(A) All storm water storage areas must be designed to contain and safely pass storm water runoff. When platting, the land extending to the centerline of any adjacent right-of-way shall be considered part of the development for purposes of determining the runoff rate. Adequate spillway provisions shall be provided to transport peak runoff from a 100-year storm assuming the outlet to be plugged. All storm water facilities shall be designed in accordance with the design standards established by the city.
(B) All areas within the city shall comply with these regulations.
(1) The required volume of storm water detention shall be that necessary to handle the runoff of a 100-year rainfall, for the critical duration from the drainage area tributary to the storm water storage area based on full development of said tributary area, less the volume discharged during the same duration at the approved release rate. In addition, the detention basin shall be designed to restrict the peak rate of discharge from the drainage area during a two-year, 24-hour rainfall event to be at or below the predevelopment levels.
(2) For sites less than 20 acres, the required detention storage (RDS) will be that found to be the most critical resulting from the inflow from the run-off of a fully developed tributary area from a 100-year storm and the outflow of the five-year storm on a predevelopment basis with the same area. This can be expressed by the following equation:
RDS = | Inflow rate from critical 100-year storm (cfs) | - | Release rate from 5-year storm | X | Duration of the critical 100-year storm (sec) |
(3) For sites greater than 20 acres, the method of sizing of the detention storage volume must be approved by the City Engineer.
(C) Detention storage may be provided as a dry bottom or wet bottom storage area.
(1) Dry bottom storm water storage areas may be designed to serve a secondary purpose for recreation, open space parking or other types of uses that will not be adversely affected by intermittent flooding.
(a) A method of carrying the low flow through these areas shall be provided in addition to a system of drains to prevent soggy areas.
(b) Outlet control structures shall be designed as simply as possible and shall require little or no attention for proper operation. Each storm water storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 100-year frequency storm occurs. This emergency overflow facility shall be designed to function without attention and shall become part of the excess storm water passageway. Hydraulic calculations shall be submitted to the City Engineer to substantiate all design features.
(c) Both outlet control structures and emergency overflow facilities shall be designed and constructed to fully protect the public health, safety and welfare. Storm water runoff velocities shall be kept at a minimum and turbulent conditions at an outlet control structure will not be permitted without complete protection for the public safety. The use of fences shall be kept to a minimum and used only as a last resort when no other method of protection is feasible.
(d) Paved surfaces that are to serve as storm water storage shall be designed with permanent-type control outlets. Emergency overflow areas shall be provided. The maximum storm water ponding depth in any paved area shall not exceed six inches.
(2) (a) Wet bottom storm water storage areas shall be designed with all of the items required for dry bottom storm water storage areas, except that the provisions of division (C)(1)(a) above shall not be required.
(b) However, the following additional conditions shall be complied with:
1. Water surface areas shall not exceed one-fifteenth of the tributary drainage area.
2. Facilities shall be provided to lower the pond elevation by gravity flow for cleaning purposes and shoreline maintenance. Shoreline protection shall be provided to prevent erosion from wave action.
3. Minimum normal water depth shall be four feet. If fish are to be maintained, some portion of the pond area should be a minimum of nine feet deep.
4. Control structures for storm water release shall be designed to operate with only a minor increase in the water surface level when the design storage capacity has been exceeded. Hydraulic calculations shall be submitted to the City Engineer to substantiate all design features.
(D) The city may, in lieu of the detention requirements detailed in the ordinance, accept infiltration practices designed in accordance with the Iowa Storm Water Management Manual. Engineering calculation demonstrating infiltration rates shall be included with the application.