(A) It is recognized that the smaller streams and drainage channels serving the county may not have sufficient capacity to receive and convey stormwater runoff resulting from continued urbanization. Accordingly, the storage and controlled release rate of excess stormwater runoff shall be required for any development, redevelopment and new construction located within the county.
(B) Possible exceptions to the requirement are minor subdivisions as described in § 155.01. The County Surveyor together with the County Plan Director, after thorough investigation and evaluation, may waive the requirement of controlled runoff for minor subdivisions.
(1) The release rate of stormwater from development, redevelopment and new construction may not exceed the stormwater runoff from the land area in its present state of development. The developer must submit to the Plan Commission detailed computations of runoff before and after development, redevelopment or new construction which demonstrates that runoff will not be increased.
(2) These computations must show that the peak runoff rate after development for the 100-year return period storm of critical duration must not exceed the ten-year return period pre-development peak runoff rate. The critical duration storm is that storm duration that requires the greatest detention storage.
(C) Computations for areas up to and including 200 acres may be based on the rational method; typical runoff coefficients are listed herein. For areas larger than 200 acres, hydrograph techniques and/or computer drainage modeling methods may be used. Hydrograph techniques and computer modeling methods used to determine stormwater runoff shall be proven methods, subject to approval of the Plan Director, County Surveyor or if appropriate, the Technical Review Committee.
(D) In the event the natural downstream channel or storm sewer is inadequate to accommodate the release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the receiving downstream channel or storm sewer system. Additional detention as determined by the County Surveyor’s office may be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(Ord. 2011-03, passed 4-4-2011)