Predicting Design Storm Runoff for the Six Mile Creek Watershed: Effects of the Potential Sprawl-Is-Us, Inc. Development

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1 Predicting Design Storm Runoff for the Six Mile Creek Watershed: Effects of the Potential Sprawl-Is-Us, Inc. Development Ross Guida Faulkner Consulting Engineers, Inc. December 19,

2 Table of Contents Section Page Introduction..3 Methods 4 Results...6 Discussion.8 Conclusion 9 Appendix.10 2

3 Introduction This report focuses on estimating the total amount of runoff for a 100-year, 24-hour design storm in the Six Mile Creek Watershed, located in Eau Claire County, Wisconsin (Fig. 1). The Six Mile Creek Watershed is located in a rural setting approximately 5 miles east of the University of Wisconsin- Eau Claire (Fig. 2). Much of the current watershed land use is agricultural, but Sprawl-Is-Us intends to develop the area south of U.S. Highway 12 for residential and commercial purposes (Fig. 1). Since state law requires the developer build a detention basin to hold all extra runoff resulting from the urbanization of a watershed, runoff values for the current land use of the watershed will be compared to the proposed development watershed runoff values. If the proposed development runoff value is higher than the current runoff value, a detention basin will be required for Sprawl-Is-Us to build using the given Six Mile Creek development plan (Fig. 4). Figure 1: Six Mile Creek Watershed Location Legend Wisconsin Counties Miles Legend 4 Proposed Development SixMileWShed Miles Figure 1: Location Map of Six Mile Creek within the state of Wisconsin 3

4 Figure 2: Six Mile Creek Watershed Location Map Methods To predict runoff for the Six Mile Creek watershed, the Soil Conservation Service Curve Number Method will be used to calculate the curve number for the current land use of the watershed and then the proposed land use of the watershed. A curve number is an empirical runoff index based on: land use and land cover (LULC); hydrologic condition of the land cover; infiltration properties of the soils; and antecedent soil moisture conditions. A higher curve numbers means there is more runoff generated. Generally, urban areas have higher curve numbers than well-maintained agricultural areas (Charts 1 and 2) These steps, which will be outlined in more detail, are used to determine the composite curve number of a watershed: 1. Map land use/land cover (LULC) types 2. Determine and map hydrologic properties 4

5 3. Determine and map soils based on their hydrologic soil groups 4. Using Curve Number (CN) Tables (Charts 1,2), determine the CN for each mapped combination of: a. LULC b. Hydrologic condition c. Hydrologic soil group 5. Calculate the weighted CN of the watershed a. Composite CN = [N i (A i /A d )] 6. Determine storm runoff depth from a design storm a. Runoff (in in.) = [P-0.2S] 2 [P+0.8S] The curve number for each land use/land cover is different (Charts 1 and 2). To get the curve number for the current land use in the Six Mile Creek watershed, LULC is digitized over a 2008 mosaic of orthorectified aerial photographs from the National Agriculture Imagery Program of the USDA Farm Security Administration. Digitizing is done using a 1:2,000 scale in ArcMap GIS 9.3. A geodatabase is created in ArcCatalog with layers for each LULC so area of each digitized land use/land cover is automatically calculated in ArcMap. Agricultural areas where crops aren t currently being grown are digitized as fallow because it appears the land and soil is not well maintained. Row crops include corn and dark green row vegetation is considered to be alfalfa. Open areas covered in grass are considered to be meadow. Wooded or forested areas are those with more than several trees together. When digitizing is complete for the current watershed land use (Fig. 3), the Eau Claire soils layer is intersected with each LULC layer so each digitized layer has a soil type of A, B, or D (there is no C in the study area). Now, each LULC is assigned the correct curve number for 5

6 each soil group using the Field Calculator in ArcMap for a newly created CN field (Charts 1, 2). These steps are repeated for the proposed development south of U.S. Highway 12 (the development is a supplied geodatabase that is added to ArcMap after all digitizing south of Hwy. 12 is removed) (Fig. 4). The resulting curve numbers and areas for each land use/land cover layer are then exported into two Microsoft Excel spreadsheets, current land use (Fig.3, Chart 3) and proposed land use (Fig. 4, Chart 4) for further calculations. In Excel, a formula is then entered to calculate the weighted (composite) curve number ( each LULC curve number as a percent of total watershed area) for each the developed and proposed watershed land uses: Composite watershed curve number = ( CN of each LULC * Area of each LULC) Area of the watershed) (Charts 3 and 4). The weighted watershed curve number can then be used to calculate runoff using the Soil Conservation Service formula: Runoff (in in.) = [P-0.2S] 2 /[P+0.8S], where S= (1000/CN), P= the total design storm precipitation depth (in in.). CN is the composite watershed curve number. For the study area, Six Mile Creek in Eau Claire County, a design storm of 24-hr. duration and 100 yr. frequency yields a storm depth of 5.8 inches so P =5.8. Results are now calculated. Results The results differ between current land use in the Six Mile Creek watershed and proposed land use in the watershed (Table 1). The curve number is higher for the proposed land use when compared to the current land use curve number. Therefore, runoff volume is higher with current Table 1: Results Current Land Use Proposed Land Use Curve Number Curve Number P=Total Precip of 100 yr. 24 hr. storm 5.8 P=Total Precip of 100 yr. 24 hr. storm 5.8 S=(1000/CN) S= (1000/CN) Runoff (in in.)=[p-.2s]^2/[p+.8s] 2.17 Runoff (in in.)=[p-.2s]^2/[p+.8s] 2.04 Cubic Feet of Runoff Cubic Feet of Runoff Current Acre-Feet of Runoff Proposed Acre Feet of Runoff

