(A) All subdivisions shall be developed with adequate surface drainage. Surface water drainage improvements shall consist of storm sewers and/or open channels, inlets, catch basins and manholes designed and constructed to conform with standards established by the Village Engineer and shall adequately drain the area being developed, including drainage from other areas which naturally drain through the area being developed.
(B) If, as the result of subdivision development, surface water is deposited in existing roadside ditches in quantities exceeding their capacity, the developer shall improve the ditches and replace culverts as needed to handle the flow.
(C) Unless engineering evidence is presented to the Village Engineer warranting exceptions, storm sewers which drain 20 acres or less for residential development shall be designed and constructed to meet the following criteria:
(1) Major streets: ten-year storm frequency; and
(2) All other area: five-year storm frequency.
(D) It shall be the responsibility of the subdivider to provide grade control for rear lot drainage to each lot owner. Swales are not public works which shall be dedicated to and accepted by the village; nevertheless, no construction surety shall be fully released until the Village Engineer has approved the swales in the subdivision. The Village Engineer will not approve any swale until adequate turf and/or grass has been established at the elevations set forth in the construction plans. Upon approval by the Village Engineer, it shall be the lot owner’s responsibility to maintain the drainage course and to keep it free from features that restrict natural drainage.
(E) The controlled release and storage of excess storm water runoff shall be required in combination for all of the areas indicated on the preliminary plan.
(1) The controlled release of storm water runoff shall not exceed the release or discharge rate which existed at the site prior to development. This rate shall be known as the pre-developed discharge rate. In the case of multiple discharge locations, no location shall discharge at a rate higher than the pre-developed discharge rate for that location under any set of conditions. The controlled release rate, in any case, shall not exceed the rated capacity of the existing natural downstream outlet channel or storm sewer system as determined by the Village Engineer. The rate at which storm water runoff is transported into a designated storage area may be as determined by the design engineer and is unrestricted.
(2) A natural or surface channel system shall be designed with adequate capacity to convey the storm water runoff from all tributary upstream areas through the development. This by-pass channel shall be designed to carry the peak rate of runoff from a 50-year storm assuming all storm sewers are blocked and that the upstream areas are fully developed and have been saturated with antecedent rainfall. No habitable structures shall be constructed within this channel; however, streets and parking or playground areas and utility easements shall be considered compatible primary uses.
(3) Storm water runoff capacity of detention facilities and discharge rates from such facilities shall be calculated by analyzing volume and rate of runoff during pre- and post-development conditions for the ten-year and the 100-year recurrence intervals.
(a) Storage capacity and discharge rates shall be based on the maximum calculated volume and peak flow of storm waters, respectively.
(b) Storage facilities shall be designed using the Soil Conservation Service (SCS) method of calculating runoff discharge rate and total volume of storage. The rational method of calculating runoff may be used for subdivision less than 20 acres.
(c) The storage volume shall be provided for the fully developed watershed that is tributary to the area designated for detention purposes. The control structure for discharge shall maintain the release rate at or below the rate established in division (E)(1) above for all rainfall events of 100 year or less frequency.
(4) (a) Detention reservoirs or dry bottom storm water storage areas shall be designed to serve a secondary purpose for recreation, open space or other types of uses that will not be adversely affected by occasional or intermittent flooding. A method of carrying the low flow through these areas shall be provided in addition to a system of drains, and both shall be provided with a positive gravity outlet to a natural channel or other storm sewer facility with adequate capacity. The combination of storage of the water from a 100-year storm and the design release rate shall not result in a storage duration in excess of 72 hours. Maximum depths of planned storm water storage shall not exceed four feet unless the existing natural ground contours and other conditions lend themselves to greater storage depth, which shall be approved by the Village Engineer. Minimum grades for turf areas shall be 0.6% and maximum slopes shall be 10%. Storage area side slopes shall be kept as close to the natural land contours as practical and a 10% slope or less shall be used whenever possible. If slopes greater than 10% are necessary to meet storage requirements or area restrictions, approval shall be obtained from the Village Engineer, and suitable erosion control shall be provided in addition to the protection required to ensure public health and safety.
