Dane County Stormwater Manual

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stormwater_management [2022/05/05 11:31] – created adminstormwater_management [2024/02/29 16:52] (current) – admin
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 Urbanization dramatically affects the hydrologic cycle by altering the relative percentage of precipitation that contributes to groundwater, evapotranspiration, and runoff relative to the natural ground cover.  Specifically, urbanization increases runoff by decreasing the amount of water that infiltrates into the ground and is taken up and transpired by plants.  This is because water cannot infiltrate into, and plants cannot grow on, impervious surfaces such as pavement and rooftops.  Figure 3.1-2 illustrates how watershed imperviousness affects the magnitude of each of the hydrologic cycle components.  Increased stormwater runoff not only decreases baseflow and groundwater recharge, but also increases the amount of water that runs off the surface, picking up and carrying pollutants to lakes, streams, rivers and wetlands.  The increased surface runoff increases flooding frequency and severity while the increased input of pollutants degrades water quality and aquatic habitat. Urbanization dramatically affects the hydrologic cycle by altering the relative percentage of precipitation that contributes to groundwater, evapotranspiration, and runoff relative to the natural ground cover.  Specifically, urbanization increases runoff by decreasing the amount of water that infiltrates into the ground and is taken up and transpired by plants.  This is because water cannot infiltrate into, and plants cannot grow on, impervious surfaces such as pavement and rooftops.  Figure 3.1-2 illustrates how watershed imperviousness affects the magnitude of each of the hydrologic cycle components.  Increased stormwater runoff not only decreases baseflow and groundwater recharge, but also increases the amount of water that runs off the surface, picking up and carrying pollutants to lakes, streams, rivers and wetlands.  The increased surface runoff increases flooding frequency and severity while the increased input of pollutants degrades water quality and aquatic habitat.
  
-[[File:/Images/Impervious_Runoff.PNG]]+{{ :wiki:images:impervious_runoff.png?600 |Figure 2: Impact of impervious area on the hydrologic cycle fluxes}}
  
-'''Figure 2: Impact of impervious area on the hydrologic cycle fluxes (Adapted from Prince Georges County Department of Environmental Resources Programs and Planning Division. 1999. "Low-Impact Development Design Strategies: An Integrated Design Approach. Department of Environmental Resources", Prince Georges County, Maryland.''' +{{ :wiki:images:baseflow.png?600 |Figure 3: Baseflow is groundwater that constantly supplies rivers and streams with high quality water. When imperviousness decreases infiltration to groundwater, baseflow decreases.}}
- +
-[[File:/Images/Baseflow.PNG]] +
- +
-'''Figure 3: Baseflow is groundwater that constantly supplies rivers and streams with high quality water. When imperviousness decreases infiltration to groundwater, baseflow decreases. (Adapted from "Water Resources Management Practicum, 2000, Dam Repair or Removal: A Decision-making Guide.")'''+
  
 Stormwater hydrographs are plots of runoff discharge versus time.  They illustrate a site’s response to a storm event.  The highest point on a hydrograph represents the peak flow rate following a storm, and the area under the graph represents the total volume of runoff generated by the storm.  Figure 4 shows the significant difference between a pre- and post- development hydrograph, specifically, that development increases the volume, peak flow rate and duration of stormwater runoff following a storm event. Stormwater hydrographs are plots of runoff discharge versus time.  They illustrate a site’s response to a storm event.  The highest point on a hydrograph represents the peak flow rate following a storm, and the area under the graph represents the total volume of runoff generated by the storm.  Figure 4 shows the significant difference between a pre- and post- development hydrograph, specifically, that development increases the volume, peak flow rate and duration of stormwater runoff following a storm event.
  
-[[File:/Images/Development_Discharge.PNG]] +{{ :wiki:images:development_discharge.png?600 |Figure 4: Hydrograph showing site discharge for pre-development and post-development conditions}}
- +
-'''Figure 4: Hydrograph showing site discharge for pre-development and post-development conditions'''+
  
 The increase in impervious surfaces increases the volume of runoff produced because it reduces infiltration, thus reducing baseflow.  The impacts of these changes include increased flooding, erosion, channel widening, habitat loss, and streambed erosion. The increase in impervious surfaces increases the volume of runoff produced because it reduces infiltration, thus reducing baseflow.  The impacts of these changes include increased flooding, erosion, channel widening, habitat loss, and streambed erosion.
  
