Volume 2-05, Chapter 3 - City of Wichita

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Section 3.4.1 - Conventional Dry Detention Pond 3.4.1.1 General Description Conventional dry detention ponds are surface facilities intended to provide for the temporary storage of stormwater runoff to control peak flood flows. These facilities temporarily detain stormwater runoff, releasing the flow over a period of time. They are designed to completely drain following a storm event and are normally dry between rain events. Conventional dry detention ponds may be used to control peak flows on-site, at the project boundary, and downstream of the project. They can be designed to control the full range of flood flows, from the 2-year event up to the 100-year event. They provide limited pollutant removal benefits due to the typically short detention time and resuspension sediments in the pond. Conventional detention facilities must be used in a treatment train approach with other controls that provide treatment of the WQ v . 3.4.1.2 Design Criteria and Specifications Location • Conventional dry detention ponds are to be located downstream of other structural stormwater controls providing treatment of the WQ v and control of the CP v . General Design • Conventional dry detention ponds are sized to temporarily store the volume of runoff required to provide flood protection up to the 100-year storm. • Routing calculations must be used to demonstrate that the storage volume and outlet structure configuration are adequate. See Chapter 4 for procedures on the design of detention storage. • Conventional dry detention ponds 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). • The design may be subject to the requirements of the Kansas dam safety program based on the volume, dam height, and level of hazard. • Earthen embankments shall have side slopes no steeper than 4:1 (horizontal to vertical) maximum. • Vegetated slopes shall be less than 20 feet in height and shall have side slopes no steeper than 4:1 (horizontal to vertical). Riprap-protected slopes shall be no steeper than 3:1. Geotechnical slope stability analysis is required for slopes greater than 10 feet in height. • Areas above the normal high water elevations of the detention facility should be sloped toward the basin to allow drainage and to prevent standing water. Careful finish grading is Page 3 - 182 Volume 2, Technical Guidance
Section 3.4.1 - Conventional Dry Detention Pond required to avoid creation of upland surface depressions that may retain runoff. The bottom area of storage facilities should be graded toward the outlet to prevent standing water conditions. A low flow or pilot channel across the facility bottom from the inlet to the outlet (often constructed with riprap or a concrete flume) is recommended to convey low flows and prevent standing water conditions. • To the extent practicable, the long axis of ponds should be oriented east-west to minimize wind wave bank erosion. • 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. Inlet and Outlet Structures • The discharge from inflow channels or pipes is to be stabilized with flared riprap aprons, or the equivalent. A sediment forebay or equivalent upstream pretreatment sized to 0.1 acreinch per impervious acre of contributing drainage should be provided for in-line dry detention basins that are located downstream of off-line water quality treatment controls. • For a dry detention basin, the principal spillway or outlet structure (usually a concrete riser with orifices and an outlet pipe, or just an outlet pipe) is sized for its flood control functions (based on hydrologic routing calculations). Small outlet openings that will be subject to clogging or are difficult to maintain are not acceptable. Trash racks, hoods, or other protection against debris blockage of principal spillway outlets are required. See Section 3.2.1 and Chapter 5. • See Chapter 4 and Chapter 5 for more information on the design of outlet works. • Seepage control or anti-seep collars should be provided for all outlet pipes through earth dams. • Riprap, plunge pools or pads, or other energy dissipators are to be placed at the end of the pond outlet pipe to prevent scouring and erosion. If the basin discharges to a channel with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a vegetated riparian zone in the shortest possible distance. • A concrete emergency spillway is to be included in the stormwater pond design to safely pass the 100-year, 24-hour rainfall flood event. The spillway prevents pond water levels from overtopping the embankment. When applicable, the emergency spillway must be designed to State of Kansas requirements for dam safety and must be located so that downstream structures will not be impacted by spillway discharges. • A minimum of 1 foot of freeboard must be provided, measured from the top of the water surface elevation for the 100-year flood, to the lowest point of the dam embankment not counting the emergency spillway. The 100-year flood elevation for emergency spillway Volume 2, Technical Guidance Page 3 - 183
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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 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 3.2.2 - 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 3.2.5 - Enhanced Swale 3.2.
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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.
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Section 3.2.6 - Infiltration Trench
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Section 3.2.7 - Soakage Trench 3.2.
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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 3.2.10 - Stormwater Wetland
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Page 217 and 218: 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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