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--- erosion_control [2022/05/13 16:57] – admin
+++ erosion_control [2022/05/19 12:14] (current) – admin
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 Dane County construction sites are highly susceptible to erosion for several reasons.  First, soil is easily detached from the land surface because vegetation and the surface layer of organic soil are stripped.  Second, heavy machinery compacts the soil causing it to lose infiltration capacity, which increases the volume of water that becomes runoff and the potential to wash soil downhill.  Third, since there is no vegetation to spread runoff into sheet flow, it is more likely to concentrate and cause gully erosion.  Fourth, Dane County soils are comprised of large amounts of silt, which is easily detached from the other soil particles.
-[{{:wiki:images:conceptual_erosion.png?400|**Conceptual model of erosion**}}]
+[{{:wiki:images:conceptual_erosion.png?400|Figure 1: Conceptual model of erosion}}]
-There are a variety of strategies for minimizing soil loss from construction sites.  These include preventing soil detachment, diverting runoff around disturbed areas, and trapping sediment carried by runoff before it leaves the site.  The most important strategy for controlling construction site erosion is preventing soil particle detachment through soil stabilization.  Vegetation should be re-established as soon as possible after land is disturbed.  In the meantime, other erosion control practices, such as [[polymer and other additives|polymers]], [[erosion matting]], and [[mulching]] must be in place.  A second line of defense is to prevent runoff from contacting detached soil particles by diverting runoff around disturbed areas.  Diversions minimize the opportunity for runoff to entrain detached soil particles and carry them offsite.  Finally, when soil particles are detached and carried by runoff, practices that slow and/or trap sediment must be installed to prevent suspended sediment from leaving the site and entering water bodies.
+\\
+There are a variety of strategies for minimizing soil loss from construction sites.  These include preventing soil detachment, diverting runoff around disturbed areas, and trapping sediment carried by runoff before it leaves the site.  The most important strategy for controlling construction site erosion is preventing soil particle detachment through soil stabilization.  Vegetation should be re-established as soon as possible after land is disturbed.  In the meantime, other erosion control practices, such as [[polymer and other additives|Polymers]], [[erosion matting]], and [[mulching]] must be in place.  A second line of defense is to prevent runoff from contacting detached soil particles by diverting runoff around disturbed areas.  Diversions minimize the opportunity for runoff to entrain detached soil particles and carry them offsite.  Finally, when soil particles are detached and carried by runoff, practices that slow and/or trap sediment must be installed to prevent suspended sediment from leaving the site and entering water bodies.
 =====Performance Standards=====
 To minimize erosion from construction sites and protect the county’s lakes and streams from sediment pollution, it's required that plans for all construction sites include practices that meet the standards in the table below.
-^Standard Name^Performance Requirement^Purpose^Applicability^
+^Standard^Performance Requirement^Purpose^Applicability^
 |Sheet and Rill Erosion|Limit cumulative sediment deposition to 5 tons/acre/year|Minimize soil loss and prevent water quality and aquatic habitat degradation|All sites requiring an erosion control plan|
 |Gully and Streambank Erosion|Prevent gully and streambank erosion|Minimize soil loss and prevent water quality and aquatic habitat degradation|All sites requiring an erosion control plan|
@@ Line 23: / Line 25: @@
 ====Table 1: Non-structural 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^
-|'''[[Construction Scheduling]]'''|Sheet and Rill Erosion|Widely Applicable|Low|None|Can greatly reduce erosion from a site|
+|[[Construction Scheduling]]|Sheet and Rill Erosion|Widely Applicable|Low|None|Can greatly reduce erosion from a site|
-|'''[[Deep Tilling]]'''|Sheet and Rill Erosion|Widely applicable on sites where heavy grading has occurred|Very Low|None|Should be timed after grading has occurred; Buried Utilities|
+|[[Deep Tilling]]|Sheet and Rill Erosion|Widely applicable on sites where heavy grading has occurred|Very Low|None|Should be timed after grading has occurred; Buried Utilities|
-|'''[[Mulching]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate|Limited effectiveness on steep slopes depending on the type of mulch|Must be reapplied/replaced frequently and crimped|
+|[[Mulching]]|Sheet and Rill Erosion|Widely Applicable|Moderate|Limited effectiveness on steep slopes depending on the type of mulch|Must be reapplied/replaced frequently and crimped|
