Dane County Stormwater Manual

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green_roof [2023/02/16 14:38] – theresa_nelsongreen_roof [2024/02/08 15:33] (current) – admin
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 No credit shall be granted for Total Suspended Solids (TSS) or Total Phosphorus (TP) reduction for water passing through or over a green roof system; however, reduction on a site-wide basis can be realized through a reduced overall volume of stormwater due to retention in the green roof. No credit shall be granted for Total Suspended Solids (TSS) or Total Phosphorus (TP) reduction for water passing through or over a green roof system; however, reduction on a site-wide basis can be realized through a reduced overall volume of stormwater due to retention in the green roof.
  
-Peak rate and volume calculations should follow the steps below to determine an adjusted curve number for use in hydrology calculations. Volume reduction on a green roof is achieved primarily through the drainage layer and growing medium components, which absorb and retain a portion of the rainfall that lands on it making it available for plant uptake and evapotranspiration. For the calculation of runoff retention in the growing medium, the maximum available water capacity for runoff reduction is the difference in the water content between the field capacity and the wilting point.+====Adjusted Curve Number==== 
 + 
 +Peak rate and volume calculations should follow the steps below to determine an adjusted curve number for use in hydrology calculations. Volume reduction on a green roof is achieved primarily through the drainage layer and growing medium components, which absorb and retain a portion of the rainfall that lands on it making it available for plant uptake and evapotranspiration. 
 + 
 +For the calculation of runoff retention in the growing medium, the maximum available water capacity for runoff reduction, or **𝜂<sub>1</sub>**, is the difference in the water content between the field capacity and the wilting point.
  
   * Field capacity is the water content of the soil after free drainage has ceased.   * Field capacity is the water content of the soil after free drainage has ceased.
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 Values for field capacity and the wilting point of the selected growing medium must be obtained either from a published research article or tested in accordance with either the ASTM D-6836 method or the Soil Survey Investigations Report No. 42, Kellogg Soil Survey Laboratory Methods Manual, published by NRCS. Values for field capacity and the wilting point of the selected growing medium must be obtained either from a published research article or tested in accordance with either the ASTM D-6836 method or the Soil Survey Investigations Report No. 42, Kellogg Soil Survey Laboratory Methods Manual, published by NRCS.
  
-The following calculation, adapted from the [[https://www.nj.gov/dep/stormwater/bmp_manual/NJ_SWBMP_9.4-gree-roofs.pdf|New Jersey Stormwater Best Management Practices Manual]], can be used to estimate the reduction in runoff volume by a green roof. Additional information and example calculations can also be found in this document.+The following calculation, adapted from the [[https://dep.nj.gov/wp-content/uploads/stormwater/bmp/nj_swbmp_9.4-gree-roofs.pdf|New Jersey Stormwater Best Management Practices Manual]], can be used to determine the adjusted curve number which can then be used in approved hydrology models to determine the reduction in runoff peak rates and volume provided by a green roof.
  
-**Step 1 - Calculate storage volume of green roof**+---- 
 + 
 +//Step 1 - Calculate storage volume of green roof//
  
 $$ V_s=\frac{A_\text{roof} \cdot[(d_\text{media} \cdot 𝜂_1)+(d_\text{drainage} \cdot 𝜂_2)]}{12 \frac{in}{ft}} \quad , where: $$ $$ V_s=\frac{A_\text{roof} \cdot[(d_\text{media} \cdot 𝜂_1)+(d_\text{drainage} \cdot 𝜂_2)]}{12 \frac{in}{ft}} \quad , where: $$
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-**Step 2 - Calculate runoff volume**+//Step 2 - Calculate runoff volume//
  
 Calculate the runoff from a traditional roof (CN=98) of equivalent area and subtract the storage volume from Step 1 to determine the green roof's runoff volume. Perform this calculation for each design storm to determine the corresponding roof runoff volumes. Calculate the runoff from a traditional roof (CN=98) of equivalent area and subtract the storage volume from Step 1 to determine the green roof's runoff volume. Perform this calculation for each design storm to determine the corresponding roof runoff volumes.
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 $$ \text{Green Roof Runoff (cf) = Traditional Roof Runoff (cf)} - V_s $$ $$ \text{Green Roof Runoff (cf) = Traditional Roof Runoff (cf)} - V_s $$
  
-**Step 3 - Calculate direct runoff depth**+//Step 3 - Calculate direct runoff depth//
  
 Divide the green roof runoff volume from Step 2 by the green roof area to determine the direct runoff depth. Divide the green roof runoff volume from Step 2 by the green roof area to determine the direct runoff depth.
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-**Step 4 - Determine adjusted curve number**+//Step 4 - Determine adjusted curve number//
  
 Use the direct runoff, 𝑄, calculated in Step 3 and the rainfall precipitation, P, for each of the storm Use the direct runoff, 𝑄, calculated in Step 3 and the rainfall precipitation, P, for each of the storm
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 {{ :wiki:images:curve_number_table.png?direct&1000 |}} {{ :wiki:images:curve_number_table.png?direct&1000 |}}
 +
 +====Adjusted Curve Number Calculator====
 +
 +The calculator below can be used to determine the adjusted curve number, with the source spreadsheet available {{:dane_county_green_roof_-_adjusted_cn_calculator.xlsx| here}}.
 +
 +{{url>https://onedrive.live.com/embed?resid=BF5D650CB7A4C098%212747&authkey=%21ABidxGax_xZhCvk&em=2&wdAllowInteractivity=False&AllowTyping=True&Item='Sheet1'!A3%3AD52&wdHideGridlines=True&wdInConfigurator=True&wdInConfigurator=True" 450px}}
  
 ====References==== ====References====
-  *[[https://www.nj.gov/dep/stormwater/bmp_manual/NJ_SWBMP_9.4-gree-roofs.pdf|New Jersey Stormwater Best Management Practices Manual - Green Roofs]]+  *[[https://dep.nj.gov/wp-content/uploads/stormwater/bmp/nj_swbmp_9.4-gree-roofs.pdf|New Jersey Stormwater Best Management Practices Manual - Green Roofs]] 
 +  *{{ :green_roof_guidance.pdf |Green Tier's Technical Guidance Document for Green Roofs}} 
 +  *{{ :dane_county_green_roof_-_adjusted_cn_calculator.xlsx | Adjusted Curve Number Calculator}}
green_roof.1676576291.txt.gz · Last modified: by theresa_nelson
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