A. Dry Bottom Storm Water Storage Area: A dry bottom storm water storage area is a facility that is designed to be normally dry and contains water only when excess storm water runoff occurs. These are commonly known as detention areas or ponds.
1. Secondary Purpose: 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, if deemed necessary by the Village, and both shall be provided with a positive gravity outlet to a natural channel or storm sewer.
2. Maximum Depth: Maximum depth of planned storm water storage shall not exceed four feet (4'), unless the existing natural ground contours and other conditions lend to greater storage depth, and a greater depth is approved by the Village Engineer.
3. Minimum Grades: Minimum grades for the bottom of the basin shall be two percent (2%). Storage area side slopes shall be kept as close to the natural land contours as practical and a 4:1 slope or less shall be used wherever possible. If slopes greater than 4:1 are necessary to meet storage requirements or area restrictions, approval shall be obtained from the Village Engineer and suitable erosion control provided, in addition to the protection required to ensure public health, safety and welfare, and provided that the slopes shall not exceed 3:1.
4. Dams And Impoundment Structures: Dam for water impoundments, pools, ponds, reservoirs and small lakes shall be planned, designed and constructed under the supervision of a registered professional engineer and/or registered structural engineer and shall meet the approval of the Village Engineer. Minimum requirements for earth dams capable of impounding water shall be as follows:
a. Foundations: A minimum depth of at least twenty four inches (24") of impervious clay layer or equivalent thereof is required for dams. The entire area in which the dam is located shall be free from all top soil, roots, vegetation, stones rock or debris, and shall be plowed and roughened to provide an interlocking tight bond with new fill.
b. Clay Core: A clay core shall be constructed within the middle third of the dam, and shall extend into a ten foot (10') wide trench excavated at least twelve inches (12") into impervious clay. The clay core shall be compacted to ninety five percent (95%) standard density.
c. Crest: The crest shall be at least ten feet (10') wide and shall have sufficient height above the high water level to prevent waves from going over the dam. The height of the free board shall not be less than one and one-half feet (11/2') above the high water level.
d. Emergency Overflow: The emergency overflow shall be located one foot (1') below the crest.
5. Outlet Control Structures: Outlet control structures shall be designed to require no, or as little, maintenance as is reasonably possible 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 a 100-year frequency storm occurs. This emergency overflow facility shall be designed to function without maintenance and shall become part of the "natural" or surface channel system. Hydraulic calculations shall be submitted to the Village Engineer to substantiate all design features. Both outlet control structures and emergency overflow facilities shall be designed and constructed to fully protect the public health, safety, and welfare. Storm water runoff velocities shall be kept at a minimum and turbulent conditions at an outfall control structure will not be permitted without complete protection for the public safety and rip-rap or similar measures to protect against erosion. The use of restrictive fences shall be kept to a minimum and used only as a last resort when no other method is feasible.
B. Wet Bottom Storage Areas: Wet Bottom Storage Area is a facility that is designed to be maintained as a free water surface or pond. These are commonly known as combination storm water detention-retention areas or ponds. Wet bottom storm water storage areas shall be designed in accordance with all of the requirements for dry bottom storm water storage areas. A safety ledge, consistent with sound engineering practices, shall be required for each such pond. The following additional conditions shall be satisfied:
1. Shoreline protection shall be provided to prevent erosion from wave action.
2. Minimum normal water depth shall be four feet (4'). A minimum of twenty five percent (25%) of the pond area shall have a minimum depth of ten feet (10').
3. Aeration facilities to prevent pond stagnation may be required. Design calculations to substantiate the effectiveness of these aeration facilities shall be submitted with final engineering plans. Agreements for the perpetual operation and maintenance of aeration facilities shall be prepared to the satisfaction of the Village. (Ord. 96-445, 7-2-1996)