-|'''[[Polymer and Other Additives]]'''|Sheet and Rill Erosion|Applicable on sites that are not actively being graded|Moderate|Risk of adverse impacts if over applied|Must be re-applied if site is disturbed after initial application|
+|[[Polymer and Other Additives]]|Sheet and Rill Erosion|Applicable on sites that are not actively being graded|Moderate|Risk of adverse impacts if over applied|Must be re-applied if site is disturbed after initial application|
-|'''[[Seeding]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate; Low once established|Possible erosion during establishment; fertilizer runoff|Must match seed mix with the time of year and site conditions; Requires > 3" of topsoil|
+|[[Seeding]]|Sheet and Rill Erosion|Widely Applicable|Moderate; Low once established|Possible erosion during establishment; fertilizer runoff|Must match seed mix with the time of year and site conditions; Requires > 3" of topsoil|
-|'''[[Sod]]'''|Sheet and Rill Erosion|Widely Applicable|Low after establishment|Fertilizer runoff, Overwatering|May need to be staked on steep slopes & channels; Proper selection of species; Requires > 3" of prepared topsoil|
+|[[Sod]]|Sheet and Rill Erosion|Widely Applicable|Low after establishment|Fertilizer runoff, Overwatering|May need to be staked on steep slopes & channels; Proper selection of species; Requires > 3" of prepared topsoil|
-|'''[[Surface Roughening]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate|Erosion may increase if not done on the contour of the slope|Need a specially selected tracked or wheeled vehicle|
+|[[Surface Roughening]]|Sheet and Rill Erosion|Widely Applicable|Moderate|Erosion may increase if not done on the contour of the slope|Need a specially selected tracked or wheeled vehicle|
 ====Table 2: Structural 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^
-|**[[Vegetated Buffer Strip]]**|Sheet and Rill Erosion|Applicable when already installed|Low|None|Sufficient/suitable land area; Must be used in conjunction with other practices|
+|[[Buffer Strip]]|Sheet and Rill Erosion|Applicable when already installed|Low|None|Sufficient/suitable land area; Must be used in conjunction with other practices|
-|'''[[Permanent Diversion]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load| Must be carefully designed to prevent property damage|
+|[[Permanent Diversion]]|Sheet and Rill Erosion|Widely Applicable|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load| Must be carefully designed to prevent property damage|
-|'''[[Temporary Diversion]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load|May need frequent repair/replacement; Channel must be stabilized on slopes with a grade of >2%|
+|[[Temporary Diversion]]|Sheet and Rill Erosion|Widely Applicable|Moderate|Possible erosion of diversion structure if diverted runoff carries a large sediment load|May need frequent repair/replacement; Channel must be stabilized on slopes with a grade of >2%|
-|'''[[Erosion Matting]]'''|Sheet and Rill Erosion|Widely applicable on low to moderate slopes|Low|Limited effectiveness on steep slopes|Proper installation|
+|[[Erosion Matting]]|Sheet and Rill Erosion|Widely applicable on low to moderate slopes|Low|Limited effectiveness on steep slopes|Proper installation|
-|'''[[Gabion]]'''|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales|Low|Does not remove smaller suspended particles|Usually used in conjunction with other practices|
+|[[Gabion]]|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales|Low|Does not remove smaller suspended particles|Usually used in conjunction with other practices|
-|'''[[Sediment Basin]]'''|Sheet and Rill Erosion|Applicable on sites with a drainage area of <100 acres|Low|Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays|May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
+|[[Sediment Basin]]|Sheet and Rill Erosion|Applicable on sites with a drainage area of <100 acres|Low|Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays|May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
-|'''[[Sediment Trap]]'''|Sheet and Rill Erosion|Applicable on sites with a drainage area of <5 acres|Low|Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays|May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
+|[[Sediment Trap]]|Sheet and Rill Erosion|Applicable on sites with a drainage area of <5 acres|Low|Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays|May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
-|'''[[Silt Fence]] and [[Silt Sock]]'''|Sheet and Rill Erosion|Widely Applicable|High|Sediment transport; High rates of failure if not properly installed and maintained; Disposal|Longevity, proper installation|
+|[[Silt Fence]] and [[Silt Sock]]|Sheet and Rill Erosion|Widely Applicable|High|Sediment transport; High rates of failure if not properly installed and maintained; Disposal|Longevity, proper installation|
-|'''[[Slope Drain]]'''|Sheet and Rill Erosion; Gully Erosion|Applicable on sites that are vulnerable to convey runoff downslope|Moderate|Possible erosion around inlet & outlet|Pipe size|
