrent Flooding

The Brandywine Creek and its tributaries typically experience out-of-bank flooding approximately once every 3 to 5 years. Areas of recurrent flooding have been identified through the use of hydrologic and hydraulic (H&H) computer models, TR-55 and HEC-RAS, to characterize existing conditions and future flood scenarios (incorporating climate change and development projections) within the watershed. The project team also examined the FEMA floodplain maps and profiles, news media reports, and published reports by the USCOE, USGS, and others. The analysis identified 22 specific flood hazard areas: 5 along the main stem of the Brandywine (BR), 5 along the East Branch (EB) and West Branch (WB), and 12 sites along the tributaries (TY) (see Figures 12 – 14).

Birdell Road in Honey Brook Township.

Flood waters from Hurricane Ida in 2021 at Brecks Mill in New Castle County, DE.

Figure 12: Flooding Hazard in the Main Stem Brandywine Creek Watershed

Figure 13: Flooding Hazard in the East Branch Brandywine Creek Watershed

Figure 14: Flooding Hazard in the West Branch Brandywine Creek Watershed

Chapter 4

Engaging the Public and Key Watershed Stakeholders

From the commencement of the study, robust public engagement was a priority, to ensure that the public’s experience was documented and incorporated. The study aimed to offer diverse engagement options to receive feedback directly from the communities that experience flooding impacts.

Initial Public Outreach Efforts

Aligned with the existing local efforts to address flooding within the community, multiple locations around the watershed were identified, so public meetings were easily accessible to community members. Public meetings were held in the City of Coatesville, Downingtown Borough, Chadds Ford Township, and the City of Wilmington. These sessions combined informational presentations with active listening and information gathering from attendees.

Additional methods of encouraging public engagement in the planning process included:

Public Outreach Events – Flood Study team members attended over 38 events, some hosted directly by the Flood Study team and other events hosted by municipal or nonprofit entities. Through these events, information about the study was presented to over 1,000 individuals throughout the bi-state region. These events incorporated presentations, flyers, active tabling, focused meetings, and targeted discussion regarding the Brandywine Flood Study, as well as access to the Public Survey and Interactive Flood Map.

Flood Study Website – The study launched a website that has been an essential tool for keeping partners and the public informed and updated throughout the study. The Brandywine Flood

Study website (www.brandywine.org/flood-study) (Technical Compendium Section 6) included: links to the Public Input Survey and the Interactive Flood Map, frequently asked questions (FAQ); a link to the Flood Study Communications Toolkit, and previous and upcoming public meetings, among other information and resources.

Flood Study Communications Toolkit – A toolkit was published via the website, including resources that partners and the public could use to promote and increase engagement with the study from residents and on social media. The toolkit included a general information flyer, a flood study survey flyer with a QR code that linked directly to the survey, a sample flood study article, and flood study graphics, including partner logos and a geographic coverage map.

Survey for Public Engagement and Feedback –A 22-question survey was distributed by partners through public platforms such as Facebook, the Brandywine Flood Study website, five public meetings, and 35 public outreach events. The survey garnered 175 responses, and select questions were extracted and developed into posters for interaction with attendees at the public meetings. The survey results and comments are included in Section 6 of the Technical Compendium.

Ford, PA on February 8, 2024.

Interactive Public Input Web Map – Available on the Brandywine Flood Study website, the interactive map provided a platform for community members to report and view flood-related issues in the Brandywine watershed (see Figure 15). Users could mark locations of concern, such as flood damage, infrastructure problems, or environmental impacts, on a shared map. Users could also directly add photos, videos, and comments to the marked locations. This tool encouraged collaboration and transparency by integrating public observations into the flood study, helping prioritize mitigation efforts (Technical Compendium Section 6).

Left Photo: Residents mark flooding locations and discuss concerns with a member of the flood study team at a public meeting in Chadds

The Flood Study team member updated residents at a public meeting in Coatesville City on March 5, 2024.

Figure 15 Public Input Web Map

Only 28% report having flood insurance coverage

79% have had flooding greatly affect their ability to travel for work, recreation, and essential services

26% have experienced flood damage to their homes and 37% experienced damage to other private property

51% have experiences property damage due to flooding

More than 66% have experienced some degree of financial losses due to flooding

95% are worried about increased frequency and/or intensity of future flooding

Media and Press Coverage – Flood study efforts were supported by significant coverage from 20 local and regional media outlets, highlighting the study’s crucial role in addressing flooding issues in the region and emphasizing the collaborative nature of the study among local governments, conservation organizations, and academic institutions.

Advisory Committee – An Advisory Committee was established at the start of the project and open to any interested stakeholders. Five meetings of this group were held, attended by representatives and specialists from municipalities, community groups, conservation organizations, county departments, federal and state agencies, water utilities, consultants, and other stakeholders. The Advisory Committee participants were engaged in this capacity as technical experts and stakeholders.

The Flood Advisory Committee holds a working meeting in May 2024.

Summary of Public Comments

Through vigorous and sustained efforts to engage the public and gain insight into localized flooding impacts, the flood study survey, the Interactive Flood Map, ongoing public meetings, and continued initiatives to participate in public events, this study engaged with over 1,500 individuals to gather public comments and reports across all mediums. All findings from the public engagement efforts are included in Section 6 of the Technical Compendium. Below is a summary of representative public comments by prominent themes.

Communication and Safety

• Increased local notification systems and frequency of notifications before and during severe precipitation events.

• Uniform, simplified messaging about flood forecasts, risks, etc.

• More tools to make historical and projected flood information available, meaningful, and actionable for the general public.

• Additional support to emergency services to assist vehicles in unsafe flood situations.

• Additional and more rapid installation of barricades, signage, and communication about blocked or closed roadways prior to and during flood events.

Structural Improvements

• Green stormwater infrastructure installed to manage areas where impervious surfaces cannot be removed (roadways and existing development), coupled with education, signage, and green stormwater infrastructure and landscaping installation guides for home and business owners.

• Ensuring disadvantaged communities are not left behind in future flood mitigation efforts.

• Evaluation and repair of municipal stormwater and sewer infrastructure systems.

• Evaluation, repair, and retrofits made to roadways, bridges, and culverts that experience frequent flooding.

Non-Structural Actions

• Additional municipal comprehensive planning and required review of stormwater management plans with each development application.

• Prioritization of open space preservation in headwater regions, as well as flood prone areas.

• Enhancement of standards in, and enforcement of, flood-specific zoning ordinances for both proposed and existing development.

The feedback collected throughout the planning process underlines the interconnectedness of individual, community, and systemic responses in addressing flooding challenges. It also highlights the public’s desire for collaborative, cost-effective, and environmentally sustainable solutions.

Additional Public Outreach

Brandywine Flood Study partners gathered with local watershed stakeholders at the Celebrating Successes & Flowing Forward: Brandywine-Christina Watershed Conference on January 16, 2025. This was a one-day conference celebrating the William Penn Foundation’s Delaware River Watershed Initiative (DRWI) in the Brandywine-Christina watershed. The last session of the conference featured the official release of the first draft of the Brandywine Flood Study. Simultaneously, drafts of the report and its extended technical version were released via the study website and made available for a 45-day public comment period.

During the 45-day comment window, six public meetings were held in various locations across the watershed. Two

“Notification of roads that are shut down due to flooding--better support on the highways when flooding is happening to get vehicles off the road as quickly as possible.”

“Storm water management must include adequate, functioning stormwater retention basins.”

“Can we require developer regulations to be more stringent than the current 100-year flood maps?”

meetings were held in Wilmington, Delaware, at the Police Athletic League and the Delaware Center for Horticulture. Public meetings were held in Pennsylvania at the Coatesville City Hall, Downingtown Borough Hall, the Chadds Ford Township building, and the Brandywine Museum of Art. These meetings consisted of presentations by study partners on the findings of the report, including recommendations for structural and non-structural methods for flood mitigation.

In-depth discussion followed with a formal questionand-answer session, as well as opportunities for attendees to speak directly with study partners, review discussion questions, and leave comments on posters within the room. One hundred thirty-four individuals attended at least one of the six meetings. Feedback from both this suite of public meetings and the online comment solicitation form were reviewed and incorporated into this final report.

Municipal Engagement

A thorough outreach effort was conducted in all municipalities within the Brandywine watershed, in order to incorporate feedback from local officials and staff. Dozens of municipalities were engaged through individualized meetings between flood study partners and municipal representatives. The roles and responsibilities of these representatives varied, encompassing municipal managers, elected officials, directors of public works, directors of emergency services, and staff or consultant engineers.

Outreach meetings were conducted from September 2023 through December 2024 and were organized around a set of guiding questions regarding specific impacts and locations of flooding to identify potential solutions.

A member of the Flood Study Team provides an update on the draft report at a public meeting held in March 2025 in Chadds Ford.

Standard topics of conversation included flood-related road closures and evacuation routes, existing stormwater basins, and infrastructure issues such as planned or expected retrofitting of culverts, bridges, and roadways. Emergency notification systems were identified, as well as popular methods of municipal communication, such as email, text, or social media alerts, and newsletters for residents. Planned or in-progress efforts related to the municipality’s MS4 permit or the County Hazard Mitigation Plan were reviewed, along with any recently completed stormwater or flood mitigation projects. In addition, long-term flood resiliency, potential climate mitigation plans, and conservation easements were discussed.

Technical Advisory Committee

At the outset of the study, a Technical Advisory Committee was convened to supplement the expertise, local knowledge, and public connections of the Flood Study partners. The committee was open to all watershed stakeholders, and included representatives from municipalities, academic institutions, local community organizations, private businesses, and government agencies, as well as individuals involved with other flood study projects in the region. The following organizations were engaged through the committee:

• 2nd Century Alliance

• Arcadis

• Brandywine Red Clay Creek Alliance

• Brandywine River Restoration Trust

• CDM Smith

• Center for Watershed Protection

• Chadds Ford Township

• Chester County Conservation District

• Chester County Department of Emergency Services

• Chester County Planning Commission

• Christina Watersheds Municipal Partnership

• City of Coatesville

• City of Wilmington

• Collaborate Northeast

• Delaware County Department of Emergency Services

• Delaware County Heritage Commission

• Delaware County Planning Commission

• Delaware Department of Natural Resources and Environmental Control

• Delaware Nature Society

• Delaware Sea Grant

• Downingtown Borough

• Downingtown Flood Advisory Committee

• Eleventh Street Bridge Community Long-Term Recovery Group, Inc.

• Gaadt Perspectives, LLC

• Gannett Fleming

• Greater Wilmington Housing Providers

• Green Building United

• Hagley Museum and Library

• Kirkwood Community Center

• Natural Resources Conservation Service

• Partnership for the Delaware Estuary

• Pennsbury Township

• Pennsylvania Department of Conservation and Natural Resources

• The Nature Conservancy, Pennsylvania/Delaware Chapter

• United States Geological Survey

• University of Delaware Institute for Public Administration

• West Chester University Department of Geography and Planning

• Wilmington Area Planning Council

The Advisory Committee was regularly engaged throughout the study process to provide guidance on both the technical analyses and the process of public engagement.

Chapter 5 Summary of Technical Analyses

The Brandywine Flood Study benefitted from a strong technical team with experience in multiple tools that provided perspective on the interaction of the land cover with the streams and floodplains, the operation and function of the five flood control dams, and how floodwaters move quickly and with great force through the steep valleys and then spread out as slower mowing floodwaters ranging over wide areas.

To further understand flooding in the Brandywine Watershed, the project team used:

• Hydrologic and hydraulic (H&H) computer models to characterize existing conditions and future scenarios in the watershed to further identify flood risk areas in the watershed.

• Future build-out assessment to better understand how future development scenarios might impact flooding events.

