As a minimum, the following criteria shall be followed when designing stormwater management facilities and control structures:
(A) Design storm.
(1) Water quantity controls shall be implemented so that post-development peak discharges for three (3) design storms are no more than the pre-development peak discharge. The 1-year, 2- year, 10-year, 25-year, and 100-year 24-hour duration storm events must be analyzed for any proposed construction and no increase in discharge rate from the pre-developed site must take place. Additionally, the 100-year event must be analyzed to ensure no adverse impacts on other properties occur. In the event a development is upstream of a structure located in a special flood hazard area as defined by FEMA FIRM Maps adopted by the city, the post development run off rates shall be reduced from the pre-development run off rate by 10%.
(2) For those areas of development and re-development that result in a new or expanded discharge from the MS4 to high-quality waters, the Design Engineer must perform a critical storm analysis by considering multiple durations (1-hour, 3-hour, 6-hour, 12-hour, and 24-hour) for different return intervals (1-year, 2-year, 10-year, 25-year and 100-year), to establish the worst case scenario for the given circumstance under consideration. Engineering judgement may be used in some cases to make determinations and will be coordinated with the City Engineer.
(B) Emergency spillways. The emergency spillway shall be sized to accommodate a flow equal to the design overflow of the 100-year storm post-development discharge, assuming all other outlets are
completely blocked, without overtopping the dam. Discharge must be conveyed to a public outlet of sufficient capacity. Erosion protection must be provided for the spillway and receiving stream and energy
dissipation must be employed.
(C) Design calculations. Design calculations submitted must include, but are not limited to, the
following:
(1) Contributing drainage area, in acres show pre-development and post-development watershed areas and how they differ.
(2) A breakdown of surface type for pre-development and post-development conditions (such as grassed, paved, roofed, and the like) as the weighted curve numbers are calculated.
(3) Stage-storage curve for the proposed stormwater management facility.
(4) Stage-discharge curve for the outlet structure of the proposed stormwater 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.
(D) Gradient. All detention basins having a vegetative cover shall be designed, constructed, and
maintained equal to or greater than 1% throughout to the point of discharge.
(E) Stormwater management plan. The final stormwater management plan for the entire development shall include, but not be limited to, the following:
(1) All calculations, assumptions and criteria used in the design of the stormwater management facilities and 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) Location, dimensions and design details required for the construction of all facilities.
(4) A description of the operation and maintenance needs for the stormwater management facilities.
(5) All information relative to the design and operation of emergency spillways.
(6) Project specifications relative to erosion and sedimentation control.
(7) All deed restrictions, easements and rights-of-way.
(8) The ownership and maintenance responsibilities for all stormwater management control structures during and after development. The identity of the responsible individual, corporation, association or other specific entity and the specific maintenance activities and schedule must be outlined on the plan.
(9) Stormwater management and control devices shall be designed to minimize water quality impacts and shall attempt to maintain pre-development runoff conditions.
(10) Project narrative and summary from existing conditions to proposed conditions showing that applicable ordinances are met.
(F) Exemptions. Exemptions from the stormwater management requirements contained herein shall be granted to the following:
(1) Land-disturbing activities on property used for agricultural, horticultural or botanical production of plants and animals useful to man, including but not limited to: forages and sod crops, grains and feed crops, tobacco, cotton and peanuts; dairy animals and dairy products; poultry and poultry products; livestock, including beef cattle, sheep, swine, horses, ponies, mules or goats, including the breeding and grazing of these animals; bees; fur animals and aquaculture, except that the construction of a structure used for agricultural purposes of one or more acres, such as broiler houses, machine sheds, repair shops and other major buildings and which require the issuance of a building permit shall require the submittal and approval of a stormwater management plan prior to the start of the land-disturbing activity.
(2) Land-disturbing activities undertaken on forestland for the production and harvesting of timber and timber products.
(3) Minor land-disturbing activities such as residential gardens, individual residential or commercial landscaping, minor home repairs, or maintenance work, and construction or maintenance of individual underground utility connections.
(4) Activities undertaken by local governments or special purpose or public service districts relating to the emergency repair and maintenance of existing facilities and structures. These activities will be carried out using appropriate best management practices to minimize the impact on the environment and surrounding properties.
(5) Any nonresidential development for which the area paved and under roof is less than 10,000 square feet.
(6) 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 Engineer, based upon interpretation of the documentation presented by the developer in conjunction with his or her knowledge of the relationship of the proposed development to the adjacent property.
(G) Design certification. Design of all stormwater management, conveyance facilities and water quality control structures shall be prepared and stamped by a licensed professional engineer (Kentucky registration required).
(H) Construction certification. Prior to final approval of the development, the licensed professional engineer must submit certification that the stormwater management, conveyance facilities and water quality control structures were constructed in accordance with the approved plan. Final approval shall also provide evidence of the recording of all stormwater 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 Engineer in writing for approval.
(I) Ownership, operation and maintenance of stormwater management facilities. For all developments requiring stormwater management facilities, ownership and maintenance responsibilities remain with the property owner/developer. The Stormwater Management/BMP Facilities Long-term Maintenance Agreement must be signed, notarized, and submitted with the stormwater quality management plan.
(Ord. O-2019-20, passed 12-16-19)