Chapter 5: Stormwater Management and Other Watershed Management Practices

Upper Broad Run Watershed Stormwater Management and Other Watershed Management Practices May 2014 CHAPTER 5: STORMWATER MANAGEMENT AND OTHER WATERSHED MANAGEMENT PRACTICES Loudoun County has implemented stormwater control measures (SCMs) and other watershed management practices since the 1980s. The initial focus of stormwater management was detention of large flows to reduce flooding. Subsequent designs addressed water quality treatment and stream channel protection. Most recently, “green” SCMs known as Low Impact Development (LID), Environmental Site Design (ESD), or green stormwater infrastructure are being encouraged for new development and to facilitate restoration of watersheds. New Virginia stormwater regulations for new and re-development will require that stormwater management provide for control of water quantity and quality using the latest guidelines. 5.1 Stormwater Control Measures for Urban/Suburban Areas The following categories of stormwater and watershed management practices were considered in this watershed management plan as the major strategies to address the effects of urban/suburban development in Upper Broad Run. Each has the potential to yield quantifiable benefits in stormwater quality and in quantity control for channel protection and flooding, with the exception of urban nutrient management, which affects only the former. Urban nutrient management Conversion of dry detention ponds (DP) to extended detention dry ponds (ED DP) Conversion of dry ponds to infiltration practices Conversion of dry ponds to extended detention wet ponds / wetlands (ED WP) Addition of pretreatment or post treatment SCMs within existing dry or wet pond boundaries New SCMs retrofits outside of existing dry or wet pond boundaries but which would drain into an existing pond or capture and treat stormwater just outside of the existing pond. Reforestation of stream buffers Reforestation of upland areas Stream restoration for erosion control and nutrient processing New Micro-SCMs (LID) such as bioretention, bioswales, urban filtration practices, etc. not associated with an existing dry or wet pond Downspout disconnection Impervious cover removal Note that stormwater control measure options chosen here are all recognized by the EPA Chesapeake Bay Program and the Virginia Assessment Scenario Tool (VAST) and have vetted nitrogen, phosphorus and pollutant removal efficiencies. 5-1 Upper Broad Run Watershed Stormwater Management and Other Watershed Management Practices May 2014 Urban nutrient management involves the reduction of fertilizer for grass lawns and other urban and suburban manicured pervious areas through efficient application. The implementation of urban nutrient management relies on public education and awareness, targeting urban and suburban residences and businesses, with an emphasis on proper application of fertilizer in order to both reduce excessive fertilizer application and to prevent fertilizer discharge into waterways via precipitation by deliberate timing of fertilizer applications. Urban nutrient management is a source reduction solution and therefore highly cost-effective, particularly in terms of phosphorus which tends to be the limiting element to algal growth in freshwaters. New stormwater management ponds involves placing new stormwater management ponds, including extended detention dry ponds, urban infiltration ponds, and constructed wetlands (Figure 5-1) and wet ponds at locations that currently have no stormwater quantity or quality controls or where existing SCMs are inadequate and where space is available for a new SCM. Ponds are the traditional method of controlling stormwater flows and the opportunity to retrofit new SWM ponds is not common in the developed environment. However, the resulting benefits to flow volume, velocity control, and water quality improvement can be significant. Benefits may vary depending on the specific design features of the individual ponds. Figure 5-1: Constructed Wetland Standard Concept Design from Virginia DEQ Design Specification No. 13 5-2 Upper Broad Run Watershed Stormwater Management and Other Watershed Management Practices May 2014 Stormwater pond conversions can include the following general options for the re-design of existing stormwater ponds to provide additional water quantity control or water quality treatment: Increasing storage capacity by additional excavation. Providing water quality treatment features at facilities that currently have only water quantity control, if the space is available. Examples include micropools, sediment forebays, or constructed stormwater wetlands. Modifying or replacing existing outlet controls to reduce the discharge rate from the stormwater management facility. Where soil types are appropriate, adding infiltration (sometime referred to as exfiltration) features to promote groundwater recharge, and improve pollutant removal. Where water quality flows can be split or separated from larger events, vegetated areas with engineered soils and underdrain, referred to as bioretention, can sometimes be retrofit into an existing pond as pretreatment or post treatment and yield a significant increase in pollutant removal efficiency. Installing proprietary settling, filtering or hydrodynamic devices in parking lots or other areas with a large percentage of impervious area to trap sediments and petroleum products before they flow into a pond. These tend to have low pollutant removal efficiencies but can be good options in the highly urban context, particularly where subterranean treatment is the only option. They have the added benefit of offering inherent quality control during construction by their manufacturers as opposed to SCMs and ponds which are custom built for each application. Specifically, the following types of conversions are recommended. Conversion of Dry Stormwater Management Detention ponds to extended detention dry ponds or extended detention wet pond/wetlands or conversion to ponds with infiltration capability, where soils permit. These SCMs typically treat the largest area of impervious cover because they have the largest drainage areas and were originally built as a low cost option for flood control, channel protection and / or water quality control. Conversion of these existing devices is among the most cost effective of pollutant reduction measures because the existing ponds don’t require acquisition of new property, the pipe infrastructure is already in place, most of the excavation is already complete, maintenance responsibilities and easements have already been established and because stormwater flows already concentrate at these devices. Pollution reduction credits may depend on specific design characteristics affecting both runoff time and treatment. - Dry and Wet Extended Detention (ED) Basins are depressions that temporarily store (“detain”) runoff and release it at a prescribed rate via surface flow or groundwater infiltration following storms. Dry ED basins are designed to dry out between storm events, in contrast with wet ED ponds, which contain standing water permanently. As such, they (ED type) are similar in construction and function to simple dry or wet detention basins 5-3 Upper Broad Run Watershed Stormwater Management and Other Watershed Management Practices May 2014 which are primarily for flood control or channel protection, except that the duration of detention of stormwater is designed to be longer, theoretically improving treatment effectiveness by increasing residence time of pollutants which encourages settling of sediments and allows more time for biological and physical processing of nutrients. - Urban Infiltration Practices are depressions created to allow the collection and infiltration of stormwater in order to trap sediments and nutrients in soil media and simultaneously recharge groundwater aquifers (Figures 5-2 and 5-3). No underdrains are associated with infiltration basins and trenches, because by definition these systems provide complete infiltration. Infiltration basins and trenches cannot be constructed on poor soils, such as C and D soil types. These urban infiltration practices may include vegetation and sand which increases the removal of phosphorus by 5 percent on average compared to infiltration practices without sand or vegetation. Figure 5-2: Residential Infiltration Trench, Taken from Virginia DEQ Stormwater Design Specification No. 8 5-4 Upper Broad Run Watershed Stormwater Management and Other Watershed Management Practices May 2014 Figure 5-3: Standard Section for Infiltration Trench, Taken from Virginia DEQ Storm- water Design Specification No. 8 Reforestation consists of the following two types of tree planting, both of which provide ancillary benefits of enhancing wildlife and amenity values. Planting trees reduces runoff through inter- ception and uptake/transpiration of precipitation, while also providing soil stability, heat island reduction and wildlife habitat benefits. - Riparian Forest Buffers are areas of trees, shrubs, and other vegetation adjacent to a body of water. The riparian area, typically at least 35 feet wide (on each side of a stream), is managed to maintain the integrity of stream channels, and to reduce the impacts of upland sources of pollution by trapping, filtering, and converting sediment, nutrients, and other chemicals. Planting trees and enhancing existing streamside vegetation with native varieties of trees, shrubs, and wildflowers restores many of the water quality, wildlife, and aesthetic benefits associated

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