• Climate impact analysis to incorporate potential changes in rainfall and streamflow into H&H modeling of streams to evaluate future risks.

Left: The West Branch Brandywine Creek as it enters City of Coatesville, PA.

Above: Housing construction underway in Chester County.

Hydrologic and Hydraulic Models

The project team used the USDA Technical Release 55 (TR-55) hydrologic model and ArcView GIS to estimate rainfall-runoff relationships in the 320-square mile Brandywine watershed in Pennsylvania and Delaware. This model includes land cover, soils, topography, and slopes to determine flood waters during different sized storms, including major storms such as Hurricane Ida. Actual measurements from USGS stream gages throughout the watershed were used to refine and validate the hydrologic model results.

The project team used HEC-RAS, a hydraulic model developed by the USACE, and USGS LIDAR elevation data to examine riverine flooding and evaluate existing conditions and proposed flood mitigation strategies. Existing FEMA HEC-RAS hydraulic models were available for: (1) Brandywine main stem in Delaware; (2) Brandywine main stem in Pennsylvania; (3) East Branch Brandywine near Downingtown Borough; (4) West Branch Brandywine near Embreeville and the City of Coatesville; and (5) Beaver Creek, a major tributary that flows into the East Branch Brandywine at Downingtown.

Photos identifying high water marks during Hurricane Ida proved valuable in verifying and calibrating the USDA TR-55 hydrologic and FEMA HEC-RAS hydraulic models (see Figures 16 and 17).

Figure 17: A USGS scientist marks the high water mark from Hurricane Ida. Photo by U.S. Geological Survey.

This scientist from the Pennsylvania Water Science Center measures the height above ground of a seedline high water mark left on a barn near Chadds Ford, PA. The property owner marked high water marks from previous flooding, visible in the left half of the picture.

The Northeastern United States has already seen a greater increase in extreme precipitation than any other region, with a 60% increase in storms between 1958 and 2022.

Figure 16: High water reference mark.

Photo by U. S. Geological Survey

Watershed Build-Out Assessment

To better understand how future development scenarios might impact flooding events, a watershed-wide build-out assessment was completed. A build-out analysis predicts the development potential allowed by existing zoning regulations and can demonstrate the strengths and weaknesses of existing municipal regulations. Build-out analyses can be effective tools for demonstrating the importance of strong natural resource ordinances, appropriate zoning, and proactive land preservation efforts at the municipal, landscape, or watershed level. Not only can they reveal the effectiveness of existing zoning in guiding future development, but they also highlight land that may be more appropriate for land preservation. For the Brandywine Flood Study, a build-out analysis was utilized to assess where in the watershed development may occur, and how land use changes during development may impact stormwater runoff volumes, leading to potential increases in flooding conditions.

The build-out analyses provide a clear picture of the development potential over the entire watershed. If all parcels are developed according to the existing zoning, an estimated 25,760 acres of impervious surface could be added to the watershed and approximately 23,747 new parcels could be added through subdivisions. This development activity could also cause the loss of about 2,066 acres of woodland and an estimated 16,319 acres of agricultural land (Table 2).

Climate Impact Analysis

As temperatures increase, the atmosphere can hold more water vapor, leading to a greater potential for precipitation. There have already been noticeable changes in precipitation patterns over the past 20 years compared to the prior century, and even more changes are projected in this century. These increases in rainfall exacerbate flood risk, which already has caused billions of dollars in damages across the U.S. in recent years. Thus, it is critical for flood studies to incorporate potential changes in rainfall and streamflow into hydrologic and hydraulic modeling of streams to evaluate future risks.

Precipitation change factors estimate the increase in precipitation that may occur in the future for storms of varying frequency and duration. The study incorporated two different methods to calculate precipitation change factors for the Brandywine watershed, which are documented in Section 3 of the Technical Compendium.

The TR-55 hydrologic models discussed previously were used to estimate peak discharge for the 2-, 10-, 50-, 100-, 500-, 800- (Ida), and 1000-year flood events for four modeled scenarios:

1. Existing 2020 land use conditions.

2. Existing land use conditions with climate change induced increase in precipitation.

3. Full build-out land use conditions with existing precipitation.

4. Full build-out conditions with climate change induced increase in precipitation.

Potential Impervious Coverage 25,760 acres

Number of Additional Parcels 23,747 parcels

Potential Woodland Loss - 2,066 acres

Potential Loss of Agricultural Lands -16,319 acres

Based on climate change and warming of the atmosphere, the 1% chance storm, which is currently 7.6 inches, is projected to increase to 8.3 inches in 2050. The estimated increased precipitation volume due to climate change ranges from 8% to 10% for all of the modeled storms.

Table 2: Maximum Development Impact

Chapter 6

Structural Recommendations

To address the scope and breadth of flooding challenges in the Brandywine watershed, a variety of structural improvements are recommended. Potential improvements vary in terms of scale, complexity, capacity, and expense, but each can play a role in mitigating the impacts of flooding in local communities. Once in place, these structural improvements typically provide relatively immediate relief from flood risk to both people and property by physically manipulating the way water moves through the landscape. However, it is important to note that many of the structural improvements described here must be considered in terms of timeframe for implementation and their potential impacts both up- and downstream, as to whether alleviation of flooding in one community may or may not exacerbate flooding in another.

The types of structural flood mitigation projects considered in this study include:

• Modifications to existing flood control facilities and reservoirs.

• Replacement, rehabilitation, or removal of bridges, culverts, or dams.

• Floodplain restoration.

• Flood control dry dams.

• Stormwater basin retrofits.

What does the term “peak flow rate reduction” mean?

In engineering and scientific terms, stream flow is often measured by the volume of water flowing past a location over a period of time. Common approaches are to measure the flow rate in million gallons per hour, or cubic feet per second (cfs). In this example, 1 million gallons per hour is around 45 cubic feet per second.

This Brandywine Flood Study prioritized finding improvements that reduce the rate of flood waters since reducing that rate is critical to both how high the flood waters rise to and how frequently we see flooding events.

Potential flood mitigation strategies were identified in each of the categories listed above. Of the numerous sites assessed, Table 3 and Figure 18 identifies those that this study recommends be pursued for their potential flood mitigation benefits. Generally, flood storage capacity, peak flow rate reduction, and water surface elevation reduction were the primary factors that determined the study’s recommendation for each site. Each category of structural improvements incorporated additional evaluation criteria, including existing and potential future risk to vulnerable populations and historically marginalized communities.

Right Photo: Bridge on U. S. Route 30/Lancaster Avenue over the East Branch Brandywine Creek in Downingtown Borough.

Left Photo: Low head dam and U. S. Route 1 Bridge in Chadds Ford Township and Pennsbury Township.

Table 3: Primary Structural Recommendations for Flood Mitigation

Figure 18: Brandywine Flood Study Structural Recommendation Sites

Evaluation of Existing Flood Control Facilities

Historic floods along the Brandywine in the first half of the 20th century drove initial flood control strategies identified in the Brandywine Watershed Work Plan. Beginning in the 1950s, the Brandywine Creek Watershed Work Plan outlined 12 flood control priorities, including multipurpose reservoirs, flood control structures, and forest and agricultural conservation measures to increase infiltration and reduce sedimentation.

After several amendments to the Plan, the 5 projects shown in Figure 19 were constructed in the upper Brandywine watershed between 1970 and 1994:

• Struble Dam - flood control, fishing

• Barneston Dam - flood control

• Marsh Creek Dam - water supply, flood control, recreation, flow augmentation

• Beaver Creek Dam - flood control

• Chambers Lake/Hibernia Dam on Birch Runwater supply, flood control, recreation

The Brandywine Flood Study conducted an initial assessment of additional flood storage potential at these 5 existing dams.

The 1952 Work Plan for the Brandywine Creek outlined 12 flood control priorities

and recreation.

Chambers Lake/Hibernia Dam at Hibernia County Park in West Caln Township is a multi-purpose dam that provides flood control, water supplies

Figure 19: Flood Control Facilities in the Brandywine Watershed

Barneston Dam Rehabilitation

Barneston Dam, one of the five flood control dams in the Upper Brandywine Creek watershed, is located in Wallace Township across the East Branch Brandywine Creek. The dam was built in 1983 by CCWRA and USDA Soil Conservation Service, now the Natural Resources Conservation Service (NRCS). Barneston Dam is owned and operated by CCWRA and provides flood protection for residents in Chester County. It is 43 feet high and is maintained as a “dry” dam, which means there is no lake or impoundment behind the dam during normal, sunny days. However, during storm events the dam detains floodwaters behind it, mitigating downstream flooding on the upper portion of the East Branch Brandywine Creek. The discharge of floodwater is constricted by the small size of the culvert spillway at Barneston Dam. This principal spillway is a 4-foot by 4-foot box culvert at the same elevation as the stream.

The dam has two additional spillways, a 240-foot-wide concrete drop spillway approximately 33.5 feet above the stream, and a vegetated auxiliary spillway that is 39.5 feet above the stream. The flood storage pool to the crest of the concrete drop spillway is approximately 1,520 acre-feet (or 495 million gallons). From 1983 through 2024, there has never been any flow through these overflow spillways — all flood waters have been detained and routed through the 4-foot by 4-foot box spillway.

An initial engineering review and modeling evaluation identified an opportunity to reconstruct the 240-footwide concrete drop spillway with a labyrinth weir, which can provide additional flood storage between the 50-year

and 100-year storm. By modifying the concrete drop spillway, the elevation of the weir may be elevated by a few feet to provide additional storage for large storms, while still meeting state and federal regulations for dam safety. This modification will not reduce flood waters downstream of the dam for smaller storms, such as the 5-year, 10-year or 25-year events. However, for very large storms, the 50-year, 100-year, and 500-year storm events, this modification could either fully control or at least delay any additional flood waters continuing in the East Branch Brandywine Creek (see Table 4).

Beaver Creek Dam – Change of Operation to Provide Additional Flood Storage

Beaver Creek Dam was operated from 1992 to 2020 with an impoundment of approximately 7.2 million gallons within an 11-acre sediment pool. During rehabilitation work on the dam from 2021 to 2022, the reservoir was drained. As of 2024, the Chester County Water Resources Authority has submitted an application to the Pennsylvania Department of Environmental Protection (PADEP) to change the operation of Beaver Creek Dam to a dry dam. This change in operation will provide additional flood storage upstream of the dam embankment, where the normal impoundment has been dewatered. The change in operation also provides environmental benefits for wetlands and native wildlife habitat. As a result of this rehabilitation project and operational change, Beaver Creek Dam meets current Pennsylvania Dam Safety standards and restricts discharge up to the 100-year storm, so no additional modifications to the structure are recommended at this time.

Left: Barneston Dam in Wallace Township on the East Branch Brandywine Creek is a dry dam with a standard weir control on the auxiliary spillway.

Right: A labyrinth weir constructed at Lake Williams Dam in York, PA. (Photo courtesy of Gannett Fleming)

Table 4: Preliminary Analysis of Proposed Labyrinth Weir Spillway and Drop Spillway Structure, Peak Outflow in CFS

* The slightly higher out flow during PMP events ensures that the water surface elevation within the reservoir remains under the top of the dam and significantly reduces the likelihood of dam failure during extreme flood events.

Review of Other Dams and Reservoirs

Additional review is proposed for dam modification potential and/or operations at Struble Dam, Marsh Creek Dam, and Rock Run Dam. Struble Dam is owned and operated by Chester County Water Resources Authority, and the lake is managed by Pennsylvania Fish and Boat Commission. Marsh Creek Reservoir and Dam are owned and operated by the Pennsylvania Department of Conservation and Natural Resources. Rock Run Reservoir and Dam, owned and operated by Pennsylvania American Water Company, is a water supply reservoir. The project team will coordinate with the responsible agencies for review of dam operations.