-'''Table 1: Effects of Imperviousness (Adapted from "Urbanization of Streams: Studies of Hydrologic Impacts", EPA 841-R-97-009, 1997)''' +**Table 1: Effects of Imperviousness** 
- +{{ :wiki:images:effects_of_imperviousness.png?600 |}}
-[[File:/Images/Effects_Of_Imperviousness.PNG]] +
- +
-The Dane County Erosion Control and Stormwater Management Ordinance sets management standards to attenuate the adverse impacts of stormwater.  Specifically, stormwater management practices must be designed and installed at new developments to meet ordinance requirements.  Management practices must be designed to maintain predevelopment peak flow for the 2- and 10-year, 24-hour storm events, so that the post-development hydrograph is similar to Figure 5.  In order to attenuate the adverse impacts of sediment loading, the ordinance also requires that the stormwater management practices be designed to trap the 5 µm particle for the 1-year, 24-hour storm event.+
  
-[[File:/Images/Discharge_With_Controls.PNG]]+The Dane County Erosion Control and Stormwater Management Ordinance sets management standards to attenuate the adverse impacts of stormwater.  Specifically, stormwater management practices must be designed and installed at new developments to meet ordinance requirements.  Management practices must be designed to maintain predevelopment peak flow for the 1, 2, 10, 100 and 200-year, 24-hour storm events, so that the post-development hydrograph is similar to Figure 5.  In order to attenuate the adverse impacts of sediment loading, the ordinance also requires that the stormwater management practices be designed to trap the 5 µm particle for the 1-year, 24-hour storm event.
  
-'''Figure 5: Hydrograph showing site discharge for pre-development and post-development with detention practices installed'''+{{ :wiki:images:discharge_with_controls.png?600 |Figure 5: Hydrograph showing site discharge for pre-development and post-development with detention practices installed}}
  
 Note from Figure 5 that conventional, stormwater detention practices can affect the timing and magnitude of the peak flow rate, but do not equate the volume of pre- and post- development runoff.  This is because these management practices retain water and release it at a peak rate equal to predevelopment conditions, but do not facilitate infiltration and evapotranspiration.   Note from Figure 5 that conventional, stormwater detention practices can affect the timing and magnitude of the peak flow rate, but do not equate the volume of pre- and post- development runoff.  This is because these management practices retain water and release it at a peak rate equal to predevelopment conditions, but do not facilitate infiltration and evapotranspiration.  
  
-In order to decrease runoff and partially mitigate the adverse impacts of increased imperviousness, the county ordinance requires that a percentage of the average annual predevelopment infiltration (stay-on) be infiltrated.  Both residential developments and nonresidential developments must achieve 90 percent of the average annual predevelopment infiltration (stay-on).  When more than 2 percent of a site is needed to meet the stay-on performance standard, a performance standard aimed at meeting a recharge goal may be utilized.  The recharge standard requires that 100 percent of predevelopment recharge is maintained on an average annual basis.  Predevelopment recharge is determined using the Wisconsin Geological and Natural History Survey’s 2009 report, Groundwater Recharge in Dane County, Estimated by a GIS-Based Water Balanced Model (WGNHS Report) or subsequent updates to this report.  An in-depth explanation of the county infiltration standards, practices and modeling guidance can be found on the [[Infiltration]] page.  The county also strongly recommends infiltration practices be used to meet thermal impact standards, where appropriate, since they have the added benefit of decreasing runoff.  Finally, site planners should use techniques that minimize imperviousness and reduce runoff, as discussed on the [[Erosion Control and Stormwater Management Ordinance]] page.+In order to decrease runoff and partially mitigate the adverse impacts of increased imperviousness, the county ordinance requires that a percentage of the average annual predevelopment infiltration (stay-on) be infiltrated.  Both residential developments and nonresidential developments must achieve 90 percent of the average annual predevelopment infiltration (stay-on).  When more than 2 percent of a site is needed to meet the stay-on performance standard, a performance standard aimed at meeting a recharge goal may be utilized.  The recharge standard requires that 100 percent of predevelopment recharge is maintained on an average annual basis.  Predevelopment recharge is determined using the Wisconsin Geological and Natural History Survey’s 2009 report, Groundwater Recharge in Dane County, Estimated by a GIS-Based Water Balanced Model (WGNHS Report) or subsequent updates to this report.  An in-depth explanation of the county infiltration standards, practices and modeling guidance can be found on the [[Infiltration Modeling]] page.  The county also strongly recommends infiltration practices be used to meet thermal impact standards, where appropriate, since they have the added benefit of decreasing runoff.  Finally, site planners should use techniques that minimize imperviousness and reduce runoff, as discussed on the [[Erosion Control and Stormwater Management Ordinance]] page.
  
