(A) (1) Release rates. The drainage system for new developments or redevelopments shall be designed to control the peak rate of discharge from the property for the two-year, 24-hour and 100-year, 24-hour events to discharge rates at or below those which existed prior to development. Additionally, the discharge from a storm water detention facility shall not cause an increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities.
(2) Detention basin outlet design. The detention basin outlet control structure shall be designed to account for observed or anticipated downstream tailwater elevations. The tailwater elevations used in the detention model shall be for the particular storm frequency being routed through the detention basin. An emergency spillway or overflow device shall be provided and set at an elevation equivalent to the 100-year design high water. A calculation shall be made to determine the water elevation in the detention basin that would result from a 100-year storm with the outflow control structure openings blocked. The discharge rate flowing through the emergency spillway shall not exceed the 100-year pre-development flow rate. The top of bank for the detention basin shall be set at least one foot above this elevation. The lowest finished floor elevation of adjacent structures shall also be at least one foot above the detention basin top of bank.
(B) Detention storage requirements. See division (A) above.
(C) Drainage system design and evaluation. The following criteria should be used in evaluating and designing the drainage system. The design will provide capacity to pass the ten-year peak flow rate in the minor drainage system and an overload flow path for flows in excess of the design capacity.
(1) Design methodologies. Detention basin design shall be calculated using NRCS TR-55 methods. Basins with drainage areas of ten acres or less may be calculated using the rational method as approved by the Illinois Department of Transportation. Other applicable methods, i.e., HEC-1, TR-20, SWMM and the like shall be used for large watersheds.
(2) Positive drainage. When practical, all developments must be provided an overland flow path that will pass the 100-year, 24-hour event flow at a stage at a stage at least one foot below the lowest grade, adjacent to a structure, in the vicinity of the flow path. Street ponding and flow depths shall not exceed curb heights.
(D) Rainfall. Unless a continuous simulation approach to drainage system hydrology is used, all design rainfall events shall be based on the Illinois State Water Survey’s Bulletin 70.
(1) The first quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations less than or equal to six hours.
(2) The second quartile distribution shall be used for storms with durations greater than six hours and less than or equal to 12 hours.
(3) The third quartile point rainfall distribution shall be used for the design and analysis of detention basins and conveyance system with critical durations greater than 12 and less than or equal to 24 hours.
(4) The fourth quartile distribution shall be used in the design and analysis of systems with durations greater than 24 hours.
(5) The first, third and fourth quartile distributions described by Huff are presented in Table 37 of Bulletin 70. Refer to Table 13 of Bulletin 70 for rainfall depth, duration and frequency. The NRCS Type II distribution may be used as an alternate to the Huff distributions. The total rainfall value for the design storm shall be adjusted for the “St. Louis Urban Effect” as given in Table 4, Illinois State Water Survey Circular 172.
(E) Antecedent moisture. Average antecedent moisture conditions shall be assumed when calculating runoff curve numbers for use in the NRCS TR-55 method.
(F) Wet detention basin design. Wet detention basins shall be designed to remove storm water pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for recreational use.
(1) Wet basin depths. Wet basins shall be at least three feet deep, excluding nearshore banks and safety ledges. If fish habitat is to be provided they shall be at least eight feet deep over 25% of the bottom area to prevent winterkill.
(2) Wet basin shoreline slopes. The side slopes of wet basins at the normal pool elevation shall not be steeper than three to one (3 to 1 horizontal to vertical). It is recommended that aquatic vegetation be established around the perimeter to provide protection from shorelines erosion. For basins in excess of five acres, riprap shoreline protection shall be provided.
(3) Permanent pool volume. The permanent pool volume in a wet basin at normal depth shall, at a minimum, be equal to the runoff volume from its watershed for the two-year, 24-hour event (calculated during dry weather conditions).
(4) Wet basin inlet and outlet orientation. The distance between detention inlets and outlets shall be maximized. Inlets and outlets shall be at opposite ends of the basin providing that the orientation does not create undue hardship based on topography or other natural constraints.
(G) Dry detention basin design. In addition to the other requirements of this chapter, dry basins shall be designed to remove storm water pollutants, to be safe, to be aesthetically pleasing and as much as feasible to be available for multiple uses. Paved low flow channels may be used in a dry basin provided provisions are made to prevent ponding.
