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 (peak rate) resulting from a proposed development should be detained on site by the provisions of appropriate wet or dry bottom reservoirs, by storage on flat roofs, 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 ten (10) year and one hundred (100) year return period. Control devices shall limit the discharge to a rate no greater than that prescribed by this ordinance (see Sections (F) and (G)).
(B) Time of Concentration. All storm water management projects within the City of Mt. Vernon must be done using the time of concentration methodology outlined in the SCS TR-55 manual. The TR-55 method examines the factors which affect time of concentration including surface roughness, channel shape and flow patterns along with watershed slope. Through the examination of sheet, shallow, concentrated and open channel flows, a more refined time of concentration may be determined. The methodology represents the best attempt of a Federal Agency to standardize times of concentration procedures.
(C) Design Storm. Design of storm water detention facilities shall be based on a return period of once in 10 years and 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 Sections (F) and (G). Rainfall depth duration frequency relationships and intensity duration frequency relationships shall be those given in Tables 5 and 5A.
(D) Allowable Release Rate.
(1) Design of storm water detention facilities shall be based on the allowable release rate of storm water originating from a proposed development and shall not exceed the amount specified in § 53.05 Storm Water Control Policy, and as described in Sections (F) and (G).
(2) In the event the natural downstream channel or storm sewer system is inadequate to accommodate the release rate provided in Table 5A, 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 as determined by the Council shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(3) The area will be considered an impact drainage area subject to the provisions of § 53.18 of this Ordinance.
(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.
(E) 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 upstream land in its undeveloped state. 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 Council and evidence of its construction can be shown.
(F) Determination of Storage Volume Rational Method.
(1) The Rational Method may be used to determine the 10 year return period undeveloped release rate for sites of less than five (5) acres of commonly owned contiguous property where no depressional storage exists.
(2) Step Procedure.
(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.
(d) Determine rainfall intensity “Iu” in inches per hour, based on time of concentration and using Figure 1 or from data given in Table 5A for the ten (10) year return period.
(e) Compute runoff based on existing land use (undeveloped), and ten (10) year return period:
Qu = CuIuA
(f) Determine composite runoff coefficient “Cd” based on developed conditions and a one hundred (100) year return period.
(g) Determine the one hundred (100) year return period rainfall intensity “Id” for various storm durations “td” up through the time of concentration for the developed area using Table 5A.
(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” up through the time of concentration of the developed area.
Std = Qd Qu
(j) Compute required storage volume “SR” in acre feet for each storm duration “td”. This assumes a triangular hydrograph of duration (2td) hours with the peak flow of Std and td hours.
SR = Std (td/12)
(k) Select the largest storage volume computed in step 10 for detention basin design.
(G) Determination of Storage Volume Hydrograph Methods.
(1) 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 SCS TR-20 computer model with the SCS TR-55 time of concentration and curve number calculation methodologies, and Huff Third Quartile (50%) rainfall distribution (Table 6) must be used to determine the 10 year return period pre development release rate for sites of five (5) acres or more and for sites with existing depressional storage. The SCS TR-20, and SCS TR-55 models are approved by the Council 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 an eight step procedure to determine the required storage volume of the detention basin.
(2) Step Procedure.
(a) Calibrate the hydrologic/hydraulic model that is to be used for prediction of runoff and routing of storm water.
(b) Determine the critical storm duration. The critical duration storm for computer modeling shall be equal to or greater than the time of concentration for the watershed being modeled.
(c) Determine the ten (10) year, undeveloped peak flow. Denote this flow by Qu10.
(d) Determine the one hundred (100) year runoff hydrograph (Hd100) for developed conditions.
(e) Determine the hydrograph that must be stored (Hs100) by subtracting a flow up to Qu10 from the hydrograph (Hd100) found in step (d).
(f) Determine the volume of water (Vs) to be stored by calculating the area under the hydrograph Hs100.
(g) The detention basin must be designed to store the largest volume (Vs) found for any storm duration analyzed in step (f).
(h) Approved routing techniques may be used to determine the final detention storage required.
(H) 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 Ordinance. The volume of such 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 one hundred (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 25 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, etc. Detention pond side slopes no steeper than 6 horizontal to 1 vertical for safety, erosion control, stability and ease of maintenance shall be permitted.
(e) 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.
(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 Council.
(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) Hydraulic calculations shall be submitted to substantiate all design features.
