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Post Construction Runoff Control & BMPs J. C. Hayes, Ph.D., P.E. & D. Hitchcock, Ph.D. South Carolina Stormwater Forum May 8, 2007 Columbia, SC Post-Construction Water Quality BMPs 2 Post-Construction Water Quality BMPs • PCWQ BMP Design Criteria 3 1. Non-Structural Controls 2. Limited Application Structural Controls 3. General Application Structural Controls Pre-Fabricated Devices Selection Process for PCWQ BMPs Non-Structural Controls Natural Stream Buffer Natural Infiltration Area 4 Natural Stream Buffer The general function of stream buffer is to: – – – – – 5 Provide shade for the stream Provide habitat around the stream Remove pollutants, bacteria, sediments, and excess nutrients through infiltration and filtering Help detain high flow rates from developed areas Provide a setback to prevent damage to structures due to flooding or stream channel changes Natural Stream Buffer 6 Natural Infiltration Area 7 Undisturbed land area covered with natural vegetation accepts runoff from new development and infiltrates runoff into soil. Natural Infiltration Area 8 Limited Application Structural Controls 9 Vegetated Filter Strip Vegetated Channels and Swales Vegetative Filter Strips 10 Rely on vegetation to slow runoff velocities and filter out sediment and other pollutants from urban runoff. To be effective, sheet flow must be maintained across the entire filter strip. Once runoff flow concentrates, it short circuits filter strip and reduces water quality benefits. A flow spreader is normally part of the design. Vegetative Filter Strip 11 Vegetated Channels and Swales 12 Can be designed and installed as alternative to curb and gutter and hard piping storm water conveyances. Improve water quality by providing partial pollutant removal as water is filtered by vegetation and by infiltration into soil. Can also reduce flow velocities when compared to hard piping systems. Vegetated Channels and Swales Maintenance requirements include: – – – – 13 Periodic inspection for erosion or formation of gullies. Removal of sediment and debris from channel bottom. Grass lined swales should be mowed to maintain height Grass should be mowed once a year. Vegetated Channels and Swales 14 Vegetated Channels and Swales 15 General Application Structural Controls Infiltration Trench Wet Detention Pond Sand Filters Storm Water Wetlands Bioretention Areas Dry Detention Pond Enhanced Grasses Swale 16 Infiltration Trench 17 Excavated trench filled with stone aggregate used to capture and infiltrate runoff into surrounding soils. Preserves natural water balance by recharging groundwater. Removes wide variety of pollutants through adsorption, precipitation, filtering, and bacterial and chemical degradation. Infiltration Trench 18 Infiltration - Pervious Materials, Pavers and Concrete Alternatives to conventional surface materials Increases chances for infiltration Can be used in combinations Pavers can be expensive Maintenance…!!!. Weeds and clogging 19 Wet Detention Pond 20 Improve storm water quality by detaining runoff for an extended period of time to allow for pollutants suspended in the runoff to settle out. Designed to treat both storm water quantity and quality. Among the most cost-effective and widely used storm water practices. Can be created by excavating an existing depression or through embankments. Wet Detention Pond 21 Wet Detention Pond Benefits include: – – – Limitations include: – – – 22 Moderate to high removal rates of urban pollutants, High community acceptance, Opportunity for wildlife habitat. Potential for thermal impacts/downstream warming, Dam height restrictions for high relief areas, Drainage problems for low relief areas. Wet Detention Pond 23 Sand Filters 24 Multi-chamber structure designed to treat storm water runoff through filtration, using a sediment forebay, a sand bed as its primary filter media, and an underdrain collection system. Typically require 2-6 feet of head. Should be used on sites that are well-stabilized to prevent excess sediment from clogging the filters. Sand Filters 25 Sand Filter 26 Storm Water Wetlands 27 Constructed shallow marsh-like systems designed to treat both urban storm water and control runoff volume. As runoff flows through the facility, pollutant removal is achieved through settling and uptake by vegetation. Storm Water Wetlands Benefits include: – – – Limitations include: – – – 28 Good nutrient removal, Wildlife habitat, Low maintenance costs. Large land requirement, Continuous base flow needed, Sediment regulation is critical to sustain wetland. Storm Water Wetlands 29 Bioretention Cell (BRC) 30 BRCs developed in early 1990’s in Prince George’s County, MD. Utilizes planting mixture with both woody and herbaceous plants to remove pollutants. Runoff is conveyed as sheet flow to treatment area, which consists of a grass buffer strip, ponding area, mulch layer, planting soil mix, and plants. Bioretention Cell 31 Bioretention Cells 32 Dry Detention Pond 33 Surface facilities intended to provide temporary storage of storm water runoff to reduce downstream water quantity impacts. Temporarily detains runoff, releasing the flow over a period of time. Designed to completely drain following a storm event and are normally dry between storm events. Less costly than wet ponds because less excavation is required. Dry Detention Pond 34 Enhanced Grassed Swale 35 Include filter bed of prepared soil that overlays underdrain system. Install berms, check dams, weirs & other structures perpendicular to flow to promote settling and infiltration. Predominantly dry, preferred in residential settings, or along roads and highways. Designed to capture water quality runoff volume, and safely pass larger flows. Enhanced Grassed Swale 36 Pre-Fabricated Control Devices 37 Pre-Fabricated stormwater management control systems may be utilized if adequate pollutant removal efficiency is demonstrated. Type of system to be used should be based on ability to remove pollutants of concern (i.e. bacteria, hydrocarbons, etc.). Pollutant removal efficiency data is necessary and preferably from a third party testing company. Maintenance is critical on these systems so a detailed long-term maintenance plan is required. Pre-Fabricated Control Devices • Used in urban areas • Installed with other stormwater infrastructure • Usually sized based on impervious area • Inspection and maintenance required 38 Pre-Fabricated Control Devices 39 Pre-Fabricated Control Devices 40 Pre-Fabricated Catch Basin Inserts 41 Pre-Fabricated Catch Basin Inserts 42 BMPs in Series or “Treatment Train” Some BMPs are better than others for certain pollutants Consider the connectivity of BMPs Front-end sediment collection; i.e., forebays or traps Vegetated BMPs can be overloaded so should be toward the end of train 43 Design difficulty is that little info is available to design BMPs to meet effluent standard Urban Land Use Buffer Extended Swale Pond (w/ Forebay) Wetland Receiving Waters Selection Process for Post Construction Water Quality BMPs 44 References for Additional Info DHEC OCRM Stormwater BMP Handbook http://www.scdhec.gov/environment/ocrm/pubs/ docs/SW/BMP_Handbook/BMP_Handbook.pdf DHEC OCRM BMP Field Manual http://www.scdhec.gov/environment/ocrm/pubs/ docs/SW/Field_Manual/OCRM_DHEC_FIELD_ MANUAL.pdf 45 Questions?