Modeling Urban Forest Scenarios and Hydrology in Grand Rapids, Michigan Prepared by Plan-It Geo, LLC for the City of Grand Rapids, Michigan Completed November 2015
1|Page Modeling Urban Forest Scenarios and Hydrology in Grand Rapids, Michigan
Overview Tree canopy in Grand Rapids provides a host of benefits to the City and its residents. In recent years, Grand Rapids has initiated laudable efforts to assess, grow, and maintain a healthy urban forest through outreach, education, monitoring, policy tools, and planning. In 2009, the City set a goal of attaining 40% average urban tree canopy (UTC). In 2015, the City contracted with Plan-It Geo to complete an updated assessment of UTC in collaboration with Friends of Grand Rapids Parks. Results show the City is currently at 34% average UTC. With shared funding from the Environmental Services and Parks & Recreation Departments, this pilot project study used i-Tree Hydro (beta) to model the stormwater management impacts of the current and projected canopy for the 40% canopy goal in Grand Rapids. Preserving and enhancing tree canopy regulates streamflow by slowing down the rate at which stormwater reaches surface waters through interception, stemflow, evapotranspiration, and soil infiltration. Trees along the banks of rivers and streams stabilize soils and mitigate erosion during large storm events. Urban Tree Canopy also helps to promote soil structure and quality through nutrient cycling and providing decomposed organic matter for topsoil (Dunne & Leopold, 1978). Hydro is a stand-alone application designed to simulate the effects of changes in tree and impervious cover characteristics within a defined watershed on stream flow and water quality. It was designed specifically to handle urban vegetation effects so urban natural resource managers and urban planners can quantify the impacts of changes in tree and impervious cover on local hydrology to aid in management and planning decisions. Preliminary results for three study areas were presented at an EPA climate resiliency charrette in October 2015. Feedback received helped to refine the modeling approach and choose a local monetary value to apply to the modeling results. Finally, benefits were compared to the cost of planting and establishing enough trees to reach an increase of 6% canopy in each area. This report outlines the model inputs, assumptions, methods, study areas, results, and possible next steps.
Figure 1: Various processes of the water cycle during a storm.
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Study Areas Three areas in Grand Rapids were selected for land cover replacement (tree canopy increase) scenarios at the sub-watershed scale. 1) The Plaster Creek Watershed extends beyond the City limits and was modeled in its entirety, based on the provided sub-watershed data. This area was of particular interest due to development pressures and watershed health. 2) A collective area coined Core City Sub-Watersheds was chosen due to its proximity to the Grand River and dominant flow direction of streams in the area. The selected sub-watersheds were clipped to the City’s boundary, aligning with data from the 2015 land cover and UTC assessment. 3) First Street Sub-Watershed was chosen in order to apply the i-Tree Model in a single unit.
First Street
Figure 2: Study boundaries for the i-Tree Hydro analysis
Methods i-Tree Hydro Requirements and Assumptions Due to the complexity of urban runoff modelling, many assumptions had to be made in this study:
Using hourly streamflow and weather data. Impervious flow may be discharged at outlets other than a treatment plant. Impact to stormwater structures was treated as impervious runoff only. The chosen weather station accurately represents the entire study area. Ideally, the station would be centrally located, at an average elevation, and have an extensive and verified data record. Local data on stream flow and weather data was insufficient to use the calibration method in iTree Hydro where stream flow can be reconstructed and predicted.
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Monetary benefit values (avoided stormwater construction costs) were provided using local values (see page 7). Soil texture/structure was uniform across the entire study area as an input in i-Tree Hydro.
