1119.05   STORM WATER RETENTION/DETENTION.
   (a)   Maximum Discharge Levels. All of the sections of this chapter are hereby qualified to the extent that in no event shall the post development storm discharge exceed the pre-development 125-year storm frequency discharge quantity at any point along the boundary of the development.
   (b)   Calculations. The volume, in cubic feet, of a retention/detention pond, shall be calculated as follows:
      (1)   Calculate existing site (pre-developed) run-off based on a twenty-five year storm curve (in cfs) (Q25).
      (2)   Calculate proposed ultimate developed run-off based on a fifty-year storm curve (in cfs) (Q50).
      (3)   Provide a volume of one-site retention/detention of storm water equal to Q50 minus Q25 in cfs., calculated for a thirty-minute period. The method of retention/detention is subject to approval by the City.
         Example:   Retention/Detention Volume Calculation
         Where:   V   =   (Q50 - Q25) 30 min. (60 sec/1 min)
            V   =   Required volume in cubic feet (ft3) of a retention/ detention pond
            Q50   =   Developed site run-off for a 50-year storm
            Q25   =   Pre-developed site run-off for a 25-year storm
      (4)   Retention/detention storage outlet pipe shall be designed for discharge based on the run-off as calculated in paragraph (b)(1) hereof.
      (5)   No on-site drainage shall outlet downstream of the main retention/ detention facility without providing supplemental retention/detention as per the above criteria, or an equivalent reduction in the retention/ detention outlet.
      (6)   An overflow outlet shall be incorporated in the design of detention/retention basins to protect the surrounding structures in the event of an overflow.
      (7)   Maximum slope of the basin shall be a 3:1 grade.
(Ord. 4360. Passed 4-18-94.)