3 PCWQ BMP Design Criteria 1. Non-Structural Controls 2. Limited Application Structural Controls 3. General Application Structural Controls Pre-Fabricated Devices Selection Process for PCWQ BMPs
4 Non-Structural Controls Natural Stream Buffer Natural Infiltration Area
5 Natural Stream Buffer The general function of stream buffer is to: – 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
10 Vegetative Filter Strips 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.
12 Vegetated Channels and Swales 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.
13 Vegetated Channels and Swales Maintenance requirements include: – 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.
16 General Application Structural Controls Infiltration Trench Wet Detention Pond Sand Filters Storm Water Wetlands Bioretention Areas Dry Detention Pond Enhanced Grasses Swale
17 Infiltration Trench 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.
19 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
20 Wet Detention Pond 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.
22 Wet Detention Pond Benefits include: – Moderate to high removal rates of urban pollutants, – High community acceptance, – Opportunity for wildlife habitat. Limitations include: – Potential for thermal impacts/downstream warming, – Dam height restrictions for high relief areas, – Drainage problems for low relief areas.
24 Sand Filters 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.
27 Storm Water Wetlands 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.
28 Storm Water Wetlands Benefits include: – Good nutrient removal, – Wildlife habitat, – Low maintenance costs. Limitations include: – Large land requirement, – Continuous base flow needed, – Sediment regulation is critical to sustain wetland.
30 Bioretention Cell (BRC) 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.
33 Dry Detention Pond 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.
35 Enhanced Grassed Swale 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.
37 Pre-Fabricated Control Devices 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.
38 Used in urban areas Installed with other stormwater infrastructure Usually sized based on impervious area Inspection and maintenance required Pre-Fabricated Control Devices
43 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 Design difficulty is that little info is available to design BMPs to meet effluent standard Extended Swale Buffer Pond (w/ Forebay) Wetland Urban Land Use Receiving Waters
44 Selection Process for Post Construction Water Quality BMPs
45 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