A rehabilitation project was completed at Hibernia Dam, owned and operated by the Chester County Water Resources Authority, to meet current Pennsylvania Dam Safety standards in 2020. As the dam currently restricts discharge up to the 100-year storm, additional modifications to the structure are not recommended at this time.

While the Icedale Dam on the upper West Branch originally proposed in the 1952 Brandywine Watershed Work Plan was not built (Chambers Lake/Hibernia Dam was built instead), a smaller dam was ultimately constructed on the site. This dam, which is owned by the Pennsylvania Fish and Boat Commission, has been breached for several years and West Brandywine Township is in the process of replacing the bridge just

downstream from the dam. This study conducted initial assessments of the flood control potential at the dam, and though no currently viable opportunities were identified, future analysis may be warranted.

Beaver Creek Dam in East Brandywine Township before (with a 7.2 million gallon pool) and after (operating as dry dam with no permanent pool) the rehabilitation project, completed in 2022.

Bridges, Culverts, and Dams –Stream Crossings

With the abundance of streams in the Brandywine watershed are the numerous bridges, culverts, and dams along the Brandywine and tributaries that raise flood elevations during storms because the hydraulic openings of these structures are too small to convey larger flows. In many cases, flood levels back up upstream, overtop bridge decks, and take transportation arteries out of service. Addressing inadequately sized infrastructure generally requires rebuilding the structures with larger waterway openings, or removing obstructions such as low head dams that are not being maintained or utilized. The replacement of bridges and culverts due to age, natural hazard damage, or other factors can provide opportunities for flood mitigation by reconstructing inadequately-sized structures to support flood reduction objectives.

However, care should be taken when considering the upstream/downstream impacts of any structural project. Any existing waterway obstruction may provide downstream flood retention benefits, and so projects should be modeled and sequenced in such a way as to minimize any downstream increases in flood volume or discharge rates.

Through the use of H&H computer models and 2017 FEMA Flood Insurance Study (FIS) flood profiles, this study identified bridges, culverts, and dams throughout the watershed that hypothetically raise flood elevations for the 10-, 50-, 100-, and 500-year floods. The analysis examined close to 300 bridges, culverts, and dams along the main stem, East and West Branches of the Brandywine, and tributaries in Pennsylvania. A comprehensive list of all

analyzed structures and the study’s associated findings are available in Section 8 of the Technical Compendium.

As shown in Table 5, of the 269 structures reviewed, 119, or 44%, were found to be hydraulically undersized. This means that floodwaters are potentially obstructed by and/or backed up behind these structures. HECRAS computer models were only available for 122 of the 269 structures along the main stem, East Branch, West Branch, and Beaver Creek. Of those, 13 (11%) were found to create increased upstream water surface elevations during floods that could result in meaningful risks to nearby residences or infrastructure. An additional 25 structures were found to be hydraulically undersized, but analysis was inconclusive about any impacts on surrounding communities.

Of the 147 structures for which only FEMA FIS profiles were available, 81 (54%) were found to be hydraulically undersized. However, without computer modeling capabilities, the study was unable to fully assess the impact of the structure on localized or regional flooding. One exception is the Rockland Road bridge/Dam No. 11 along the main stem in northern Delaware, for which models are currently being developed and plans are underway to remove the dam.

While some of these may be technically undersized to pass larger flood volumes, they may not create additional risk to people or buildings, and therefore may not be a priority for retrofitting. In fact, in some cases, these obstructions, typically in undeveloped areas, may provide flood control benefits for downstream communities by holding back and slowing the movement of flood waters.

Bridge for East Glencrest Rd over the West Branch Brandywine Creek in Valley Township.

Bridge on Lloyd Avenue over Beaver Creek in Caln Township

Table 5: Hydraulically Undersized Bridges, Culverts, and Dams in the Brandywine Watershed

The hydraulically undersized structures in the Brandywine watershed that were identified as having meaningful flood mitigation potential and are recommended for further analysis related to retrofits, reconstruction, or removal by this study are listed in Table 6.

A brief description of each site is included in the section that follows.

Table 6: Bridges, Dams, and Culverts in the Brandywine Watershed Identified for Structural Flood Mitigation Potential

Main Stem Brandywine Creek

U.S. Route 13/Northeast Boulevard Bridge –Wilmington, DE

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 1.9 and 2.9 feet respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of roughly 0.75 feet for nearby structures during the 100-year flood.

Figure 20 shows modeled conditions for the 100-year flood if the bridge were replaced. The green represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Bancroft Dam No. 4 – Wilmington, DE

HEC-RAS models indicate that the existing dam elevates the immediate upstream WSE for the 100- and 500-year floods by 4.8 and 4.9 feet respectively. Removal of the dam could result in WSE reductions of up to 3 feet for nearby structures during the 100-year flood.

Figure 21 shows modeled conditions for the 100-year flood if the dam were removed. The orange represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions

DuPont

Experimental Station Dam No.

6 – Wilmington, DE

HEC-RAS models indicate that the existing dam elevates the immediate upstream WSE for the 100- and 500-year floods by 5.2 and 2.3 feet respectively. Removing the dam could result in WSE reductions of up to 3 feet for nearby structures during the 100-year flood.

Figure 22 shows modeled conditions for the 100-year flood if the dam were removed. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Rockland Dam No. 11 – New Castle County, DE

Though HEC-RAS models are not currently available for the stream section of the main stem where Rockland Dam no. 11 is located, the 2017 FEMA FIS flood profile indicates that the existing dam elevates the immediate upstream WSE for the 100- and 500-year floods by 4.5 and 5 feet respectively. The development of a HEC-RAS model for this stream section is currently underway at the University of Delaware’s Water Resources Center to further evaluate the benefits of removing this dam on surrounding infrastructure.

Figure 20: Water Surface Elevation Model Output at U.S. Route 13/Northeast Boulevard Bridge

Figure 21: Water Surface Elevation Model Output at Bancroft Dam No. 4

Figure 22: Water Surface Elevation Model Output at DuPont Experimental Station Dam No. 6

East Penn Railroad Bridge – Chadds Ford/ Pennsbury, PA

HEC-RAS models indicate that the existing railroad bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500year floods by 3.3 and 0.7 feet respectively. Removing the bridge (or replacing it with a larger structure, if future use is planned) could result in WSE reductions of roughly 1 to 3 feet for numerous structures in Chadds Ford during the 100-year flood.

Figure 23 shows modeled conditions for the 100-year flood if the bridge and its abutments were removed. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

U.S. Route 1 – Chadds Ford/Pennsbury, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 2.4 and 0.7 feet respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of roughly 0.3 feet for a handful of nearby structures upstream of the bridge during the 100-year flood.

Figure 24 shows modeled conditions for the 100-year flood if the bridge were replaced. The pink represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

PA Dam No. 1 – Chadds Ford/Pennsbury, PA

HEC-RAS models indicate that the existing dam elevates the immediate upstream WSE for the 100- and 500-year floods by 2.2 and 0.6 feet respectively. Removing the dam could potentially improve the reductions in WSE provided by replacing the Route 1 bridge and the East Penn Railroad Bridge (which are located immediately downstream) during the 100-year flood. It would also provide environmental co-benefits, including improved fish passage.

Figure 25 shows modeled conditions for the 100-year flood if the dam were removed. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Figure 23: Water Surface Elevation Model Output at East Penn Railroad Bridge

Figure 24: Water Surface Elevation Model Output at U.S. Route 1

Figure 25: Water Surface Elevation Model Output at PA Dam 1

East Branch Brandywine Creek

Route 322 Bridge – East Bradford, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 0.7 and 1 foot respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of roughly 0.2-0.6 feet for nearby structures, including the Downingtown Area Regional Authority’s wastewater treatment plant, during the 100-year flood.

Figure 26 shows modeled conditions for the 100-year flood if the dam were removed. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Business

Route 30/Lancaster Avenue Bridge –Downingtown, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 0.3 and 0.4 feet respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of roughly 0.2-0.4 feet for upstream structures and roadways during the 100-year flood.

Figure 27 shows modeled conditions for the 100-year flood if the dam were removed. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

West Branch Brandywine Creek

Reading Wilmington Northern Railroad Bridge –Modena, PA

HEC-RAS models indicate that the existing railroad bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500year floods by 2.8 and 1.3 feet respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of roughly 2 feet for several nearby structures during the 100-year flood.

Figure 28 shows modeled conditions for the 100-year flood if the bridge were replaced. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Figure 26: Water Surface Elevation Model Output at Route 322 Bridge

Figure 27: Water Surface Elevation Model Output at Business Route 30/Lancaster Avenue Bridge

Figure 28: Water Surface Elevation Model Output at Reading Wilm. Northern Railroad Bridge

Cleveland Cliffs Rail Bridge – South Coatesville, PA

HEC-RAS models indicate that the existing railroad bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500year floods by 1.9 and 0.5 feet respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of 1 to 4 feet for nearby structures, including the campus of the Pennsylvania American Water wastewater treatment plant, during the 100-year flood.

Figure 29 shows modeled conditions for the 100-year flood if the bridge were replaced. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

East Glencrest Road Bridge – Valley, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 1.5 and 1.7 feet respectively. Replacing the bridge to remove flow obstructions could potentially reduce roadway flooding along East Glencrest Road, Manor Road, and Wagontown Road during the 100-year flood.

Figure 30 shows modeled conditions for the 100-year flood if the bridge were replaced. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Beaver Creek

Manor Avenue Bridge – Downingtown, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 1 and 2.1 foot respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of 0.4 feet for approximately 60 upstream structures, during the 100-year flood.

Figure 31 shows modeled conditions for the 100-year flood if the bridge were replaced. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

Figure 29: Water Surface Elevation Model

Output at Cleveland Cliffs Rail Bridge

Figure 30: Water Surface Elevation Model

Output at East Glencrest Road Bridge

Figure 31: Water Surface Elevation Model

Output at Manor Avenue Bridge

Lloyd Avenue Bridge – Caln, PA

HEC-RAS models indicate that the existing bridge and its abutments elevate the immediate upstream water surface elevation (WSE) for the 100- and 500-year floods by 1 and 2.1 foot respectively. Replacing the bridge to remove flow obstructions could result in WSE reductions of 0.4 feet for approximately 60 upstream structures, during the 100-year flood.

Figure 32 shows modeled conditions for the 100-year flood if the bridge were replaced. The purple represents the existing 100-year floodplain, while the blue shows the extent and depth of modeled future conditions (where darker blue indicates greater depth).

It should be noted that the bridges, dams, and culverts included in this study were assessed exclsively through H&H analyses. Other analyses, including feasibility studies and environmental assessments, would likely be required before any implementation project could proceed.

Maintaining bridges and culverts over their lifespans can be just as important as properly sizing them when they are initially installed. Both public and private structure owners should develop regular maintenance plans to ensure that issues like debris, sediment buildup, or damage do not compromise the structure’s ability to safely pass storm flows. A helpful resource to inform the development of these plans (and what may or not require a permit) is PADEP’s 2018 booklet titled “Guidelines for Maintaining Streams in Your Community” (discussed in further detail in Chapter 7).

In some cases, particularly with public bridges and culverts, this might be achievable through volunteer stream cleanup events. Within the Brandywine watershed, there are multiple community organizations and municipalities that organize and host stream clean-ups. These events are typically suitable for both adults and kids (with supervision) to engage in removing trash and other small debris that could otherwise cause jams or clogs under bridges or in culverts. These events are often hosted by environmental groups, including watershed associations and nature centers, but are also organized and hosted by homeowners associations, community and faith organizations, and municipal Parks and Recreational committees or Environmental Advisory Councils.

