Volume 2-05, Chapter 3 - City of Wichita

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Section 3.2.9 - Bioretention Areas and channel slope. The minimum grassed channel length should be 20 feet. Design guidance on grass channels for pretreatment can be found in Section 3.2.4. • The mulch layer should consist of 2 to 4 inches of commercially available fine shredded hardwood mulch or shredded hardwood chips. • The sand bed (optional) should be 12 to 18 inches thick. Sand should be clean and have less than 15% silt or clay content. • Gravel for the 4” to 9” thick layer above the gravel bedding (and diaphragm and curtain, where used), should be ASTM D 448 size No. 6 (1/8 inch to ¼ inch). • The underdrain collection system is equipped with a 6-inch perforated PVC pipe (AASHTO M 252) in an 8-inch gravel layer. The pipe should have 3/8-inch perforations, spaced at 6- inch centers, with a minimum of 4 holes per row. The pipe is spaced at a maximum of 10 feet on center and a minimum grade of 0.5% must be maintained. • A narrow 24 inch wide permeable filter fabric is placed on the gravel layer directly above the perforated pipes to limit piping of soil directly into the pipe. Filter fabric is also placed along the vertical or sloping outer walls of the bioretention system to limit vertical infiltration prior to filtration through the soil. D. PRETREATMENT / INLETS • Adequate pretreatment and inlet protection for bioretention systems is provided when all of the following are provided: (a) grass filter strip below a level spreader, or grass channel, (b) gravel diaphragm and (c) an organic or mulch layer. E. OUTLET STRUCTURES • Outlet pipe is to be provided from the underdrain system to the facility discharge. Due to the slow rate of filtration, outlet protection is generally unnecessary. F. EMERGENCY SPILLWAY • An overflow structure and nonerosive overflow channel must be provided to safely pass flows from the bioretention area that exceeds the storage capacity to a stabilized downstream area or watercourse. If the system is located off-line, the overflow should be set above the shallow ponding limit. • The high flow overflow system within the structure typically consists of an area drain catchbasin, though any number of conventional systems could be used. The throat of the catch basin inlet is normally placed 6 inches above the mulch layer. It should be designed as a domed grate or a covered weir structure to avoid clogging with floating mulch and debris, and should be located at a distance from inlets to avoid short circuiting of flow. It may also be placed into the side slope of the structure maintaining a neat contoured appearance. Page 3 - 118 Volume 2, Technical Guidance
Section 3.2.9 - Bioretention Areas G. MAINTENANCE ACCESS • The local jurisdiction may require that the facility be placed in a reserve and/or establishment of a drainage easement 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. H. SAFETY FEATURES • Bioretention areas generally do not require any special safety features. Fencing of bioretention facilities is not generally desirable. I. LANDSCAPING • Landscaping is critical to the performance and function of bioretention areas. Native species are encouraged. • A dense and vigorous vegetative cover should be established over the contributing pervious drainage areas before runoff can be accepted into the facility. Side slopes should be sodded to limit erosion of fine particles onto the bioretention surface. • The bioretention area should be vegetated to resemble a terrestrial ecosystem, • With a mature tree canopy, subcanopy of understory trees, scrub layer, and herbaceous ground cover. Three species each of both trees and scrubs are recommended to be planted. • With native prairie grasses. • For bioretention areas with trees, the tree-to-shrub ratio should be 2:1 to 3:1. On average, the trees should be spaced 8 feet apart. Plants should be placed to replicate a natural forest. Woody vegetation should not be specified at inflow locations. • For bioretention areas with prairie grasses, native grasses should be well established, with a mixture of species. Wildflowers may be incorporated into the design to add color and visual interest. • After the trees and shrubs or grasses are established, the ground cover and mulch should be established. • Choose plants based on factors such as whether native or not, resistance to drought and inundation, cost, aesthetics, shallow or deep rooted, maintenance, etc. Planting recommendations for bioretention facilities are as follows: • Native plant species should be specified over non-native species. • Vegetation should be selected based on a specified zone of hydric tolerance. • A selection of trees with an understory of shrubs and herbaceous materials or a mixture of prairie grasses should be provided. Volume 2, Technical Guidance Page 3 - 119
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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 •
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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 3.2.2 - Extended Dry Detent
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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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Page 73 and 74: Section 3.2.4 - Grassed Channel max
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.
Page 101 and 102: Section 3.2.7 - Soakage Trench •
Page 103 and 104: Section 3.2.8 - Surface Sand Filter
Page 117 and 118: Section 3.2.9 - Bioretention Areas
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Page 133 and 134: Section 3.2.10 - Stormwater Wetland
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Page 153 and 154: Section 3.3.1 - Proprietary Treatme
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Page 157 and 158: Section 3.3.2 - Gravity (Oil-Water)
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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 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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