The following shall govern the design of any improvement with respect to the detention of stormwater runoff.
(A) Acceptable detention methods.
(1) The increased stormwater runoff resulting from a proposed development should be detained on-site by the provisions of appropriate wet or dry bottom reservoirs (basins) by storage on flat roofs, parking lots, streets, lawns, or other acceptable techniques.
(2) Measures which retard the rate of overland flow and the velocity in runoff channels shall also be used to control the runoff rate partially. Detention basins shall be sized to store excess flows from storms with a 50-year return period. Control devices shall limit the discharge to a rate no greater than that prescribed by this subchapter.
(B) Design storm. Design of stormwater detention facilities shall be based on a return period of once in 50 years. The storage volume and outflow rate shall be sufficient to handle stormwater runoff from a critical duration storm.
(C) Allowable release rate. The allowable release rate of stormwater originating from a proposed development shall not exceed the amount specified in § 153.04.
(D) Determination of storage volume; rational method.
(1) For areas of 200 acres or less, the rational method may be used to determine the required volume of stormwater storage.
(2) The following 11-step procedure may be used to determine the required volume of storage. Table 6 in Appendix A of the ordinance codified herein shall be used to compute the required storage volume. Other design methods may also be used, subject to approval of the Board:
(a) Determine total drainage area in acres (“A”);
(b) Determine composite runoff coefficient (“Cu”) based on existing land use (undeveloped);
(c) Determine time of concentration (“tc”) in minutes based on existing conditions using the TR55 method (see Appendix B of the ordinance codified herein);
(d) Determine rainfall intensity (“Iu”) in inches per hour, based on time of concentration and using data given in Table 3 in the ordinance codified herein for the ten-year return period;
(e) Compute runoff based on existing land use (undeveloped), and ten-year return period (Qu = Cu Iu A);
(f) Determine composite runoff coefficient (“Cd”) based on developed conditions and a 50-year return period;
(g) Determine the 50-year return period rainfall intensity (“Id”) for various storm durations (“td”) for the developed area using Table 3 (see Appendix A of the ordinance codified herein);
(h) Determine developed inflow rates (“Qd”) for various storm durations (“td”), measured in hours (Qd = CdIdA);
(i) Compute a storage rate (“Std”) for various storm durations (“td”) until a peak storage volume is determined (Std = Qd-Qu);
(j) Computer required storage volume (“SR”) in acre-feet for each storm duration (“td”). This assumes a triangular hydrograph of duration (“2*td”) hours with the peak flow of “Std” at “td” hours (SR = Std (td/12)); and
(k) Select the largest storage volume computed in division (D)(2)(j) above for detention basin design.
(E) Determination of storage volume; other methods. Methods, other than the rational method for determining runoff and routing of stormwater, may be used to determine the storage volume required to control stormwater runoff. The procedures or methods used must receive the prior approval of the Board.
(F) General detention basis design requirements.
(1) The maximum planned depth of stormwater stored (without a permanent pool) shall not exceed four feet unless a six-foot high fence protects the basin.
(2) All stormwater detention facilities shall be separated by not less than 25 feet from any occupied building or structure.
(3) Safety screens having a maximum opening of four inches shall be provided for any pipe or opening to prevent children or large animals from crawling into the structures.
(4) 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 Board.
(5) 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 stormwater release rate.
(6) Emergency overflow facilities, such as a weir or spillway, shall be provided in case the normal discharge devices become totally or partially inoperative. The overflow facility shall be of such design that its operation is automatic, and does not require manual attention.
(7) Grass, or other suitable vegetative cover, shall be provided throughout the entire detention storage basin area. Grass shall be cut regularly at approximately monthly intervals during the growing season, or as required to maintain the facility.
(8) Debris and trash removal, and other necessary maintenance, shall be performed on a regular basis to assure continued operation in conformance to design.
(9) No detention facility or other water storage area, permanent or temporary, shall be constructed under or within ten feet of any pole or high voltage electric line. Likewise, poles or high voltage electric lines shall not be placed within ten feet of any detention facility, or other water storage facility.
(10) Detention facilities must maintain setbacks from private and public water supply facilities as prescribed by the State Board of Health and the Recommended Standards For Waterworks by the Great Lakes-Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers.
(G) Dry bottom facility design requirements. 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) The detention facility shall, whenever possible, be designed to serve as a secondary or multipurpose function.
(3) In excavated detention facilities, side slopes of 3:1 or flatter shall be provided for stability. In the event of valley storage, natural slopes may be considered stable.