In addition to debris and trash removed by volunteer based stream clean-ups, there are circumstances where flood debris in the streams or floodplains is either very large, like picnic tables or building materials, or potentially hazardous, such as metal drums, fuel containers, or fertilizer/herbicide containers. For these scenarios, it is important to have qualified professionals for the retrieval, handling, and disposal of large and/or potentially hazardous debris in the floodplain that cannot be safely handled by volunteers. Part of a successful maintenance plan should include points of contact for these types of contractors so that hazards can be removed as quickly as possible after they are identified.

Figure 32: Water Surface Elevation Model Output at Lloyd Avenue Bridge

Flood debris being cleared from the inlet of Barneston Dam

Floodplain Restoration

Floodplains are nature’s buffer zones between waterways and adjacent lands. They provide space for streams to rise and spread out of their channels, naturally slowing and storing flood waters. They also offer numerous other ecological benefits, including wildlife habitat, pollution filtering, and carbon sequestration. However, development in and around floodplains over the past 100 years has greatly compromised their functionality.

Floodplain restoration at two different sites in Scotland by the Tweed Forum added meanders back into what was a straightened channel. The site at Lake Wood (upper right photo) has a re-meandered channel that increases the stream length by 30%. The two lower photos show the stream at Cringletie as it naturalized over time following the restoration of the meander. Photos courtesy of Tweed Forum, Melrose, Scotland

The project team identified and assessed numerous sites in the watershed for floodplain restoration potential. Floodplain restoration that is focused on downstream flood reduction involves providing additional storage for out-of-bank flooding in the adjoining floodplains, as well as providing for slower flow of these out-of-bank flows, so that the water does not race through the area to downstream communities. Floodplain restoration will often have small wetlands or depressions in the floodplains, which provide additional storage and even shallow groundwater infiltration following storms.

Two-dimensional (2D) HEC-RAS models were utilized to identify relatively flat and wide floodplain areas that may attenuate the flood waters along stream reaches, providing substantial flood storage. Along the 52.5 miles of the Brandywine and its tributaries, the floodplain area technically has 16.5 billion gallons of storage capacity.

Factors used to determine potentially viable projects included modeled impacts to existing nearby buildings and infrastructure (in terms of upstream and downstream water surface elevation reduction), estimated cost of grading and hauling floodplain material, and downstream peak flow and volume reduction estimates. Where available, the models used bathymetry data, bridge and dam geometry from FEMA effective HEC-RAS models. Table 7 and Figure 33 show findings from initial analyses at these sites. Additional information regarding the analysis for each site can be found in Section 9 of the Technical Compendium.

Due to the very steep banks, wide disconnected floodplain, and presence of a single landowner (the Brandywine Conservancy), the floodplain restoration project in Chadds Ford is recommended for implementation. By itself, this floodplain restoration project would likely only result in reduced flood elevations during smaller storm events. However, it would complement and enhance the impact of other recommended projects in the immediate area (including the removal/replacement of the East Penn Railroad Bridge, the replacement of the U.S. Route 1 bridge, and the removal of PA Dam no. 1), as well as ongoing efforts to improve flood resiliency of nearby structures in Chadds Ford.

The sites at Mary Street in Downingtown and at the confluence of Valley Run and Beaver Creek in Caln are recommended for further analysis to assess potential impacts on localized flooding during smaller flooding events, although they are unlikely to provide significant reductions in flood elevations or volumes for the 100-year flood or greater.

All other sites assessed showed minimal potential impact on localized and regional peak flow reduction or water surface elevation during modeled flood events.

Table 7: Initial Analysis of Potential Regional Floodplain Restoration Sites

Site

Municipality Watershed

Brandywine Conservancy Properties Chadds Ford Township Main Stem Brandywine

Icedale Lake Honey Brook and West Brandywine Townships West Branch Brandywine

Mary Street Riparian Corridor Downingtown Borough Beaver Creek

Wedgewood at Valley Run/ Beaver Creek Confluence Caln Township Beaver Creek

Brandywine Picnic Park East Bradford and Birmingham Townships Main Stem Brandywine

Caln Woodlands Caln Township Beaver Creek

Creek Road above Downingtown East Brandywine Township East Branch Brandywine

East Fallowfield Park East Fallowfield Township Dennis Run

Ingleside Golf Course Caln Township Valley Run

Johnsontown Park Downingtown Borough East Branch Brandywine

Manor Road/Kings Highway City of Coatesville West Branch Brandywine

Paradise Valley Nature Area East Bradford Township Valley Creek

Parkside Soccer Fields Downingtown Borough East Branch Brandywine

Poorhouse Road Reservoirs West Bradford Township Beaver Creek

South Coatesville on Dennis Run South Coatesville Borough Dennis Run

Tabas Memorial Park Downingtown Borough East Branch Brandywine

Floodplain restoration/ Legacy dam removal Recommended for implementation

Floodplain restoration/ Storage capacity improvements Further analysis recommended

Floodplain/ Streambank restoration Further analysis recommended

Floodplain/Stream restoration Further analysis recommended

Floodplain restoration/ Storage capacity improvements Not recommended –minimal regional impact

Floodplain/Streambank restoration Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

Floodplain/Streambank restoration Not recommended –minimal regional impact

Floodplain/Streambank restoration Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

Floodplain/Streambank restoration Not recommended –minimal regional impact

Floodplain/Streambank restoration Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

Floodplain restoration/ storage capacity improvements Not recommended –minimal regional impact

33: Sites Assessed for Floodplain Restoration Potential

Figure

Flood Control Dry Dams

Dry dams are flood control structures that only activate during flooding conditions. This means that there is no permanent reservoir or standing pool behind the dam. During normal or dry weather conditions, the stream moves naturally through the channel without any impediments to flow or fish passage. During intense precipitation, the structure of the dam holds back floodwater and allows it to be released downstream at a controlled rate. These flood control facilities can be implemented at a variety of scales, depending on the size and topography of the drainage area above the structure, anticipated discharge during storm events, the underlying geology of the site, and other factors. Barneston Dam and Beaver Creek Dam are two examples of dry dams already installed within the Brandywine watershed.

Several sites throughout the watershed were analyzed to determine their suitability for dry dams to mitigate downstream flooding (see Table 8 and Figure 34). Ultimately, none of the sites were recommended for further analysis, as shown in the table below. In most cases, this was because there was insufficient space (either due to topography constraints, existing development, or both) to create enough flood storage to meaningfully reduce flow rates during major storms. Details for each site can be found in Section 9 of the Technical Compendium.

Road East Fallowfield

near Brandywine Drive Newlin West Branch Brandywine

Buck Road near Rte 322 East Brandywine

Unnamed Trib below Chester County Public Safety Training Campus South Coatesville West Branch Brandywine

Unnamed Trib near Union Street South Coatesville West Branch Brandywine

Branch above Valley Station Road Coatesville West Branch Brandywine

- minimal impact for watershed flow rate reduction

- minimal impact for watershed flow rate reduction

recommended - minimal impact for watershed flow rate reduction

Buck Run near Laurel Forge Road

Dennis Run near Hephzibah

Embreeville

Table 8: Initial Analysis of Potential Flood Mitigation Dry Ponds

Quaker Mill Flood Mitigation Pond in Ellicott City, MD, includes a dam structure that holds back flood waters during large storm events. Photo courtesy of the Office of the Maryland Governor, managed by the Maryland State Archives

34: Sites Assessed for Flood Control Dry Dam Potential

Figure

Stormwater Reduction Measures

Stormwater runoff contributes to local flooding during both small and large storm events. Local improvements and investments made in each municipality will provide benefits to nearby neighborhoods as well as downstream communities. The project team compiled stormwater infrastructure geospatial data for municipalities in the Brandywine Creek watershed to identify potential flood mitigation measures to better protect residents living along the Brandywine Creek and its tributaries in Pennsylvania and Delaware.

Stormwater infrastructure includes Best Management Practices (BMPs) like detention basins, wet ponds, infiltration facilities, stormwater inlets, pipes, and outfalls. These features convey, reduce peak flowrates and volume, and control stormwater runoff before it is released to surface waters throughout the drainage area. Stormwater infrastructure is typically constructed during land development and is regulated by local municipalities in accordance with state law.

For this study, data collection focused on basin data from various available sources to compile a comprehensive geodatabase of stormwater basins in the Brandywine Creek watershed. Attributes for basins in the data set include parcel ownership, area, depth, storage volume, age, and condition (where available). This study was not able to collect comprehensive information on subsurface infrastructure (storage area, inlets, pipes, and outlets), but future data collection efforts are in the planning stage in Chester County to better understand how stormwater is conveyed through existing infrastructure to the stream and impacts local flooding.

Stormwater Basin Retrofits

A desktop analysis identified 1,232 stormwater basins in the Brandywine watershed. These were primarily concentrated in areas that have been developed over more recent decades and therefore are subject to local stormwater management regulations. In total, it is estimated that these basins (assuming they are functioning as designed) have a collective maximum capacity of 5.4 million cubic feet, or about 40 million gallons.

That volume is equal to roughly 1% of the capacity of the existing five major flood control facilities in the upper watershed. Therefore, it is unlikely that investments in storage capacity upgrades to these smaller, distributed systems would have any measurable impact on regional flooding. However, in certain areas, retrofitting existing stormwater basins may make very meaningful

contributions to localized flood reduction efforts, particularly in areas near the smaller, flood-prone tributaries to the larger main stem, East Branch, and West Branch stretches of the Brandywine Creek.

As minimal data on subsurface stormwater infrastructure was available to inform how each basin connects to the local hydrologic regime, assessing more localized opportunities for flood mitigation through basin retrofits was beyond the scope of this study. However, the Chester County Water Resources Authority currently has plans to work collaboratively with municipalities across the county to compile that information in a GIS database, which would allow for a more robust analysis of basin retrofit opportunities in the near future.

A subsurface infiltration gallery installed on a residential property during redevelopment can help reduce stormwater reaching the stream.

Stormwater basin retrofits can provide both additional volume control as well as water quality benefits.

One site that was assessed for potential stormwater basin retrofits was the Route 113 cloverleaf basin, which straddles the border between East Caln Township and Downingtown Borough. Given the size and depth of the basin, the study team analyzed whether additional runoff from adjacent areas with minimal stormwater infrastructure could be rerouted or piped into it to increase local flood storage capacities. Preliminary analysis determined that this would ultimately be infeasible.

Municipalities should regularly inspect their stormwater basins, and all stormwater infrastructure, to ensure they are functioning as designed. Faulty or failing basins have the potential to exacerbate community flooding issues, so frequent monitoring and timely repair is important.

Finally, many older developments and communities lack critical stormwater infrastructure, as they were constructed prior to the adoption of stormwater management regulations. Especially in these areas, redevelopment presents opportunities to install stormwater infrastructure that will help address runoffrelated challenges, such as flooding.

Reducing Impervious Cover

Rainfall runs off impervious cover, contributing to increased stormwater and local flooding. Removing impervious surfaces and replacing them with either natural vegetation or pervious pavement/pavers can help reduce the amount of stormwater runoff.

Drainage Improvement Projects

Municipalities are responsible for maintenance of their stormwater drainage system. Inspection of inlets, catch basins, manholes, pipes, and related stormwater infrastructure helps to identify malfunctioning components of the stormwater collection system. Incorporating additional factors, such as useful life estimates and local flood frequency, can help municipalities prioritize stormwater infrastructure in need of repair or replacement.

Backflow Prevention Device Installation

During heavy storm events, stormwater infrastructure that is outdated to meet current capacity, under-designed, improperly maintained, or simply overwhelmed for extreme storm events may experience floodwaters backing up through the system, resulting in localized flooding. For example, this can happen when flood elevations in the stream are higher than stormwater outfalls. Backflow prevention devices, like gates, flaps, or valves, may be installed at various points within the stormwater system to prevent backwater from contributing to flooding.

