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--- stormwater_management [2022/05/16 15:47] – admin
+++ stormwater_management [2024/02/29 16:52] (current) – admin
@@ Line 25: / Line 25: @@
 {{ :wiki:images:effects_of_imperviousness.png?600 |}}
-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.
+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.
 {{ :wiki:images:discharge_with_controls.png?600 |Figure 5: Hydrograph showing site discharge for pre-development and post-development with detention practices installed}}
@@ Line 58: / Line 58: @@
 **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|
 |[[Native Plants]]|Infiltration, Rate Control|Widely applicable|Low|None|Careful selection of native species; Requires a cover crop during establishment|
@@ Line 66: / Line 66: @@
 **Table 3: Structural stormwater management practices**
-^ Structural Practices ^ Applicable Standard ^ Site Applicability ^ Maintenance Requirement ^ Environmental Concerns ^ Special Consideration ^
+^Practice^ 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|
 |[[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|
 |[[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 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|
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 \\
-====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.
+**NRCS MSE4 Storm Distribution**
-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.
+^Frequency (Year)^Rainfall (Inches)^
-*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.
+|1|2.49|
-*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.
+|2|2.84|
+|10|4.09|
-====Oil and Grease Control====
+|100|6.66|
-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.
+|200|7.53|
+|500|8.94|
-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.
-====Runoff Rate====
-The ordinance requires that all stormwater facilities be designed, installed and maintained to effectively accomplish the following:
-*Maintain predevelopment peak runoff rates for the 1-year, 24-hour storm event (2.49 inches over 24 hours)
-*Maintain predevelopment peak runoff rates for the 2-year, 24-hour storm event (2.84 inches over 24 hours)
-*Maintain predevelopment peak runoff rates for the 10-year, 24-hour storm event (4.09 inches over 24 hours)
-*Maintain predevelopment peak runoff rates for the 100-year, 24-hour storm event (6.66 inches over 24 hours)
-*Maintain predevelopment peak runoff rates for the 200-year, 24-hour storm event (7.53 inches over 24 hours)
 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.
@@ Line 137: / Line 119: @@
 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====
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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.
+====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====
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 ====Redevelopment to Green Infrastructure====
-Sites with [[redevelopment]] are required to treat the first 1/2" of runoff from redeveloped impervious surfaces with 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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