(H) (1) Dry basin drainage. Dry basins shall be designed so that 80% of their bottom area shall have standing water no longer than 72 hours for any runoff event less than the 100-year, 24-hour event. Grading plans shall clearly distinguish the wet portion of the basin bottom. Underdrains directed to the outlet may be used to accomplish this requirement.
(2) Velocity dissipation. Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize resuspension of pollutants.
(3) Dry basin inlet and outlet orientation. Shall be the same as division (F)(4) above.
(I) Existing depressional areas. Existing depressional storage volume will be maintained and the volume of detention storage provided to meet the requirements of this chapter shall be in addition to existing storage.
(J) Minimum detention outlet size. Where a single pipe outlet orifice plate is to be used to control discharge, it shall have a minimum diameter of 12 inches for larger basins. Smaller basins may install a smaller rectangular or v-notch weir to control discharge. If this minimum orifice size permits release rates greater than those specified in this section, and regional detention is not a practical alternative, outlets, structures such as perforated risers, or flow control orifices shall be used.
(K) (1) Detention in floodplains. The placement of detention basins within the floodplain is strongly discouraged because of questions about their reliable operation during flood events. However, the storm water detention requirements of this chapter may be fulfilled by providing detention storage within flood fringe areas on the project site provided the following provisions are met as well as compliance with § 153.002.
(2) Detention in flood fringe areas. The placement of a detention basin in a flood fringe area shall require compensatory storage for one and one-half times the volume below the base flood elevation occupied by the detention basin including any terms. The release from the detention storage provided shall still be controlled consistent with the requirements of this section. The applicant shall demonstrate its operation for all streamflow and floodplain backwater conditions.
(a) Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention.
(b) All floodplain storage lost below the existing ten-year flood elevation shall be replaced below the existing ten-year elevation. All floodplain storage lost above the existing ten-year elevation shall be replaced above the existing ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse and comply with § 153.002.
(3) Detention on prime farmland. The placement of detention basins shall avoid the utilization of prime farmland. All detention basin construction shall examine potential impacts to adjacent agricultural land and shall address measures that will be implemented to eliminate such impacts and comply with § 153.002.
(4) Detention in floodways. Detention basins shall be placed in the floodway only in accordance with division (K)(1) above.
(5) On-stream detention. On-stream detention basins are discouraged but allowable if they provide regional public benefits and if they meet the other provisions of this chapter with respect to water quality and control of the 100-year 24-hour events from the property. Further criteria are presented in § 153.039. If on-stream detention is used in watersheds larger than one square mile, the applicant will use hydrographic modeling to demonstrate that the design will not increase the water level for any properties upstream or downstream of the property. Also, impoundment of the stream as part of on-stream detention shall:
(a) Not prevent the migration of indigenous fish species, which require access to upstream areas as part of their life cycle, such as for spawning;
(b) Not cause or contribute to the degradation of water quality or stream aquatic habitat;
(c) Include a design calling for gradual bank slopes, appropriate bank stabilization measures and a pre-sedimentation basin;
(d) Not involve any stream channelization or the filling of wetlands;
(e) Require the implementation of an effective non-point source management program throughout the upstream watershed which shall include as a minimum: runoff reduction best management practices (BMPs) consistent with § 153.036; two-year, 24 hour detention/sedimentation basin for all development consistent with this division (K)(5);
(f) Not occur downstream of a wastewater discharge;
(g) Not contribute to the duration or flood frequency of any adjacent land; and
(h) Comply with § 153.002.
(L) Drainage into wetlands, rivers, streams, lakes, ponds and depressional storage areas. Wetlands, lakes, ponds and depressional storage areas shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments. In addition to the other requirements of this chapter, the following requirements shall be met for all developments whose drainage flows into wetlands, rivers, lakes, ponds or depressional storage areas.
(1) Detention in wetlands, rivers, streams, lakes, ponds or depressional storage areas. Existing wetlands, rivers, lakes, ponds or depressional storage areas shall not be modified for the purpose of storm water detention unless it is demonstrated that the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions and shall comply with § 153.002. Existing storage and release rate characteristics of wetlands, rivers, lakes, ponds or depressional storage areas shall be maintained and the volume of detention storage provided to meet the requirements of this section shall be in addition to this existing storage.
(3) Alteration of drainage patterns. Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetlands, rivers, lakes, ponds or depressional storage areas.
