Technical Compendium: Section 4 - Brandywine Flood Study by Brandywine Conservancy & Museum of Art

Watershed Build-Out Assessment

4.1 Overview

To better understand how future development scenarios might impact flooding events, a watershed-wide buildout 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 this tool reveal the effectiveness of existing zoning in guiding future development, but it also highlights land that may be more appropriate for land preservation. The Brandywine Flood Study utilized a build-out analysis to assess where in the watershed development may occur and how the land use changes during development may impact stormwater runoff volumes, leading to potential increases in flooding conditions.

4.2 Data Sources

The following data layers are utilized to complete the Flood Study’s build-out analysis.

• Floodplain – FEMA (2017)

• Impervious Surface – Chesapeake Conservancy (2017)

• Intensive Land Cover – Delaware Valley Regional Planning Commission (2015) (residential, commercial, utilities, ROWs, industrial, and related non-impervious supporting lands (i.e. lawns))

• Woodlands – Chesapeake Conservancy (2017)

• Wetlands – National Wetland Inventory (NWI 2019)

• Hydric Soils – National Resources Conservation Service (NRCS 2013)

• Slopes – Chester County GIS (Moderate 15%–25% and Steep >25%); New Castle County, Wilmington, Delaware County, and Lancaster County slopes were calculated from USGS Digital Elevation Models (DEMs)

• Utility Right-of-Ways (ROWs) – Delaware Valley Regional Planning Commission (2015)

• Protected Lands – Chester County GIS 2023, Lancaster County GIS 2023, Delaware County GIS 2023, New Castle County GIS 2023

• Streams and Water Bodies – National Hydrography Dataset 2023

• Tax Parcels – Chester County GIS 2023, Lancaster County GIS 2023, New Castle County 2023, Delaware County GIS 2023.

• Roads – Chester County GIS 2022, Delaware County GIS 2022, Lancaster County GIS 2023, New Castle County 2023

• Proposed Developments – Chester County Planning Commission

• Zoning – Chester County GIS (2016) – cross-referenced with local zoning maps

4.3 Analysis of Developable Lands

The first step of this analysis required the removal of all lands currently developed or permanently protected, leaving parcels that have development potential.

• Permanently protected lands include land preserved through conservation easement, agricultural easement, deed restriction, publicly owned land, and Homeowners Association (HOA) open space.

• Developed land was defined as parcels greater than 4 acres, parcels that have more than 30% of the land in intensive land uses, such as impervious surfaces, utility rights-of-way, and irregular parcels with existing development and/or limited access or opportunity for additional subdivision.

Once parcels with development potential were identified, natural resources and other constrained land were removed. Since this analysis was conducted for the entire watershed, natural resource restriction baselines were determined based on best practices and recommendations from the Chester County Planning Commission. The following baselines were assumed for this analysis:

Natural Resource Regulation – Baselines

• 100-Year Floodplain – 0% disturbance

• Riparian Buffer (75 ft.) – 0% disturbance

• Wetlands – 0% disturbance

• Wetland Buffer (50 ft.) – 20% disturbance

• Slopes

• Steep Slopes (25%+) – 15% disturbance

• Moderate Slopes (15-25%) – 30% disturbance

• Woodlands – 50% disturbance

These baselines were removed from the developable acres at the parcel level.

4.4 Municipal Zoning Analysis

The third step in this analysis required the review of municipal zoning for all parcels included in this analysis, to determine the maximum impervious surface allowed by-right based on the local zoning district. Table 4.1 provides a summary of the existing municipal natural resource ordinances.

Municipality

Table 4.1 Summary of Exis ng Municipal Natural Resource Ordinances

Table 4.1 Summary of Existing Municipal Natural Resource Ordinances

Chester County guidance 15% disturbance

Birmingham Township 15% disturbance

Caln Township

Charlestown Township Low disturbance

Coatesville Township 15% disturbance

Chester County, PA

disturbance Limit to 20% disturbance of woodlands which overlap other natural resource areas. Limit to 50% disturbance of woodlands which do not overlap other natural resource areas.

