(A) Dry bottom stormwater storage areas shall be designed as recreation or open areas or other types of uses that will not be adversely affected by occasional or intermittent flooding. A method of carrying the low flow through these areas shall be provided, in addition to a system of drains, and both shall be provided with a positive gravity outlet to a natural channel or storm sewer with adequate capacity.
(B) The combination of storage of the water from a 100-year storm and the design release rate shall not result in a storage duration in excess of 72 hours. The maximum depth of planned stormwater storage shall not exceed four feet, unless existing natural ground contours and other conditions lend to a greater storage depth, which shall be approved by the City Engineer. Minimum grades for turf areas shall be two percent and maximum slopes shall be ten percent. Storage area side slopes shall be kept as close to the natural land contours as practical. If slopes of greater than ten percent are necessary to meet storage requirements or area restrictions, approval shall be obtained from the City Engineer and suitable erosion control shall be provided.
(C) Outlet control structures shall be designed to operate automatically. Each stormwater storage area shall be equipped with facilities for emergency overflow in the event that a storm in excess of the 100-year frequency storm occurs. The emergency overflow facility shall be designed to function automatically and shall become part of the natural or surface channel system. Hydraulic calculations shall be submitted to substantiate all design features. Stormwater runoff velocities shall be kept at a minimum and turbulent conditions at an outfall control structure will not be permitted without complete protection for the public safety. No outlet control device (restrictor) shall be less than three inches in diameter, unless required by the City Engineer.
(2000 Code, § 15.84.050)