(A) The County understands that many streams and drainage channels serving the county may not have sufficient capacity to receive and convey storm water runoff resulting from existing and continued urbanization. Accordingly, the storage and controlled release rate of excess storm water runoff shall be generally required for any development, re-development and new construction on real estate which lies within the jurisdiction of the Board, unless specifically exempted under this section. Drainage plans and calculations for the detention and controlled runoff for a specific development site shall comply with all provisions of this chapter and be submitted to the Board for approval prior to application for an improvement location permit.
(B) The release rate of storm water from development, re-development and new construction shall not exceed the storm water runoff rate from the land area in its present condition. The developer must submit detailed computations of runoff before and after development, re-development or new construction, that demonstrate that the runoff rate will not be increased.
(C) These computations must show that the peak runoff release 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. Furthermore, the peak runoff rate after development for the ten-year return period storm of critical duration must not exceed the two-year return period pre-development peak runoff rate.
(D) Special cases may exist where the downstream drainage capacity may be limited in a way that strict adherence to this policy may result in undue property damage. In these cases, the Board may further restrict the peak runoff release rate. As determined by the Board, the allowable release rate may be reduced so that additional detention shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(E) Special cases may also exist where the detention of runoff from a development would be detrimental to the overall watershed and drainage outfall. In these cases, the Board may waive or reduce detention requirements.
(F) The Board, after thorough investigation and evaluation, may waive or reduce the requirement of controlled runoff for a specific development site if hydrologic modeling of the watershed and receiving stream prove that the reduction or waiver of controlled runoff is warranted. It shall be the responsibility of the developer to have the modeling prepared and submitted to the Board for approval under the same procedures and requirements outlined in this chapter for a drainage plan.
(Ord. 8-2014, passed 7-21-2014)