§ 26-176. Stormwater detention.

In general stormwater detention and retention facilities shall be designed in accordance with the provisions of section 5606 of the APWA criteria and Chapter 8 of HEC 22 as modified in this Code.

(1)

As discussed in the section 26-174, the 100-year storm detention basin inflow hydrograph shall be determined using methods described in SCS (NCRS) TR No. 55.

(2)

The principal spillway shall be a weir, pipe, or orifice and shall be designed to release no more than 1.8 cfs per acre during the 100-year 24-hour storm unless otherwise stipulated by the community development director. It shall be capable of functioning without an attendant, and shall not require the use of tools, equipment or mechanical devices.

(3)

The principal spillway shall be capable of discharging at least 80 percent of the detention storage volume within 24 hours.

(4)

Either the principal spillway or other outlet works in the case of retention basins shall be capable of draining the facility within 72 hours.

(5)

The emergency spillway may be combined with the principal spillway or may be a separate structure. It shall be designed to operate at an elevation 0.5 feet above the level of the 100-year storage volume.

(6)

The principal spillway, outlet works, and conveyance system entrances to the detention basin shall be provided with energy dissipation devices as required to control erosion in accordance with the provisions of section 26-175.

(7)

The inflow hydrograph shall be routed through the basin using the storage indication or modified Puls method as expressed by the equation:

DS/Dt = (I ₁ + I ₂ )/2 - (O ₁ +O ₂ )/2

where:

DS = change in storage, ft.

Dt = time interval, min.

I = inflow, ft.

O = outflow, ft.

(8)

The following design information shall be submitted as part of the drainage study or stormwater management plan:

a.

Detention basin stage-storage-volume curves with indications of volumes allocated to detention, sediment and permanent pool storage in the case of wet basins.

b.

Superimposed inflow and outflow hydrographs for the enclosed system design-year and 100-year, 24-hour storms.

c.

Stage discharge rating curves for the principal and emergency spillways, and wet basin outlet works.

d.

Cumulative inflow volume, cumulative storage volume, cumulative discharge and stage elevation versus time routing curves for the design year and 100-year 24 hour storms.

(9)

Detention and retention basin dams greater than ten feet in height shall be designed as class C structures in accordance with the policies and procedures of the Water Resources Division of the Kansas Department of Agriculture.

(10)

Underground or oversized storm sewer storage may be provided subject to the foregoing design criteria. However, rooftop storage and parking storage are not encouraged, but will be evaluated by the community development director on a case by case basis.

(11)

The developer shall enter into a legally enforceable agreement with the city providing for the perpetual maintenance of the detention facilities and authorizing the city to enter the property for maintenance purposes at the owner's expense in the case of default.

(Code 1967, ch. 7, art. I, § 7-7; Ord. No. 1400, § 1, 3-26-2001; Ord. No. 1689, § 5(7-7), 7-9-2012; Ord. No. 1729, § 5, 1-12-2015 )