(A) General. The following shall govern the design of any improvement with respect to the detention of stormwater runoff. Basins shall be constructed to temporarily detain the stormwater runoff which exceeds the maximum peak release rate authorized by this chapter. The required volume of storage provided in these basins, together with storage as may be authorized in other onsite facilities, shall be sufficient to control excess runoff from the 10-year or 100-year storm as explained below in division (D) below.
(B) Additional information. Also, basins shall be constructed to provide adequate capacity to allow for sediment accumulation resulting from development and to permit the pond to function for reasonable periods between cleanings.
(C) Acceptable detention facilities. The increased stormwater runoff resulting from a proposed development should be detained onsite by the provisions of appropriate wet bottom or dry bottom detention facilities, parking lots or other acceptable techniques. Measures which retard the rate of overland flow and the velocity in runoff channels shall also be used to partially control runoff rates.
(D) Allowable release rates. Control devices shall limit the discharge to a rate so that the release rate from the site is no greater than the 2-year predeveloped rate for 0-10 year return interval storms and the 10-year predeveloped rate for 11-100 year return interval storms. That is, all storms up to and including the 10-year return period storm must be detained at a release rate below the predeveloped peak 2-year return period stormwater runoff rate. The release rate for developments and redevelopments for the 11-100 year return period storms shall not exceed the predeveloped peak 10-year return period rate. As stated in § 51.06, the allowable release rates may be reduced from these levels if downstream restrictions exist. (E) Drainage system overflow design. Drainage systems, including all ditches, channels, conduits, swales and the like, shall have adequate capacity to convey the stormwater runoff from all upstream tributary areas (off-site land areas) through the development under consideration for a 100-year return period design storm calculated on the basis of the upstream land use in its present state of development. Swales between privately owned residential lots shall not be used to convey the above referenced stormwater runoff unless the discharge paths are confined within the drainage easements and/or common areas. In the case of existing upstream detention, an allowance equivalent to the reduction in flow rate provided may be made for upstream detention only when:
(1) The detention and release rate have previously been approved by the City of Brazil official charged with the approval authority at the time of the approval; and
(2) Evidence of its construction and maintenance can be shown.
(F) General detention basin design requirements. Basins should be designed to collect sediment and debris in specific locations so that removal costs are kept to a minimum.
(1) The maximum volume of water stored and subsequently released at the design release rate shall not result in a storage duration in excess of 48 hours from the start of the storm unless additional storms occur within the period.
(2) All stormwater detention facilities shall be separated by not less than 25 feet from any building or structure to be occupied.
(3) No detention facility or other water storage area, permanent or temporary, shall be constructed under or within 20 feet of any pole or high voltage electric line. Likewise, poles or high voltage electric lines shall not be placed within 20 feet of any detention facility or other water storage area.
(4) All stormwater detention facilities shall be separated from any road by no less than 1 right-of-way width, measured from the top of the bank, using the most restrictive right-of-way possible. If the width of the right-of-way is less than 50 feet, then the minimum distance between top of bank and road shall be increased to 50 feet.
(5) No slopes steeper than 3 horizontal to 1 vertical (3:1) for safety, erosion control, stability and ease of maintenance shall be permitted.
(6) Safety screens having a maximum opening of 4 inches shall be provided for any pipe or opening to prevent children or large animals from crawling into the structures.
(7) Danger signs shall be mounted at appropriate locations to warn of deep water, possible flood conditions during storm periods and other dangers that exist. Fencing shall be provided if deemed necessary by the city engineer.
(8) Outlet control structures shall be designed to operate as simply as possible and shall require little or no maintenance and/or attention for proper operation. They shall limit discharges into existing or planned downstream channels or conduits so as not to exceed the predetermined maximum authorized peak flow rate.
(9) Emergency overflow facilities such as a weir or spillway shall be provided for the release of exceptional storm runoff or in emergency conditions should the normal discharge devices become totally or partially inoperative. The overflow facility shall be of a design so that its operation is automatic and does not require manual attention.
(a) Off-site flows greater than the rate produced by the 10-year storm in the post-developed condition shall be conveyed through the emergency spillway, not through the primary outlet structure.
(b) Emergency overflow facilities shall be designed to handle 1-1/4 times the peak discharge and peak flow velocity resulting from the 100-year design storm event runoff from the entire contributing watershed, assuming post-development condition, draining to the detention/retention facility.
(10) Grass or other suitable vegetative cover shall be provided along the banks of the detention storage basin. Grass should be cut regularly at approximately monthly intervals during the growing season or as required to maintain the facility.
(11) Debris and trash removal and other necessary maintenance shall be performed on a regular basis to ensure continued operation in conformance to design.