+|[[Slope Drain]]|Sheet and Rill Erosion; Gully Erosion|Applicable on sites that are vulnerable to convey runoff downslope|Moderate|Possible erosion around inlet & outlet|Pipe size|
-|'''[[Stone Check Dam]]'''|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales, Drainage areas <2 acres| Low to Moderate|Does not remove smaller suspended solids||
+|[[Stone Check Dam]]|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales, Drainage areas <2 acres| Low to Moderate|Does not remove smaller suspended particles| |
-|'''[[Stone Tracking Pad]]'''|Sheet and Rill Erosion|Widely Applicable|Low to High|None|Cost effective; Must use >3" clear stone|
+|[[Stone Tracking Pad]]|Sheet and Rill Erosion|Widely Applicable|Low to High|None|Cost effective; Must use >3" clear stone|
-|'''[[Stone Weeper]]'''|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales, Drainage areas <2 acres|Low to Moderate|Does not remove smaller suspended particles| |
+|[[Stone Weeper]]|Sheet and Rill Erosion; Gully Erosion|Applicable to vegetated ditches and swales, Drainage areas <2 acres|Low to Moderate|Does not remove smaller suspended particles| |
-|'''[[Inlet Protection]]'''|Sheet and Rill Erosion|Widely Applicable|Moderate to High|Ineffective for large storm events; Limited effectiveness with large sediment loads| May need frequent cleaned/replacement; used in conjunction with other practices|
+|[[Inlet Protection]]|Sheet and Rill Erosion|Widely Applicable|Moderate to High|Ineffective for large storm events; Limited effectiveness with large sediment loads| May need frequent cleaned/replacement; used in conjunction with other practices|
 ======The 5 Tons/Acre/Year Standard======
+Research has shown the average soil loss on uncontrolled construction sites is approximately 30 tons/acre/year. Limiting sediment deposition to 5 tons/acre/year (equivalent to 7.5 tons/acre/year soil loss) provides an average reduction of approximately 80%, compared to uncontrolled conditions. By analyzing the USDA Natural Resources Conservation Service’s definition of particle size distribution for Plano Silt loam (Figure 2), a soil that is similar to the majority of Dane County’s soils, it is shown that in order to achieve a trapping efficiency of 80% during construction, the 5 micron (µm) particle will need to be trapped.
+[{{ :wiki:images:trapping_efficiency_1_.png?600 |Figure 2: Trapping efficiency based on Plano silt loam soil distribution}}]
+\\
+In Dane County, it is not feasible to trap particles smaller than 5 µm from a cost/benefit and engineering standpoint.  In a pond that is two feet deep, trapping the 5 µm particle requires a settling time of 6 hours, which is feasible for pond design.  However, the particle settling time increases exponentially with decreasing particle size (Figure 3).  For example, trapping the 3 µm instead of the 5 µm particle increases the required settling time from 6 to 24 hours, but only increases the expected trapping efficiency by 5%.  Designing a pond with a settling time of 24 hours would be much more costly and require a larger land area.  Thus, a soil loss standard lower than 5 tons/acre/year would provide small additional benefit at a very high cost.  Dane County’s approach is equivalent to the intent of the performance standards for construction sites in the Department of Natural Resources’ Chapter [[https://docs.legis.wisconsin.gov/code/admin_code/nr/100/151/III/11/6m/b/2|NR 151]].
+[{{ :wiki:images:particle_settling_time.png?600 |Figure 3: Particle settling time in 2 feet of water}}]
+\\
 ======Calculating Soil Loss and Sediment Discharge From Construction Sites======
+The Wisconsin Department of Natural Resources has developed an Excel spreadsheet that calculates soil loss and sediment discharge from construction sites.  This spreadsheet uses the Universal Soil Loss Equation (USLE) to determine whether the combination of proposed erosion control practices will limit sediment discharge from sheet and rill erosion to 5 tons/acre/year.  Guidance on how to show compliance with this standard can be found on the [[Soil Loss and Sediment Discharge]] page.  Note that the spreadsheet estimates soil loss from sheet and rill erosion only.  It does not predict soil loss resulting from high channel velocities, gully erosion or streambank erosion.
 ======Gully and Streambank Erosion======
+Gully erosion is caused by concentrated overland flow of surface water in depressions and drainage ways.  The surface water’s erosive force removes topsoil while increasing energy as it moves downslope.  Once an unprotected gully begins to form, lateral erosion takes place, widening the gully and undercutting the sides where additional soil is removed.  Preventive practices and proper management of gullies are required on construction sites.
+Streambank erosion removes soil along the banks and bed of a channel. The erosion is the result of high flow within the stream channel after rain events.  The erosive force of the flow causes undercutting of the banks, which deposits large amounts of sediment directly into the stream channel.  The sediment is then carried and deposited downstream.