Downingtown Borough

Township

Table 4.1 (continued)

Highland Township 15% disturbance 30% disturbance 35% disturbance for residential 50% disturbance of non-residential

Honey Brook Borough NA

Honey Brook Township 0% disturbance 30% disturbance 10% disturbance

Kennett Township 15% disturbance 30% disturbance 25% disturbance

Limits disturbance within a 25-foot buffer

35-ft. buffer-no disturbance

35 to80-ft. buffer allows 20% disturbance

80 to100-ft. buffer allows 20% disturbance

Limits disturbance within a 50-ft. buffer

Limits disturbance within a 10-ft. buffer

Limits disturbance within a 50-ft. buffer

Londonderry Township NA NA NA NA

Modena Borough 30% disturbance NA Parcels less than 1 ac.: allowed 75% disturbance

Parcels 1-5 ac.: allowed 65% disturbance

Parcels greater than 5 ac.: 50% disturbance

Newlin Township No disturbance Limited disturbance NA

Limits disturbance within a 50-ft. buffer

Limits disturbance within a 75-ft. buffer

30-ft. buffer: no disturbance

30 to70-ft. buffer allows 20% disturbance

Limits disturbance within a 25-ft. buffer

Limits disturbance within a 50-ft. buffer

Dwellings must be located 100 ft. away

Parkesburg Borough 20% disturbance 40% disturbance NA NA NA

Pennsbury Township 15% disturbance 30% disturbance 35% disturbance for Residential 50% disturbance of non-residential

Limits disturbance within a 50-ft. buffer

Pocopson Township 10% disturbance 25% disturbance 20% disturbance Limits disturbance within a 50-ft. buffer

Sadsbury Borough Limited disturbance Limited disturbance 50% disturbance

Limits disturbance within a 50-ft. buffer

35-ft. buffer: no disturbance

35 to100-ft. buffer allows 20% disturbance

25-ft. buffer: no disturbance

25 to100-ft. buffer allows 20% disturbance

50-ft. buffer:no disturbance

50 to100-ft. buffer allows 20% disturbance

South Coatesville Borough Limited disturbance Limited disturbance NA NA NA

Thornbury Township No disturbance 60% disturbance Up to 10,000 sq. ft.

Limits disturbance within a 100-ft. buffer

Limits disturbance within a 50-ft. buffer

Municipality

Table 4.1 (continued)

Upper Uwchlan Township Limited disturbance Limited disturbance 25%-50% disturbance

Uwchlan Township 15% disturbance Low intensity uses 25% disturbance

Valley Township 25% disturbance 30% disturbance Conservation zone: 35% disturbance

Residential: 50% disturbance for parcels less than 1 ac. and 35% disturbance on parcels more than 1 ac.

Agricultural uses: 75% disturbance

Non-residential:50% disturbance

Industrial:85% disturbance

Wallace Township 20% disturbance 10% disturbance

Class 1 10%

Class 2 15% Class 3 25%

West Bradford Township 5% disturbance 25% disturbance 10% disturbance of forest interior 20% disturbance of forest

West Brandywine Township Limited disturbance Limited disturbance

West Caln Township 15% disturbance 30% disturbance 35% disturbance of residential 50% disturbance of non-residential

West Chester Borough

West Fallowfield Township

West Goshen Township 0% disturbance 20% disturbance Replacement requirements

Limits disturbance within a 25-ft. buffer

Limits disturbance within a 50-ft. buffer

Limits disturbance within a 75-ft. buffer

Limits disturbance within a 50-ft. buffer

100-ft. Buffer: no disturbance 150-ft. buffer for streams listed as impaired

Limits disturbance within a 50-ft. buffer

Inner 50-ft. buffer: no disturbance Outer 50-ft. buffer: allows 20% disturbance

Limits disturbance within a 15-ft. buffer and 25-ft. buffer in wetland headwaters 100-ft. buffer on headwaters streams no disturbance 75-ft. buffer on other waterways

Limits disturbance to 20% within a 100-ft. buffer

disturbance within a 25-ft. buffer

Zone 1: no disturbance within 35-ft. buffer Zone 2: 35 to 100- ft. buffer allows 20% disturbance

Limits disturbance within a 100-ft. buffer

Table 4.1 (continued)

Municipality

West Marlborough Township Require study methods to preserve Require study methods to preserve 20% disturbance NA

West Nantmeal Township 10% disturbance 50% disturbance Permit required

West Sadsbury Township 10% disturbance 20% disturbance 34% disturbance

West Vincent Township Maximum impervious surface 5 to 30% depending on use Maximum impervious surface 8 to 40% depending on use 35% disturbance of residential 50% disturbance of non-residential

West Whiteland Township Limited disturbance Limited disturbance NA

Limited disturbance in the following zones:

Zone 1:15- ft. buffer

Zone 2: 65-ft. buffer

Zone 3: 75- ft. buffer

Limits disturbance within a 50-ft. buffer NA

Limits disturbance within a 100-ft. buffer

Zone 1: no disturbance within 35-ft. buffer

Zone 2: 80-ft. buffer allows limited disturbance

Zone 3: 100-ft. buffer allows stormwater discharge

Limits disturbance to 20% within a 100-ft. buffer

Limits disturbance within a 25-ft. buffer

Limits disturbance within a 150-ft. buffer

Westtown Township Limited disturbance Limited disturbance 10% disturbance

Delaware County, PA

Chadds Ford Township 70% excluded from lot calculations NA To the maximum extent possible, mature, healthy trees of 12 in. or greater in caliper and other significant existing vegetation shall be retained and protected.

