In order to minimize storm runoff damage to downstream properties and overloading of existing drainage courses the following criteria shall be followed on all developments of parcels one acre or larger in size.
(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 predevelopment 25-year storm frequency discharge quantity at any point along the boundary of the development.
(b) Definition.
(1) "Retention." Storage areas that maintain a planned permanent level of water for a considerable length of time for aesthetic, agriculture, consumptive, public safety, or other uses, released at a controlled rate.
(2) "Detention." Storage of drainage for a short period of time then released at a controlled rate. The storage area may remain dry during non-drainage periods.
(c) Criteria.
(1) Calculate existing site (predeveloped) runoff based on a 25-year storm curve (in cfs.)
(2) Calculate proposed ultimate developed runoff based on a 50-year storm curve (in cfs.)
(3) Provide a volume of on-site retention/detention of storm water equal to (2)-(1) in cfs., calculated for a thirty (30) minute period. The method of retention/detention is subject to approval by the Village Engineer and Planning Commission.
Example: V = (Q50-Q25) 30 min.
(1800 seconds)
Where V = Required volume in cubic feet
Q50 = Developed site runoff
for a 50-year storm
Q25 = Predeveloped site
runoff for a 25-year storm
(4) Retention/Detention storage outlet pipe shall be designed based on the runoff as calculated in subparagraph (c)(1) hereinabove.
(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.
(Ord. 19-86. Passed 2-20-86; Ord. 36-86. Passed 5- 15-86; Ord. 65-86. Passed 8-21-86.)