Volume 2-05, Chapter 3 - City of Wichita

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Section 3.2.3 - Vegetative Filter Strip 3.2.3.1 General Description Filter strips are uniformly graded and densely vegetated sections of land engineered to treat runoff and remove pollutants through vegetative filtering and infiltration. Filter strips are best suited to treating runoff from roads and highways, roof downspouts, small parking lots, and other pervious surfaces. They are also ideal components of the "outer zone" of a stream buffer, or as pretreatment for another structural stormwater control. Filter strips can serve as a buffer between incompatible land uses, be landscaped to be aesthetically pleasing, and provide groundwater recharge in areas with pervious soils. Filter strips rely on the use of vegetation to slow runoff velocities and filter out sediment and other pollutants from urban stormwater. There can also be a reduction in runoff volume for smaller flows that infiltrate pervious soils while contained within the filter strip. To be effective, however, sheet flow must be maintained across the entire filter strip. Once runoff flow concentrates, it effectively short-circuits the filter strip and reduces any water quality benefits. Therefore, a flow spreader is often included at the top of the filter strip design. There are two different filter strip designs: a simple filter strip and a design that includes a permeable berm at the bottom. The presence of the berm increases the contact time with the runoff, thus reducing the overall width of the filter strip required to treat stormwater runoff. Filter strips are typically an on-line practice, so they must be designed to withstand the full range of storm events without eroding. 3.2.3.2 Pollutant Removal Capabilities Pollutant removal from filter strips is highly variable and depends primarily on density of vegetation and contact time for filtration and infiltration. These, in turn, depend on soil and vegetation type, slope, and presence of sheet flow. The following pollutant removal rates are average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or “treatment train” approach. • Total Suspended Solids – 50% • Total Phosphorus – 20% • Total Nitrogen – 20% • Heavy Metals – 40% • Fecal Coliform – insufficient data For additional information and data on pollutant removal capabilities, see the National Pollutant Removal Performance Database (2nd Edition) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org. Page 3 - 54 Volume 2, Technical Guidance
Section 3.2.3 - Vegetative Filter Strip 3.2.3.3 Design Criteria and Specifications General Criteria • Filter strips may be used to treat small drainage areas. Flow must enter the filter strip as overland flow spread out over the width (long dimension; perpendicular to flow) of the strip. Flow usually concentrates within a maximum of 75 feet for impervious surfaces, and 150 feet for pervious surfaces (Claytor & Schueler, 1996). These are the maximum allowable contributing flow paths for filter strips unless special provisions are made to ensure design flows spread evenly across the filter strip, instead of becoming concentrated. • Filter strips should be integrated within site designs. • Filter strips should be constructed outside any natural stream buffer area whenever possible to maintain a more natural buffer along the channel. • Filter strips should be designed for slopes between 2% and 6%. Greater slopes than this would encourage the formation of concentrated flow. Flatter slopes would encourage standing water. • The depth of sheet flow along filter strips should normally be limited to approximately 1 inch to discourage flow concentrations. A depth of 0.5 inches is ideal, where practicable. • Filter strips should not be used on soils that cannot sustain a dense grass cover with high retardance. Designers should choose a grass that can withstand relatively high velocity flows at the entrances, and both wet and dry periods. • The flow path should be at least 15 feet across the strip to provide filtration and contact time for water quality treatment. A minimum of 25 feet is preferred (where available). • Both the top and toe regions of the slope should be as flat and even as possible to encourage sheet flow and prevent erosion. • An effective flow spreader is a gravel diaphragm at the top of the slope (ASTM D 448 size 6, 1/8” to 3/8”). The gravel diaphragm (a small trench running along the top of the filter strip) serves two purposes. First, it acts as a pretreatment device, settling out sediment particles before they reach the practice. Second it acts as a level spreader, maintaining sheet flow as runoff flows over the filter strip. Other types of flow spreaders include a concrete sill, curb stops, or curb and gutter with “sawteeth” cut into it. • Ensure that flows in excess of design flow move through the strip without damaging it. A bypass channel or overflow spillway with protected channel section may be required to handle flows above the allowable as defined below. • Pedestrian and vehicular traffic across the filter strip should be limited. • The local jurisdiction may require that the facility be placed in a reserve and/or establishment of a drainage easement to the facility, which is accessible from a public road or other accessible easement. When required, the drainage easement should be at least 20 feet wide, provide a minimum traversable width of 15 feet, have a maximum slope of no more than 10%, and be appropriately stabilized to withstand maintenance equipment and vehicles. Volume 2, Technical Guidance Page 3 - 55
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CHAPTER 3 STORMWATER CONTROLS TABLE
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Chapter 3 - Table of Contents 3.2.8
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Chapter 3 - Table of Contents LIST
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Section 3.1 - Stormwater Management
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Page 31 and 32: Section 3.2.1 - Stormwater Pond 3.2
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Page 35 and 36: Section 3.2.1 - Stormwater Pond For
Page 37 and 38: Section 3.2.1 - Stormwater Pond •
Page 39 and 40: Section 3.2.1 - Stormwater Pond 1'
Page 45 and 46: Section 3.2.1 - Stormwater Pond 3.2
Page 47 and 48: Section 3.2.1 - Stormwater Pond Ste
Page 53 and 54: Section 3.2.2 - Extended Dry Detent
Page 59: Section 3.2.3 - Vegetative Filter S
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Page 65 and 66: Section 3.2.3 - Vegetative Filter S
Page 67 and 68: Section 3.2.4 - Grassed Channel 3.2
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Page 75 and 76: Section 3.2.5 - Enhanced Swale 3.2.
Page 77 and 78: Section 3.2.5 - Enhanced Swale Chan
Page 79 and 80: Section 3.2.5 - Enhanced Swale B. G
Page 81 and 82: Section 3.2.5 - Enhanced Swale High
Page 83 and 84: Section 3.2.5 - Enhanced Swale Side
Page 85 and 86: Section 3.2.5 - Enhanced Swale 3.2.
Page 87 and 88: Section 3.2.6 - Infiltration Trench
Page 99 and 100: Section 3.2.7 - Soakage Trench 3.2.
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Page 103 and 104: Section 3.2.8 - Surface Sand Filter
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Section 3.2.8 - Surface Sand Filter
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Section 3.2.9 - Bioretention Areas
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Section 3.2.10 - Stormwater Wetland
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Section 3.3 - Secondary TSS Treatme
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Section 3.3.1 - Proprietary Treatme
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Section 3.3.1 - Proprietary Treatme
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Section 3.3.2 - Gravity (Oil-Water)
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Section 3.3.3 - Alum Treatment 3.3.
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Section 3.3.3 - Alum Treatment •
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Section 3.3.4 - Underground Sand Fi
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Section 3.3.5 - Organic Filter 3.3.
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Section 3.3.5 - Organic Filter •
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Section 3.4 - Other Stormwater Mana
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Section 3.4.1 - Conventional Dry De
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Section 3.4.2 - Underground Dry Det
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Section 3.4.3 - Porous Pavement 3.4
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Section 3.4.3 - Porous Pavement The
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Section 3.4.3 - Porous Pavement com
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Section 3.4.3 - Porous Pavement Fig
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Section 3.4.4 - Modular Porous Pave
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Section 3.4.5 - Green Roof 3.4.5 Gr
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Section 3.4.5 - Green Roof Signific
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Section 3.4.5 - Green Roof 3.4.5.3
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Volume 2-05, Chapter 3 - City of Wichita

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