(A) The city reserves the right to develop or adopt other guidance documents to serve as design and implementation standards. Other guidance documents distributed by the city should be reviewed and considered when preparing design and maintenance plans. Checklists, technical manuals, administrative, or procedural matters may be modified as needed to meet the objectives of the stormwater ordinance, provided said modifications are consistent with the intent of this chapter and the requirements of the State.
(B) The following criteria, along with accompanying technical manuals and/or guidance documents as mentioned above, shall be implemented when designing improvements with respect to managing storm water runoff:
(1) Methods of determining storm water runoff discharge rate and volume. The volume of required storm water storage and discharge rate for drainage areas totaling 100 acres or less, shall be calculated on the basis of the runoff from a 2, 10, 25-year frequency storm event with a 24 hour duration for pre and post development conditions. The calculations can be made in accordance with the instantaneous runoff factor method, the rational method, soil conservation service (SCS) method or other methods that may be deemed appropriate by the City Civil Engineer. The Intensity Duration Curves for Louisville MSP may be used for Bardstown. For larger drainage systems, the SCS hydrologic methods or the "Regional Method" of the Kentucky Transportation Cabinet, Department of Highways shall be used to determine peak runoff rates.
(B) Release rate.
(1) All developments undertaken as outlined in that chapter shall be done in such a way as to insure that stormwater falling on a given site shall be absorbed or detained on site to the extent that the controlled release rate of stormwater runoff from all developments described in § 156.15(A) shall not exceed the pre- development stormwater runoff rate, unless it can be shown that no significant adverse downstream impacts will result from higher rates. The rate at which stormwater runoff is delivered to a designated stormwater storage area shall be unrestricted.
(2) In the event that the City Civil Engineer determines that 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) Where it can be demonstrated by the developer that a higher stormwater release rate will not be contrary to the purpose and intent of this chapter and where such proposed release rate will not adversely affect properties in the downstream portion of the watershed, the City Civil Engineer may permit such release to be used as deemed appropriate.
(2) 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.
(D) Excess stormwater passage.
(1) An excess stormwater passage shall be provided for all stormwater areas. Such passage shall have the capacity to convey through the proposed development the excess stormwater. The capacity for a passage shall be such that it will be able to transport the peak rate of run-off from a 100-year, 24-hour return frequency storm.
(2) There shall be no buildings or structures constructed within excess stormwater passage or within the elevation of the flood of record. Parking lots, playgrounds and park areas, which shall not impair or endanger the water holding capability of a development shall be considered compatible uses.
(3) Appropriate land planning shall be undertaken to preserve the existing natural drainage of a proposed development as part of the excess stormwater passage.
(4) Open channels shall be protected from erosion by appropriate vegetative cover, lining or other treatment and earthen channel side slopes shall be no steeper than three to one (3:1). Open channels with lining shall have a maximum gradient on side slopes of 67% and channel side slopes steeper than 67% shall be designed as structural retaining walls. Refer to the Kentucky Transportation Center’s BMP Manual for controlling erosion, sediment, and pollutant runoff to protect channels with steep side slopes or set on a steep gradient.
(E) Stormwater storage/detention areas. 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, or streets. Storage areas shall be designed to the satisfaction of the City Civil Engineer and if possible to provide secondary purposes for recreation, open spaces, parking lot or other types of use that will not be adversely affected by intermittent flooding. The following shall govern the design of detention facilities:
(1) Storage volume.
(a) All stormwater storage areas must be designed to contain and safely pass stormwater runoff. The combined capacity of these storage areas shall be sufficient to contain the stormwater from the development. The detention facility must be designed for periodic maintenance and energy dissipators shall be provided at points necessary.
(b) The ponding of stormwater runoff shall not exceed a depth of one foot on a pedestrian mall area or 1-1/2 feet maximum in parking lots. Where these areas are used for ponding the maximum depth should occur in the most remote and least used areas.
(c) The drainage and grading design shall be prepared to insure that in a 100-year storm the depth of water run-off in any street, alley, or pedestrian mall will not exceed the level of the first floor of any building. The finished floor elevation shall be set two (2) feet above the 100-year flood elevation for all new development and the minimum finished floor elevations for homes shall be stated on the record plat.
(d) For wet pond storage areas when calculating the storage capacity, only the volume available to store excess stormwater shall be considered. Permanent water storage does not constitute control of excess storm runoff.
(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) of this section. Detention basins shall be fully discharged within 36 hours of the end of the storm event.
(3) Release velocity. Detention facilities shall release stormwater at a nonerosive velocity. The protected channel receiving the detention discharge shall incorporate features to reduce velocity to nonerosive 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.
(a) Overflow for each stormwater storage area shall be provided in the event a storm in excess of the design capacity occurs. Such overflows hall be constructed to function without specific attention and shall become part of the excess stormwater passage.
(b) 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. The spillway dimensions shall be clearly dimensioned and protected from erosion.
(c) Where rooftop storage or excess stormwater is provided, the building shall be provided with adequate structural design to insure that roof failure does not occur. Overflow areas shall be provided so that the weight of stored stormwater will not exceed the structural capacity of the roof.
(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.
(6) Dam. The dam slopes shall not exceed three to 1 (3:1) on the interior of the pond. The minimum width of the top of dam shall be two (2) feet, and the back side of the dam shall not exceed a slope of two to one (2:1). Detention/retention pond dams built with the maximum allowable slope may be required to utilize erosion control blankets or other means beyond seed straw to stabilize the slopes.
(7) Basin slope. Dry basins shall have a minimum 1% bottom drainage slope to allow for positive drainage during and immediately following a rain event. For dry basins with bottom slopes less than 2%, a paved channel is required.
(F) Sinkholes and subterranean water channels.
(1) The use of sinkholes or subterranean water channels for direct drainage of excess stormwater shall not be permitted although they may be used to drain a stormwater storage area. The introduction of any foreign matter or the filling, clogging or interfering with the natural drainage capabilities of the sinkholes shall not be permitted.
(2) Any person, firm or corporation proposing alterations, improvements or other disturbances of any sinkholes or known subterranean water channel must submit plans to the City Civil Engineer showing that said alterations, improvements or disturbance would not interfere with the drainage capability. Also, erosion control methods must be shown for any activities which might create erosion or sedimentation and must be included in the plans.
(3) Sinkholes shall not be altered in any way which would negatively affect the drainage capabilities of the sinkhole. Development within the 100-year floodplain of a sinkhole shall not be permitted.
(Ord. 432, passed 5-16-90; Am. Ord. B2006-8, passed 5-23-06; Am. Ord. B2011-19, passed 9-27-11) Penalty, see § 156.99