Differences

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--- stormwater_management [2022/06/21 15:37] – 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 70: / Line 70: @@
 |[[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|
@@ Line 89: / Line 89: @@
 ====Runoff Rate Control====
-The ordinance requires that all stormwater facilities be designed, installed and maintained to effectively accomplish the following:
+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.
-  *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)
+**NRCS MSE4 Storm Distribution**
-  *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)
+^Frequency (Year)^Rainfall (Inches)^
-  *Maintain predevelopment peak runoff rates for the 200-year, 24-hour storm event (7.53 inches over 24 hours)
+|1|2.49|
+|2|2.84|
+|10|4.09|
+|100|6.66|
+|200|7.53|
+|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.
@@ Line 132: / Line 137: @@
 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 grassed waterway, vegetated or paved area, grade stabilization structure, underground outlet, rock chute, rock lined channel or stable watercourse.
+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.
@@ Line 138: / Line 143: @@
 ===Conveyance===
-To prevent [[grassed swales|grassed swale]] 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.
+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.