Difference between revisions of "Contributing drainage area to stormwater BMPs"
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| − | Contributing area is defined as the total area, including pervious and impervious surfaces, contributing to a BMP. It is assumed that in most cases impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected. The recommended contributing area to a BMP may be modified for the following conditions. | + | Contributing drainage area is defined as the total area, including pervious and impervious surfaces, contributing to a <span title="One of many different structural or non–structural methods used to treat runoff"> '''best management practice'''</span> (BMP). It is assumed that in most cases, with the exception of <span title="Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention."> '''[https://stormwater.pca.state.mn.us/index.php?title=Green_roofs green roofs]'''</span> and many <span title="Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir for temporary storage and/or infiltration. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP)."> '''[https://stormwater.pca.state.mn.us/index.php?title=Permeable_pavement permeable pavement]'''</span> systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is <span title="Impervious areas that are hydraulically connected to the conveyance system (e.g streets with curbs, catch basins, storm drains) and to the watershed outlet point without flowing over pervious areas."> '''directly connected impervious'''</span>. The recommended contributing area to a BMP may be modified for the following conditions. |
| − | *The recommended contributing area may be increased if pervious surfaces constitute the majority of the contributing area and soils are [https://stormwater.pca.state.mn.us/index.php?title=Design_infiltration_rates hydrologic soil group] (HSG) A or B | + | *The recommended contributing area may be increased if pervious surfaces constitute the majority of the contributing area and soils are <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> (HSG) A or B |
*The recommended contributing area should be decreased if impervious surfaces contribute more than 80 percent of the contributing area or if the entire impervious surface is directly connected and routed to the BMP | *The recommended contributing area should be decreased if impervious surfaces contribute more than 80 percent of the contributing area or if the entire impervious surface is directly connected and routed to the BMP | ||
*The recommended contributing area should be decreased or may be increased based on pollutant loading (decrease with higher pollutant loads) | *The recommended contributing area should be decreased or may be increased based on pollutant loading (decrease with higher pollutant loads) | ||
| − | Runoff coefficients may be calculated for an area contributing to a BMP. Runoff coefficients greater than about 0.55 are typical of urban areas having 50 percent or more impervious surface. Typical runoff coefficients are shown on these pages ([https://stormwater.pca.state.mn.us/index.php/Runoff_coefficients_for_5_to_10_year_storms], [https://stormwater.pca.state.mn.us/index.php/Runoff_coefficients_by_land_use_and_soil_type]) and discussed [https://stormwater.pca.state.mn.us/index.php/The_Simple_Method_for_estimating_phosphorus_export#Runoff_coefficient_.28Rv.29 here]. | + | <span title="The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)."> [https://stormwater.pca.state.mn.us/index.php?title=Runoff_coefficients_for_5_to_10_year_storms '''Runoff coefficients''']</span> may be calculated for an area contributing to a BMP. Runoff coefficients greater than about 0.55 are typical of urban areas having 50 percent or more impervious surface. Typical runoff coefficients are shown on these pages ([https://stormwater.pca.state.mn.us/index.php/Runoff_coefficients_for_5_to_10_year_storms], [https://stormwater.pca.state.mn.us/index.php/Runoff_coefficients_by_land_use_and_soil_type]) and discussed [https://stormwater.pca.state.mn.us/index.php/The_Simple_Method_for_estimating_phosphorus_export#Runoff_coefficient_.28Rv.29 here]. To see how runoff <span title="The SCS curve number method is a widely used method for determining the approximate amount of runoff from a rainfall even in a particular area. The curve number is based on the area's hydrologic soil group, land use , treatment and hydrologic condition."> '''curve number'''</span> is associated with impervious percentages, see [https://www.chijournal.org/Journals/PDF/R200-23 at this link]. |
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| + | [[Category:Level 2 - Best management practices/Guidance and information]] | ||
Latest revision as of 18:32, 29 December 2022
Contributing drainage area is defined as the total area, including pervious and impervious surfaces, contributing to a best management practice (BMP). It is assumed that in most cases, with the exception of green roofs and many permeable pavement systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected impervious. The recommended contributing area to a BMP may be modified for the following conditions.
- The recommended contributing area may be increased if pervious surfaces constitute the majority of the contributing area and soils are hydrologic soil group (HSG) A or B
- The recommended contributing area should be decreased if impervious surfaces contribute more than 80 percent of the contributing area or if the entire impervious surface is directly connected and routed to the BMP
- The recommended contributing area should be decreased or may be increased based on pollutant loading (decrease with higher pollutant loads)
Runoff coefficients may be calculated for an area contributing to a BMP. Runoff coefficients greater than about 0.55 are typical of urban areas having 50 percent or more impervious surface. Typical runoff coefficients are shown on these pages ([1], [2]) and discussed here. To see how runoff curve number is associated with impervious percentages, see at this link.
This page was last edited on 29 December 2022, at 18:32.