Volume 2-05, Chapter 3 - City of Wichita

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Section 3.2.10 - Stormwater Wetland • Permanent micropool; and, • Overlying zone in which runoff control volumes are stored. • Pond/wetland systems also include a stormwater pond facility (see Section 3.2.1 for pond design information). • In addition, all wetland designs must include a sediment forebay or equivalent upstream pre-treatment (providing a volume of 0.1 inch over the contributing impervious area) at the inflow to the facility to allow heavier sediments to drop out of suspension before the runoff enters the wetland marsh. The pre-treatment storage volume is part of the total WQ v . • Wetlands functioning as flood control facilities located in floodplains or backwater areas must perform as specified for peak flow control for any tailwater condition, up to the Base Flood Elevation (BFE). The potential for back flow into the pond must be addressed with flap gates or by providing sufficient volume to receive backflow up to the BFE, and still provide peak flow control surcharge volume in the pond (above the BFE). • Additional wetland design features include an emergency spillway, maintenance access, safety bench, wetland buffer, and appropriate wetland vegetation and native landscaping. Figure 3-41 through Figure 3-43 provide plan and profile schematics for the design of a shallow wetland, extended detention shallow wetland and pond/wetland system, respectively. C. PHYSICAL SPECIFICATIONS / GEOMETRY In general, wetland designs are unique for each site and application. However, there are a number of geometric ratios and limiting depths for the design of a stormwater wetland that must be observed for adequate pollutant removal, ease of maintenance, and improved safety. Table 3-8 provides the recommended physical specifications and geometry for the various stormwater wetland design variants, (Schueler, 1992). Table 3-8 Recommended Design Criteria for Stormwater Wetlands Design Criteria Shallow Wetland ED Shallow Wetland Pond/Wetland Length to Width Ratio (minimum) Extended Detention (ED) Allocation of WQv Volume (pool/marsh/ED) in % Allocation of Surface Area (deepwater/low marsh/high marsh/semiwet) in % Forebay/Upstream Pretreatment 2:1 2:1 2:1 No Yes Optional 25/75/0 25/25/50 20/35/40/5 10/35/45/10 70/30/0 (includes pond volume) 45/25/25/5 (includes pond surface area) Required Required Required Page 3 - 132 Volume 2, Technical Guidance
Section 3.2.10 - Stormwater Wetland Design Criteria Shallow Wetland ED Shallow Wetland Pond/Wetland Micropool Required Required Required Outlet Configuration Reverse-slope pipe or hooded broadcrested weir Reverse-slope pipe or hooded broadcrested weir Reverse-slope pipe or hooded broadcrested weir • The stormwater wetland should be designed with the recommended proportion of “depth zones.” Each of the three wetland design variants has depth zone allocations which are given as a percentage of the stormwater wetland surface area. Target allocations are found in Table 3-8. The four basic depth zones are: Deepwater zone From 1.5 to 6 feet deep. Includes the outlet micropool and deepwater channels through the wetland facility. This zone supports little emergent wetland vegetation, but may support submerged or floating vegetation. Low marsh zone From 6 to 18 inches below the normal permanent pool or water surface elevation. This zone is suitable for the growth of several emergent wetland plant species. High marsh zone From 6 inches below the pool to the normal pool elevation. This zone will support a greater density and diversity of wetland species than the low marsh zone. The high marsh zone should have a higher surface area to volume ratio than the low marsh zone. Semi-wet zone Those areas above the permanent pool that are inundated during larger storm events. This zone supports a number of species that can survive flooding. • A minimum dry weather flow path of 2:1 (length to width) is required from inflow to outlet across the stormwater wetland and should ideally be greater than 3:1. This path may be achieved by constructing internal dikes or berms, using marsh plantings, and by using multiple cells. Finger channels are commonly used in surface flow systems to create serpentine configurations and prevent short-circuiting. Microtopography (contours along the bottom of a wetland or marsh that provide a variety of conditions for different species needs and increases the surface area to volume ratio) is encouraged to enhance wetland diversity. • A 4 to 6 foot deep micropool must be included in the design at the outlet to prevent the outlet from clogging and resuspension of sediments, and to mitigate thermal effects. Volume 2, Technical Guidance Page 3 - 133
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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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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 B. G
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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 3.2.5 - Enhanced Swale 3.2.
Page 87 and 88: Section 3.2.6 - Infiltration Trench
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Page 103 and 104: Section 3.2.8 - Surface Sand Filter
Page 117 and 118: Section 3.2.9 - Bioretention Areas
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 181 and 182: Section 3.3.5 - Organic Filter 3.3.
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Page 185 and 186: Section 3.4 - Other Stormwater Mana
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Section 3.4.1 - Conventional Dry De
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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 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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