(a) When deemed necessary by the city engineer, flood detention/retention systems shall be employed in lieu of or in combination with structural flood control measures to reduce flooding potential or restrict it to a level no greater than pre-platting and/or pre-development conditions. The amount of reduction is stipulated within the Stormwater Detention/Retention Manual, and subsequent amendments, which was approved for use by the city engineer as development standard in the Technical Standards Manual section 4-03.0.
All proposed residential net densities of three (3) or more units per acre and all proposed commercial and industrial developments greater than one (1) acre in size shall provide some method of peak and volumetric runoff reduction. The amount of reduction is stipulated within the Stormwater Detention/Retention Manual, and subsequent amendments, which was approved for use by the city engineer as development standard in the Technical Standards Manual section 4-03.0.
(b) Basins which have been identified by the city engineer as unsuitable for additional development because of the high probability of increased flooding, or flooding of existing improvements or properties not previously studied, may be developed further only upon the incorporation of adequate detention/retention systems as reviewed and approved by the city engineer. The following criteria shall be considered:
(1) If a drainage basin is determined to be a balanced basin, detention or retention systems shall be employed. These systems shall maintain the existing balance within the basin by limiting the flood peak discharges from the site to values no greater than pre-developed conditions.
(2) If a drainage basin is determined to be a critical basin where potential flood problems currently exist, detention or retention systems shall be employed. The purpose of such systems in a critical basin shall be to reduce the potential flood hazard through the detention or retention of storm runoff in fair and equitably apportioned increments.
(c) The design of a detention or retention system, as reviewed and approved by the city engineer, shall include consideration of the degree of existing development within the basin and the capacity of the downstream drainage facilities. The systems will be designed with strict conformance to the public's health, safety and welfare. The effects of recharging storm runoff and possible pollution of the groundwater shall be evaluated for all systems employing infiltration systems, such as dry wells, in order to prevent contamination of the groundwater aquifer.
(d) Structural flood control measures may be utilized in conjunction with or in place of a detention/retention system if it can be clearly demonstrated that such measures shall accomplish, with an equal or greater degree of success, the function of such system, which includes preservation of the water and sediment equilibrium in the affected watercourse and mitigation of the environmental impacts. Appropriate structural flood control measures, such as channelization to a logical conclusion downstream of the proposed development and/or improvements to existing off- site flood control systems within the affected drainage or stream reach, shall be completed in accordance with plans reviewed and approved by the city engineer.
(e) A fee may be utilized in place of a detention/retention system, at the request of affected persons, when it can be clearly demonstrated that detention at the site does not provide off- site flood relief due to the parcel size, location within the drainage basin, or other factors. The fees collected will be used to construct public flood control improvements which will be designed to mitigate the potential damage of floodwaters associated with the property from which the fees are contributed. In balanced and critical basins, and where development is less than three (3) units to the acre, use of a fee system may be considered appropriate in lieu of a detention system in order to preserve the natural drainage patterns. The amount of the fee shall be proportional to the cost of the otherwise required detention/retention system.
(f) The city engineer shall prepare and retain for public inspection and use an official map designating critical and balanced basins within the city.
(g) All repairs and maintenance of detention/retention systems shall conform to the city engineer approved design drawings and specifications. Any deviation shall occur only if approved by the city engineer.
(Ord. No. 7407, § 5, 6-25-90; Ord. No. 11396, § 1, 8-9-16; Ord. No. 11801, § 1, 12-8-20)