 If all of these techniques are utilized, the volume of post-development runoff will approach the volume of predevelopment runoff, reducing the effects of development on lakes and streams. If all of these techniques are utilized, the volume of post-development runoff will approach the volume of predevelopment runoff, reducing the effects of development on lakes and streams.
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 There are three types of management practices that can be used to attenuate stormwater impacts. Dane County recommends utilization of these three methods in the order listed below: There are three types of management practices that can be used to attenuate stormwater impacts. Dane County recommends utilization of these three methods in the order listed below:
-##Site planning to minimize the volume of runoff originating from the site. +  * Site planning to minimize the volume of runoff originating from the site. 
-##Nonstructural techniques, including “good housekeeping” practices, to minimize the amount of pollutants that come into contact with runoff. +  * Nonstructural techniques, including “good housekeeping” practices, to minimize the amount of pollutants that come into contact with runoff. 
-##Construction and maintenance of structural management practices to capture and treat stormwater runoff. +  * Construction and maintenance of structural management practices to capture and treat stormwater runoff. 
  
 Incorporating these management techniques into the site planning process requires that project proponents identify the site’s physical characteristics, use models and other analyses to determine if applicable standards are being met, and consider the cost and feasibility of maintaining the proposed management practices. Incorporating these management techniques into the site planning process requires that project proponents identify the site’s physical characteristics, use models and other analyses to determine if applicable standards are being met, and consider the cost and feasibility of maintaining the proposed management practices.
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 The Dane County Erosion Control and Stormwater Management Ordinance requires that all sites needing a stormwater plan and permit install practices that comply with the following standards. The Dane County Erosion Control and Stormwater Management Ordinance requires that all sites needing a stormwater plan and permit install practices that comply with the following standards.
  
-[[File:/Images/Stormwater_Standards.PNG]] +  * Runoff Rate Control 
- +  * Sediment Control 
-Submitted plans must also satisfy all items on the [[https://wred-lwrd.countyofdane.com/documents/Forms/Stormwater%20Management%20Permit%20Application.pdf|stormwater management application checklist]].+  * Infiltration 
 +  * Stable Outlet 
 +  * Oil & Grease Control 
 +  * Thermal Control 
 +  * [[Redevelopment]] to Green Infrastructure
  
 In order to assist in meeting the ordinance requirements, Tables 2 and 3 list practices that could be used to achieve the stormwater performance standards.  The table briefly describes where management practices should be used along with maintenance requirements, environmental concerns and any special considerations for the practice. Other practices may be used to meet erosion control or stormwater management standards if first approved by the Dane County LWRD Director. In order to assist in meeting the ordinance requirements, Tables 2 and 3 list practices that could be used to achieve the stormwater performance standards.  The table briefly describes where management practices should be used along with maintenance requirements, environmental concerns and any special considerations for the practice. Other practices may be used to meet erosion control or stormwater management standards if first approved by the Dane County LWRD Director.
  