Municipalities should inventory existing stormwater infrastructure to identify places where flood backflows may cause or exacerbate localized flooding. Those facilities should be prioritized for the installation of these types of devices.

Additional Opportunities

The structural opportunities recommended for further investigation and implementation in this chapter were determined through this study to be the most likely to have a positive impact on localized and/or regionalized flooding in the Brandywine watershed. However, it is not an exhaustive list of all projects that could potentially provide a public benefit. Several flood mitigation and stormwater studies are ongoing in targeted areas within the watershed, including the cities of Coatesville and Wilmington, as well as the greater Downingtown area. (Section 12 of the Technical Compendium includes a description of all complementary initiatives active at the time of this study.) The results of those studies may produce additional structural recommendations beyond what has been identified here.

It is important to note that structural solutions of any kind take time to implement. The processes of project design, permitting, fundraising, bidding, and construction each have their own time horizons that can vary based on numerous factors. In light of this, the following chapter outlines a variety of non-structural recommendations, many of which can be implemented more rapidly. These structural and non-structural strategies can be pursued simultaneously to reduce community flood risks in both the short and long term.

Chapter 7 Non-Structural Recommendations

Along with structural improvements to mitigate flood damage, this study identified numerous nonstructural recommendations that help to achieve maximum flood mitigation and prevention benefits to the community and the natural landscape of the Brandywine watershed during flood occurrences. Non-structural actions include mitigation measures that do not require a physical structure, such as a levee or dam, exist in many forms, and can be led by many different entities. Some strategies may be more appropriate for areas with significant development, and others better suited for areas with more open space and undeveloped land.

Non-Structural Recommendations for Developed Areas

In urban and suburban areas, much of the landscape, especially along waterways, is already developed. While this may present challenges for implementing larger-scale structural projects, non-structural recommendations offer communities the opportunity to reduce flood risks through policy, planning, public education, and emergency management efforts. Many of the non-structural solutions recommended in this study for developed areas are actively utilized by municipalities throughout the Brandywine watershed. So, expanding these efforts by incorporating new techniques, best practices, and information may be the lowest hanging fruit for many communities to implement.

Emergency Preparedness Planning

As flooding is often unpredictable, robust emergency preparedness planning and training are critical tools for ensuring that first responders and emergency management personnel are adequately equipped to respond as waters rise during severe storm events. Each municipality, within its jurisdiction, is responsible for emergency management, response, and recovery, including developing and updating the

local disaster emergency management plan. For flood hazards, these plans should:

• Give special attention to roadways and access points that may be cut off by flood waters, preventing emergency services from reaching those in need.

• Identify bridge crossings and low-lying roads that are particularly vulnerable.

• Determine communities bisected by a waterway may require two emergency response plans: one for each side of the stream, in the event that first responders are unable to cross from one side to the other. Care should be taken to identify these potential problem sites, along with concrete steps to maneuver around them during a flood event.

• Ensure emergency response plans, evacuations routes, and related resources are easily accessible by the public and promoted regularly throughout communities prior to an emergency.

In addition, municipalities should consider:

• Conducting flood simulation tabletop exercises, during which local officials and the public run through protocols and procedures to train for addressing real world crises.

Left: Shaw’s Bridge Park along the Main Stem Brandywine Creek in East Bradford Township.

Emergency responders review emergency preparedness documents and maps during a tabletop exercise in 2019.

• Identify residents and communities that are particularly vulnerable to flood hazards (due to age, language barriers, health concerns or disabilities, etc.) and make special considerations for supporting those individuals before, during, and after a flood.

• Proactively closing flood-prone roads during a storm to keep the public safe. Less than two feet of rushing water can carry away the average car, and many drivers are likely to underestimate water depths and the risks they pose. In high hazard areas where cones or standard barricades might not be enough to dissuade drivers, some communities have installed roadway closure gates. The Flood Study partners strongly recommend that municipalities assess their jurisdictions for high hazard areas and work with local and state emergency services, along with other community partners, to identify opportunities to fund and install these closure gates in high-risk areas. One option, discussed in the following bullet point, is to include this strategy for particular roadways in municipal submissions for County Hazard Mitigation Plans.

• Participating in the development of, and subsequently adopting, the County Hazard Mitigation Plan. Identifying areas of recurrent flooding and mitigation opportunities in hazard mitigation plans opens the door for communities to access funding, both before and in the aftermath of a flood event.

Revised Flood Stage Levels for Stream Gages

Within the Brandywine Creek watershed, there are 12 stream gages, 4 precipitation gages, and 2 lake gages maintained and operated by the USGS. These gages support flood preparedness, water supply management, and water quality monitoring, as well as provide key information for flow rates for the design of bridges and culverts.

One of the principal purposes of many of the stream gages is to support flood preparedness by tracking the real-time stream heights. The National Weather Service (NWS) publishes plots of these real-time stream heights and includes the NWS Flood Categories for each location as a reference for whether the water elevations are approaching or within a flood category. Ideally, these reference heights should be correlated with flooding impacts on nearby roads, businesses, and bridges.

Road closure gate in Montgomery County which can be quickly rotated into position.

The following stream gages in the Brandywine Creek watershed have existing Flood Stage elevations included on NWS and USGS flood alert websites:

• West Branch Brandywine Creek near Honey Brook

• West Branch Brandywine Creek at Coatesville

• West Branch Brandywine Creek at Modena

• East Branch Brandywine Creek below Downingtown

• Brandywine Creek at Chadds Ford

• Brandywine Creek at Wilmington

t is recommended that the existing Flood Stage elevations at the six gages listed above be reviewed and revised as needed to correlate flood impacts—such as road flooding or impacts to structures—to current water heights and to provide sufficient lead time for road closures or evacuations to take place safely.

In addition, the following three gages do not have existing NWS Flood Stage elevations, and it is recommended that Flood Stage elevations be developed and submitted to NWS and USGS for these three gages:

• East Branch Brandywine Creek near Downingtown

• Beaver Creek at Mary Street Field at Downingtown

• Buck Run at Doe Run

Public Alerts and Readiness

Early warnings ahead of major storms play a critical role in protecting and saving lives. Existing resources that can connect the public to flood alerts and preparedness information include:

• ReadyChesCo – Severe weather and flood alerts are sent out through the ReadyChesCo system. In Chester County, the County’s Department of Emergency Services offers the ReadyChesCo program, for which individuals can register for free to receive emergency and non-emergency alerts for their community.

• DelCo Alert - Delaware County residents have a mass notification system called DelCo Alert. The emergency notification system sends alerts directly to residents through their mobile devices, email, and/ or landline phones. Residents may be alerted before, during, and after a major emergency and are provided with critical information when minutes and even seconds might make a difference.

Forecast of peak height of the Brandywine Creek at Chadds Ford from the National Weather Service.

• Delaware Emergency Notification System (DENS) – DENS is public warning and emergency protective action system available for those who live and work in the State of Delaware. It allows local 911 centers or emergency managers to send messages to a specific street orneighborhood, and larger areas affected by an emergency event like a flood. Cell phones are not automatically a part of the DENS contacts database, so individuals must register their cell phone numbers with DENS to receive messages to those devices.

• Delaware News Weather Notifications – Delaware residents can sign up for real-time, email-based weather alerts from the state’s news updates website.

• Flood Tools – The Chester County Water Resources Authority hosts a web-based “Flood Tools” portal (www.chesco.org/floodtools) with current and forecasted flood conditions across the county. One of the portal’s features includes instructions and links for individuals to sign up for rainfall and stream height and flow alerts for their area based on data directly from the local USGS monitoring network.

• Reverse 9-1-1 Systems – A “Reverse 9-1-1” is a tool which emergency services can use to call telephone numbers in a specific geographic area to notify residents of an emergency and any actions that they may need to take. “Reverse 9-1-1” is used to supplement other tools, such as ReadyChesCo, DelCoAlert or DENS. It is recommended that the counties continue to incorporate multiple emergency alert tools, including “Reverse 9-1-1,” to keep the community aware of forecasted and actual conditions, particularly as conditions can change rapidly if rainfall is more intense or centered over a different region than the initial forecast.

• Ready.gov – Officials and disaster assistance personnel recommend that individuals and families assemble an emergency kit and have an established plan for what to do in a variety of emergency situations. In the case of a flood, this may mean being without electricity for a period of time or evacuating to higher ground. The U.S. Department of Homeland Security maintains the www.ready. gov website, which includes information on what to keep in an emergency kit and how to develop an emergency plan.

Flood Insurance

Flood insurance through the National Flood Insurance Program (NFIP) can help individuals recover losses and rebuild their lives after flood events. Those with federally backed mortgages and other loans may be required to carry a flood insurance policy on their property, and it is sensible for anyone owning property with an elevated risk of flooding to consider getting a policy. Municipal officials and community organizations can help educate the public on the value of flood insurance and dispel related common myths. For example, NFIP flood insurance is available for anyone (including renters), regardless of whether their property is located within Special Flood Hazard Areas (SFHA).

Figure 35 illustrates the distribution of FEMA repetitive loss flood insurance claims within the Brandywine watershed. NFIP-insured structures that have at least two paid flood losses of more than $1,000 each in any 10-year period since 1978 are considered repetitive loss properties.

FLOOD INSURANCE MANUAL

Reverse 9-1-1 alert used during Hurricane Ida in 2021.

NATIONAL FLOOD INSURANCE PROGRAM

35: Repetitive Loss Property Claims in the Brandywine Watershed

Figure

Enforcing and Enhancing Local Floodplain Regulations

All municipalities in Pennsylvania are required to participate in NFIP and adopt local floodplain ordinances. These ordinances are critical tools to helping build safer, more resilient communities. Floodplain ordinances require municipalities to:

• Designate a Floodplain Administrator to oversee the implementation of the local floodplain management program and enforcement of the ordinance.

• Adopt FEMA flood maps and use the official SFHAs to identify boundaries within which floodplain regulations are enforced.

• Develop and implement a floodplain permitting program requiring permits for all development activities (including grading/earth moving, smallscale projects, etc.) within the floodplain.

• Identify construction standards specific to structures and development within the floodplain.

• Adhere to stringent requirements for issuing variances from the floodplain ordinance (based on unusual characteristics of the site that are not shared by adjacent parcels and pertain exclusively to the land, not to any specific structures or property owners).

• Enforce code requirements for new structures and for structures determined to be “substantially improved” (when the market value of improvements to a structure is greater than or equal to 50% of the value of the structure) or “substantially damaged” (when the market value of necessary repairs to a structure after it is damaged is greater than or equal to 50% of the value of the structure).

Communities may choose to implement higher standards to further reduce local flood risk, such as:

• Increased freeboard requirements in construction standards (e.g., USACE-24 guidance says to require the design flood elevation to be 2 feet higher than the FEMA base flood elevation).

• Cumulative substantial improvement rules.

• Compensatory storage requirements to offset fill placement in the floodplain.

Communities who elect to adopt higher standards may be eligible to participate in FEMA’s Community Rating System (CRS) program. This program points to a municipality for activities and regulations that go beyond the minimum requirements, which translate to lowered NFIP insurance premiums for their residents.

Municipalities may also supplement their floodplain ordinances with the adoption of flood overlay districts. These are zoning overlay districts which can be used to further address flooding concerns and regulate land use in flood prone areas beyond the official 100-year floodplain as delineated by FEMA. Considerations for development of the flood overlay district might include:

• Areas adjacent to or within a specific distance of a stream, swale, or waterway which may be subject to periodic flooding or inundation.

• Known high-water marks from past flooding events.

• Areas of alluvial soils based on local, state, or federal surveys (or site testing).

• The 500-year floodplain as delineated by FEMA flood maps.

