§ 26-171. Performance criteria.

(a)

Secondary or major system protective levels. Regardless of land use, the secondary or major system shall be designed to limit water surface elevations from runoff produced by the 100-year storm in the vicinity of buildings or structures to the lowest elevation at which water may enter the structure less one foot of freeboard (i.e., low entry elevation minus one foot). Regardless of street classification, the maximum 100-year flood ponding elevation at the low point in streets (measured at the crown) shall be seven inches or 0.58 feet. Primary system collection and conveyance elements shall be proportioned as required to maintain the foregoing secondary system protective levels.

(b)

Primary or minor system design. Primary or minor system collection and conveyance elements including inlets and conduits shall be proportioned to limit the design storm in-system hydraulic grade line (HGL) to an elevation no less than six inches or 0.5 feet below any opening to the surface. A minimum of one foot of freeboard shall be maintained between the top of bank of open channels and the design stormwater surface elevation; and between the right-of-way line low point and the design storm headwater pool at culverts. Primary system collection and conveyance elements (including open channels) shall be sized for design storms according to land use or function as indicated below:

(1)

Residential districts: 10-year storm.

(2)

Commercial districts: 25-year storm.

(3)

Industrial districts: 25-year storm.

(4)

Government and institutional districts: 25-year storm.

(5)

Bridges, culverts or pipes crossing arterial streets: 50-year storm.

(6)

100-year floodway: 100-year storm.

(c)

Stormwater detention and retention.

(1)

Unless adverse downstream conditions dictate a lower rate, detention facilities shall be designed to limit the total site discharge from the 100-year storm to no more than 1.8 cfs per acre with a minimum of 0.5 feet of freeboard to the crest of the emergency spillway. The full 100-year storm discharge from offsite tributary areas may be added to the allowable on-site release rate if downstream conditions permit. In addition to the water storage volume needed to meet detention requirements, facilities shall be designed to provide for sedimentation storage as determined by the following formula:

S = 15YIA

where:

S = sediment storage in cubic feet.

Y = number of years before sediment is removed.

I = ratio of impervious surface to total tributary area based on land use.

A = total area tributary to the detention facility.

(2)

The number of years of sediment storage to be provided will be based on a maintenance plan and agreement giving the city the right to enter the property and remove sediment at the owner's expense in the event of failure to maintain. The agreement will run with the land and will be a condition of stormwater management plan and final plat approval.

(d)

Existing drainage system components. Existing drainage system collection, conveyance and storage elements may be retained as elements of an improved system providing:

(1)

They are in sound structural condition.

(2)

Their hydraulic capacity, including surcharge, is not less than 80 percent of the capacity required for the land use and function stipulated in subsection (c) of this section.

(3)

Adequate easements exist or will be provided for their operation and maintenance.

(e)

Existing upstream drainage systems. In determining discharges from off-site sources, it shall be assumed that upstream collection and conveyance elements function in accordance with these design criteria, regardless of their actual condition and performance. However, should the designer elect to use lower flows from an upstream detention system, that system's operating characteristics for each design storm considered; physical condition; maintenance history; and long term viability must be evaluated and reported in the drainage study.

(f)

Existing downstream drainage systems. The impact of the development and proposed drainage system on downstream improvements will be evaluated and made part of the drainage study. The city may, at its discretion, require lower on-site detention facility release rates or participation in improvements to the downstream system including a regional detention facility to relieve existing flooding conditions.

(g)

Detention waivers. Detention may be waived, at the discretion of the community development director, in the event that the developer demonstrates in the drainage study that the delayed release rate will increase a downstream peak discharge from the larger overall drainage basin concentrating at a later time than the included subject sub-basin. However, the developer will be required to either propose or participate in alternate mitigating measures if the increase in runoff from the development will cause or aggravate downstream flooding or overtax existing inadequate or obsolete drainage facilities.

(h)

Curb inlets and street gutter capacity. Curb inlets shall be located to control street spread ("T" width of water surface normal to the gutter as measured from a point in the gutter 0.5 feet from the back of the curb) for discharges calculated at the curb inlet's sub-basin time of concentration according to the following:

Street Width (back of curb to back of curb-ft.)	Spread (0.5 ft.from back of curb-ft.)	Depth of Flow (ft.)
28 or less	10.0	0.20
28+ to 36	11.0	0.22
36+	11.5	0.23
Divided roadways as above for each direction
Arterial and collector intersections	5.5	0.12
Pedestrians crosswalks	5.5	0.12

 

(i)

Spread. Spread shall be determined at the beginning of the gutter flare to the curb inlet using Izzard's equation or appropriate nomographs. Spread at the ends of flares to sump inlets located in the low point of sag vertical curves shall be calculated using a minimum street grade (S) of 0.3 percent. Curb inlet capacity shall be determined using the methods and formulas or appended nomographs found in any of the following: or appended nomographs Chapter 4 of HEC 22; Hydraulic Performance of Setback Curb Inlets (Report No. K-TRAN: KU-98-3, Bruce M. McEnroe, and Reuben P. Wade, University of Kansas, Lawrence, Kansas, June 1998); and Hydraulic Performance of Curb and Gutter Inlets (Report No. K-TRAN: KU-99-1, Bruce M. McEnroe, Reuben P. Wade, and Andrew K. Smith, University of Kansas, Lawrence, Kansas). Curb inlets shall be rated at 80 percent of their calculated capacity as a clogging allowance.

(j)

Field or area inlets. Field or area inlets shall be designed as a broad crested weir (Q=CLH ^3/2 , where C=2.8) to a depth equal to the top of the opening and as an orifice (Q=Ca(2gh) ^1/2 , where C=0.6) when the opening is submerged. Field or area inlets shall be rated at 50 percent of their calculated capacity as a clogging allowance. Although steel frames or bars spaced at no less than four-inch intervals may be placed in the inlet throats for safety purposes, grates will not be permitted.

(Code 1967, ch. 7, art. I, § 7-2; Ord. No. 1400, § 1, 3-26-2001; Ord. No. 1689, § 2(7-2), 7-9-2012; Ord. No. 1729, § 3, 1-12-2015 )