Drainage schemes for control of storm water shall comply with the following regulations.
(A) The controlled release and storage of excess storm water runoff shall be required in combination for all developments except single-family residential developments that contain an area less than one acre.
(B) The controlled release rate of storm water runoff from all developments described in subsection (A), hereof, shall not exceed the existing "safe" storm drainage capacity of the natural downstream outlet channel or storm sewer system. The rate at which storm water runoff is delivered to a designated storm water storage area shall be unrestricted.
(C) Storm water channels.
(1) A "natural" or surface channel system shall be designed with adequate capacity to convey through the development the storm water runoff from all tributary upstream areas. This "by-pass" channel shall be designed to carry the peak rate of runoff from a 100-year frequency rain event, assuming all storm sewers are blocked and that the upstream areas are fully developed and have been saturated with antecedent rainfall. No habitable structures shall be constructed within this floodway, however, streets and parking or playground areas and utility easements shall be considered compatible uses.
(2) Design of this floodway system shall also take into consideration control of storm water velocity to prevent erosion or other damage to the facility which will restrict its primary use. Depths of flow shall be kept to a minimum and retention of channel configurations shall be totally under Municipal control. In the event that the area within this "by-pass" channel is reshaped or restricted for use as a floodway, the Municipality will cause to have any restrictions removed at the expense of the party or parties causing said restriction.
(3) Should the development contain an existing "natural" waterway, this land configuration shall be preserved as part of the "by-pass" channel system. Construction of a "low flow" system of storm sewers to carry the minor storm runoff and reshaping of the channel with a maximum of six horizontal to one vertical (6:1) side slopes and bottom of a width adequate to facilitate maintenance and carry the flood runoff without eroding velocities shall be included in the plans for land development.
(D) The required volume for storm water detention shall be calculated on the basis of the runoff from a 25-year frequency rain event of any duration as published by a recognized agency. This volume of storage shall be provided for the fully developed watershed that is tributary to the area designated for detention purposes. The storm water release rate shall be considered when calculating the storm water storage capacity and the control structure designed to maintain a relatively uniform flow rate regardless of the depth of storm water in the storage area.
(E) Dry bottom storm water storage areas.
(1) Dry bottom storm water storage areas shall be designed to serve a secondary purpose for recreation, open space 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 as described in § 157.002 hereof.
(2) The combination of storage of water from a 25-year storm and the design release rate shall not result in a storage duration in excess of 72 hours. Maximum depth of planned storm water storage shall not exceed four feet unless the existing natural ground contours and other conditions lend to greater storage depth, which shall be approved by the municipality. Minimum grades for turf areas shall be 2% and maximum slopes shall be 10% (ten units horizontally to one unit vertically). Storage area side slopes shall be kept as close to the natural land contours as practical and a 10% slope or less shall be used wherever possible. If slopes greater than 10% are necessary to meet storage requirements or area restrictions, approval shall be obtained from the Municipality and suitable erosion control provided in addition to the protection required to insure public health, safety and welfare.
(3) Outlet control structures shall be designed as simply as possible and shall require little or no attention for proper operation. Outlet control structures shall also be designed in consideration of the safety of adjacent persons and property. When necessary, said structures will have features to restrict access to unwanted parties. Each storm water storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 25-year frequency storm occurs. This emergency overflow facility shall be designed to function without attention and shall become part of the "natural" or surface channel system described in § 157.002 hereof. Hydraulic calculations shall be submitted to substantiate all design features.
(4) 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 outfall control structure will not be permitted without complete protection for the public safety. The use of restrictive fences shall be kept to a minimum and used only as a last resort when no other method is feasible.
(F) Rooftop and paved storm water storage areas.
(1) Rooftop storage shall be designed with permanent-type control inlets and parapet walls to contain runoff on the rooftop. Emergency overflow areas shall be provided to insure that the weight of water stored will not exceed the structural capacity of the roof.
(2) Paved surfaces that are to serve as storm water storage areas shall have minimum grades of 1% and shall be restricted to storage depths of one foot maximum. Release rates and storage volume requirements for paved storage areas remain the same as outlined in § 157.002 hereof. If a portion of an area within a storm water storage area is to be paved for parking or recreational purposes, the paved surface shall be placed at the highest elevation within the storage area as possible. Maximum parking lot grades shall not exceed normal design parameters of 3% to 5%.
(G) Plans and approval required. Plans, specifications and all calculations for storm water runoff control as required hereunder shall be submitted to the city for review and approval prior to the approval of a final plat, in the case of subdivisions, or issuance of a building permit, in the case of planned unit developments, commercial, industrial, or qualifying residential construction. Final engineering plans and calculations shall show complete details for all of the items covered in this chapter and shall be submitted for review and approved prior to the start of construction.
(H) Control facilities constructed first. Where development of a property presents the threat of flooding or damage by flash runoff to downstream residents, the facilities for storm water runoff control shall be constructed prior to any earthmoving or drainage construction on the project site.
(Ord. 3270, passed 8-28-00)