(b) Outlet control structures shall be designed as simply as possible and shall require little or no maintenance and/or attention for proper operation. Each storm water storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 100-year return frequency storm occurs. The emergency overflow facility shall be designed to function without maintenance and/or attention and shall become part of the natural or surface channel system described in a preceding paragraph. Hydraulic calculations shall be submitted to substantiate all design features. Both outlet control structures and emergency overflow facilities shall be designed and constructed to fully protect the public health and safety. Storm water runoff velocities shall be kept to a minimum and turbulent conditions at an outfall control structure will not be permitted without complete protection for the public safety. The use of restrictive fences shall be kept to a minimum and shall be used only as a last resort when no other method is feasible.
(5) Retention reservoir or wet bottom storm water storage areas shall be designed with all of the items required for detention reservoir storage areas, except that a low flow conduit and a system of drains with a positive gravity outlet shall not be required; however, the following additional conditions shall be complied with:
(a) Water surface area shall not exceed one-tenth of the tributary drainage area;
(b) Shoreline protection shall be provided to prevent erosion from wave action;
(c) Minimum normal water depth shall be six feet. If fish are to be used to keep the pond clean, a minimum of one-fourth of the pond area shall be a minimum of ten feet deep;
(d) Facilities shall be available, if possible, to allow the pond level to be lowered by gravity flow for cleaning purposes and shoreline maintenance;
(e) Control structures for storm water release shall be designed to operate at full capacity with only a minor increase in the water surface water level. Hydraulic calculations shall be submitted to substantiate all design features; and
(f) Aeration facilities to prevent pond stagnation shall be provided, if necessary. Design calculations to substantiate the effectiveness of these aeration facilities shall be submitted with construction plans. Agreement for the perpetual operation and maintenance of aeration facilities shall be prepared to the satisfaction of the Village Engineer and Plan Commission.
(6) Where developments form only a portion of the watershed or contain portions of several watersheds, the requirements for providing storage shall be based upon that portion of the area being developed as compared to the total watershed tributary to the storage area. Compensating storage will be acceptable whenever it is justified and feasible. As a watershed is developed with a series of storm water storage facilities, due consideration will be given for calculations of the allowable release rate and capacity of the natural or surface channel system, as described in division (E)(2) above.
(7) Where development of a property presents the threat of flooding or damage by flash flood runoff to downstream residents, the facilities for storm water runoff control shall be constructed prior to any earthmoving or drainage construction on the project site.
(8) The construction of the storm water control systems shall be accomplished as part of the cost of land development. If the amount of storage capacity can be increased to provide certain benefits to the surrounding properties, negotiations for public participation in the cost of such development may be feasible.
(9) The ability to retain and maximize the ground water recharge capacity to the area being developed is encouraged. Design of the storm water runoff control system as provided in division (E)(4) above shall give due consideration to providing ground water recharge to compensate for the reduction in the percolation that occurs when the ground surface is paved and/or roofed over. The use of natural gravel deposits for the lower portions of storm runoff storage areas, the flattening of drainage slopes and the retention of existing topography are samples of possible recharge methods.
(10) During the construction phase of land development, facilities shall be provided by appropriate stock-pile design, to prevent the erosion and washing away of the earth. Silting of downstream areas shall be prevented through the strategic use of silting basins, sodding of runoff channels and by limiting the period of time during which the earth is stripped of vegetation.
(11) Final engineering plans shall show complete details for all items covered in this section. Plans, specifications and all calculations for storm water runoff control and storm sewers shall be submitted for review and approval as part of the construction plan submittal or as part of the site plan submittal for large scale developments.
(Prior Code, § 153.136) (Ord. 06-01-01, passed 1-9-2006)