Detention of storm water runoff shall be required. Retention ponds or reservoirs may be constructed. Such retention facilities shall be designed to retain a certain water level at all times and may, in addition to serving as part of the storm water drainage system of the village, be used for scenic and recreation purposes. Detailed engineering plans and specifications for retention or detention facilities shall be prepared by the design engineer for review by the Village Engineer and Plan Commission and approval by the Village Board of Trustees. The design of retention or detention facilities shall be in accordance with the following guidelines:
(A) There must not be any pollution of any kind entering the water area during or after construction.
(B) Access. A plat of subdivision containing a lake, pond, or other body of water shall provide for a 20 foot easement encircling the lake, pond, or other body of water and terminating at a dedicated street. Such easement shall be deemed dedicated to the village for inspection and related purposes.
(C) A mechanical control gate may be used at the outlet pipe.
(D) The bottom slope of the pool shall be a maximum of four to one horizontal to vertical from the maximum flood stage elevation to a point 20 feet out from the normal pool elevation.
(E) The entire area shall be graded, seeded and mulched, and the soil shall be stable enough to support mowing equipment and shall be landscaped. Minimum five trees per acre of 2½ inch diameter.
(F) Storm drainage control shall be dedicated by the village.
(G) Dams and constructed walls. All dams and constructed walls shall be designed by a registered engineer and, if of earth, shall have a maximum slope of 2.5 to 1, be constructed of compacted clean earth fill and have an interior core wall of concrete on sheet steel. An IDOT-DWR dam permit may be required.
(H) Embankments. Shaped earth embankments shall have a maximum slope of three to one and shall be compacted to remove all voids exposed by cutting and shall be cleared of all extraneous material and foliage below the designed water line. An IDOT-DWR dam permit may be required.
(I) Depth. Retention facilities shall maintain a minimum constant water depth of ten feet over 30% of the surface water area, in addition to the depth of occasional water detained in the facility in storm situations.
(J) Detention facilities shall be normally dry.
(K) Area. Retention facilities smaller in area than three acres shall not be permitted. Such area shall be measured at the constant water level. Area detention facilities shall be sized to handle all calculated run-off to the facility.
(L) Overflow outlets. Overflow outlets shall be provided, as necessary, to keep the water level within a retention facility within its design limits. Material and methods of construction for overflow piping and maximum overflow rate shall be designed according to village requirements as stipulated by the Village Engineer. Overflow spillways shall be of concrete and so constructed that no erosion of the dam or outfall shall occur.
(M) Overflow outfalls. Overflow outfalls shall be as specified by the Village Engineer.
(N) Bank stabilization and landscaping. All cut embankments, earth dams, or other surfaces shall be seeded or sodded or in accordance with this section, to the designed occasional water level of the retention facility. From this point to the constant water level, sand or gravel may be used. Stabilization with stone, concrete, or other material may be used if specifically approve by the Village Board of Trustees. Trees shall not be planted on or within 20 feet of any earth dam.
(O) The release rate of storm water from all developments shall not exceed the storm water runoff from the area in its natural undeveloped state. Therefore, the allowable release rate shall not be greater than that calculated from a storm of three year frequency with a runoff rate coefficient of 0.15, unless it can be shown through detailed calculations acceptable to the Village Engineer that the discharge rate of the natural outlet channel serving the area is greater.
(P) Drainage systems shall have adequate capacity to bypass through the development the flow from all upstream areas for a storm of design frequency assuming that the land is in a fully developed state under present zoning or zoning proposed under a comprehensive plan. The bypass flow rate shall be computed utilizing a runoff coefficient of not less than 0.35. An allowance will be made for upstream detention when such upstream detention and release rate has previously been approved by the village and that evidence of its construction can be shown.
(Q) The live detention storage to be provided will be calculated on the basis of the 100 year frequency rainfall as published by the U.S. Weather Bureau for this area. The detention volume required will be that necessary to handle the runoff of a 100 year rainfall, for any and all durations, from the fully developed drainage area tributary to the reservoir, less that volume discharged during the same duration at the approved release rate.
(R) Storm water runoff in excess of the capacity of the storm sewer is to be directed to the detention pond by way of rear yard swales and along streets.
(S) Grades of the subdivision shall be set so that excessive flooding depths are avoided and under no conditions shall the water reach an elevation of one foot below the top of the lowest foundation opening. The developer's engineer shall furnish to the Village Engineer a copy of the routing of this excess water.
(T) All ponds shall have a freeboard of one foot minimum above the high water elevation.
(U) All ponds shall have inlet and outlet structures that have either a concrete headwall or a precast reinforced concrete flared end section that are securely tied to avoid washouts.
(V) All ponds shall have rip-rap installed at inlets and outlets to prevent erosion.
(W) All ponds shall be accessible by a paved roadway that has been approved by the Superintendent of Public Works on a dedicated right-of-way from an existing roadway.
('70 Code, § 8.1.20D.) (Ord. 79-08, passed 11-26-79) Penalty, see § 154.999