The following requirements shall be applicable and shall be satisfied prior to the construction, improvement, or development of any structure, project, or land which is subject to the provisions of this chapter:
(A) Maximum controlled release rate. The maximum controlled stormwater runoff release rate shall not exceed the natural safe stormwater drainage capacity of the downstream system, which has been found to be 0.2 cubic feet per second per acre in the county. Pipe outlets of less than 6 inches in diameter shall not be allowed. Multiple outlets from a stormwater storage area shall be avoided if they are designed to be less than 12 inches in diameter. An orifice or a restriction may be approved in circumstances when the allowable discharge is less than the capacity of a 12-inch diameter pipe.
(B) Excess stormwater passageway.
(1) An excess stormwater passageway shall be provided, which shall have adequate capacity to convey the excess stormwater from the tributary watershed. The capacity of this excess stormwater passageway shall be adequate to transport the peak rate of runoff from the 100-year return frequency storm, assuming all up-stream areas are fully developed for uses specifically listed as permitted uses by existing zoning and antecedent rainfall has saturated the tributary watershed. The increase due to saturation shall be calculated by a ratio of the amount of runoff of the Antecedent Moisture Condition III to Antecedent Moisture Condition II of the 24-hour 100-year return frequency storm as outlined by the U.S. Soil Conservation Service for the tributary watershed. No structures shall be constructed within this passageway; however, streets, parking lots, playgrounds, park areas, pedestrian walkways, open green space, and utility and sewer easements shall be considered compatible uses.
(2) Design of the excess stormwater passageway shall include control of stormwater velocity to eliminate problems of soil erosion or other damage which could detract from the primary use of the area. The entire area of the excess stormwater passageway shall remain under the jurisdiction of the Village. In the event this passageway is reshaped or its capacity to transport excess stormwater is otherwise restricted, the Village may cause to have any restrictions removed at the expense of the agency, party, or parties causing or permitting the restrictions. Should a natural drainage exist, it shall be preserved and used as a part of the excess stormwater passageway.
(C) Stormwater storage. When the maximum controlled stormwater runoff release rate shall be exceeded, any or all of the following stormwater storage methods shall be provided and constructed:
(1) Dry bottom stormwater storage.
(a) Dry bottom stormwater storage areas must be designed to serve a secondary purpose for recreation, open space, or similar type or uses which will not be adversely affected by occasional intermittent flooding.
(b) The combination of storage of excess stormwater runoff from a 100-year return frequency storm and the allowable release rate shall not result in a storage duration in excess of 48 hours.
(c) Minimum grades for turf areas shall be 0.5% (200 units horizontal to 1 unit vertical) and maximum side slopes shall be 25% (4 units horizontal to 1 unit vertical). Storage area side slopes shall follow the natural land contours as closely as practicable, and a minimum of earth excavation shall be used to create the storage facility.
(d) Temporary seeding or other soil stabilization measures shall be established in the stormwater storage area and excess stormwater passageway immediately following the construction or reconstruction of these areas. During the construction of the overall development, it is recognized that a limited amount of sediment buildup may occur in the stormwater storage area due to erosion. In no case shall the volume of the storage area be reduced to less than 3/4 of the required volume during the construction phase of the development.
(e) Permanent erosion control measure such as mulching, hydroseeding, conventional seeding, nurse crops, fertilizing, or sod installation shall be utilized to control soil movement and erosion within the storage area and excess stormwater passageway. These measures shall meet or exceed the standards established by the Winnebago County Soil and Water Conservation District. The installation of these permanent measures shall take place only after the majority of construction and other silt- and sediment-producing activities have been completed. Prior to the establishment of the permanent erosion control measures, the required capacity of the stormwater storage area and the excess stormwater passageway shall be restored.
(f) The control structure shall be provided with an interceptor for trash and debris, and it shall be designed and constructed to prevent soil erosion and not require manual adjustments for its proper operation. An inlet design that will produce turbulent flow conditions during any portion of the stormwater storage cycle will not be acceptable.
(g) Adequate impact stilling basins shall be provided to ensure that downstream soil erosion is alleviated and the regime of the downstream drainage facility is not disturbed.