-'''Table 2: Non-structural stormwater management practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)''' +**Table 2: Non-structural stormwater management practices** 
-^Non-Structural Practices^Applicable Standard^Site Applicability^Maintenance Requirement^Environmental Concerns^Special Consideration^+^Practice^Applicable Standard^Site Applicability^Maintenance Requirement^Environmental Concerns^Special Consideration^
 |[[Minimizing Impervious Areas]]|Thermal, Rate Control, Infiltration|Limited application to retrofit sites|Low|None|May reduce improvement costs| |[[Minimizing Impervious Areas]]|Thermal, Rate Control, Infiltration|Limited application to retrofit sites|Low|None|May reduce improvement costs|
 |[[Native Plants]]|Infiltration, Rate Control|Widely applicable|Low|None|Careful selection of native species; Requires a cover crop during establishment| |[[Native Plants]]|Infiltration, Rate Control|Widely applicable|Low|None|Careful selection of native species; Requires a cover crop during establishment|
-|[[Parking Lot and Street Sweeping]]|20% TSS Goal|Widely applicable|Moderate|Sediment and debris collected may be contaminated with heavy metals|Hi-Vac trucks are more efficient|+|[[Street Sweeping]]|20% TSS Goal|Widely applicable|Moderate|Sediment and debris collected may be contaminated with heavy metals|Hi-Vac trucks are more efficient|
 |[[Tree Planting]]|Thermal|Widely applicable (excluding berms and streambanks)|Low|Canopy may shade out ground level vegetation|Careful selection of native species; Size; Proper spacing| |[[Tree Planting]]|Thermal|Widely applicable (excluding berms and streambanks)|Low|Canopy may shade out ground level vegetation|Careful selection of native species; Size; Proper spacing|
  
