The following shall govern the design of any improvement with respect to the detention of stormwater runoff.
(A) Acceptable detention methods. The increased stormwater runoff resulting from a proposed development should be detained onsite by the provisions of appropriate wet bottom retention or dry bottom detention facilities, 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 partially control runoff rates. Detention/retention facilities 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 subchapter.
(B) Design storm. Design of stormwater detention/retention facilities shall be based on a storm with a 1% chance of occurrence every year, also know as a 100-year storm. The storage volume and outflow rate shall be sufficient to handle stormwater runoff from a 24-hour duration storm.
(C) Allowable release rate.
(1) Only methods approved by the County Surveyor shall be used to determine the five-year return period pre-development release rate for development sites controlled by this subchapter.
(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 as determined by the County Surveyor.
(3) If more than one detention/retention facility is involved in the development of the area upstream of the limiting restriction, the allowable release rate off of the site 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, including all ditches, channels, conduits, swales, and the like, shall have adequate capacity to convey the stormwater runoff from all upstream tributary areas (offsite land areas) through the development under consideration for a 100-year return period design storm calculated on the basis of the upstream land 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. An allowance, equivalent to the reduction in flow rate provided, may be made for upstream detention when such upstream detention and release rate have previously been approved by the Drainage Board and evidence of its construction and maintenance can be shown.
(E) Acceptable outlet.
(1) Design and construction of the stormwater facility shall provide for the discharge of the stormwater runoff from offsite land areas as well as the stormwater from the area being developed (onsite land areas) to an acceptable outlet(s) having capacity to receive upstream (offsite) and onsite drainage.
(2) The acceptable outlet for stormwater discharge shall be a regulated open (ditch) drain or a “Blue Line” open drain as shown on the USGS Quadrangle Maps. “Blue Line” open drains shall have adequate available capacity to handle design runoff; Roadside ditches may be acceptable provided permission is granted from right-of-way owner, roadside ditch is in a maintained condition, and the outlet for the roadside ditch is a regulated open drain or “Blue Line” open drain.
(3) In case of extreme hardship, the Drainage Board has final authority over the acceptable outlet.
(4) Where the outfall from the stormwater drainage system of any developer flows through real estate owned by others prior to reaching a regulated drain or natural waterway, no approval shall be granted for such stormwater drainage system until all impacted owners consent in writing to such use of their real estate or are notified of such use in writing of a hearing by the Drainage Board to review such use. The notification shall include the time and place of a hearing and be delivered, either personally or by certified mail, at least five days prior to the hearing thereon, and proof of such notice to each landowner shall be filed with the Drainage Board prior to such hearing, which proof shall be by affidavit.
(F) Determination of storage volume. The required volume of stormwater storage for development sites shall be approved by the County Surveyor.
(G) General detention basin design requirements. Basins shall be constructed to temporarily detain the stormwater runoff which exceeds the maximum peak release rate authorized by this subchapter. The volume of storage provided in these basins, together with such storage as may be authorized in other onsite facilities shall be sufficient to control excess runoff from the 100-year storm. The following design principles shall be observed.
(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) All excavated excess spoil to be spread onsite shall be spread so as to provide for aesthetic and recreational features such as sledding hills, sports fields, decorative mounds, and the like. Slopes no steeper than six horizontal to one vertical for safety, erosion control, stability, and ease of maintenance shall be permitted.
(4) 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.
(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 predetermined maximum authorized peak flow rate.
(6) Emergency overflow facilities such as a weir or spillway shall be provided for the release of at least 100-year storm runoff (or, if applicable, the minimum required under the IDNR dam safety criteria) 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.
(7) Grass or other suitable vegetative cover shall be provided throughout the entire detention storage basin area. Grass should 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) Hydraulic calculations shall be submitted to substantiate all design features.
(10) 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 area.
(11) 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.
(H) 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 may, whenever possible, be designed to serve as a secondary or multipurpose function. Recreational facilities, aesthetic qualities (open spaces), or other types of use shall be considered in planning the detention facility.
(3) In excavated detention facilities, a minimum side slope of three to one shall be provided for stability. In the case of valley storage, natural slopes may be considered to be stable.
(I) Wet bottom facility design requirements. Where part of a detention facility 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 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, if fish are to be used to keep the pond clean, have a minimum depth of approximately eight feet over at least 25% of the pond area.
(2) In excavated ponds, 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 at a slope of five to one is required and shall be installed in all ponds.
(4) 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.
(5) 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.
(J) 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 structural engineer.
(K) 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 shall 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.
(L) Facility financial responsibilities. The construction cost of stormwater control systems and required facilities which are identified in the Stormwater Ordinance of the county shall be accepted as part of the cost of land development.
(M) 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 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.
(N) Inspections. All public and privately owned detention storage facilities may be inspected during construction by representatives of the county.
(O) Corrective measures. If deficiencies are found by the inspector prior to final approval of the drainage facility, the owner of the detention/retention facility will be required to take the necessary measurements to correct such deficiencies. If the owner fails to do so, the Drainage Board will undertake the work and collect the cost of maintenance or repair from the owner using lien rights if necessary.
(P) 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.
(Q) 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.
(Ord. 1997-16, passed 12-8-1997; Ord. 2005-01, passed 2-17-2005) Penalty, see § 153.999