Data Inputs One challenge of modeling the effects of UTC and land cover on urban hydrology with i-Tree Hydro is data availability. Ideally, stream gauge (flow) data and weather station data would be available for any size watershed or sub-watershed in a study area. However, these sources are very limited in Grand Rapids (see Figure 3). The following stations were chosen and used for the three study areas: USGS Stream Gauging Station: GRAND RIVER AT GRAND RAPIDS, MI 04119000 Weather Station: GRAND RAPIDS/KENT C 726350-94860 Period of Record: 6/27/2012 0:00 – 6/27/2012 23:00
Weather Stations:
A
C
A. Muskegon B. Tulip City C. Grand Rapids/Kent County
B
USGS Stream Gauge Stations: A. ROGUE RIVER NEAR ROCKFORD, MI 04118500 B. GRAND RIVER AT GRAND RAPIDS, MI 04119000
A
B
Figure 3: Locations of available weather and stream gauge stations
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Case-Specific Inputs In order to refine cost-benefit estimates of impervious runoff, a design storm was used in place of an annual report. To estimate runoff, a design storm was used to replicate a 100-year, 24-hour storm event based on Atlas 14 projections for the year 2050. The use of a 100 year design storm is consistent with stormwater mitigation best management practices (BMPs) and is intended to be a preparation for a statistically probable event within the lifetime of the structure (it is financially impossible to prepare for the worst when designing stormwater structures). Predicted rainfall for this design storm was 6.27 inches, with a Type II rainfall distribution typical for the area (Figure 4).
Figure 4: NRCS SCS TR 55 Rainfall Distributions
The duration of the storm, as stated above, was 24 hours, taking place at the end of June in 2012 (data availability restricted a more recent year from being used). A summer month was chosen to ensure that all precipitation was rainfall and that frozen conditions did not exist on the river (which have a tendency to make collecting valid data difficult). At the suggestion of the i-Tree staff, the weather and evaporation data input files were manipulated to replicate a Type II storm and then re-entered into Hydro (Coville, 2015). Detailed information on the rainfall distribution can be found in the Assessment Spreadsheet.
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Case-Specific Inputs (continued)
Figure 5: Land Cover classification completed by Plan-It Geo. Included are land cover points for Plaster Creek as well as the three Hydro boundaries.
Land cover percentages are an additional requirement for the i-Tree Hydro model. Since the 2015 land cover / UTC assessment did not cover all of Plaster Creek, i-Tree Canopy (https://www.itreetools.org/canopy/index.php) was used to estimate land cover percentages based on sampling of 1,000 random points locations and aerial imagery. Five land cover classes were classified: tree canopy, vegetation, water, soil, and impervious. For the First Street and Core City Sub Watershed study areas, Plan-It Geo’s existing UTC study data were used (Figure 5). 6|Page Modeling Urban Forest Scenarios and Hydrology in Grand Rapids, Michigan
Study Results Results from this pilot project using i-Tree Hydro (beta) are summarized for the three study areas below. Included are estimates for avoided runoff costs, projections for planting projects to increase each area’s UTC by 6%, and a cost benefit analysis comparing the costs of construction with stormwater, air quality, and carbon sequestration benefits. Canopy Increase Scenarios Results in all three areas highlight the importance of tree canopy in Grand Rapids and potential for the City’s 40% UTC goal to contribute environmentally and economically. As shown in Table 1 below, an increase of 6% to each study area resulted in decreases in runoff of about 5% in all areas. This resulted in an increase in savings ranging from $2.0M to $52.7M. Monetary values were approximated using a localized estimated construction cost of $1/gal for stormwater mitigation structures and a $0.25/gal estimated environmental impact costs (Mike Lunn, Director Environmental Services Division – Grand Rapids MI).
Table 1: Runoff and Total Savings comparisons for the three study areas in % change and dollars, respectively.
Study Area
% Decrease in Impervious Runoff
Plaster Creek Watershed Core City Sub-Watersheds First Street Sub-Watershed
-5% -5% -5%
Total Savings (Avoided Cost) $ $ $
Cost Estimate Comparisons
Cost Estimate Comparisons $900,000,000 $800,000,000
$32,000,000
$700,000,000
Cost Estimates ($)
Cost Estimates ($)
52,720,171 36,505,533 2,039,770
$600,000,000 $500,000,000 $400,000,000 $300,000,000 $200,000,000 $100,000,000
$31,500,000 $31,000,000 $30,500,000 $30,000,000 $29,500,000
$Plaster Creek Base Case
Core City SubWatersheds
Canopy Increase of 6%
$29,000,000 First Street Base Case
Canopy Increase of 6%
Figure 6: Comparisons of cost estimates ($) in the three study areas.