Structural Elevations, Floodproofing, and Property Buyouts

Structures built in the floodplains along the Brandywine Creek include industrial sites, commercial businesses, and residences. A good portion of these structures and the people who rely upon them tend to be the most vulnerable to damages from flood events. Table 9 lists the total acres within the 100-year floodplain (FEMA-designated Special Flood Hazard Area Zones A, AE, AE Floodway, and AO), along with the number of parcels, structures (>500 square feet), estimated housing units, and estimated residential population by municipality.

Two main options for increasing the resilience of buildings already located within the floodplain are structural elevation and floodproofing. For residential properties, the standard protocol is to elevate the house above the base flood elevation (also known as the 100year flood height). Structural elevation may be achieved in several forms, including elevating the building on fill or abandoning the bottom floors. For non-residential structures, floodproofing is an acceptable strategy by dry floodproofing (when materials are used to make the exterior of a building watertight) or wet floodproofing (when flood damage–resistant materials are used to minimize damage in the lower portion of a structure, which is intentionally allowed to flood). Structural elevation and floodproofing have numerous benefits; however, they do not eliminate the risk to life and property entirely. For example, an elevated home might keep the residents high and dry, but first responders may still be cut off from accessing them when the land around the structure floods.

For homes subject to frequent, hazardous floods, some communities have chosen to pursue voluntary property buyouts. In these cases, the municipality offers the owner of the flood-prone property to pay fair market value, and then the site is completely cleared. This eliminates future risk to loss of life from flooding at the site, and when coupled with floodplain restoration, can even reduce potential flood damage to nearby areas.

Unfortunately, despite the benefits, buyout programs are not without downsides. Many residents may be unwilling to participate, and those who do may choose to move out of the municipality, which has potential ramifications for the tax base and overall fabric of the community. It can also be an expensive process, although there are several federal and state programs that can provide funding to support property buyouts (particularly after disaster declarations). Ultimately, it is up to the community to weigh the risks and benefits of a buyout program before initiating one.

For municipalities in Pennsylvania, one alternative to a traditional buyout program would be to utilize an official map to identify flood prone parcels that could be acquired by the municipality in the future. In Pennsylvania, an “official map” is a combined map and ordinance designed to aid in proactively planning for future growth by reserving land for public uses. This is different from

a municipality’s zoning map or other maps included in a comprehensive plan. Under Article IV of the PA Municipalities Planning Code (MPC), municipalities can adopt official maps to designate sites of interest for possible future right of ways, like roads or trails, or public grounds, such as parks or flood control basins, floodways and floodplains, storm water management areas and drainage easements. An official map does not, in and of itself, constitute an active attempt to acquire specific property rights. Instead, it serves as a guiding document that can assist municipalities with effectively prioritizing areas for future acquisition as those parcels become available, enabling it to focus its resources on properties that provide the greatest benefit to the community. Municipalities should conduct a comprehensive analysis of structures within the delineated 100-year floodplain and consult with the affected property owners. The analysis should assess the value and structural soundness of the buildings to determine whether they are fit for elevation or floodproofing, compared to persistent flood risks and NFIP claims. Special attention should be paid to residential structures and critical infrastructure (including hospitals, daycare facilities, utilities, etc.). County Departments of Emergency Services may be able to provide assistance with these inventory efforts.

Official Map adopted by Uwchlan Township notes potential future park development among other features.

* Includes FEMA Special Flood Hazard Area zones A, AE, AE Floodway, and AO

** Structures greater than 500 square feet, based on 2012 or 2015 aerial imagery

Sources: FEMA Flood Map Service Center, https://msc.fema.gov/portal/advanceSearch DVRPC, https://catalog.dvrpc.org/dataset/impervious-surfaces-2015-chester-county DVRPC, https://catalog.dvrpc.org/dataset/impervious-surfaces-2015-delaware-county Lancaster County, https://www.pasda.psu.edu/uci/DataSummary.aspx?dataset=1257 New Castle County GIS Services, https://apps-nccde.hub.arcgis.com/

Strengthening Steep Slope Protection Ordinances

Steeper slopes generate more stormwater runoff than flatter areas, leading to flooding and problems with erosion. Most municipalities in Chester County have protective ordinances that restrict some or all development activities on slopes with a 15% or higher grade. However, even less steep slopes can generate significant runoff that can damage infrastructure and create risks for the public. To address this issue, a model ordinance should be developed to identify additional stormwater management protections and/or development restrictions for slopes with grades of 10 to 15%.

Public Education and Engagement

Consistent education and outreach are needed on “blue sky” days to help community members prepare for flood events. Popular ready-made campaigns include the NWS’s “Turn Around, Don’t Drown®” program, which educates the public on the dangers of trying to drive through floodwaters. NWS offers numerous resources, including emergency sign templates, that are available online for public use (www.weather.gov/safety/floodturn-around-dont-drown).

Through the NFIP, FEMA has a High Water Mark Initiative, aimed at encouraging community awareness of flood risk and mitigation opportunities through historic high water mark signage (https://www.fema. gov/flood-maps/products-tools/high-water-markinitiative). In places like Washington, D.C., and Carson City, Nevada, communities have gone one step further, enlisting the help of artists to visually depict the impact of floods through public art installations like murals and sculptures. A program like that could be replicated in the Brandywine watershed with relative ease, as the USGS recorded high water marks at bridges along the main stem and the East and West branches of the Brandywine during Hurricane Ida. For instance, the recorded high water marks at the U.S. Route 13 bridge in northeast Wilmington (RM 7970) was 9.7 feet, and at U.S. Route 1 in Chadds Ford (RM 23,660), the Ida high water mark was found at 171.6 feet. Installing Hurricane Ida high water mark signs or artworks at some of these locations could serve as a reminder to the public about the historic high water experienced during the largest flood in two centuries along the Brandywine and its tributaries.

Stream and Floodplain Maintenance for Riparian Landowners

Property owners with a stream running through or adjacent to their property may need to perform maintenance in the creek or within the floodplain to clear flood debris, repair a culvert of a bridge, or even protect stream banks that are eroding or at risk of collapse. Certain maintenance activities do not require either a permit or a notification to a regulatory agency, while other activities do, and it can be confusing to know whether an activity is regulated.

In Delaware, the DNREC Wetlands and Waterways Section oversees regulated activities, and in Pennsylvania, supervision of these activities is handled by the PADEP’s Bureau of Waterways Engineering and Wetlands. To help municipal officials, property owners, and others who are affected better understand the regulatory requirements that apply, PADEP has published 2 resources that provide guidance on stream maintenance work. The goal of these resources is that stream maintenance work is conducted in a responsible manner that will not inadvertently create other problems and that complies with state and federal laws.

One resource is the “Bridge & Culvert Maintenance and Repair Tool,” an online interactive tool designed to assist bridge and culvert owners in determining what, if any, permit and notification requirements are needed for bridge and culvert maintenance and repair.

The second resource published by PADEP is a 2018 booklet titled “Guidelines for Maintaining Streams in Your Community.” It outlines actions that do not require PADEP notification, those that do require notification, and actions that require permits.

Examples of actions that do not require PADEP notification include:

• Removing non-native (manmade) material, such as litter and construction debris, from the stream, banks, and riparian areas.

• Removing woody debris, such as trees, logs, or brush, from the stream while standing in the stream. (This includes the use of hand-held equipment, such as chainsaws, but not heavy equipment.)

• Removing woody debris from the stream while standing on the bank. (This includes the use of heavy equipment, as long as it remains on the bank and is “picking out” the material and not digging into the streambed.)

• Planting trees and other plants on streambanks and in riparian areas, especially native species.

Examples of actions that do require PADEP notification include:

• Streambank stabilization projects.

• Repairing a bridge or culvert.

• Redirecting the flow of a stream by reshaping gravel bars or moving gravel to the streambank.

• Armoring streambanks with concrete, construction debris, and other impervious materials.

• Dredging streams.

• Creating dikes.

Riparian landowners and managers should also keep in mind that even where an activity does not require state-level approval, it may require a local floodplain permit. The designated floodplain manager in a municipality should be able to provide guidance on local regulations and requirements.

PADEP has published a guidebook to assist property owners with stream and floodplain maintenance.

This online tool published by PADEP is designed to assist bridge and culvert owners understand permitting requirements.

Non-Structural Recommendations for Less Developed Areas

In areas with more limited development, a variety of strategies exist to utilize open space as a natural flood mitigation tool to slow, spread, and store floodwaters. Protecting these lands is critical, as development may exacerbate future flooding. While a complete development moratorium is not permitted under state law, local governments should consider implementing zoning ordinances and adopting policy changes that limit and/or heavily regulate development within floodplains or other flood-prone areas. In addition, there are opportunities for local governments and conservation organizations to protect open spaces for public and private use as an effective tactic in protecting communities from flooding. It prevents development in flood-prone areas and allows landscapes to absorb and slow the flow of water.

Protecting and enhancing natural floodplains is one of the most cost-effective methods for managing flood risk in downstream communities. The most impactful opportunities for flood storage and open space conservation are typically found in areas where the floodplain is minimally developed, wide, and mildly sloped. Examples of this in the Brandywine watershed include the stretch of stream between Chadds Ford and Lenape Park, where the floodplain ranges from 1700 to 2600 feet wide, and along the East Branch below Embreeville, where the landscape is roughly 900 feet wide.

There is a long legacy of land preservation in the Brandywine watershed by municipalities and conservation organizations. Presently, approximately a third of the watershed in Chester County is permanently preserved as open space or agricultural land. This has been achieved primarily through the use of fee simple land and conservation easement acquisitions, both of which are described in detail in the following sections. A combination of land conservation and active land management can support natural systems which are extremely effective in mitigating flood risk and creating healthier ecosystems.

Conservation easement for Camp Indian Run in Wallace Township and West Brandywine Township. Photo courtesy of Chester County Department of Parks + Preservation

Fee Simple Acquisition

Fee simple acquisition entails the outright purchase of a parcel of land. Ownership of the land allows for more flexibility in how it is managed. Deed restrictions can be placed on the land to prevent certain types of development and protect sensitive environments within the parcel. These restrictions can also limit future land use to prevent commercial, agricultural, or disruptive recreational activities. It can allow for land management techniques that may be prohibited in other forms of land conservation for wetland and floodplain restoration, such as dredging. While fee simple acquisition may be more effective in quickly protecting and managing flood-prone land, it is usually more costly, as the owning entity must have enough funding to make the purchase and provide all the equipment and labor necessary for the management and maintenance of these lands.

Conservation Easements

For more than six decades, conservation easements have protected land within the Brandywine watershed and have helped mitigate flooding within the region. A conservation easement is a legal agreement between a landowner and a conservation organization or government entity that protects the conservation values of a parcels of land in perpetuity. Conservation objectives can vary from uninterrupted public views of open space to the presence of rare habitat types. Conservation easements can be used to protect many aspects of a landscape, such as its scenic value, a sensitive ecosystem, agricultural soils, and more by extinguishing some of the development rights held by the original owner and limit the allowable activities, uses, and improvements of the landscape.

Once a conservation easement is executed, it can be extremely difficult to make any changes to it. This is very important, as it protects the land from any future landowners who may intend to develop or use the property for other means. However, easement language can also create difficulties in managing and restoring land in ways that allow for adaptation to changing conditions.

It has been noted in various studies that conservation easements or fee acquisition of land have not been used previously to preserve land specifically for its ability to mitigate flood hazards, though both tools easily provide a means to this goal. While there are flood mitigation benefits that occur along with other conservation values protected in easements, there is a lack of targeted floodrelated language (OSI, 2020, p.4). For this reason, it is important to consider the ways in which conservation easements can be written, amended, or restructured in a way that would protect flood prone land more effectively. Section 11 of the Technical Compendium includes additional details on easement language to promote flood mitigation through these tools.