  
-'''Table 3: Structural stormwater management practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)''' +**Table 3: Structural stormwater management practices** 
- +^Practice^ Applicable Standard ^ Site Applicability ^ Maintenance Requirement ^ Environmental Concerns ^ Special Consideration ^ 
-^ Structural Practices ^ Applicable Standard ^ Site Applicability ^ Maintenance Requirement ^ Environmental Concerns ^ Special Consideration ^ +|[[Bioretention Device]]|80% TSS; 40% TSS; Infiltration; Oil and Grease; Thermal; Rate Control|Widely applicable|Moderate|Potential for groundwater contamination if not designed, sited, constructed and maintained properly|Cost; Use native plus or root stock; contamination from salt; construction timing| 
-|[[Bioretention Basin]]|80% TSS; 40% TSS; Infiltration; Oil and Grease; Thermal; Rate Control|Widely applicable|Moderate|Potential for groundwater contamination if not designed, sited, constructed and maintained properly|Cost; Use native plus or root stock; contamination from salt; construction timing| +|[[Dry Pond]]|80% TSS; 40% TSS; Rate Control|Widely applicable, Larger drainage areas needed |Low to Moderate|Provides less water quality improvement than wet pond|Sufficient/suitable land area; Design considerations; Sediment forebay|
-|[[Dry Basin]]|80% TSS; 40% TSS; Rate Control|Widely applicable, Larger drainage areas needed |Low to Moderate|Provides less water quality improvement than Wet Basins|Sufficient/suitable land area; Design considerations; Sediment forebay|+
 |[[Gabion]]|80% TSS; 40% TSS; Stable Outlet|Widely applicable|Low to Moderate|Does not remove smaller suspended solids|Carefully size stone| |[[Gabion]]|80% TSS; 40% TSS; Stable Outlet|Widely applicable|Low to Moderate|Does not remove smaller suspended solids|Carefully size stone|
-|[[Grassed Swale]]|Stable Outlet|Widely applicable|Low to Moderate|Restricted use for areas with high pollution potential|Pretreatment; Check dams; Careful design|+|[[Vegetated Swale]]|Stable Outlet|Widely applicable|Low to Moderate|Restricted use for areas with high pollution potential|Pretreatment; Check dams; Careful design|
 |[[Infiltration Basin]]|Infiltration; Rate Control; Stable Outlet; Thermal|Moderately restricted to sites with suitable soils; Requires a substantial area to meet standards|Low to Moderate|Potential for groundwater contamination; Restricted use for areas with high pollution potential|Sufficient/suitable land area; Proper construction; Compaction avoidance 80% TSS pretreatment| |[[Infiltration Basin]]|Infiltration; Rate Control; Stable Outlet; Thermal|Moderately restricted to sites with suitable soils; Requires a substantial area to meet standards|Low to Moderate|Potential for groundwater contamination; Restricted use for areas with high pollution potential|Sufficient/suitable land area; Proper construction; Compaction avoidance 80% TSS pretreatment|
 |[[Infiltration Trench]]|Infiltration; Rate Control; Thermal|Highly restricted to sites with small drainage areas and proper soils; Depth to water table and bedrock; Slopes|High|Potential for groundwater contamination; Restricted use for areas with high pollution potential|Recommended with careful soils evaluation & 80% TSS pretreatment| |[[Infiltration Trench]]|Infiltration; Rate Control; Thermal|Highly restricted to sites with small drainage areas and proper soils; Depth to water table and bedrock; Slopes|High|Potential for groundwater contamination; Restricted use for areas with high pollution potential|Recommended with careful soils evaluation & 80% TSS pretreatment|
 |[[Lined Waterway or Outlet]]|Stable Outlet|Widely applicable|Low to Moderate|Alters natural cover|Sufficient/suitable land area; Runoff velocities| |[[Lined Waterway or Outlet]]|Stable Outlet|Widely applicable|Low to Moderate|Alters natural cover|Sufficient/suitable land area; Runoff velocities|
-|[[Proprietary Stormwater Filtration Device]]|Oil and Grease Control; Sediment Control|Applicable on small impervious areas|Moderate to High|Limited pollutant removal|Cost and Frequent Maintenance|+|[[Filtration Device]]|Oil and Grease Control; Sediment Control|Applicable on small impervious areas|Moderate to High|Limited pollutant removal|Cost and Frequent Maintenance|
 |[[Permanent Diversion]]|Stable Outlet|Applicable to vegetated ditches and swales|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load|Must be carefully designed to prevent property damage| |[[Permanent Diversion]]|Stable Outlet|Applicable to vegetated ditches and swales|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load|Must be carefully designed to prevent property damage|
-|[[Pervious Pavement]]|Infiltration; Thermal; Rate Control|Applicable on areas with very low traffic volumes|Moderate|Potential for groundwater contamination|Limited use in cold climates, Durability, Potential to clog|+|[[Permeable Pavement]]|Infiltration; Thermal; Rate Control|Applicable on areas with very low traffic volumes|Moderate|Potential for groundwater contamination|Limited use in cold climates, Durability, Potential to clog|
 |[[Rain Garden]]|80% TSS; 40% TSS; Rate Control; Infiltration|Applicable on sites with drainage areas less than 2 acres|Low|Susceptible to clogging|Sufficient/suitable land area, proper soils| |[[Rain Garden]]|80% TSS; 40% TSS; Rate Control; Infiltration|Applicable on sites with drainage areas less than 2 acres|Low|Susceptible to clogging|Sufficient/suitable land area, proper soils|
 |[[Stone Check Dam]]|80% TSS; 40% TSS; Rate Control; Stable Outlet|Applicable to vegetated ditches and swales|Low to Moderate|Does not remove smaller suspended solids|Use clear or washed stone| |[[Stone Check Dam]]|80% TSS; 40% TSS; Rate Control; Stable Outlet|Applicable to vegetated ditches and swales|Low to Moderate|Does not remove smaller suspended solids|Use clear or washed stone|
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 |[[Stone Weeper]]|Widely applicable to outlets|Applicable to vegetated ditches and swales|Low to Moderate|Does not remove smaller suspended solids|Carefully sized stone| |[[Stone Weeper]]|Widely applicable to outlets|Applicable to vegetated ditches and swales|Low to Moderate|Does not remove smaller suspended solids|Carefully sized stone|
 |[[Subsurface Drain]]|Thermal; Rate Control|Widely applicable|Low|Provides limited sediment and pollutant removal|Must have stable outlet| |[[Subsurface Drain]]|Thermal; Rate Control|Widely applicable|Low|Provides limited sediment and pollutant removal|Must have stable outlet|
-|[[Vegetated Buffer Strip]]|80% TSS; Rate Reduction|Widely applicable|Low|None|Sufficient/suitable land area; Careful selection of species; Must be used in conjunction with other BMPs| +|[[Buffer Strip]]|80% TSS; Rate Reduction|Widely applicable|Low|None|Sufficient/suitable land area; Careful selection of species; Must be used in conjunction with other BMPs| 
-|[[Wet Basin]]|80% TSS; 40% TSS; Rate Control|Widely applicable|Low|Possible thermal impacts; low bacteria removal; May attract undesirable wildlife|Sufficient/suitable land area; Design considerations; Sediment forebay|+|[[Wet Pond]]|80% TSS; 40% TSS; Rate Control|Widely applicable|Low|Possible thermal impacts; low bacteria removal; May attract undesirable wildlife|Sufficient/suitable land area; Design considerations; Sediment forebay|
  