(4) Detention/sedimentation. All runoff from the development shall be routed through a preliminary detention/sedimentation basin designed to capture the two-year, 24-hour event and hold it for at least 24 hours, before being discharged to the wetland, river, lake, pond or depressional storage area. This basin shall be constructed before property grading begins and shall be maintained throughout the construction process. In addition, the drainage hierarchy defined in § 153.036 should be followed to minimize runoff volumes and rates being discharged to the wetland, river, stream, lake, pond or depressional storage area and as further regulated in and this section.
(5) Loessal soils. Care must be taken to avoid open flow discharges of storm water over silt (loessal) soils due to high potential for erosion.
(6) Sinkholes, karst area. The following requirements apply for new developments or redevelopments where sinkholes are determined to be present.
(a) A storm water detention basin shall not be placed in or over a sinkhole.
(b) Storm water detention basins shall not be located closer than 100 feet from the rim of a sinkhole.
(c) The outflow from a storm water detention basin, channel, ditch or any storm water runoff generated as a result of a new development or redevelopment shall not empty into or be directed, redirected by any means into or through any sinkhole.
(d) If, after the review of the storm water drainage plan, the Code Administrator may determine that more detailed information is required, a sinkhole evaluation may be required. A sinkhole evaluation which addresses the geologic, engineering and environmental factors resulting from a new development or redevelopment be performed by a professional with experience and expertise in karst topography, whom shall certify the results of the evaluation. This evaluation shall be the responsibility of the applicant and performed at no cost to the county. After a review of this evaluation and with the consultation of the County Soil and Water Conservation District, the County Code Administrator may either approve or disapprove the drainage plan as submitted.
(e) Whenever a new sinkhole appears or it becomes apparent that the sinkhole has not yet been identified, it shall be reported to the County Soil and Water Conservation District.
(f) Compliance with § 153.002 is required.
(M) Street detention, parking lot detention and culvert drainage.
(1) Street detention. If streets are to be used as part of the minor or major drainage system, ponding depths shall not exceed curb heights and shall not remain flooded for drainage system, ponding depths shall not exceed curb heights and shall not remain flooded for more than eight hours for any event less than or equal to the 100-year, 24-hour event.
(2) Parking lot detention. The maximum storm water ponding depth in any parking area shall not exceed six inches for more than four hours.
(3) Culvert, road and driveway crossings. Sizing of culvert crossings shall consider entrance and exit losses as well as tailwater conditions on the culvert.
(N) (1) Infiltration practices. To effectively reduce runoff volumes, infiltration practices including basins, trenches and porous pavement should be located in hydrologic soil groups “A” and “B” as designated by the U.S.D.A. Natural Resources Conservation Service. Infiltration basins and trenches designed to re-charge groundwater shall not be located within 75 feet of a water supply well or building foundation and comply with § 153.002. A sediment settling basin shall be provided to remove coarse sediment from storm water flows before they reach infiltration basins or trenches. Storm water shall not be allowed to stand more than 72 hours over 80% of the dry basin’s bottom area for the maximum design event to be exfiltrated. The bottom of infiltration basins or trenches shall be a minimum of four feet above the seasonally high groundwater and bedrock level. Engineering calculations demonstrating infiltration rates shall be included with the application.
(2) Vegetated filter strips and swales. To effectively filter storm water pollutants and promote infiltration of runoff sites should be designed to maximize the use of vegetated filter strips and swales. Whenever practicable, runoff from impervious surfaces should be directed onto filter strips and swales comprised of native grasses and forbs before being routed to a storm sewer or detention basin.
(O) Safety considerations. The drainage system components, especially all detention basins, shall be designed to protect the safety of any children or adults coming in contact with the system during runoff events and shall comply with § 153.002.
(1) Slide slopes. The side slopes of all detention basins at 100-year, 24-hour capacity shall be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance. Side slopes of detention basins and open channels shall not be steeper than three to one (horizontal to vertical).
(2) Safety ledge. All wet detention basins shall have a level safety ledge at least four feet in width two and one-half to three feet below the normal water depth or must be protected by an enclosed fence, at least 48 inches in height.
(3) Velocity. Velocities throughout the surface drainage system shall be controlled to safe levels taking into consideration rates and depths of flow.
(4) Overflow structures. See division (A)(2) above.
(P) Maintenance considerations. The storm water drainage system shall be designed to minimize and facilitate maintenance. Turfed side slopes shall be designed to allow lawn mowing equipment to easily negotiate them. Wet basins shall be provided with alternate outflows which can be used to completely drain the pool for sediment removal. Pumping may be considered if drainage by gravity is not feasible. Presedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Site access for heavy equipment shall be provided.
(Ord. passed 9-8-2009)