(l) No residential lot or any parts thereof shall be used for the storage of water, either temporary or permanent, without approval of the Council.
(I) 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 multipurpose function. Recreational facilities, aesthetic qualities (open spaces) or other types of use shall be considered in planning the detention facility.
(J) Wet Bottom Basin Design Requirements. Where a 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 without 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) (a) Basins designed with permanent pools or containing permanent ponds shall have a water area of at least one half acre.
(b) If fish are to be maintained in the pond, a minimum depth of approximately 10 feet shall be maintained over at least 25 percent of the pond area. The remaining pond area shall have no extensive shallow areas, except as required by subsection (3) below.
(2) In excavated lakes the underwater side slopes in the lake 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.
(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 growth 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 plan 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 Council.
(8) The perimeter of wet bottom detention basins, defined by the high water contour which represents the high water elevation, shall be a minimum horizontal distance of 10 feet from high voltage electric lines.
(K) Roof Top Storage. Detention storage requirements may be met in total or in part by detention on flat roofs. Details of such designs are to be included in the building permit application and shall include the depth and volume of storage, details of outlet devices and downdrains, and elevations of emergency overflow provisions.
(L) Parking Lot Storage. Paved parking lots may be designed to provide detention storage of storm waters on all or a portion of their surfaces. Depths of storage must be limited to a maximum depth of six (6) inches so as to prevent damage to parked vehicles and so that access to parked vehicles is not impaired. Locate the deepest ponding zones at remote and least used portions of the parking lot.
(M) Facility Financial Responsibilities. The construction cost of storm water detention systems and facilities as required by this ordinance shall be part of the cost of land development.
(N) Facility Maintenance Responsibility.
(1) 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. This determination shall be made before the final drainage plans are approved.
(2) Storm water detention and retention basins may be donated to the City of Mt. Vernon or other unit of government approved by the City, for ownership and permanent maintenance providing:
(a) The City or other governmental unit is willing to accept responsibility and the City Council accepts operation and maintenance in writing by appropriate resolution.
(b) The facility has been designed and constructed according to all applicable provisions of this Ordinance.
(c) All improvements have been constructed, approved and accepted by the City for the land area served by the basin.
(d) Retention ponds containing a permanent pool of water have all slopes between the permanent pool and high water line sodded and the remaining land area hydroseeded using a method approved by the Council; are equipped with electrically driven aeration devices, if required to maintain proper aerobic conditions and sustain aquatic life; provide suitable access acceptable to the responsible government agency; and have the high water line not closer than 25 feet to any property line.
(e) Dry detention ponds shall have all slopes, bottom of the basin and areas above the high water line hydroseeded; and shall have the high water line not closer than 25 feet to any development boundary.
(O) Inspections. All public and privately owned detention storage facilities shall be subject to inspection by representatives of the City.
(P) Corrective Measures. If deficiencies are found upon inspection, the owner of the detention/retention facility shall be required to take the necessary measures to correct such deficiencies. If the owner fails to do so, the City may undertake the work and the owner shall be personally liable for all costs incurred by the City and the real estate shall be subject to a lien for said costs, which lien may be foreclosed by the City with all the costs and expenses therefore, including reasonable attorney’s fees being assessed against the owner and the real estate.
(Q) 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 Ordinance is maintained. Developers are encouraged to plan and construct these systems on a joint or regional basis.
(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 6 (six) percent 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 cost are kept to a minimum. The Council will require temporary and permanent erosion control plans to be submitted as a part of the construction plans.
(S) Detention Facilities in Flood Plains. If detention storage is provided within a flood plain, only the net increase in storage volume above that which naturally existed on the flood plain 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.
(T) Off Site Drainage Provision. 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 Council.
(U) Erosion Control. Erosion control plans shall be submitted as part of the construction plans and specifications and shall include the following:
(1) A complete copy of the Stormwater Pollution Prevention Control Plan filed with the Illinois Environmental Protection Agency.
(2) Temporary erosion control measures necessary during the initial construction and establishment phases up to final site grading and seeding.
(3) A permanent erosion control plan of all the graded and non-hard surface areas within the proposed development, as planned for completion, up to and including seeding of the final lot on which business or residential dwellings are to be placed.
(4) Details concerning removal of temporary erosion control devices after the initial establishment of adequate vegetative cover.
(5) Maintenance procedures, as part of the continuing plan, to keep all of the land under adequate cover and erosion at an acceptable minimum.
(Prior Code, Art. 20, § 20-15)