The following shall govern the design of any improvement with respect to the detention of storm water runoff:
(A) Acceptable detention methods. The increased storm water runoff resulting from a proposed residential major subdivision, commercial, industrial, or institutional development should be detained on-site by the provisions of appropriate wet or dry-bottom reservoirs, by storage on parking lots, streets, lawns, or other acceptable techniques. 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 100-year return period. Control devices shall limit the discharge to a rate no greater than that prescribed by this chapter (see divisions (E) and (F) of this section).
(B) Design storm. Design of storm water detention facilities shall be based on a return period of once in 100 years. The storage volume and outflow rate shall be sufficient to handle storm water runoff from a critical duration storm, as defined in divisions (E) and (F) of this section. Rainfall depth-duration-frequency relationships and intensity- duration-frequency relationships shall be those given in Appendix B, Tables 5 and 6.
(C) Allowable release rate.
(1) The allowable release rate of storm water originating from a proposed residential major subdivision, commercial, industrial, or institutional development shall not exceed the amount specified in § 157.20, Storm Water Control Policy, and as described in divisions (E) and (F) of this section.
(2) In the event the natural downstream channel or storm sewer system is inadequate to accommodate the release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the receiving downstream channel or storm sewer system and additional detention shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(3) If more than one detention basin is involved in the development of the area upstream of the limiting restriction, the allowable release rate from any one detention basin shall be in direct proportion to the ratio of its drainage area to the drainage area of the entire watershed upstream of the restriction.
(D) Drainage system overflow design. Drainage systems shall have adequate capacity to convey the storm water runoff from all upstream tributary areas through the development under consideration for a storm of 100-year design return period calculated on the basis of the upstream land in its present state of development. An allowance, equivalent to the reduction in flow rate provided, shall be made for upstream detention when such upstream detention and release rate have previously been approved by the Board and evidence of its construction can be shown.
(E) Determination of storage volume - rational method. For areas of 200 acres or less, the rational method may be used to determine the required volume of storm water storage. The following 11-step procedure may be used to determine the required volume of storage. Other design methods may also be used, subject to approval of the Board, and as described in division (F) of this section.
Step Procedure
(1) Determine total drainage area in acres "A".
(2) Determine composite runoff coefficient "CU" based on existing land use (undeveloped).
(3) Determine time of concentration "tC" in minutes based on existing conditions.
(4) Determine rainfall intensity "IU" in inches per hour, based on time of concentration from data given in Appendix B, Table 6 for the 10-year return period.
(5) Compute runoff based on existing land use (undeveloped), and 10-year return period:
QU=CUIUA
(6) Determine composite runoff coefficient "Cd" based on developed conditions and a 100-year return period.
(7) Determine the 100-year return period rainfall intensity "Id" for various storm durations "td" up through the time of concentration for the developed area using Appendix B, Table 6.
(8) Determine developed inflow rates "Qd" for various storm durations “td" measured in hours.
Qd=CdIdA
(9) Compute a storage rate "Std" for various storm durations "td" up through the time of concentration of the developed area.
Std=Qd–Qu
(10) Compute 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)
(11) Select the largest storage volume computed in step 10 for detention basin design.
(F) Determination of storage volume - other methods. Methods other than the rational method for determining runoff and routing of storm water may be used to determine the storage volume required to control storm water runoff. The procedures or methods used must receive the prior approval of the Board. The ILLUDAS, TN-20 and TR-55 models are approved by the Board for appropriate use in analysis of the runoff and routing of storm water. The use of these models or other approved procedures can be defined in a seven-step procedure to determine the required storage volume of the detention basin.
Step Procedure
(1) Calibrate the hydrologic/hydraulic model that is to be used for prediction of runoff and routing of storm water.
(2) For each storm duration listed in Appendix B, Table 5, perform, steps three through six.
(3) Determine the 10-year, undeveloped peak flow. Denote this flow by Q10U
(4) Determine the 100-year runoff hydrograph (H100d) for developed conditions.
(5) Determine the hydrograph that must be stored (H100S) by subtracting a flow up to Q10U from the hydrograph (H100d) found in step 4.
(6) Determine the volume of water (VS) to be stored by calculating the area under the hydrograph (H100S).
(7) The detention basin must be designed to store the largest volume (VS) found for any storm duration analyzed in step 6.
(G) General detention basin design requirements.
(1) Basins shall be constructed to detain temporarily the storm water runoff which exceeds the maximum peak flow rate authorized by this chapter. The volume of storage provided in these basins, together with such storage as may be authorized in other on-site facilities, shall be sufficient to control excess runoff from the 100-year storm.
(2) The following design principles shall be observed:
(a) 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 unless additional storms occur within the period.
(b) The maximum planned depth of storm water stored (without a permanent pool) shall not exceed four feet.
(c) All storm water detention facilities shall be separated by not less than 50 feet from any building or structure to be occupied.
