The State of Tennessee issues to the Town of Collierville an NPDES General Permit for Discharges from Small Municipal Separate Storm Sewer Systems, which requires permanent stormwater management. Site designs for all new and redevelopment projects approved after the State established implementation date shall require management measures that are designed, built and maintained to infiltrate, evapotranspire, harvest and/or use, at a minimum, the first inch of every rainfall event preceded by 72 hours of no measurable precipitation. This first inch of rainfall must be 100% managed with no discharge to surface waters, except as provided herein.
(A) Application of runoff reduction requirements may not be appropriate in all circumstances. Alternate treatment approaches may be approved where there is a potential for introducing pollutants into the groundwater, where pre-existing soil contamination is present in areas subject to contact with infiltrated runoff, or where there is the presence of sinkholes or other karst features.
(B) Pre-development infiltrative capacity of soils at the site must be taken into account in selection of runoff reduction management measures.
(C) For developments that cannot meet 100% of the runoff reduction requirement, the remainder of the stipulated amount of rainfall must be treated prior to discharge with a technology documented to remove 80% total suspended solids (TSS) unless an alternative provided under this ordinance is approved. The treatment technology must be designed, installed and maintained to continue to meet this performance standard.
(D) For developments that cannot meet 100% of the runoff reduction requirements, alternative runoff reduction measures may be implemented at another location, preferably within the same USGS 12-digit hydrologic unit code (HUC) as the original development. Off-site mitigation must be a minimum of 1.5 times the amount of water not managed on site. The off-site mitigation location (or alternative location outside the 12-digit HUC) and runoff reduction measures must be approved by the Town’s Engineering/Development Department. The Town’s Engineering/Development Department shall aid in identifying priority areas within the watershed in which mitigation projects can be completed. The Town’s Engineering/Development Department shall create an inventory of appropriate mitigation projects, and develop appropriate institutional standards and management systems to value, evaluate and track transactions. Mitigation can be used for retrofit or redevelopment projects, but should be avoided in areas of new development.
(E) For developments that cannot meet 100% of the runoff reduction and pollutant removal standards and cannot provide for off-site mitigation, the owner may make payment into a public stormwater project fund to be established by the Town. The payment into the fund by the owner shall be 1.5 times the estimated cost of on-site runoff reduction controls.
(1) Funds from the payment in lieu of runoff reduction and pollutant removal shall be used by the Town to construct and maintain detention and conveyance systems within the town.
(2) Acceptance of payment in lieu of runoff reduction and pollutant removal is not automatic and must be considered on a case-by-case basis.
(3) If payment in lieu of runoff reduction and pollutant removal is approved, the developer is still responsible for conveying and managing stormwater runoff from the development to the existing storm drainage system in accordance with the requirements of this section.
(F) Stormwater discharges to critical areas with sensitive resources (e.g., recharge areas, wetlands) may be subject to additional performance criteria as determined by the town’s Engineering/Development Department, or may need to utilize or restrict certain stormwater management practices.
(G) Stormwater discharges from hot spots may require the application of specific structural BMP’s and pollution prevention practices. Stormwater from a hot spot land use may not be infiltrated.
(H) Prior to or during the site design process, applicants shall consult with the town’s Engineering/Development Department to determine if they are subject to additional stormwater requirements.
(Ord. 2012-18, passed 11-12-12; Am. Ord. 2015-03, passed 3-23-15)