As a minimum, the following criteria shall be followed when designing a storm water management facility.
(A) The rainfall events shall be analyzed using the Soil Conservation Service TR-55 method, Rational Method or other methods only as pre-approved by the city.
(B) The storm water discharge point onto adjoining property may not be relocated without the permission of the affected adjoining landowner and the city.
(C) If the storm water discharge onto adjoining property is of a sheet flow nature before development, the storm water discharge onto adjoining property after development of the property may not be changed to a concentrated discharge point without the written agreement of the affected adjoining landowner.
(D) The initial reference conditions for an undeveloped site shall be the conditions that existed on that site as of April 1, 1998. This date refers to the aerial photography on file at the City Engineering Department and available on request.
(E) When a property to be developed experiences upstream storm water runoff onto the property, the effects of that runoff under current conditions shall be included in the storm water analysis. If the off-site runoff onto the property is not isolated from the detention system, the effects of routing the off-site runoff through the detention facilities shall be included in the analysis (Routed Through Design).
(F) Design storm. Storm water management facilities shall be designed to retain the difference in the pre-development and post-development 5-year, 10-year and 25-year, 24-hour storm event. Stormwater management facilities that discharge to high quality waters shall be designed to retain the difference in the pre-development and post-development 2-year, 24-hour storm event. High quality waters are categorized by the Kentucky Division of Water as high quality pursuant to the requirements of 401 KAR 10:030, § 1(3).
(G) Emergency spillways. Emergency spillways shall be designed to pass the 100-year storm. The effect of the 100-year storm must be accommodated and documented in the design of all storm water management facilities.
(H) Design Calculations. Design calculations submitted must include, but not be limited to, the following:
(1) Contributing drainage area, in acres. Indicate if pre-development and post-development areas differ.
(2) A breakdown of surface type for pre-development and post-development conditions (such as grassed, paved, roofed, and the like).
(3) Stage-storage curve for the proposed storm water management facility.
(4) Stage-discharge curve for the outlet structure of the proposed storm water management facility.
(5) Inflow and outflow hydrographs for pre-development and post-development conditions.
(6) Emergency spillway design calculations.
(7) Embankment design criteria as it relates to slope stability and compaction requirements during construction.
(I) Storm water management and water quality plan. The final storm water management and water quality plan shall include, but not be limited to, the following:
(1) All calculations, assumptions and criteria used in the design of the storm water management facilities and water quality control structures.
(2) All plans and profiles of proposed storm sewers and open channels including horizontal and vertical controls, elevations, sizes, slopes and materials.
(3) All plans will depict all contributing areas on the plans.
(4) Location, dimensions and design details required for the construction of all facilities.
(5) A description of the operation and maintenance needs for the storm water management facilities and water quality control structures.
(6) All information relative to the design and operation of emergency spillways.
(7) Project specifications relative to erosion and sedimentation control. (Refer to Best Management Practice for Construction Activities prepared by the Kentucky Natural Resources and Environmental Protection Cabinet for design guidelines associated with erosion and sediment control.)
(8) All deed restrictions, easements and rights-of-way.
(9) The ownership and maintenance responsibilities for all storm water management and water quality control structures during and after development. The identity of the responsible individual, corporation, association or other specific entity and the specific maintenance must be outlined on the plan.
(a) Storm water detention facilities and water quality control structures that are not maintained in proper working condition will be subject to corrective action by city forces along with appropriate fees and fines.
(b) The property owner shall be responsible for maintaining the storm water detention facilities and water quality control structures on the property, unless a maintenance agreement exists with multiple property owners for a regional detention facility.
(J) Exemptions. Exemptions from the storm water management requirement contained herein shall be granted to the following:
(1) All existing residentially subdivided property developments excluding sites to be used or developed as a residential planned development project.
(2) Residential subdivisions or residential planned development projects where minimum lot size is greater than five acres.
(3) Any nonresidential development for which the area paved and under roof is less than 7,500 square feet.
(4) Waivers may also be granted if, in other cases, the developer can provide sufficient documentation that the proposed development will not result in an adverse impact either upstream or downstream of the proposed site. Waivers shall be granted solely at the discretion of the city plan review staff, based upon interpretation of the documentation presented by the developer in conjunction with staff knowledge of the relationship of the proposed development to the adjacent property.
(K) Design certification. Design of all storm water management and conveyance facilities and water quality control structures shall be prepared and stamped by a licensed professional engineer (Kentucky registration required).
(L) Construction certification. Prior to final approval of the development or issuance of certificate of occupancy, the licensed professional engineer must submit certification that the storm water management and conveyance facilities were constructed in accordance with the approved plan. Final approval shall also provide evidence of the recording of all storm water conveyance, management, and water quality facilities deed restrictions, easements and rights-of-way. Any request for deviation from the approved plan during construction shall be submitted to the city plan review staff in writing for approval.
(M) Ownership, operation and maintenance of detention systems and water quality management facilities:
(1) For commercial, industrial and multifamily residential developments, ownership and maintenance responsibilities remain with the property owner/developer.
(2) For single family residential subdivisions, the city may at its discretion accept ownership and maintenance responsibilities; provided, that:
(a) Construction and certification is in accordance to the approved plan; and
(b) Appropriate land dedication and easements are provided, including adequate public ingress and egress from the facility to a public street.
(N) Additional treatment and monitoring may be required. The City reserves the right to require for new and redeveloped properties superseding or additional treatment criteria or objectives for specific pollutant(s) as necessary to meet overall stormwater quality management program objectives or directives under a watershed improvement or total maximum daily load (TMDL) program or KPDES/NPDES permit program as administered by the USEPA or Commonwealth of Kentucky.
(O) Self-inspection required. The property owner shall provide self-inspection documentation for water quality management facilities. Stormwater management staff will periodically inspect water quality management facilities for the purpose of identifying maintenance and structural deficiencies and if required proof of monitoring. If additional treatment and or monitoring is required, the property owner shall be fully responsible for monitoring their stormwater management and water quality facilities in accordance with the most recent directives under a watershed improvement or Total Maximum Daily Load Program or KPDES/NPDES permit program as administered by the USEPA or Commonwealth of Kentucky.
(P) In lieu of fee, an off site mitigation program may be established. The City of Murray Stormwater Management Department may develop a payment-in-lieu program to allow property owners/developers to make payment to the City in lieu of constructing stormwater quality management structures. The in lieu of fee funds shall be applied to public stormwater projects. Another option is to allow the property owner/developer to provide off site mitigation in the same watershed. Both of these options may be developed by the City of Murray Stormwater Management Department following the permit requirements of the KPDES permit for small municipal separate storm sewer systems.
(Ord. 2001-1253, passed 8-23-01; Am. Ord. 2005-1373, passed 3-24-05; Am. Ord. 2006-1429, passed 11-21-06; Am. Ord. 2014-1636, passed 4-10-14)