Difference between revisions of "Infiltration design guideline - underdrains"
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| − | The following are ''RECOMMENDED'' for infiltration practices with underdrains. | + | The following are ''RECOMMENDED'' for infiltration practices with <span title="An underground drain or trench with openings through which the water may percolate from the soil or ground above"> '''underdrains'''</span>. |
*The minimum pipe diameter is 4 inches. | *The minimum pipe diameter is 4 inches. | ||
*Install 2 or more underdrains for each infiltration system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area. | *Install 2 or more underdrains for each infiltration system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area. | ||
| − | *Include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap. | + | *Include at least 2 observation/cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap. |
| − | *Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe. | + | *Construct underdrains with Schedule 40 or SDR 35 smooth wall [https://www.homestratosphere.com/pvc-pipe-and-fittings-sizes/ PVC pipe]. |
| − | *Install underdrains with a minimum slope of 0.5 percent, particularly in [ | + | *Install underdrains with a minimum slope of 0.5 percent, particularly in <span title="A soil classification system (Natural Resource Conservation System) based on runoff potential. Groups include A soils (coarse textured with very low runoff potential), B soils (medium coarse textured with low runoff potential), C soils (fine to moderate textured with moderate runoff potential), and D soils (fine textured with high runoff potential)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates hydrologic soil group]'''</span> D soils (Note: to utilize [http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Manning_s_Equation.htm#:~:text=The%20Mannings%20equation%20is%20an,flow%20area%20and%20channel%20slope.&text=Under%20the%20assumption%20of%20uniform,and%20the%20water%20surface%20slope. Manning’s equation] the slope must be greater than 0). |
*Include a utility trace wire for all buried piping. | *Include a utility trace wire for all buried piping. | ||
*For underdrains that daylight on grade, include a marking stake and animal guard; | *For underdrains that daylight on grade, include a marking stake and animal guard; | ||
| − | *For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating the system as either an [ | + | *For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating the system as either an <span title="Infiltration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium and into underlying soil, where it may eventually percolate into groundwater. The filtering media is typically coarse-textured and may contain organic material, as in the case of bioinfiltration BMPs."> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices '''infiltration''']</span> system, <span title="Filtration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium, such as sand or an organic material. They are generally used on small drainage areas (5 acres or less) and are primarily designed for pollutant removal. They are effective at removing total suspended solids (TSS), particulate phosphorus, metals, and most organics. They are less effective for soluble pollutants such as dissolved phosphorus, chloride, and nitrate."> [https://stormwater.pca.state.mn.us/index.php?title=Filtration '''filtration''']</span> system, or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the underdrain discharge rate equal a 48 hour <span title="The length of time, usually expressed in hours, for ponded water in a stormwater practice to drain. For stormwater practices where water is stored in media, there is no clear definition of drawdown, but an acceptable assumption is the time for water to drain to field capacity"> '''drawdown time'''</span>. |
| − | *Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade. | + | *Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below <span title="a level space, shelf, or raised barrier (usually made of compacted soil) separating two areas"> '''berms'''</span>, down steep slopes, makes a connection to a drainage structure, or daylights on grade. |
*Spacing of collection laterals should be less than 25 feet. | *Spacing of collection laterals should be less than 25 feet. | ||
| − | *Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage. | + | *Underdrain pipes should have a minimum of 3 inches of washed [https://www.dirtconnections.com/what-is-57-stone/ #57 stone] above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage. |
| − | *Avoid filter fabric. | + | *Avoid <span title="A type of materials that collects sediments while permeable to water."> '''filter fabric''' </span>. |
*Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric. | *Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric. | ||
The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the [https://ncdenr.s3.amazonaws.com/s3fs-public/Water%20Quality/Surface%20Water%20Protection/SPU/SPU%20-%20BMP%20Manual%20Documents/BMPMan-Ch05-BMPDesignElem-20090716-DWQ-SPU.pdf North Carolina Department of Environment and Natural Resources Stormwater BMP Manual]. Underdrain spacing can be calculated using the following spreadsheet, which utilizes the vanSchilfgaarde Equation. The spradsheet includes an example calculation. | The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the [https://ncdenr.s3.amazonaws.com/s3fs-public/Water%20Quality/Surface%20Water%20Protection/SPU/SPU%20-%20BMP%20Manual%20Documents/BMPMan-Ch05-BMPDesignElem-20090716-DWQ-SPU.pdf North Carolina Department of Environment and Natural Resources Stormwater BMP Manual]. Underdrain spacing can be calculated using the following spreadsheet, which utilizes the vanSchilfgaarde Equation. The spradsheet includes an example calculation. | ||
[[file:Underdrain spacing calculation.xlsx]] | [[file:Underdrain spacing calculation.xlsx]] | ||
Revision as of 17:04, 6 June 2022
The following are RECOMMENDED for infiltration practices with underdrains.
- The minimum pipe diameter is 4 inches.
- Install 2 or more underdrains for each infiltration system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area.
- Include at least 2 observation/cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap.
- Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe.
- Install underdrains with a minimum slope of 0.5 percent, particularly in hydrologic soil group D soils (Note: to utilize Manning’s equation the slope must be greater than 0).
- Include a utility trace wire for all buried piping.
- For underdrains that daylight on grade, include a marking stake and animal guard;
- For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating the system as either an infiltration system, filtration system, or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the underdrain discharge rate equal a 48 hour drawdown time.
- Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade.
- Spacing of collection laterals should be less than 25 feet.
- Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage.
- Avoid filter fabric .
- Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric.
The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual. Underdrain spacing can be calculated using the following spreadsheet, which utilizes the vanSchilfgaarde Equation. The spradsheet includes an example calculation. File:Underdrain spacing calculation.xlsx