 \\ \\
  
-====Sediment Control Requirements==== +====Runoff Rate Control==== 
-For new development, the ordinance requires stormwater practices be designed to retain all soil particles greater than 5 microns (80% reduction) for the 1-year, 24-hour storm event. +The ordinance requires that all stormwater facilities be designed to maintain predevelopment peak runoff rates for the 1, 2, 10, 100 and 200-year 24-hour design storms shown below.
- +
-For redevelopment resulting in exposed surface parking lots and associated traffic areas, the ordinance requires that stormwater practices be designed to retain soil particles greater than 20 microns (40% reduction) for the 1-year, 24-hour storm event. +
- +
-Although not required by the ordinance, the following goals should be met whenever possible.  The design, suggested location, and implementation of proposed practices should be included in the plans. +
-*For existing development, design practices to retain soil particles greater than 40 microns on the site, resulting from a 1-year, 24-hour storm event. +
-*For street reconstruction, design practices to retain soil practices greater than 20 microns on the site, resulting from a 1-year, 24-hour storm event. +
- +
-====Oil and Grease Control==== +
-The ordinance requires that all stormwater plans for commercial and industrial developments and all other areas where the potential for oil or grease exists must include practices to treat oil and grease in the first 0.5 inches of runoff.  The best available oil and grease removal technology must be used. +
- +
-Oil and grease removal practices are generally combined with other stormwater runoff management practices and are obtained through commercial sources.  Information regarding choice, installation and maintenance of these management practices is best obtained from the manufacturer. +
- +
-Sites that must control the first half-inch of runoff for oil and grease include: +
-*vehicle fueling and service areas +
-*commercial buildings with drive-through areas +
-*parking lots with more than 40 stalls +
-*convenience stores +
-*other areas that are determined to have the potential for oil and grease pollution+
  
-Additional guidance and approved treatment practices can be found on the [[Oil and Grease Control|Oil and Grease Control]] page.+**NRCS MSE4 Storm Distribution**
  
-====Runoff Rate==== +^Frequency (Year)^Rainfall (Inches)^ 
-The ordinance requires that all stormwater facilities be designed, installed and maintained to effectively accomplish the following: +|1|2.49| 
-*Maintain predevelopment peak runoff rates for the 1-year, 24-hour storm event (2.49 inches over 24 hours) +|2|2.84| 
-*Maintain predevelopment peak runoff rates for the 2-year, 24-hour storm event (2.84 inches over 24 hours)  +|10|4.09| 
-*Maintain predevelopment peak runoff rates for the 10-year, 24-hour storm event (4.09 inches over 24 hours) +|100|6.66| 
-*Maintain predevelopment peak runoff rates for the 100-year, 24-hour storm event (6.66 inches over 24 hours) +|200|7.53| 
-*Maintain predevelopment peak runoff rates for the 200-year, 24-hour storm event (7.53 inches over 24 hours)+|500|8.94|
  
 The ordinance requirements for water quantity apply to individual sites and not the entire watershed.  It is more difficult to control the larger storms with the practices installed on an individual site. The ordinance requirements for water quantity apply to individual sites and not the entire watershed.  It is more difficult to control the larger storms with the practices installed on an individual site.
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 Impervious surfaces such as roofs (including overhangs), roads, sidewalks, patios, driveways, and parking lots, including gravel surfaces, should be modeled with curve number of 98. Water body areas (including permanent pools and infiltration facility bottoms) should be modeled with curve number of 100. Impervious surfaces such as roofs (including overhangs), roads, sidewalks, patios, driveways, and parking lots, including gravel surfaces, should be modeled with curve number of 98. Water body areas (including permanent pools and infiltration facility bottoms) should be modeled with curve number of 100.
  
-====Stable Outlets==== +====Sediment Control==== 
-The ordinance requires that discharges from new construction sites have a stable outlet capable of carrying designed flow at a non-erosive velocity.  Outlet design must consider both flow capacity and duration.  This requirement applies to both the site outlet and the ultimate outlet to stormwater conveyance or water body.+For new development, the ordinance requires stormwater practices be designed to retain all soil particles greater than 5 microns (80% reduction) for the 1-year, 24-hour storm event.
  
