(A) Plan submittal procedures. For village approval of retention basins, the applicant shall furnish the Village Clerk with three sets of retention basin plans and one set of calculations and charts, along with any required fees and deposits. Detailed cost estimates shall be submitted concurrently. The Clerk shall stamp two sets as authorized to be reviewed, and the applicant will submit these copies along with the set of calculations and charts to the Village Engineer for review, comments and/or approval. The Village Engineer shall check the plans for conformity to the standards set forth herein and certify that they are consistent with the design standards of the village, after which, he or she will return one of the sets with the appropriate comments to the applicant. The applicant, after making any changes requested on the set of plans returned to him or her, shall submit three sets of the revised plans to the Engineer for final approval. The Village Engineer shall review these revised plans for conformity to the comments mentioned heretofore and, if they have been properly made, will return one approved copy to the applicant. The applicant shall obtain approval of the Oakland County Road Commission, the DNR and any other agency where applicable.
(B) Design standards.
(1) Storm water retention is necessary for all developments in the village. Waiver of this requirement will be considered by the Village Council upon submittal of a request for waiver, in report form, stating the reasons why a retention basin should not be necessary. Such report shall include maps, charts and calculations prepared by a registered professional engineer, registered in Michigan.
(2) Retention basins shall be designed to retain storm water for the developed site. The applicant is not required to retain water from off-site areas in the drainage district.
(3) Retention basins shall be designed to retain ten-year storm for all sites. The Oakland County Drain Commission method shall be used. The basin shall have a one-foot freeboard above the high water elevation and an emergency high water overflow spillway.
(4) The basin shall be designed to store all runoff in excess of agricultural runoff. A rule of thumb for determining agricultural runoff is 0.2 cfs/acre of imperviousness. It is recommended that the project engineer meet with the Village Engineer before design is begun.
(5) The retention basin shall be completely fenced if side slope is one on six or steeper. The fence shall be a six-foot high chain link. The gate shall be 12 feet wide, double opening. The fence specifications should have all detail information.
(6) The minimum side slope and beret requirements are:
(a) Three on one with ten feet beret area at the fence; or
(b) Four on one with a two feet beret area at the fence.
(7) If the three on one side slope is used for the retention basin, a ramp will be necessary from the beret area to the bottom of the basin. The minimum slope on the ramp shall be four on one. The width of the ramp shall be 20 feet at the beret area and 12 feet at the bottom of the basin.
(8) The bottom of the basin shall have a minimum slope of 1.0%. As an alternate method, a 12-foot wide cunette may be constructed. A CUNETTE is a small channel to concentrate small flow to obtain desirable velocities. The minimum slope of the cunette shall be 0.30. The cunette shall be constructed of six-inch thick unreinforced grade “A” concrete. The cunette shall have a cross slope of three feet from the edge to the center. A two-foot deep footer shall be installed along both outer edges. The cunette shall extend from inlet pipe(s) to outlet pipe.
(9) The entire retention basin must be seeded or sodded. The village will not approve the basin until turf is established. Soil erosion control measures shall remain in place at the basin’s outlet until the turf is established.
(10) Concrete rip-rap or approved equal is required at all pipe entrances to the basin. The minimum width of the riprap shall be twice the outside diameter of the pipe. The rip-rap shall extend from bottom of the basin to the top of slope. Three types of materials may be used:
(a) Fieldstone or broken concrete of four-inch minimum thickness and one-square foot minimum area. Broken concrete or stone shall be mortared to form a monolithic slab with minimum thickness of four inches;
(b) Poured Grade “A” concrete of four-inch minimum thickness; or
(c) Maccaferri gabions, river type, reno mattress, size as designated on the plans and installed per manufacturer’s recommendations.
(11) All pipe entering a retention basin shall have either a headwall or end section at the end of the pipe. Bar screens must be installed on all open ends of pipe 12 inches or larger in diameter.
(12) An overflow system must be provided. When possible, the overflow shall outlet into an adequate storm sewer. There are two alternate methods:
(a) Using an overflow pipe. The invert elevation of this pipe shall be above the maximum storage elevation of the basin;
(b) A low corner overflow. One corner of the basin shall be set at an elevation higher than the maximum storage elevation of the basin. A defined overflow route shall be shown, located on applicant’s property. All gates constructed directly in front of a paved roadway shall have a “Road Ends” sign fastened securely to the gate. Scotchlite on 0.080 aluminum Michigan Manual of Uniform Traffic-Control Devices, sign W14-2a (30-inches by 30-inches), is required.
(13) Where the drain outlet for the retention basin is not deep enough to completely dewater the basin, pumps must be installed. The pumps shall be installed in duplicate, with each pump capable of handling the flow. Controls shall be set in the receiving water to regulate the flow. The controls may be:
(a) Electrodes placed inside a galvanized pipe stilling well at a location adequately protected from the back water curve during discharge;
(b) A bubbler system in a stilling well protected as in division (1) above. The operating controls and pump shall be set in a pump house with adequate dimensions for working area. The pump house and wetwell must be located inside the fenced area. Complete specifications for the pumps and controls and performance curves for the pumps called for and must be submitted to the Village Engineer for approval. A building permit is required for all electrical work and for concrete structures. A manhole with inside diameter of six feet is required between the lift station and the outlet. The discharge lines shall be ductile iron. They shall enter the manhole, and a storm sewer shall be installed from the manhole to the outlet. Flap gates shall be installed on the lines from the lift station. The manhole cover shall be E.J.I.W. No. 8247A hinged type or equivalent. A two-foot deep sump in the manhole is required.
(14) The pump house and the gate(s) to the retention basin shall be locked at all times. It shall be the owner’s responsibility to provide a lock and keys necessary to ensure that the basin is closed until final acceptance by the village and then furnish three key sets to the village when completed.
(15) A silt trap and bar screen shall precede a pumped outlet from the retention basin. The screen clear opening shall be two-inches minimum.
(16) Aboveground, enclosed pump stations are preferred. Belowground stations will be considered, using either fiberglass or concrete chambers. Metal structures below ground are not to be used.
(17) Duplex pumps must be designed to a peak discharge equaling the maximum outlet rate established by the Village Engineer when the retention area is at the overflow elevation (point of minimum static head).
(O.C. § 5.705) (Am. Ord. 268, passed 9-19-22)