(d) All excavated excess spoil may be spread so as to provide for aesthetic and recreational features such as sliding hills, sports fields, and the like. Slopes no steeper than 4 horizontal to 1 vertical for safety, erosion control, stability and ease of maintenance shall be permitted.
(e) 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.
(f) Danger signs shall be mounted at appropriate locations to warn of deep water, possible flooding conditions during storm periods and other dangers that exist. Fencing shall be provided if deemed necessary by the Board.
(g) 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.
(h) Emergency overflow facilities such as a weir or spillway shall be provided for the release of exceptional storm runoffs or in emergency conditions, should 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.
(i) Grass or other suitable vegetative cover shall be provided throughout the entire basin area. Grass should be cut regularly at approximately monthly intervals during the growing season or as required.
(j) Debris and trash removal and other necessary maintenance shall be performed on a regular basis to assure continued operation in conformance to design.
(k) A report shall be submitted to the Board describing (a) the proposed development; (b) the current land use conditions; (c) the method of hydraulic and hydrologic analysis used, including any assumptions or special conditions; (d) the results of the analysis; and (e) the recommended drainage control facilities. Hydraulic and hydrologic calculations, including input and output files, shall be included as appendices to the report.
(H) Dry-bottom basin design requirements. Detention basins which will not contain a permanent pool of water shall comply with the following requirements:
(1) Provisions shall be incorporated to facilitate complete interior drainage of dry-bottom basins, to include the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, paved gutters, or the installation of subsurface drains.
(2) The detention basin shall, whenever possible, be designed to serve a secondary or multi-purpose function. Recreational facilities, aesthetic qualities (open spaces) or other types of use shall be considered in planning the detention facility.
(I) Wet-bottom basin design requirements. Where part of a detention basin will contain a permanent pool of water, all the items required for detention storage shall apply except that the system of drains with a positive gravity outlet required to maintain a dry-bottom basin will not be required. A controlled positive outlet will be required to maintain the design water level in the wet-bottom basin and provide required detention storage above the design water level. However, the following additional conditions shall apply:
(1) Basins designed with permanent pools or containing permanent ponds shall have a water area of at least one-half acre. If fish are to be maintained in the pond, a minimum depth of approximately ten feet shall be maintained over at least 25% of the pond area. The remaining pond area shall have no extensive shallow areas, except as required by division (I)(3) below.
(2) In excavated ponds, the underwater side slopes in the pond shall be stable. In the case of valley storage, natural slopes may be considered to be stable.
(3) A safety ledge four to six feet in width is required and must be installed in all ponds 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 pond is required in all cases and shall have a minimum width of 20 feet and exit slope of 6 horizontal to 1 vertical. The ramp shall be of a material that will prevent its deterioration due to vehicle use and/or wave action
(5) Periodic maintenance is required in ponds to control weed and larval growth. The pond shall also be designed to provide for the easy removal of sediment which will accumulate during periods of pond operation. A means of maintaining the designed water level of the pond during prolonged periods of dry weather is also required.
(6) For emergency use, basin cleaning or shoreline maintenance, facilities shall be provided or plans prepared for auxiliary equipment to permit emptying and drainage.
(7) Facilities to enhance and maintain pond water quality shall be provided, if required to meet applicable water quality standards. 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.
(J) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of storm waters on all or a portion of their surfaces. Outlets will be designed so as to empty the stored waters slowly. Depths of storage must be limited to a maximum depth of seven 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.
(K) Facility financial responsibilities. The construction cost of storm water control systems and facilities as required by this chapter 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.
(L) Facility maintenance responsibility.
(1) See § 157.28(C) concerning detention basins within residential major subdivisions.
(2) On commercial, industrial and institutional developments, maintenance of detention facilities during construction and thereafter shall be the responsibility of the land developer/owner.
(M) Inspections. All public and privately-owned detention storage facilities shall be inspected not less often than once every five years. For commercial, industrial and institutional sites, the property owner shall provide a certified inspection report by a licensed professional engineer or land surveyor covering physical conditions, available storage capacity and operational condition of key facility elements to the Board. For inspections within major subdivisions, see § 157.28(C)(3).
(N) 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 city will undertake the work and collect from the owner using lien rights, if necessary.
(O) Joint development of control systems. Storm water control systems may be planned and constructed jointly by two or more developers as long as compliance with this chapter is maintained.
(P) 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.
(Q) Detention facilities in floodplains. If detention storage is provided within a 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.
(R) Off-site drainage provisions. When the allowable runoff is released in an area that is susceptible to flooding, the developer may be required to construct appropriate storm drains through such area to avert increased flood hazard caused by the concentration of allowable runoff at one point instead of the natural overland distribution. The requirement of off-site drains shall be at the discretion of the Board.
(Ord. 2000-22, passed 12-28-00) Penalty, see § 157.99