-Stable outlets are an integral part of well-designed erosion control and stormwater management practices.  Stable outlets allow stormwater and erosion control structures to function properly and provide a way for runoff to be discharged without causing damage to downstream properties or water bodies.  A stable outlet can be a grassed waterway, vegetated or paved area, grade stabilization structure, underground outlet, rock chute, rock lined channel or stable watercourse. +For [[redevelopment]] resulting in exposed surface parking lots and associated traffic areas, the ordinance requires that stormwater practices be designed to retain soil particles greater than 20 microns (40% reduction) for the 1-year, 24-hour storm event.
  
-Stable outlets must have the capacity to handle the designed outflow from the stormwater or erosion control structures they serve.  If the outlet is to be vegetated, it should be constructed and established before installation of other stormwater or erosion control structures.  Verify that the channel lining is adequate to carry the design to velocity and volume.+Although not required by the ordinance, the following goals should be met whenever possible.  The design, suggested location, and implementation of proposed practices should be included in the plans.
  
-===Channel Lining=== +  *For existing development, design practices to retain soil particles greater than 40 microns on the site, resulting from a 1-year, 24-hour storm event. 
-To prevent channels from eroding, an analysis of the channel velocity must be performed to determine the required control practice(s).  Where velocities are higher than 5 feet per second or where the channel must carry prolonged flow, the channel should be lined with riprap or other armoring material.  Channel linings shall be designed based on the expected channel velocity from the 10-year, 24-hour storm event.+  *For street reconstruction, design practices to retain soil practices greater than 20 microns on the site, resulting from a 1-year, 24-hour storm event.
  
 ====Infiltration==== ====Infiltration====
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 The ordinance requires that a percentage of the average annual rainfall be infiltrated unless the applicant can demonstrate that the practice is likely to result in groundwater contamination.  Infiltration is all precipitation that does not leave the site as surface runoff, and is referred to as “stay-on.”  For both residential and non-residential developments, 90 percent of what infiltrated in the predevelopment condition (predevelopment infiltration) must be infiltrated.  If more than two percent of a site is needed to meet the infiltration standard, infiltration practices may be alternatively designed to meet an average annual recharge goal determined by the WGNHS Report.  If the ordinance requirement is met with the recharge methodology, a minimum of two percent of the site  must be dedicated to the infiltration practices. The ordinance requires that a percentage of the average annual rainfall be infiltrated unless the applicant can demonstrate that the practice is likely to result in groundwater contamination.  Infiltration is all precipitation that does not leave the site as surface runoff, and is referred to as “stay-on.”  For both residential and non-residential developments, 90 percent of what infiltrated in the predevelopment condition (predevelopment infiltration) must be infiltrated.  If more than two percent of a site is needed to meet the infiltration standard, infiltration practices may be alternatively designed to meet an average annual recharge goal determined by the WGNHS Report.  If the ordinance requirement is met with the recharge methodology, a minimum of two percent of the site  must be dedicated to the infiltration practices.
 +
 +====Stable Outlet====
 +The ordinance requires that discharges from new construction sites have a stable outlet capable of carrying designed flow at a non-erosive velocity.  Outlet design must consider both flow capacity and duration.  This requirement applies to both the site outlet and the ultimate outlet to stormwater conveyance or water body.
 +
 +Stable outlets are an integral part of well-designed erosion control and stormwater management practices.  Stable outlets allow stormwater and erosion control structures to function properly and provide a way for runoff to be discharged without causing damage to downstream properties or water bodies.  A stable outlet can be a [[vegetated swale]], vegetated or paved area, grade stabilization structure, underground outlet, rock chute, rock lined channel or stable watercourse. 
 +
 +Stable outlets must have the capacity to handle the designed outflow from the stormwater or erosion control structures they serve.  If the outlet is to be vegetated, it should be constructed and established before installation of other stormwater or erosion control structures.  Verify that the channel lining is adequate to carry the design to velocity and volume.
 +
 +===Conveyance===
 +
 +To prevent [[vegetated swale|vegetated swales]] from eroding, an analysis of the channel velocity must be performed to determine the required control practice(s).  Where velocities are higher than 5 feet per second or where the channel must carry prolonged flow, the channel should be lined with riprap or other armoring material.  Channel linings shall be designed based on the expected channel velocity from the 10-year, 24-hour storm event.
 +
 +Culverts under private drives must be designed to convey the 10-year design storm, while those under public roadways must be designed to convey the 25-year design storm.
 +
 +====Oil and Grease Control====
 +The ordinance requires that all stormwater plans for commercial and industrial developments and all other areas where the potential for oil or grease exists must include practices to treat oil and grease in the first 0.5 inches of runoff.  The best available oil and grease removal technology must be used.
 +
 +Oil and grease removal practices are generally combined with other stormwater runoff management practices and are obtained through commercial sources.  Information regarding choice, installation and maintenance of these management practices is best obtained from the manufacturer.
 +
 +Sites that must control the first half-inch of runoff for oil and grease include:
 +  *vehicle fueling and service areas
 +  *commercial buildings with drive-through areas
 +  *parking lots with more than 40 stalls
 +  *convenience stores
 +  *other areas that are determined to have the potential for oil and grease pollution
 +
 +Additional guidance and approved treatment practices can be found on the [[Oil and Grease Control]] page.
  