(12) No residential lots or any part thereof, shall be used for any part of a detention basin or for the storage of water, either temporary or permanent, unless that part of a lot is located within an easement. The easement must be of sufficient width, as determined by the city engineer, to perform routine maintenance activities.
(G) Additional requirements for wet-bottom facility design.
(1) Where part of a detention facility will contain a permanent pool of water, all of the items required for detention storage, and in divisions (I) and (J) below, shall apply except that the system of drains with a positive gravity outlet required to maintain a dry bottom facility will not be required.
(2) A controlled positive outlet will be required to maintain the design water level in the wet bottom facility and provide required detention storage above the design water level. However, the following additional conditions shall apply.
(a) Facilities designed with permanent pools or containing permanent lakes shall have a water area of at least 1/2 acre. If fish are to be used to keep the pond clean, a minimum depth of approximately 10 feet shall be maintained over at least 25% of the pond area. The remaining lake area shall have no extensive shallow areas, except as required by division (3) below.
(b) A safety ledge for 4 to 6 feet in width is required and shall be installed in all lakes approximately 30 to 36 inches below the permanent water level. In addition, a similar maintenance ledge 12 to 18 inches above the permanent water line shall be provided. The slope between the 2 ledges shall be stable and of a material such as stone or riprap which will prevent erosion due to wave action.
(c) A safety ramp exist from the lake shall be required in all cases and shall have a minimum width of 20 feet and exit slope of 6 horizontal to 1 vertical (6:1). The ramp shall be of a material that will prevent its deterioration due to vehicle use or wave action such as a concrete block chute or a cellular confinement system filled with #53 aggregate with a geotextile fabric underlay. Any other construction materials shall receive prior approval from the city engineer.
(d) Periodic maintenance is required in lakes to control weed and larval growth. The facility shall also be designed to provide for the easy removal of sediment which will accumulate during periods of reservoir operation. A means of maintaining the designed water level of the lake during prolonged periods of dry weather may also be required.
(e) Aeration facilities to prevent pond stagnation shall be considered in all designs. Design calculations to substantiate the effectiveness of any aeration facilities shall be submitted with final engineering plans. Agreements for the perpetual operation and maintenance of aeration facilities shall be prepared to the satisfaction of the city engineer.
(f) For visual clarification, refer to Figures 4 and 5.
(H) Additional requirements for dry-bottom facility design. Detention facilities which will not contain a permanent pool of water shall comply with the following requirements:
(1) Provisions shall be incorporated into facilities for complete interior drainage of dry bottom facilities including the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facility, paved gutters or the installation of subsurface drains;
(2) For residential developments, the maximum planned depth of stormwater stored shall not exceed 4 feet; and
(3) In excavated detention facilities, a minimum side slope of 3:1 shall be provided for stability. In the case of valley storage, natural slopes may be considered to be stable.
(1) Paved parking lots may be designed to provide temporary detention storage of stormwaters on all or a portion of their surface.
(2) Outlets for parking lot storage of stormwaters will be designed so as to empty the stored waters slowly. Depths of stored water shall be limited to a maximum depth of 7 inches so as to prevent damage to parked vehicles and so that access to parked vehicles is not impaired. Ponding should, in general, be confined to those positions of the parking lots farthest from the area served.
(J) Detention facilities in floodplains. If detention storage is provided within a 100-year floodplain, only the net increase in storage volume above that which naturally existed on the floodplain shall be credited to the development. No credit will be granted for volumes below the elevation of the regulatory flood at the location unless compensatory storage is also provided.
(K) Facility financial responsibilities. The construction costs of stormwater control systems and required facilities which are identified in the Subdivision Chapter of the City of Brazil shall be accepted as part of the cost of land development.
(L) Facility maintenance responsibilities. Maintenance of detention/retention facilities during construction and thereafter, shall be the responsibility of the land developer/owner. Assignment of responsibility for maintaining facilities serving 1 or more lots(s) or holding(s) shall be documented by appropriate covenants to property deeds and documented in sales agreements, unless responsibility is formally accepted by a public body.
(M) Joint development of control systems. Stormwater control systems may be planned and constructed jointly by 2 or more developers as long as compliance with this chapter is maintained.
(N) Diffused outlets. When the allowable runoff is released in an area that is susceptible to flooding or erosion, the developer may be required to construct appropriate storm drains through the area to avert increased flood hazard caused by the concentration of allowable runoff at 1 point instead of the natural overland distribution. The requirements of diffused outlet drains shall be at the discretion of the city engineer.
(O) IDNR requirements. All designs for basins to be constructed in the floodway of a stream with a drainage area of 1 square mile or more must also satisfy IDNR permit requirements.
(Ord. 01-2000, passed 7-18-2000)