River Corridor Easements

Another method to protect flood-prone lands is through an altogether different kind of conservation easement. The Vermont Rivers Program, under the Vermont Agency of Natural Resources, works to protect flood-prone land through River Corridor Easements (RCEs).

In the program’s sample easement, it states the following Conservation Purposes:

“The purposes of this grant are to allow the ___________ River to re-establish its natural slope and meander pattern, have banks stabilized by a buffer of native, predominantly woody vegetation, and access to natural floodplains in order to reduce flood and erosion hazards, improve water quality through capture and storage of flows, sediment and nutrients, and to conserve and enhance aquatic and wildlife habitats and the natural processes associated with the Protected Property now and in the future.”

This is a passive means of restoration and bars most active management practices within the river corridor. In these easements, the channel management rights are either purchased by the holding entity or donated by the landowner as a standalone easement or as an amendment to another conservation easement. A designated area of land adjacent to either side of the river is protected and contains a 50-foot riparian buffer consisting of native, woody plants. If this buffer does not exist upon acquisition of the easement, it must be planted and managed. The

50-foot riparian buffer must then be maintained so that as the river moves (as many rivers and streams naturally do), the riparian buffer is permitted to move with it, within the designated easement boundaries. Though the adoption of a new kind of easement may seem daunting to many conservation organizations and other easement holders, this easement language has seen notable success in Vermont, where flooding has created decadeslong safety, environmental, and agricultural issues. Details about river conservation easement language can be found in Section 11 of the Technical Compendium.

Developing Municipal Open Space Funds

An extremely useful tool that many municipalities in the region already employ to fund both land acquisition and easements is municipal open space funding programs. These programs are implemented by local governments through a small increase in earned income taxes. An example of this can be seen in Elk Township (Chester County, PA), which, in 2006, proposed an open space funding referendum and passed by a vote of township residents. The referendum allowed a 0.5% earned income tax increase for resident wage earners, to be used to fund the purchase of agricultural lands and open space in the township. Between 2006 and 2016, the township’s protected lands grew from 14% to 37% and raised about $90,000 each year at very little cost to individual residents. Open space funds allow municipalities to prioritize their own land conservation goals, such as prime agricultural soils or recreational spaces. Hence, municipalities may use open space funds to protect lands for other public benefits, such as flood mitigation. Overall, the implementation of an open space fund is an effective tool that municipalities may use for flood management initiatives.

Ingrams Mill Nature Area is a 36-acre preserved park along the East Branch Brandywine Creek owned by East Bradford Township.

Chapter 8

Moving into Implementation

Study Limitations and Recommendations

While this study involved a robust assessment of flood hazards and potential mitigation opportunities within the Brandywine watershed, it is not without its limits, such as:

• Results included in this report are based on the best available data, public/partner input, and the computer modeling software used.

• Study partners worked with available HEC-RAS models from FEMA, which were not available for many of the smaller tributaries.

• Generally, the scale of analysis was based on subwatersheds and not at an individual site/ project scale.

Structural recommendations included in this study are conceptual in nature, and project design was not within the scope of this study. For that reason, the development of cost estimates for mitigation projects was also omitted. Engineering designs and their associated site analyses will need to be completed as projects are selected for implementation. Future partners for implementation are welcome to the available data and models used in this study, which can be requested directly from the Chester County Water Resources Authority.

Fortunately, further analyses of potential localized mitigation projects are currently underway in several areas of the watershed. Ongoing studies in the Cities of Coatesville, Downingtown, and Wilmington will likely produce additional sites to supplement those identified in this study. The Flood Study partners are committed to supporting these efforts as they come to fruition.

Roles for Implementation

Achieving full implementation of this study’s potential will require engagement from individuals, municipalities, and organizations throughout the watershed. This section outlines potential implementation roles for different stakeholders, based on the recommendations outlined in Chapters 6 and 7. Though not exhaustive, this list is meant to serve as a starting point for those looking to reduce flood risks in their communities.

Chester County Water Resources Authority

• Begin the preliminary stages of design and preparation for the rehabilitation of Barneston Dam in the East Branch Brandywine watershed to comply with updated state requirements and improve flood storage capacity.

• Coordinate with county facilities to assess opportunities for impervious cover reduction and stormwater control/flood storage projects on county-owned properties within the watershed.

• Identify opportunities to support municipalities with the implementation of floodplain ordinances and participation in the FEMA Community Rating System (CRS).

• Maintain operations of Struble Lake, Beaver Creek Dam, and Hibernia Dam to ensure ongoing flood control benefits for downstream communities.

• Maintain the Flood Tools website to provide public information on current and forecasted flooding conditions.

Left Photo: Bridge replacement on Route 322 in West Bradford Township and East Bradford Township. Photo courtesy of Chester County Planning Commission

Right Photo: Barneston Dam on the East Branch Brandywine Creek.

County Departments of Emergency Services/ Emergency Management

• Coordinate with municipalities throughout the watershed to identify and incorporate flood hazards and projects into the updated County Hazard Mitigation Plans.

• Support municipal and multi-municipal emergency preparedness and planning efforts.

• Support municipal and multi-municipal grant applications for pre-disaster mitigation funding.

• For Chester County, continue to broadcast storm and flood alerts to subscribers of the Ready ChesCo alert system.

• For Delaware County, continue to broadcast storm and flood alerts to subscribers of the DelCo Alert system.

Municipalities

• Inspect, maintain, rehabilitate, and upgrade stormwater infrastructure to improve flood storage capacity.

• Support training and certification opportunities for municipal floodplain administrators and explore implementing higher standards in local floodplain ordinances as recommended in Chapter 7.

• Prioritize replacement or upgrades of municipally owned bridges, culverts, or other obstructions identified in Chapter 6 to reduce local flood risks.

• Identify properties in the floodplain that are subject to high risk to life and damages and consider offering voluntary property buyouts.

• Work with the County Planning Commission/ Department and/or regional metropolitan planning organization to submit bridge repair and replacement projects to the state Transportation Improvement Program (TIP) list.

• Participate in their county’s Hazard Mitigation Plan update processes and adopt the plan upon its completion to ensure future eligibility for state and federal hazard mitigation funding.

• Review community emergency response plans to ensure they account for major flood scenarios, especially in streamside communities or those bisected by waterways.

• Educate community members on flood preparedness tools and resources from local, regional, and national sources.

Delco Alert and Ready ChesCo send alerts directly to residents of severe storms, road closures and other emergency situations.

• Educate municipal staff, elected officials, and the public about the importance of proper enforcement of the local floodplain ordinance.

• Consider participation in the FEMA CRS program to reduce local flood insurance premium costs and encourage residential participation in the NFIP.

PennDOT/DelDOT

• Prioritize replacement or upgrades of state-owned bridges, culverts, or other obstructions identified in Chapter 6 and design beyond the 100-year storm to reduce long-term local flood risks.

• Coordinate with municipalities to implement bridge and culvert maintenance programs to reduce flow obstructions caused by the buildup of debris and sediment.

• Inspect, maintain, rehabilitate, and upgrade stormwater infrastructure to improve flood storage capacity.

Bridge on Route 322/Manor Road over Beaver Creek in Downingtown Borough.

Conservation Organizations

• Prioritize parcels with natural floodplains for preservation.

• Explore floodplain restoration on owned and/or eased lands to improve flood storage, particularly in areas where floodplains are flat, wide, and vertically disconnected from the stream channel.

• Educate municipal representatives and the public about the importance of floodplain protection.

• Provide technical assistance for municipalities, homeowners associations, and others on issues related to stormwater management, riparian buffers, etc.

• Coordinate with academic and local government partners as opportunities arise to seek funding for project implementation.

Community Groups

• Help identify strategies to improve community preparedness and prevention, including accessing and interpreting information about flooding before and after storm events.

• Elevate local concerns about the impacts of flooding to municipal and county officials, including areas of chronic flooding, barriers to individual or community resilience, etc.

• Coordinate with municipalities to support waterway cleanups to reduce litter and debris that can contribute to flooding obstructions.

• Coordinate with county, state, and federal disaster response efforts after a flood to improve the efficiency of recovery efforts.

Individuals

• Sign up for early warning alerts, like those provided by the county, the state, USGS, or NWS

• Maintain a personal emergency preparedness kit and be aware of local evacuation routes.

• For property owners and renters: consider purchasing federal flood insurance for properties within the designated floodplain and close to waterways with flood potential (even if they are not along streams with a mapped floodplain).

• Consider elevating structures within the floodplain to reduce flood risk.

• Be aware of and prepared to comply with substantial improvement and substantial damage requirements in the local floodplain ordinance as they apply to properties in the special flood hazard area.

• Remember to never drive or walk through floodwaters, even if they do not seem too deep.

Potential Funding Opportunities for Implementation

Funding is often one of the largest hurdles to implementing flood mitigation and risk reduction strategies. Fortunately, there are local, state, and federal grant funding opportunities that communities can pursue to offset the costs of these efforts. For some non-floodrelated grants, flood protection and mitigation may be incorporated as a secondary or co-benefit to the primary focus of the grant (e.g., habitat restoration or infrastructure repair).

Table 10 highlights potential grant funding opportunities that may be relevant in the implementation of this study’s recommendations. It is important to note that some grants are only open to local governments or nonprofits, while others may have broader applicant eligibility. Projects involving partnerships across sectors or jurisdictional boundaries generally tend to rank higher with grant funders, and so collaboration is strongly encouraged.

Conservation easement for the Brandywine Creek Greenway West Branch in West Bradford Township.

Photo courtesy of Chester County Department of Parks + Preservation”

Table 10: Potential Grant Funding Opportunities

Funding Entity

Chester County Department of Community Development Community Revitalization Grants NA Project implementation

Chester County Parks and Preservation

Chester County Parks and Preservation

Preservation Partnership Program Municipal Acquisition Grant

Preservation Partnership Program Conservancy Acquisition Grant

Acquisition

Acquisition

Chester County’s boroughs and the City of Coatesville are eligible applicants. Eligible activities include urban revitalization efforts for municipally owned infrastructure, including stormwater improvements, floodplain management, etc. A 25% match is required unless otherwise indicated under the program guidelines.

Funds 50% of appraised value of fee simple land purchases and conservation/trail easements that preserve significant natural, recreational, agricultural, historic, and cultural land resources. Acquisitions must enhance public access and public benefit. Available only to local government within Chester County.

Funds 50% of appraised value of fee simple land purchases and conservation/trail easements that preserve significant natural, recreational, agricultural, historic, and cultural land resources. Acquisitions must enhance public access and public benefit. Available only to nonprofit land conservation organizations in Chester County.

DE DNREC Outdoor Recreation Parks and Trails Program NA Acquisition

FEMA Hazard Mitigation Grant Program

FEMA Flood Mitigation Assistance Grant Program

NA Acquisitions, planning, project implementation

NA

Planning, project implementation

Local governments and park districts may apply. Grants may be awarded for 50% of eligible project costs for fee simple acquisition of parkland, open space, or conservation areas; planning and design of parks or trails; and outdoor recreation facility construction in the state of Delaware.

Local governments are eligible to apply when funding becomes available after a presidentially declared disaster. Eligible projects include planning and enforcement of hazard mitigation plans; acquisition of hazard prone properties; flood control structure construction; elevation; drainage improvements; and retrofits to structures, utilities, and infrastructure.

Local governments are eligible as sub-applicants under the commonwealth’s application to FEMA. Projects eligible for funding may include capacity building activities, mitigation planning, project scoping, localized flood risk reduction projects, individual flood mitigation projects, enhancing local floodplain management, and repetitive loss strategy development. Typically, a 25% match is required.