 ====Thermal Control==== ====Thermal Control====
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 These areas can also be identified by turning on the "Thermally Sensitive Areas" layer in the [[https://dcimapapps.countyofdane.com/lwrviewer/|LWRD Viewer]]. These areas can also be identified by turning on the "Thermally Sensitive Areas" layer in the [[https://dcimapapps.countyofdane.com/lwrviewer/|LWRD Viewer]].
  
-[[File:/Images/Watershed.PNG]] +{{ :wiki:images:watershed.png?600 |Figure 7: A ''watershed'' is the land area that drains to a common location (typically a water body and its tributaries).  For this reason, watershed boundaries are also called drainage divides.   Practices to reduce the temperature of runoff must be installed if a site is located anywhere within the watershed of a thermally sensitive water body.}}
- +
-'''Figure 7: A ''watershed'' is the land area that drains to a common location (typically a water body and its tributaries).  For this reason, watershed boundaries are also called drainage divides.   Practices to reduce the temperature of runoff must be installed if a site is located anywhere within the watershed of a thermally sensitive water body.'''+
  
 The increase of impervious surfaces in urban areas is a major source of thermal pollution in cold climates and threatens the health of cold-water ecosystems ([[https://www.mwcog.org/documents/1990/12/12/thermal-impacts-associated-with-urbanization-and-stormwater-management-best-management-practices/|Galli, 1990]]).  Research shows that the average stream temperature increases directly with the percentage of impervious cover in the watershed.  Impervious areas absorb energy from the sun, which causes them to become warmer.  As water runs over these areas, it absorbs some of that heat energy and is warmed, causing thermal pollution in lakes, rivers, and streams.  Impervious areas also compound the problem by reducing infiltration, which in turn increases the volume of runoff that is created, leading to higher permanent stream temperatures in the summer months.  The increase of impervious surfaces in urban areas is a major source of thermal pollution in cold climates and threatens the health of cold-water ecosystems ([[https://www.mwcog.org/documents/1990/12/12/thermal-impacts-associated-with-urbanization-and-stormwater-management-best-management-practices/|Galli, 1990]]).  Research shows that the average stream temperature increases directly with the percentage of impervious cover in the watershed.  Impervious areas absorb energy from the sun, which causes them to become warmer.  As water runs over these areas, it absorbs some of that heat energy and is warmed, causing thermal pollution in lakes, rivers, and streams.  Impervious areas also compound the problem by reducing infiltration, which in turn increases the volume of runoff that is created, leading to higher permanent stream temperatures in the summer months. 
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 Guidance on designing sites in thermally sensitive areas can be found on the [[thermal mitigation]] page. Guidance on designing sites in thermally sensitive areas can be found on the [[thermal mitigation]] page.
 +
 +====Redevelopment to Green Infrastructure====
 +
 +Sites with [[redevelopment]] are required to treat the first 1/2" of runoff from redeveloped impervious surfaces with [[green infrastructure]].
  
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stormwater_management.1651764693.txt.gz · Last modified: by admin
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