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Cost of Tree Planting and Establishment for UTC Goals A basic analysis was conducted to evaluate the cost of planting and establishment to increase canopy by 6% in each of the three areas. Given that significantly fewer large stature trees (elms, oaks, etc.) are needed to reach a canopy goal compared to smaller stature trees (apple, pear, etc.), tree species categories of small, medium and large size at maturity was also evaluated to compare the cost of planting/establishment. Three tree sizes (at maturity) were used; Small (20-foot crown spread, Medium (30-foot), and Large (40foot, or a 20-foot radius). The number of trees needed to increase canopy by 6% in each area was calculated. The source of average cost for planting and establishment of each tree size was taken from the US Forest Service’s Community Tree Guides. Finally, total costs to establish tree canopy were approximated for each area (Table 2). The scope of this study and this exercise were not intended to provide a comprehensive cost/benefit analysis but does provide a framework and initial view of how the costs of canopy goals play out against canopy benefits, with an emphasis on stormwater management. The results in Table 2 below illustrate the savings by planting and maintaining fewer large stature tree species compared to small and medium sized species. For example, in Plaster Creek it would cost an estimated $99.3 million to increase the tree canopy by 6% establishing small trees over 20 years whereas it would cost an estimated $42.1 million by establishing large trees. Note that canopy increases from natural regeneration (which can be significantly lower in cost than individual tree planting) were not included in this simple analysis.
Table 2: Cost approximations to increase canopy by 6% using different tree sizes.
Study Area
Area (km2)
Plaster Creek
120.7
Core City Sub Watersheds
80.0
First Street
4.0
Tree Size
Crown Diameter (ft)
Trees/acre
Original Canopy
S M L S M L S M L
20 30 40 20 30 40 20 30 40
138.7 61.1 34.6 138.7 61.1 34.6 138.7 61.1 34.6
29.8% 29.8% 29.8% 33.9% 33.9% 33.9% 26.6% 26.6% 26.6%
Canopy Trees Needed Projected Needed for 6% for 6% Canopy Increase (ac) Increase 35.8% 1789.5 248,208 35.8% 1789.5 109,341 35.8% 1789.5 61,918 39.9% 1186.1 164,513 39.9% 1186.1 72,471 39.9% 1186.1 41,039 32.6% 59.3 8,226 32.6% 59.3 3,624 32.6% 59.3 2,052
Cost/year * $ $ $ $ $ $ $ $ $
20.00 27.00 34.00 20.00 27.00 34.00 20.00 27.00 34.00
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Cost To Establish ($)** 99.3M 59.0M 42.1M 65.8M 39.1M 27.9M 3.3M 2.0M 1.4M
Cost/Benefit Analysis When analyzing the costs and benefits of tree planting and canopy goals, it is important to consider not only stormwater management but the provisioning of services such as carbon storage/sequestration and air quality annually and over many years. By quantifying the myriad benefits that the urban forest provides, a clearer picture of their importance can be seen. Table 3 illustrates that when only considering stormwater management benefits of urban tree canopy, planting costs outweigh the benefits they provide in many cases. However when considering added benefits such as carbon sequestration and air quality (estimated over a time period of 20 years, an average life of a stormwater structure), the benefits outweigh the costs of planting and establishing trees in all cases. It should be noted that other services like energy efficiency savings and benefits such as property values were beyond the scope of this study but if included could substantially alter the outcomes more positively. Table 3: Cost Benefit Analysis of tree planting projects, stormwater benefits, and air quality/carbon benefits.