National Fish and Wildlife Foundation Delaware Watershed Conservation Fund

National Fish and Wildlife Foundation

Delaware Watershed Conservation Fund

Implementation Grants Project implementation

Planning Grants Planning

PA DCED Act 13 Marcellus Legacy Fund Flood Mitigation Program Project implementation

PA DCED Act 13 Marcellus Legacy Fund Watershed Restoration and Protection Program Project implementation

Eligible entities include nonprofits, governmental organizations, and academic institutions. Eligible projects are those which are shovel-ready within six months and result in quantifiable benefits for fish, wildlife, and people within the Delaware River watershed. A 1:1 non-federal match is required.

Eligible entities include nonprofits, governmental organizations, and academic institutions. Eligible projects include engagement, planning, and prioritization; feasibility, suitability, or alternatives analyses; site assessment and conceptual design; and final design and permitting. A 1:1 non-federal match is required.

Eligible entities include municipalities, academic institutions, watershed organizations, and businesses. Projects authorized by a flood protection authority, PADEP, USACE, NRCS, or identified by a local government for flood mitigation are eligible for the program. A 15% match is required.

Eligible entities include municipalities, academic institutions, watershed organizations, and businesses. Projects which involve the construction, improvement, expansion, repair, maintenance, or rehabilitation of new or existing watershed protection BMPs are eligible. A 15% match is required.

Funding

PA DCED Municipal Assistance Program Community Planning Planning

PA DCED H2O PA Water Supply, Sanitary Sewer, and Stormwater Projects Project implementation

PA DCED H2O PA High Hazard Unsafe Dam Projects Project implementation

PA DCED H2O PA Flood Control Projects Project implementation

Counties and municipalities are eligible to apply. This program provides funding to assist local governments with the development of multi-municipal plans. A 50% match is required.

Municipalities and municipal authorities are eligible entities. The program provides single or multi-year funding to assist with the construction of drinking water, sanitary sewer, and stormwater projects. A 50% match is required.

Municipalities, municipal authorities, independent agencies, and the commonwealth are eligible entities. Single or multi-year projects which involve the repair, rehabilitation, or removal of all or a part of a high hazard unsafe dam are eligible. A 25% match is required.

Municipalities, municipal authorities, independent agencies, and the commonwealth are eligible entities. Single or multi-year projects which involve construction, improvement, repair or rehabilitation of all or part of a flood control system are eligible. The applicant must provide easements and rights-of-way, relocation of buildings and utilities, alterations or rebuilding of inadequate bridges, and operation and maintenance of the completed project.

PA DCED Greenways, Trails, and Recreation Program NA Planning, acquisition, project implementation

PA DCNR Community Conservation Partnerships Program Community Recreation and Conservation Planning Grant Planning

PA DCNR Community Conservation Partnerships Program Land Acquisition and Conservation Acquisition

PADEP Growing Greener Plus Watershed Restoration Project implementation

Eligible entities include municipalities, academic institutions, watershed organizations, and businesses. Projects which involve planning, acquisition, development, rehabilitation and repair of greenways, recreational trails, open space, parks and beautification projects are eligible. A 15% match is required.

Funds planning projects that study the needs, benefits and opportunities for future land acquisition, development and/or management of parks and facilities, critical habitat, open space, natural areas, greenway, and river/watershed corridors. Open to county or municipal governments, educational institutions, and nonprofits in PA.

Funds the purchase or donation of land for park and recreation areas, greenways, critical habitat areas, and open space. Open to local governments, educational institutions, and nonprofit land trusts in PA. Acquisitions can be projects that enhance public access or protect open space and critical habitats for important species and ecosystems.

Eligible entities include watershed associations, local governments, conservation districts, nonprofits, municipal authorities, and educational institutions. Floodplain restoration for flood mitigation and stormwater management projects are among the eligible project types. A 15% match is required.

PEMA Building Resilient Infrastructure and Communities Capacity and Capability Building Planning

PEMA Building Resilient Infrastructure and Communities Mitigation Projects Project implementation

Local governments are eligible as sub-applicants under the Commonwealth’s application to FEMA. Eligible projects include building code activities, project scoping, and mitigation planning. There is a 25% non-federal match requirement (10% non-federal match for identified small and impoverished communities).

Local governments are eligible as sub-applicants under the commonwealth’s application to FEMA. Eligible projects include acquisition, demolition, relocation, elevation, and other floodproofing measures. There is a 25% non-federal match requirement (10% non-federal match for identified small and impoverished communities).

Funding Entity Grant

PennVEST NA

The Land Trust Alliance (LTA) and Open Space Institute (OSI)

Land and Climate Catalyst Planning Grants

U.S Fish and Wildlife Service North American Wetlands Conservation Act

NA Project implementation

NA Planning

U.S Fish and Wildlife Service North American Wetlands Conservation Act

Standard Grant Acquisition

Though not usually a source of grant funding, PennVEST offers low-interest loans for construction or improvements to stormwater management facilities.

Funds the development of climate-informed land conservation, stewardship, or communications plans that address habitat resilience and/or community adaptation to climate impacts, such as stronger storms, flooding, drought, fire or extreme heat, and more. Open to LTA members and affiliate state land trust associations for LTA funding. OSI funding is open to nonprofits and state/federally recognized tribes within the Delaware River watershed.

Supports public-private partnership projects that further the goals of the North American Wetlands Conservation Act. These projects must involve long-term protection, restoration, and/or enhancement of wetlands and associated uplands habitats for the benefit of all wetlands-associated migratory birds. Funding available for projects requesting between $250,000 and $3,000,000.

Small Grant Acquisition

U.S Fish and Wildlife Service

U.S. Army Corps of Engineers

Acres for America

NA Acquisition

Southeastern Pennsylvania and Lower Delaware River Basin Environmental Improvements Program (566 Program) NA Design, construction

U.S. Department of Transportation Rebuilding American Infrastructure with Sustainability and Equity (RAISE) Program

U.S. Fish and Wildlife Service

NA

Planning, project implementation

Funds public-private partnership projects that further the goals of the North American Wetlands Conservation Act. Projects must involve longterm protection, restoration, and/or enhancement of wetlands and associated uplands habitats for the benefit of wetlands-associated migratory birds. Funding available for projects requesting $250,000 or less.

Funds projects that conserve critical wildlife habitats and lands that connect existing protected lands, and projects that provide public access. Open to nonprofits, state and local government agencies, tribal governments and organizations, and educational institutions. Prioritizes larger-scale conservation projects.

Funding for this authority is provided to the Corps through appropriated funding under Environmental Infrastructure and distributed to specific projects through the annual Work Plan or Congressional Earmark. The 566 Program allows USACE to provide design and construction assistance to non-federal interests for carrying out water-related environmental infrastructure, resource protection, and development projects in southeastern PA.

State and local governments, transit agencies, and publicly chartered authorities are eligible. Eligible applications for funding may include planning and implementation of highway, bridge, rail, and roadway projects, along with culvert or stormwater management projects that improve aquatic habitats.

National Fish Passage Program NA Project Implementation

USDA NRCS Small Watershed Program (PL566)

USDA NRCS Emergency Watershed Protection Program

Structural Watershed Projects Flood control operations, construction

NA Easement acquisition, project implementation

Eligible entities include nonprofits, governmental organizations, businesses, individuals, and academic institutions. Eligible projects will remove instream barriers and restore aquatic organism passage and aquatic connectivity. This includes, but is not limited to, dam removals, culvert replacements, floodplain restoration, and the installation of fishways.

Requires a state, county, or local government sponsor. Eligible projects include flood control structures (dams, levees, channels, etc.) or agricultural water supply reservoirs. Cost share amount is variable depending on project purpose.

Requires a state, county, local government, or conservation district sponsor. Eligible activities include providing financial and technical assistance to remove debris from streams, protect destabilized streambanks, establish cover on critically eroding lands, repairing conservation practices, and the purchase of flood plain easements. The program is designed for installation of recovery measures. Generally, a 25% match is required.

Final Thoughts

Anywhere there is water, there is the potential for flooding. Even with unlimited financial and technological resources, it would be impossible to eliminate all flood risks. However, the Flood Study partners are confident that implementation of the structural and non-structural recommendations laid out in this report can meaningfully reduce future flood risks to communities throughout the Brandywine watershed.

Collectively, this study analyzed roughly 300 sites for flood mitigation projects, including the 5 existing flood control structures in the headwaters, as well as bridges, culverts, low head dams, disconnected floodplains, and major stormwater facilities. Of those, 16 were identified as potential opportunities to reduce flood risk at either the regional or localized scale. These include one major flood control structure upgrade, one large floodplain restoration project, 4 low head dam removals, and 10 bridge replacements. Implementing these projects will be a long-term initiative, requiring buy-in and engagement from partners at multiple levels across numerous sectors.

While the structural recommendations described above are often the most visible types of flood mitigation efforts, planning and public education initiatives are equally as important and, in some cases, may be implemented more rapidly. Emergency preparedness planning, early warning systems, and flooded roadway closure gates can help keep people out of harm’s way during storm events. Structural elevations, property buyouts, and stronger floodplain ordinances can reduce existing and future risks for properties in flood-prone areas. Practices like land conservation are critical for ensuring that future flooding is not exacerbated by new development in sensitive areas like floodplains, which provide immense benefit for flood attenuation.

Though this study’s goal was to holistically assess flood impacts and mitigation opportunities in the Brandywine watershed, the study partners recognize that it is not an exhaustive review of every potential challenge and mitigation strategy. Fortunately, other detailed studies are underway in a number of communities that will complement this study. The information in this report will inform and supplement those efforts. This study is also intended to be broadly transferable, in terms of methodology, data collection and analyses, and flood mitigation recommendations.

Meaningful reductions in flood risk in the Brandywine watershed will require a collective effort from all stakeholders and an ongoing commitment to managing shared waterways. The study partners are committed to supporting municipalities, stakeholders, and others in implementing the strategies outlined in this report, and to continually assessing new opportunities to reduce localized and regional flooding in the future.

To access the Brandywine Flood Study Technical Compendium, which contains

all the data and analysis referenced here, please visit brandywine.org/flood-study or scan the below QR code:

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Pennsylvania Department of Environmental Protection. (2025). Pennsylvania Municipal Separate Storm Sewer Systems (MS4s), Community Status Report. Accessed January 7, 2025.

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Commonly Used Acronyms and Abbreviations

ABBREVIATION MEANING

BC Brandywine Conservancy

BFE Base Flood Elevation

BFS Brandywine Flood Study

BMP Best Management Practice

CCWRA Chester County Water Resources Authority

cfs cubic feet per second

CRS Community Rating System

DE Delaware

DE DNREC Department of Natural Resources and Environmental Control

DENS Delaware Emergency Notification System

DOT Department of Transportation

FEMA Federal Emergency Management Agency

FIRM Flood Insurance Rate Map

FIS Flood Insurance Study

GIS Geographic Information Systems

H&H Hydraulic and Hydrologic

HEC-RAS Hydrologic Engineering Center River Analysis System

HMP Hazard Mitigation Plan

LIDAR Light Detection and Ranging

mph miles per hour

MS4 Municipal Separate Storm Sewer System

ABBREVIATION MEANING

NFIP

National Flood Insurance Program

NOAA National Oceanic and Atmospheric Administration

NPDES National Pollutant Discharge Elimination System

NRCS Natural Resource Conservation Service

NWS National Weather Service

OSI Open Space Institute

PA Pennsylvania

PA DCED

PA DCNR

PA MPC

PADEP

PA Department of Community and Economic Development

PA Department of Conservation and Natural Resources

PA Municipalities Planning Code

PA Department of Environmental Protection

RCE River Corridor Easement

SFHA Special Flood Hazard Area

sq. mi. square mile

TR-55 Technical Release 55

UDWRC University of Delaware Water Resources Center

USACE United States Army Corps of Engineers

USDA

USGS

United States Department of Agriculture

United States Geologic Survey

WSE Water Surface Elevation

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