(A) When the maximum controlled stormwater runoff rate would be exceeded upon the development, redevelopment, or new construction on the subdivider’s land, stormwater storage methods shall be provided and constructed by the subdivider.
(B) The maximum controlled release rate for 50-year precipitation event for each independent watershed area within the subdivider’s land shall be equal to the peak rate of discharge from the area prior to development in a five-year return frequency precipitation event. The release rate shall be as determined by the Rational Method, TR-55 Method, or other method acceptable to the Village Engineer.
(C) Stormwater runoff from upstream off-site areas shall be directed around the stormwater storage area. However, when pass-through conditions are deemed necessary, the maximum controlled release rate for each independent watershed area within the subdivider’s land shall be determined in the following manner.
(1) For that portion of the watershed outside of a subdivider’s land, the peak rate of runoff which would have occurred for a 50-year return period storm under the state of development existing just prior to application for development of a subdivider’s land shall be calculated and determined.
(2) For that portion of the watershed within the lands of the subdivider, the peak rate of runoff which would have occurred for a five-year return period storm under a state of traditional agricultural usage shall be calculated and determined.
(3) The maximum controlled release rate for each independent watershed area within the subdivider’s land as it exits the subdivider’s land for storms up to and including the 50-year return period storm shall be the sum of divisions (C)(1) and (C)(2) above.
(D) The maximum controlled release rate for small developments with a total watershed area of five acres or less may be assumed to be 0.50 cfs per acre if the subdivider chooses to utilize the Direct Method of Calculating Required Storage for Small Developments, as outlined in (F)(4) below.
(E) The village may consider or require reduced release rates under certain circumstances:
(1) In the event that the downstream stormwater runoff drainage system is inadequate to accommodate the maximum release rate provided above, then the village, at its option, may reduce the allowable release rate to that rate permitted by the capacity of the receiving stormwater runoff drainage system and additional storage may be required to store that portion of the runoff exceeding the capacity of the receiving stormwater runoff drainage system. When the village exercises this option, the subdivider and village shall pay a proportional share of the costs of the storage, as mutually determined by the village and the subdivider; and
(2) In the event that the stormwater runoff control can be provided exceeding the requirements of this chapter and the benefit of this additional control would accrue to the village at large, the village may participate in the increased cost of the control of stormwater runoff in proportion to the additional benefits derived therefrom.
(F) The volume of required stormwater storage shall be calculated on the basis of the maximum value achieved from the runoff of a design event less the volume of water released through the outlet structure. The following standards shall apply to watersheds of various sizes:
(1) Standard reservoir routing methods shall be utilized to determine the required storage volume. SCS TR-55 or TR-20 methodologies shall be utilized to develop hydrographs of the stormwater runoff for the site. Stage/storage/discharge relationships shall be developed for the detention basin utilizing standard engineering methods and utilized to route the hydrographs through the stormwater detention facility;
(2) SCS TR-55 methods may be utilized to calculate the required storage volume for developments with watershed areas of less than 25 acres and which do not involve significant off-site drainage that must be passed through the detention basin;
(3) (a) The modified rational method may be utilized to calculate required storage volume for developments with watershed areas of less than 25 acres and which do not involve significant off-site drainage that must be passed through the detention basin;
(b) Time of concentration shall be estimated utilizing SCS TR-55 methods for developed conditions. “C” values utilized shall be set at 125% of “normal” five-year storm values to account for reduced soil infiltration capabilities during large storm events, up to a maximum “C” value of 1.0. In determining the volume of storage required when using the modified rational method, the release rate utilized in the calculations is equal to the release rate through the outlet structure when one-half of the storage volume is filled; and
(c) When using the modified rational method the critical storm duration (that requiring the largest detention volume) for any design event shall be identified and used in determining storage volume; and
(4) For small developments with a total watershed area of less than five acres, the total volume of stormwater storage required may be determined by the direct method, which does not required hydrologic analyses. When the direct method is utilized, the total storage volume provided shall be equal to two and one-half inches of water over the watershed area.
(G) An outlet structure shall be provided to discharge water from the basin at the design outlet controlled rate previously specified in divisions (B) and (C) above.
(H) Each stormwater storage facility shall also be provided with a means of emergency overflow. This emergency overflow structure shall be constructed to function without special maintenance attention and can become a part of the excess stormwater passageway for the entire development. The emergency overflow structure shall be capable of passing the 100-year return period storm event flow.
(I) The entire stormwater storage facility shall be designed and constructed to fully protect the public health, safety, and welfare. The minimum building site elevation adjacent to stormwater detention basins shall be set at least two feet above the maximum created head during the 100-year return period storm event. The maximum created head will include the energy head at the emergency overflow structure.
(J) Stormwater detention basins shall be provided with a freeboard of not less than two feet above the water surface elevation that is anticipated to occur during the 50-year return period storm event. The freeboard shall not be overtopped during the 100-year return period storm event.
(K) (1) Stormwater storage facilities shall not receive runoff from tributary areas outside the development site unless the Village Engineer determines that runoff from such areas can be accommodated in the storage area in a manner that will protect immediate downstream properties. In making this determination, the Village Engineer may require the subdivider’s engineer to provide additional data or calculations.
(2) When stormwater runoff from tributary areas outside of the development cannot reasonably be directed around the stormwater storage area, the Village Engineer may allow the use of staged release outlet structures, which allow stormwater runoff from off-site areas to pass through the stormwater storage area undetained, while simultaneously detaining and providing controlled release for the volume of excess stormwater runoff from the site.
(L) Where portions of the subdivider’s land are tributary to the same drain for an outlet, but which are within two or more tributary areas to that drain, the subdivider may construct, upon site-specific authorization of the Village Engineer, compensatory stormwater detention facilities within one tributary area which offsets the lack of construction of stormwater detention facilities in another tributary area. Such compensatory storage shall be designed and constructed such that the net effect of the facilities shall be to limit the rate at which stormwater runoff is released into the drain to that rate which would have occurred had stormwater detention been constructed for all tributary areas.
(M) Stormwater storage areas may be planned and constructed jointly by two or more landowners provided the provisions of this chapter are met. Adequate easements and provisions for future maintenance by the landowners must be provided.
(N) Where stormwater detention, retention, or depressional storage areas are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial earthwork for the development. Any eroded sediment captured in these facilities shall be removed by the applicant before project completion in order to maintain the design volume of the facilities.
(O) Construction of impervious surfaces shall not begin until the developer’s engineer has certified in writing to the Plat Officer that the necessary detention facilities are in place and operational. The Village Engineer or the Maintenance Superintendent may conduct periodic inspections of the work in progress to be certain that the drainage system is being constructed as designed.
(Ord. 97-7, passed 7-1-1997, § 5.19)