§ 10-4.302. Stormwater rate provisions.

1.

Rate control. Peak post-development stormwater discharge rates shall not exceed the peak pre-development rates for all duration storms with return period frequency of up to and including the 25-year storm period. When redevelopment is occurring on a site, the analysis of pre-development runoff shall use the current site conditions. These rate control requirements shall not apply to discharges made to off-site stormwater facilities, as provided for in section 10-4.305, or to approved discharges directly into water bodies, watercourses, wetlands and constructed conveyances which are of sufficient size and capacity to receive the discharges without significant adverse effects. Direct discharge shall also have to comply with the provisions of subsection 10-4.321(d). Stormwater software models shall be acceptable to the county administrator or designee.

2.

No off-site impact. The stormwater discharge shall not cause flooding or other adverse impacts for the downstream areas. For some sites, if there is an immediate downstream flooding problem, then an analysis of the downstream impacts may be necessary regardless of the discharge flow rate or size of project. Flooding problems may require the extent of the analysis to be moved further downstream and/or a continuous analysis be performed based on actual rainfall data. Prior to submitting a conveyance analysis, the limits of such analysis must be discussed and formally agreed to by the county administrator or designee.

a.

Conveyance analysis. If a site is greater than two acres, and its discharge is greater than two and one-half percent of the flow in the conveyance structure at the discharge point for the critical storm, then an analysis shall be completed to show that no adverse impacts occur downstream. The analysis shall include all storms up to and including the 25-year frequency. If there are flooding problems within the analysis area defined above, then an analysis of the storms up to and including the 100-year frequency may be required; or

b.

Restricted discharge. The stormwater management facility shall be designed such that post-development discharge is restricted to the critical duration two-year pre-development discharge rate for all duration and return frequencies up to and including the 25-year, 24-hour storm event. The total required detention volume shall again be available within 90 hours following a rainfall event. At the discretion of the county administrator or designee, a conveyance analysis shall be required if problem areas have been identified downstream.

(Ord. No. 07-20, § 2, 7-10-07; Ord. No. 12-07, § 6, 5-8-12)