New

Limits disturbance within a 75-ft. buffer

Identified a 25-ft. buffer; 85% of area within wetlands, and 50% of area within wetland margins, to be excluded from lot area calculation.

Castle County, DE 0% disturbance 50% disturbance Max disturbance for young forest 50%

Limits disturbance within a 50-ft. buffer

Zone 1: 25-ft. no disturbance

Zone 2: 150– ft. limited disturbance

Zone 3 :175-ft. regulated disturbance

Limits disturbance within a 75-ft. buffer

Limits disturbance within a 50-ft. buffer

Limits disturbance within a 100-ft. buffer

4.5 Impervious Surface Estimation

Finally, based on the local zoning, impervious surface estimates were calculated for each developable parcel to determine the maximum by-right development potential. Impervious surface estimates were calculated using the identified impervious coverage percentages for each zoning district within the local zoning code. Impervious coverage or lot coverage typically includes all impervious surfaces including the building footprint, accessory structures, and paved surfaces. While the specific calculations may differ by municipality, the example below calculates impervious coverage using the gross lot area; however, some municipalities calculate this number using the net lot area once constrained lands are deducted. Table 4.2 illustrates sample impervious surface calculations.

4.6 Results

The final build-out calculation provides a clear picture of the development potential over the entire municipality. The resulting map (Figure 4.1) shows potential additional acres of impervious coverage if all undeveloped parcels were developed by right. While this map shows the worst-case scenario in terms of development, it also reveals opportunities for targeted land preservation.

If all parcels are developed according to the existing zoning, an estimated 25,760 ac. of impervious surface could be added to the watershed, and approximately 23,747 new parcels could be added through subdivisions (Table 4.3). This development activity could also cause the loss of about 2,066 ac. of woodland and an estimated 16,319 ac. of agricultural land (Table 4.4). A full summary of results by municipality is provided in Table 4.5.

Table 4.2 Impervious Surface Es mates by Zoning District

Table 4.2 Impervious Surface Estimates by Zoning District

Figure 4.1 Potential for Impervious Buildout in the Brandywine Watershed

Figure 4.1 Poten al for Impervious Buildout in the Brandywine Watershed

Table 4.3 Impervious Surface Estimates by Subbasin

Table 4.3 Impervious Surface Es mates by Subbasin

Impervious Surface Es mates by Subbasin

Subbasin Es mated addi onal impervious surface (ac.)

Table 4.4 Maximum Development Impact

Maximum Development Impact

Poten al impervious coverage 25,760 ac.

Number of addi onal parcels 23,747 parcels

Poten al woodland loss - 2,066 ac.

Poten al loss of agricultural lands -16,319 ac.

Birmingham Township

Caln Township

City of Coatesville

Chester County, PA

Table 4.5 (continued)

East

Fallowfield

Township

East

Marlborough Township

East

Nantmeal

Township

East

Whiteland Township

Table 4.5 (continued)

Highland Township

Honey Brook Borough

Honey Brook Township

Kennett Township

Table 4.5 (continued)

Londonderry Township

Table 4.5 (continued)

Pennsbury Township

Pocopson Township

Sadsbury Township

South Coatesville Borough

Table 4.5 (continued)

Thornbury Township

Upper Uwchlan Township

Wallace Township

West Bradford Township

West Brandywine Township

West Caln Township

Table 4.5 (continued)

West Chester Borough

West Fallowfield Township

West Goshen Township

West Marlborough Township

Table 4.5 (continued)

West

Nantmeal Township

West

Sadsbury Township

West

Township

West

Whiteland Township

Table 4.5 (continued)

Westtown Township

Chadds Ford Township

Concord Township

Bethel Township

Table 4.5 (continued)

Delaware County,

New Castle County

Wilmington

Table 4.5 (continued)

The build-out analysis also reveals potential changes in land use if all undeveloped parcels are developed. Based on existing zoning, over 7,600 buildable ac. are in single-family residential zoning, while over 640 buildable acres are in industrial zoning and 450 buildable acres are zoned commercially. Additionally, agricultural lands are likely to see the most impact from development (Figure 4.2). The results of the watershed buildout assessment were incorporated in the hydrologic and hydraulic (H&H) models to characterize flooding impacts to current flood-prone areas and identify communities which may be subject to increases in flood risk in the future.

Figure 4.2 Potential Land Use Change