(A) Pre-developed peak flows generated from the 25-year frequency storm shall not be increased. The peak flows from the 25-year storm shall be detained in onsite stormwater detention basins with release rates equal to, or less than, the flows generated from the site for the 25-year storm event when the site was in its existing (natural) state. Detention ponds must also be designed such that the 100-year storm will not overtop the structure. The Engineer shall design an emergency spillway system that will safely discharge the 100-year storm without damage to the downstream property.
(B) The City Engineer or the Director of Public Works shall have the authority to waive the requirement for onsite detention, provided that at least one of the following conditions is met:
(1) The development is eligible to financially participate in an approved Regional Stormwater Management Program (Facility). Under this provision, the applicant shall demonstrate that the peak, post-developed runoff generated from the 100-year storm can be conveyed downstream to the Regional Facility and not impact adversely any downstream properties. An adverse impact shall be:
(a) Any impact which causes an inundation, or an increased inundation, of any building structure, roadway, or improvement.
(b) Downstream erosion and/or sedimentation, or an increase in erosion and/or sedimentation.
(2) The development is adjacent to a defined water course that has sufficient capacity to convey the site's post-developed peak discharge from the 100-year storm event without creating an adverse impact on any other properties. The discharge in the water course shall be determined by using the 100-year storm event with the post-developed site and the remainder of the watershed in an ultimate build-out state.
(3) The development is located within the watershed such that onsite detention may worsen downstream conditions of the watershed. In such cases, the developer's Engineer shall demonstrate that conveyance or a combination of detention and conveyance will provide a safer downstream condition.
(4) The applicant can demonstrate that there will be no increased adverse effect to downstream properties, or if changes made by the applicant to downstream properties can mitigate future increased adverse effects.
(Ord. 2007-18, passed 6-26-07)