The following rules shall govern the design of improvements with respect to managing stormwater runoff:
(A) Methods of determining stormwater runoff rate and volume. The volume of required stormwater storage and runoff shall be calculated on the basis of the runoff from a 25-year frequency storm with 24-hour duration. The calculations can be made in accordance with the instantaneous runoff factor method, the rational method or other methods that may be deemed appropriate by the Stormwater Manager.
(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 shall not exceed the pre-development stormwater runoff rate. The rate at which stormwater runoff is delivered to a designated stormwater storage area shall be unrestricted.
(2) In the event that the Stormwater Manager 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 storm water 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 Stormwater Manager 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 storm water passage, however, 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 covers, lining or other treatment and earthen channel side slopes shall be no steeper than two to one. 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.
(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, 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 storm from the development. The detention facility must be designed for periodic maintenance and energy dissipaters 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.
(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.
(3) Release velocity. Detention facilities shall release stormwater at a non-erosive velocity. The protected channel receiving the 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.
(a) Overflow for each stormwater storage area shall be provided in the event of a storm in excess of the design capacity occurs. Such overflows shall 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.
(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 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.
(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 introductions of any foreign matter or the filling, clogging or interfering with the natural drainage capabilities of the sinkholes shall not be permitted.
(2) 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. 2809, passed 12-10-2012) Penalty, see § 160.99