Study Area
Plaster Creek Core City Sub Watersheds First Street
Costs Total Tree Size Cost To Establish ($) S 99.3M M 59.0M L 42.1M S 65.8M M 39.1M L 27.9M S 3.3M M 2.0M L 1.4M
Benefits Estimated Stormwater Benefits ($) 52.7M 52.7M 52.7M 36.5M 36.5M 36.5M 2.0M 2.0M 2.0M
Carbon Air Quality Benefits Benefits ($) [20 ($) [20 year]* year]* 24.7M 27.9M 24.7M 27.9M 24.7M 27.9M 18.3M 20.6M 18.3M 20.6M 18.3M 20.6M .7M .8M .7M .8M .7M .8M
Net Benefits ($) [Avoided Runoff Only] -46.6M -6.3M 10.6M -29.3M -2.6M 8.6M -1.3M .1M .6M
Benefits ($) [WITH Air/Carbon] 6.0M 46.3M 63.2M 9.6M 36.3M 47.5M .3M 1.7M 2.2M
Next Steps/Discussion When considered in conjunction with many other best management practices for mitigating stormwater issues, urban tree canopy proves to be an incredibly valuable asset to the community. The findings in this pilot project study should be used to help influence decisions on the preservation and maintenance of urban trees and forested areas as well as illustrate the importance of community development and green infrastructure projects which incorporate and maximize establishment of urban tree canopy. The City’s efforts to establish tree canopy goals for broad land use types and minimum soil volume requirements in parking lot islands are positive and progressive policy planning tools to reach these environmental and economic community goals. Additionally, local level tree plans such as in the East 9|Page Modeling Urban Forest Scenarios and Hydrology in Grand Rapids, Michigan
Hills Neighborhood are incorporating a needed social capital element to preserve and expand canopy on private property where less space on public land is available. As mentioned above, the scope of this study did not analyze and encompass all of the benefits that urban tree canopy provides. Future studies can incorporate natural regeneration, tree growth, tree mortality rates and benefits from energy savings and property values to further refine this type of modeling and improve the cost-benefit analysis outcomes.
References Coville, Robert. E-mail interview. Nov. 2015. Dunne, Thomas, and Luna B. Leopold. Water in environmental planning. Macmillan, 1978. McPherson, E. Gregory, et al. Northeast Community Tree Guide: Benefits, Costs, and Strategic Planning. US Department of Agriculture, Forest Service, Pacific Southwest Research Station, 2007.] Contributors to this report include: Mike Lunn, Grand Rapids Environmental Services Division Carrie Rivette, Acting Wastewater/Stormwater Superintendent Suzanne Schulz, AICP Planning Director Joe Sulak, Parks Superintendent & City Forester
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Supplemental Information on Flash Flooding and Canopy Many of the more dire flooding situations that occur in urban areas are due in part to an abundance of impervious areas within any given city. In a natural system, surfaces are rougher and much more porous (loosely compacted soil/grass/etc.). This prohibits all of the stormwater from reaching the channel at once because of interception from tree canopy and infiltration into healthy, stable, well-structured soils (which are promoted by healthy understory and overstory vegetation). In the average American city, much of the bare soil, native grass, and large stands of trees have been replaced with parking lots, sidewalks, roads, buildings, etc. With a smoother surface and little exposed healthy soil to infiltrate, the same size/intensity storm producing the same amount of runoff will reach the channel much faster, and over a shorter period of time, putting strain not only on the channels within the city, but on riveradjacent developments and treatment facilities. As shown in Plan-It Geo’s i-Tree Hydro assessment, an increased amount of canopy cover decreases the amount of impervious flow that occurs in any given storm event, making healthy trees and understory an integral part of green infrastructure design.
Figure 7: Difference in response times in a natural system versus an urbanized system. Note that the lag time between the rainfall event and the peak stream discharge is shorter, and the rate of change in discharge much more drastic than the natural system. These kinds of response times are indicative of flash flood scenarios. Source: http://commons.wvc.edu/rdawes/g101ocl/Basics/streams.html
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