Dane County Stormwater Manual

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green_roof [2023/02/15 11:43] – admingreen_roof [2024/02/08 15:33] (current) – admin
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 **Advantages** **Advantages**
-  *Reduces runoff through increased evapotranspiration+  *Reduces runoff volume through increased evapotranspiration
   *Slows rate of runoff   *Slows rate of runoff
-  *Reduces thermal impacts of runoff+  *Mitigates thermal impacts of runoff 
 +  *Extends life of roof membrane 
 +  *Creates habitat and beautifies the built environment
  
 **Disadvantages** **Disadvantages**
   *Heavier than traditional roof, requiring additional support   *Heavier than traditional roof, requiring additional support
-  *Increased maintenance compared to traditional roof+  *Increased cost and maintenance compared to traditional roof
  
 =====Design===== =====Design=====
-An //extensive// green roof has a shallow growing medium—usually less than six (6) inches thick—with a modest roof load, limited plant diversity, and minimal watering requirements.+There are many types of green roofs, which can be categorized based on the type of assembly and system components. The primary types of systems are:
  
-An //intensive// green roof has a minimum twelve (12) inch thick growing medium, though it may range upwards to several feet.  This depth of growing medium can support a more diverse plant selection, including small trees.+  * Extensive 
 +    * Shallow growing medium (<6"), with a modest roof load, limited plant diversity, and minimal watering requirements. 
 +  * Intensive 
 +    * Minimum twelve-inch thick growing medium, which can support a more diverse plant selection, including small trees. 
 +  * Semi-intensive 
 +  * Modular (tray) systems 
 +  * Layered systems 
 +  * Enhanced systems 
 +    * Sponge roof (additional retention in mineral wool or synthetic fabric) 
 +    * Blue-green roof (sponge + detention storage) 
 +    * Purple roof (sponge + detention storage + friction layer) 
 + 
 +Any of these systems can be used to meet stormwater requirements, but must be designed to address the following elements.
  
 **Drainage Area** **Drainage Area**
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   * Select species based on the local climate and the building micro-climates. Wind, temperature, sunlight, and rainfall (climate) all impact plant growth on green roofs; and those factors can vary in different areas of the roof (micro-climate), especially around the edges. Be aware of extreme conditions caused by edge effects, prevailing wind patterns, variable sunlight conditions, exhaust fans, and reflections from adjacent windows or siding. Consider no-planting zones as appropriate.   * Select species based on the local climate and the building micro-climates. Wind, temperature, sunlight, and rainfall (climate) all impact plant growth on green roofs; and those factors can vary in different areas of the roof (micro-climate), especially around the edges. Be aware of extreme conditions caused by edge effects, prevailing wind patterns, variable sunlight conditions, exhaust fans, and reflections from adjacent windows or siding. Consider no-planting zones as appropriate.
   * Planting should be avoided during the peak summer months due to the additional irrigation required.   * Planting should be avoided during the peak summer months due to the additional irrigation required.
-  * See Appendix 2 for plant species recommendations.+ 
 +Resources for choosing green roof vegetation are provided in Appendix 2 of the Green Roof Guidance Document.
  
 **System Layer Requirements** **System Layer Requirements**
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 **Irrigation** **Irrigation**
  
-Irrigation, or the ability to irrigate when needed, is strongly recommended for all green roof systems. Irrigation is an important design parameter and considerations should be made during the early design phases to accommodate this component. 3Irrigation is required during plant establishment and during long durations without rainfall. Moisture meters can be installed and used to determine when irrigation is necessary.+Irrigation, or the ability to irrigate when needed, is strongly recommended for all green roof systems. Irrigation is an important design parameter and considerations should be made during the early design phases to accommodate this component. Irrigation is required during plant establishment and during long durations without rainfall. Moisture meters can be installed and used to determine when irrigation is necessary.
  
 **Equipment & Storage** **Equipment & Storage**
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 =====Maintenance===== =====Maintenance=====
-  *Yearly weeding +  *Inspect twice a week during vegetation establishment and at least once every season after that, as well as after every storm even exceeding 1” or major weather events. 
-  *Application of slow-release fertilizer to boost growth+  *Strongly advise using a landscaping professional to inspect vegetation and keep coverage above 80% to prevent the spread of weeds. 
 +  *Replace media and vegetation if standing water for >72 hours. 
 +  *Include access and safety recommendations for inspector and maintenance workers. 
 +  *Provide recommendations for watering
  
 =====Method to Determine Practice Efficiency===== =====Method to Determine Practice Efficiency=====
 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}}
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