(a) Sites located in more than One District - for a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the discharge requirements for that subarea as indicated in Section 937.03. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea.
(b) Off-Site Areas - Off-site areas which drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(c) On-Site Areas - On- Site Areas proposed to remain undisturbed as part of the Regulated Activity, including previously developed areas, that are not within the drainage area of any proposed BMPs shall be considered as existing conditions, without considering any reductions in cover type.
(d) "Downstream Hydraulic Capacity Analysis" - Any existing downstream hydraulic capacity analysis shall be conducted in accordance with this Ordinance.
(1) All downstream facilities impacted by the total site area of the Regulated Activity shall be studied to determine if the facility has adequate capacity to handle existing and proposed flows. An impacted downstream facility is one to which the runoff from the total site area of the Regulated Activity comprises more than 50% of the total flow to such a facility. The study shall end at a perennial stream. Downstream facilities include, but are not limited to, manmade or natural swales and open channels, pipes, inlets, culverts, bridges and roadways.
(2) If any private facility is found to be undersized, the applicant shall be responsible for updating the facility in coordination with the Regulated Activity.
(3) If any public facility is found to be undersized or inadequate, the applicant shall work with the Municipality on upgrading the facility in coordination with the Regulated Activity.
(e) Regional Detention Alternatives - For certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the calibrated model as developed for the Stormwater Management Plan.
(f) Capacity Improvements of Local Drainage Networks - In certain instances, primarily within the provisional no detention areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Any capacity improvements would be designed based upon development of all areas tributary to the proposed improvement and the capacity criteria specified in Section 937.08. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this Ordinance except that all new development within the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.
(g) Capacity improvements may also be provided as necessary to implement any regional or subregional detention alternatives.
(h) Where the potential for groundwater and/or surface water contamination exists, based on the proposed use of the Regulated Activity, safeguards shall be incorporated into the site.
(1) For industrial or commercial sites where it is possible that toxic or hazardous substances may come into contact with stormwater runoff, pretreatment of the first-flush (first 1/2 inch) runoff over areas where industrial and commercial operations take place shall be provided. Pretreatment shall include means for separating light and heavy toxic and hazardous substances from the stormwater before the stormwater is conveyed to the general stormwater management facility(ies).
(2) Infiltration systems may be used to handle runoff from commercial or industrial working or parking areas only after the first-flush stormwater from these areas has been pretreated for removal of toxic and hazardous substances.
(i) Roof drains and sump pumps shall discharge to infiltration or vegetative BMPs and to the maximum extent practicable satisfy the criteria for DIAs.
(Ord. 32-2011. Passed 10-4-11.)