Volume 2-05, Chapter 3 - City of Wichita

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Section 3.2.10 - Stormwater Wetland 3.2.10.3 Pollutant Removal Capabilities All of the stormwater wetland design variants are presumed to be able to remove 75% of the total suspended solids load in typical urban post-development runoff when sized, designed, constructed and maintained in accordance with the recommended specifications. Undersized or poorly designed wetland facilities can reduce TSS removal performance. The following design 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 – 75% • Total Phosphorus – 45% • Total Nitrogen – 30% • Heavy Metals – 50% • Fecal Coliform – 70% (if no resident waterfowl population present) 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. 3.2.10.4 Application and Site Feasibility Criteria Stormwater wetlands are generally applicable to most types of development, and can be utilized in both residential and nonresidential areas. However, due to the large land requirements, wetlands may not be practical in higher density areas. The following criteria should be evaluated to ensure the suitability of a stormwater wetland for meeting stormwater management objectives on a site or development. General Feasibility • Suitable for Residential Subdivision Usage – YES • Suitable for High Density/Ultra Urban Areas – Land requirements may preclude use • Regional Stormwater Control – YES • Hotspot Runoff – NO Physical Feasibility - Physical Constraints at Project Site Drainage Area: A minimum of 25 acres and a positive water balance is needed to maintain wetland conditions. Space Required: Approximately 3 to 5% of the tributary drainage area. Page 3 - 130 Volume 2, Technical Guidance
Section 3.2.10 - Stormwater Wetland Site Slope: There should not be a slope greater than 8% across the wetland site. Minimum Head: Elevation difference needed at a site from the inflow to the outflow: 3 to 5 feet. Minimum Depth to Water Table: The bottom of constructed wetlands must be at least 5 feet above the historical high groundwater elevation if unlined, or 2 feet if lined. Soils: Permeable soils are not well suited for a constructed stormwater wetland. Underlying soils of hydrologic group ‘C’ or ‘D’ should be adequate to maintain wetland conditions. Most group ‘A’ soils and some group ‘B’ soils will require a liner. Evaluation of soils should be based on an actual subsurface analysis and permeability tests. 3.2.10.5 Planning and Design Criteria The following criteria are to be considered minimum standards for the design of a stormwater wetland facility. Consult with the local review authority to determine if there are any variations to these criteria or additional standards that must be followed. A. LOCATION AND SITING • Stormwater wetlands should normally have a minimum contributing drainage area of 25 acres or more. • A continuous base flow is required to support wetland vegetation. A water balance must be performed to demonstrate that a stormwater wetland can withstand a 30 day drought at summer evaporation rates without completely drawing down (see Chapter 4). • Wetland siting should also take into account the location and use of other site features such as natural depressions, buffers, and undisturbed natural areas, and should attempt to aesthetically “fit” the facility into the landscape. Bedrock close to the surface may prevent excavation. • Stormwater wetlands cannot be located within navigable waters of the U.S., including natural wetlands, without obtaining a Section 404 permit under the Clean Water Act, and other applicable federal and State permits. In some isolated cases, a wetlands permit may be granted to convert an existing degraded wetland in the context of local watershed restoration efforts. • If a wetland facility is not used for flood control, it should be designed as an off-line system to bypass higher flows rather than passing them through the wetland system. • Minimum setback requirements for stormwater wetland facilities shall be a specified by local regulations. • All utilities should be located outside of the wetland site. B. GENERAL DESIGN • A well-designed stormwater wetland consists of: • Shallow marsh areas of varying depths with wetland vegetation; Volume 2, Technical Guidance Page 3 - 131
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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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Section 3.2.1 - Stormwater Pond 3.2
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Section 3.2.1 - Stormwater Pond Fig
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Section 3.2.1 - Stormwater Pond For
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Section 3.2.1 - Stormwater Pond 1'
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Section 3.2.1 - Stormwater Pond 3.2
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Section 3.2.1 - Stormwater Pond Ste
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Section 3.2.3 - Vegetative Filter S
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Section 3.2.4 - Grassed Channel 3.2
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Section 3.2.4 - Grassed Channel max
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Section 3.2.5 - Enhanced Swale 3.2.
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Section 3.2.5 - Enhanced Swale Chan
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Section 3.2.5 - Enhanced Swale High
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Section 3.2.5 - Enhanced Swale Side
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Section 3.4.3 - Porous Pavement 3.4
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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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