(h) Each stormwater storage area shall be provided with a method of overflow in the event a storm in excess of the design capacity occurs. This overflow facility shall be constructed to function without specific attention and can become a part of the excess stormwater passageway described in division (B) of this section.
(i) The entire stormwater storage area shall be designed and constructed to fully protect the public heath, safety, and welfare. If a condition occurs in the stormwater storage area which is hazardous to the public heath, safety, or welfare, the person responsible for the condition will be required to provide approved corrective measures. In the event these corrective measures are not provided, the Village may eliminate the hazard at the expense of the person responsible.
(j) Low flow conduits or channels shall be provided in stormwater storage areas; these conduits or channels shall be so constructed that they will not interfere with the secondary usage of the storage area and will reduce the frequency of time that the storage area will be covered with water.
(2) Wet bottom stormwater storage. Wet bottom stormwater storage areas shall be designed in compliance with all of the regulations which are applicable and govern the construction of dry bottom stormwater storage areas. The following additional regulations shall apply:
(a) The water surface area of the permanent pool shall not exceed 1/10 of the area of the tributary watershed;
(b) Protection of the shoreline must be provided to alleviate soil erosion due to wave action;
(c) Minimum normal water depth shall be 4 feet. If fish are to be used to keep the pond clean, at least 1/4 of the pond area shall be a minimum of 10 feet deep;
(d) Facilities shall be provided to lower the pond elevation by gravity flow for cleaning purposes and shoreline maintenance;
(e) The control structure for stormwater release shall be designed to operate at full design release rate with only a minor increase in the water depth in order to minimize the land surface wetted by frequent minor stormwater runoff conditions;
(f) Measures shall be included in the design to prevent pond stagnation. This may be accomplished by fountain aeration or some other method used to ensure aerobic pond conditions; and
(g) The volume of water permanently stored shall not be considered to be part of the required excess stormwater storage volume.
(3) Paved stormwater storage. Design and construction of the pavement base must ensure that there is no pavement damage due to flooding. Control structures in paved areas must be readily accessible for maintenance and cleaning. Vortex control devices will be required.
(4) Rooftop stormwater storage. Rooftop storage of excess stormwater shall be designed and constructed to provide permanent control inlets and parapet walls to contain excess stormwater. Adequate structural roof design must be provided to ensure that roof deflection does not occur which could cause the roofing material to fail and result in leakage. Overflow areas must be provided to ensure that the weight of stored stormwater will never exceed the structural capacity of the roof.
(5) Automobile parking in stormwater storage areas. Automobile parking facilities used to store excess stormwater must be constructed having a maximum depth of stored stormwater of 1.5 feet; and these areas shall be located in the most remote, least used areas of the parking facility.
(6) Underground stormwater storage. Underground stormwater storage facilities must be designed for easy access in order to remove accumulated sediment and debris. These facilities must be provided with a positive gravity outlet.
(D) Calculations.
(1) The volume of required stormwater storage shall be calculated on the basis of the maximum value achieved from the runoff of a 100-year return frequency storm less the volume of water released through the outlet structure. Any generally recognized and substantiated method acceptable to the Administrator may be used for these calculations. The release rate of the outlet structure when 1/2 of the storage area is filled may be used in lieu of routing techniques in small drainage areas. The control structure shall be designed to maintain as uniform a flow as possible, independent of the stormwater storage volume. Where the proposed structure, project, or land development forms only a portion of a watershed or contains portions of several watersheds, the storage volume calculations shall be based upon the area of the entire project, development, or land use change. The maximum release rate shall be established by multiplying the total acreage of the tributary watershed by 0.2 cubic feet per second per acre.
(2) Stormwater storage areas which will be filled to capacity by high frequency storms shall be designed in a manner that will protect immediate downstream properties, and all overflow structures shall be designed to function properly and effectively without the necessity of making manual adjustments. A larger outlet for a stormwater storage area may be permitted by the Administrator for the orderly management of stormwater runoff where large tributary areas are developed without detention.
(3) If the orderly management of the stormwater runoff cannot be achieved by passing the entire tributary area runoff through the stormwater storage area, then the stormwater storage area shall be constructed to exclude the runoff from the tributary area originating outside of the area to be developed.
(Prior Code, § 13-6) (Ord. 1986-8, passed 5-7-1987; Ord. 1997-20, passed 5-7-1998)