The following rules shall govern the design of improvements with respect to managing stormwater runoff.
(A) Methods of determining stormwater runoff rate and volume. For the design of drainage and storage facilities, runoff discharge rate and total volume shall be calculated using the rational method, Soil Conservation Service Unit Hydrograph method or some other accepted method as agreed in advance with the city’s Drainage Coordinator. For all detention/retention facilities, the Soil Conservation Service Unit Hydrograph method will be required. Total volume outflow must also be considered when reviewing downstream conditions. Further, the “Best Management Practices (BMPs) for Controlling Erosion, Sediment, and Pollutant Runoff from Construction Sites” Manual, most recent edition as revised from time-to-time, as adopted and approved by the Kentucky Department of Environmental Protection, Division of Water, is hereby adopted by the City of Franklin and is incorporated herein, by reference and will be relied upon and incorporated in any design and/or drainage control plans approved by the City of Franklin. A copy of the most recent edition of said manual will be maintained at the office of the City Clerk and the Planning and Zoning Office, and will be available on the city’s website.
(B) Release rate. The controlled release rate of stormwater runoff from all developments described in § 150.19 shall not exceed the pre-development peak rate for a storm event of given frequency and duration. The rate at which stormwater runoff is delivered to a designated stormwater storage area shall be unrestricted. In the event the natural downstream channel or storm sewer system is inadequate to accommodate the release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the downstream channel or storm sewer system.
(C) Development design.
(1) Streets, blocks, lots, parks and other public grounds shall be located and laid out in such a manner as to minimize the velocity of overland flow and allow maximum opportunity for infiltration of stormwater into the ground, and to preserve and utilize existing and planned streams, channels and detention basins, and include, whenever possible, streams and floodplain within parks and other public grounds.
(2) Consideration must be given in the preparation of development drainage plans to preclude adverse impacts due to higher rates and volumes of flow that will occur during construction.
(D) Detention. The increased stormwater runoff resulting from the proposed development may be accommodated by the provision of appropriate detention facilities including wet or dry bottom reservoirs, flat roofs, parking lots, underground vaults, road embankment, recreation area or streets. The following shall govern the design of detention facilities.
(1) Storage volume. The volume of storage potential provided in detention facilities shall be sufficient to control the excess stormwater runoff, from the site in its developed state for the two year, five year, ten year, 25 year, 50 year, and 100 year design storms frequency rainfalls as published by the U.S. Weather Bureau. The allowable stormwater release rate from the detention facilities shall not be exceeded regardless of the depth of stormwater contained in the required stormwater detention facility.
(2) Release rate. At no time during the design storm shall the stormwater runoff release rate exceed the allowable release rate as set forth in division (B) above.
(3) Release velocity. Detention facilities shall release stormwater at a non-erosive velocity. Protected channels receiving detention discharge shall incorporate features to reduce velocity to non-erosive levels at the point where such discharge enters the unprotected channel. If release is into a subsurface conduit, the energy gradient in the receiving facility shall not be increased beyond the slope of the conduit.
(4) Spillway. Emergency spillways shall be provided to permit the safe passage of runoff generated from a 100-year, 24-hour storm or greater if required by state law.
(5) Freeboard. Detention facilities shall have adequate capacity to contain the storage volume of tributary stormwater runoff with at least one foot of freeboard above the water surface of flow in the emergency spillway in a 100-year, 24-hour storm or as required by state law.
(Ord. 980.2-9-94, passed 8-24-1994; Ord. 2023-022, passed 6-26-2023)