(H) Wet bottom facility design requirements. Where part of a detention facility will contain a permanent pool of water, all the items required for general and dry-bottom detention storage shall apply except that the system of drains required to maintain a dry bottom facility will not be required. 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.
(1) Facilities designed with permanent pools or containing permanent lakes shall have a water area of at least one-half acre. If fish are to be used to keep the pond clean, a minimum depth of approximately ten 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 (H)(3) below.
(2) In excavated lakes, the underwater side slopes in the lake shall be stable. In the event of valley storage, natural slopes may be considered stable.
(3) A safety ledge four to six 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 two ledges shall be stable and of a material, such as stone or riprap, which will prevent erosion due to wave action.
(4) A safety ramp exit from the lake will be required in all cases, and shall have a minimum width of 20 feet and exit slope of 6:1. The ramp shall be of a material that will prevent its deterioration due to vehicle use or wave action.
(5) (a) 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.
(b) A means of maintaining the designed water level of the lake during prolonged periods of dry weather is also required.
(6) For emergency use, facility cleaning, or shoreline maintenance, additional facilities may have to be provided, or plans prepared for auxiliary equipment to permit emptying and drainage.
(7) Aeration facilities to prevent pond stagnation shall be provided, if required. Design calculations to substantiate the effectiveness of these 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 Board.
(I) Rooftop storage. Detention storage requirements may be met, in total or in part, by detention on flat roofs. Details of such design to be included in the building permit application shall include the depth and volume of storage, details of outlet devices and downdrains, elevations of emergency overflow provisions, and certification of the structural portion of the building design plans by a professional engineer or architect.
(J) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of stormwaters on all or a portion of their surfaces. Outlets will be designed so as to empty the stored waters slowly. Depths of storage shall be limited to a maximum of eight inches in vehicle parking areas so as to limit damage to parked vehicles, and so that access to parked vehicles is not impaired.
(K) Underground tanks or pipes. Detention storage requirements may be met in total or in part by detention in underground tank(s), or in a network of underground pipes. Tanks must be of reinforced concrete construction unless otherwise approved by the Board. Pipes must be reinforced concrete or dual wall (corrugated exterior/smooth interior) high-density polyethylene (HDPE) materials unless otherwise approved by the Board. Details of the underground storage system and the outlet or release structure(s) to be included in the building permit application.
(L) Retention basins. Retention basins (no outlet) are considered a last-resort system. The Board may require the construction of a suitable outlet to prevent the use of retention basins. A retention basin will not be considered unless infiltration wells, dry wells, or other subsurface absorption system is proposed in conjunction with the retention well.
(M) Facility financial responsibility. The construction cost of stormwater control systems and facilities, as required by this subchapter, shall be accepted as part of the cost of land development. If general public use of the facility can be demonstrated, negotiations for public participation in the cost of such development may be considered.
(N) Facility maintenance responsibility. 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 more than one lot or holding shall be documented by appropriate covenants to property deeds, unless responsibility is formally accepted by a public body, and shall be determined before the final drainage plans are approved. Stormwater detention and retention basins may be donated to the county for ownership and permanent maintenance; provided, that:
(1) The county is willing to accept responsibility;
(2) The facility has been designed and constructed according to all applicable provisions of this subchapter; and
(3) All improvements have been constructed, approved, and accepted by the county for the land area served by the drainage basin.
(O) Inspections. All public and privately-owned detention storage facilities will be inspected by representatives of the county or the Drainage Board not less often than once every two years. Inspection reports shall be kept on file for at least six years, and shall be retained with the records of the County Surveyor and County Engineer, or such other designee of the county.
(P) Corrective measures. If deficiencies are found by the Inspector, the owner of the detention/retention facility will be required to take the necessary measures to correct such deficiencies. If the owner fails to do so, the county will undertake the work and collect from the owner using lien rights, if necessary.
(Q) Joint development of control systems. Stormwater control systems may be planned and constructed jointly by two or more developers as long as compliance with this subchapter is maintained.
(R) Installation of control systems. Runoff and erosion control systems shall be installed as soon as possible during the course of site development. Detention/retention basins shall be designed with an additional 10% of available capacity to allow for sediment accumulation resulting from development, and to permit the pond to function for reasonable periods between cleanings. Basins should be designed to collect sediment and debris in specific locations so that removal costs are kept to a minimum.
(Ord. 2007-10, passed - -)