Volume 2-05, Chapter 3 - City of Wichita

Section 3.2.2 - Extended Dry Detention Pond
• 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 3:1 (horizontal to vertical).
• 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 recommended 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 grading is
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.
• Ponds cannot be located within a stream or any other navigable waters of the U.S.,
including wetlands, without obtaining applicable local, Kansas and federal permits.
• Dry extended 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).
Inlet and Outlet Structures
• Discharge into the pond from inflow channels or pipes are to be stabilized with flared
riprap aprons, or the equivalent. A sediment forebay or equivalent upstream pretreatment
with a volume of 0.1 inches per impervious acre of contributing drainage shall be provided
upstream of the pond. The pre-treatment storage volume is part of the total WQ v required.
• An orifice capable of detaining the WQ v for 24 hours must be provided. The orifice shall be
adequately protected from clogging using designs found in chapter 5.
• Likewise, the outlet structure must have an orifice capable of detaining the CP v for 24
hours. Orifice protection requirements are the same as for the WQ v .
• For peak flow control, discharge is controlled by a principal spillway, typically consisting of
a riser and outlet pipe. The WQ v and CP v orifices are usually incorporated into the riser.
Additional orifices and weirs are incorporated into the riser to control the 2 through 100-
year design storms.
• Small outlets that will be subject to clogging or are difficult to maintain are not acceptable.
• See Volume 2, Chapters 4 and 5 for more information on the design of outlet works.
• Seepage control or anti-seep collars should be provided for all outlet pipes.
Volume 2, Technical Guidance Page 3 - 49