7 Figure 3: Six Mile Creek Watershed Current Land Use Legend SixMileWShed MainChannel Forest_Intersect Fallow_Intersect Row_Crops_Intersect CSL_or_RMeadow_Intersect Meadow_Intersect Farmsteads_Intersect Dirt_Roads_Intersect Paved_Roads_Intersect Dirt_Roads Miles 4 Figure 3: Current land use digitized over aerial photo mosaic Figure 4: Six Mile Creek Watershed Proposed Land Use Legend MainChannel SixMileWShed Meadow_Intersect Meadow_Intersect1 Paved_Roads_Intersect_Inters PavedStreets_Lots_Intersect Roads_Hard_Intersect Row_Crops_Intersect RowCrop_SR_Intersect Dirt_Roads_Intersect Roads_Dirt_Intersect Forest_Intersect Woodlands_Intersect Fallow_Intersect Farmsteads_Intersect CSL_or_RMeadow_Intersect Farmsteads_Intersect1 Pasture_Intersect Alfalfa_Intersect Parks_Intersect1 quarteracre_lots_intersect halfacre_lots_intersect acre_lots_intersect Commercial_Intersect 4 Miles Figure 4: Imported digitized geodatabase of proposed Sprawl Is Us development south of U.S. Hwy. 12; digitized LULC north of U.S. Hwy. 12 is from the current land use map (Fig. 3) 7

8 land use than it would be with the proposed land use (Graph 1). Results naturally follow this trend since the curve number is used to calculate the total depth of runoff and then the volume of runoff Graph 1: Runoff generated by the 100 yr, 24 hr Design Storm for the Six Mile Creek Watershed Acre Feet of Runoff Current Land Use Land Use Proposed Land Use Graph 1: Acre feet of runoff generated by the design storm for the Six Mile Creek watershed Discussion Since runoff values are lower for the proposed development than they are with current runoff values, and there are two agricultural factors with higher curve number values, it must be those land use/land covers (fallow and straight row crop with poor soil conditions) that accounts for part of the difference in curve numbers. Fallow is land that isn t kept up and has been compacted from years of agriculture, contributing to high runoff numbers. The straight row 8

9 crops appear to have been farmed for years and also result in compacted soil and higher amounts of runoff. The new development proposal also includes significantly sized lots. Since it is assumed many of these lots will consist of a large area of lawns, the soil will be allowed to return to its natural state. Therefore, more water will infiltrate into the ground, creating less runoff. Paved roads and parking lots absorb almost no water and they are also part of the development. But, they have a much lower area and weight when compared to other LULC areas in the watershed. That means the urbanization land uses with the highest curve numbers don t have too much influence of this specific development. Sprawl-Is-Us, Inc. will not have to build a detention basin to store excess runoff caused by urbanization because there simply is no excess. In fact, 30 acre-feet less runoff is generated during a 100-year, 24 hour storm if the proposed development is built. As a result, the state law of building the excess runoff detention basin does not apply to this specific proposal. Conclusion Six Mile Creek is currently a rural watershed with primary land use/land cover that is agricultural. Row crops and fallow are the main agricultural land uses and the soil conditions for these land uses yield higher curve numbers and higher runoff values when compared to other rural types of land use/land cover and even urban land use/land cover. Urbanization often increases runoff values, but it s not always the case. If large lots (1/4 acre or more), open areas with more than 75% grass, and little pavement is used, urban areas can actually reduce runoff values. Using the SCS curve number method with the design storm of 100-years and 24-hours has a depth of 5.8 inches inches (current) and 2.04 inches (developed) of runoff is 9

10 generated. Less runoff in the proposed development accounts for a difference of more than 30 acre feet of runoff. Sprawl-Is-Us, Inc. is therefore not required by state law to build a detention basin for their urbanization of the Six Mile Creek Watershed. If they plan to build they can use their current proposal for a new development in the Six Mile Creek Watershed. If the proposal changes, it is important to note a new study will need to analyze any changes in land use/land cover. Specifically, more paved areas will most likely cause runoff greater than both runoff given by current land use in the watershed and the current proposal. If more paved areas become necessary changes to the development, it is likely a detention basin would need to be built. To avoid this scenario, it is suggested the current proposal not be changed. 10

11 Appendix Chart 1: Curve Numbers for Non-Urban Areas 11

12 Chart 2: Curve Numbers for Urban Areas 12

13 Graph 2: Curves for Determining Runoff 13

14 Chart 3: Six Mile Creek Current Land Use LULC Forest Row Crop Fallow Dirt Road Paved Road CSLorRW Meadow Farmstead Curve Number CN Area (m^2) CN*Area P=Total Precip of 100 yr. 24 hr. storm 5.8 S=(1000/CN) Runoff (in in.)=[p.2s]^2/[p+.8s] 2.17 Cubic Feet of Runoff Acre Feet of Runoff

15 Chart 4: Six Mile Creek with Proposed Land Use LULC CN Sum_SHAPE_ CN*Area Alfalfa Commercial CSLorRMeadow Dirt Roads Dirt Roads_P Fallow Farmsteads Farmsteads_P HalfAcreLots Meadow Meadow_P OneAcreLots Parks Pasture PavedRoads PavedStreetsLots_P QuarterAcreLots

16 RoadsHard_P StraightRowCrops StraightRowCrops_P Woodlands Woodlands_P Curve Number P=Total Precip of 100 yr. 24 hr. storm 5.8 S= (1000/CN) Runoff (in in.)=[p.2s]^2/[p+.8s] 2.04 Cubic Feet of Runoff Acre Feet of Runoff