All storm water facilities connecting directly or indirectly to County facilities shall be designed in accordance with the following minimum requirements. The County Agency may deny a connection permit for a storm water facility which is not in substantial compliance with these design and construction standards and conditions, or for which a satisfactory compliance schedule cannot be agreed upon.
(a) General Regulations.
(1) Construction shall not commence until all applicable local, State, and Federal permits have been obtained.
(2) For items not specifically covered herein, the design, in general, shall conform to the prevailing standards of the County Agency.
(b) Right-of-Way Requirements.
(1) The following minimum right-of-way widths are required for established County drains and natural watercourses that will be utilized.
A. Open drains and watercourses whose maximum bank to bank width exceeds thirty feet shall have a right-of-way to the extreme width of the drain, plus thirty feet. The easement shall be centered on the centerline of the drain or watercourse.
B. Open drains and watercourses whose maximum bank to bank width is less than thirty feet shall have a right-of-way equal to the extreme width of the drain, plus twenty-four feet. The easement shall be centered on the centerline of the drain or watercourse.
C. Enclosed drains whose internal diameter is eight feet or less shall have a right-of-way of twenty feet centered on the centerline of the enclosure.
D. Enclosed drains whose internal diameter exceeds eight feet shall have a right-of-way of twenty-five feet centered on the centerline of the enclosure.
(2) The above widths shall govern generally. However, if the County Agency determines that additional right-of-way is required for proper construction, or because of special circumstances, such facts shall be made known after a review of the preliminary layout by the County Agency. Exceptions to the above right-of-way requirements may be made only at the discretion of the County Agency.
(c) Drainage Districts. In accordance with the Drain Code, Act 40 of the Public Acts of 1956, as amended, the drainage of proposed subdivisions shall be contained within the drainage district or drainage districts of the established County drain, or if there is no established drainage district, then within the limits of the natural drainage basin. When needed, minor alterations of the legal limits of established drainage districts shall be made pursuant to the procedures set forth in the Drain Code.
(d) Design Flows.
(1) Run-off determination. The rational method shall be used and is expressed as: Q = CIA, where Q is the peak flow rate in cubic feet per second (C.F.S.), C is the runoff coefficient, I is the rainfall intensity in inches per hour (in./hr.), and A is the drainage area in acres.
Flow rates shall be based on adjusted runoff coefficients, incremented areas, and adjusted rainfall intensities applicable to those segments of the improvements which are being designed. Values for the various terms used in the rational method formula shall be determined as follows:
A. A composite runoff coefficient based on the percentage of types of surface with respect to the type of development shall be used.
B. Rainfall intensity-duration-frequency curves have been developed by the U.S. Department of Commerce, Weather Bureau, for the Detroit area. Rainfall relationships extracted from Technical Paper #25, U.S. Department of Commerce, Weather Bureau, December, 1955, shall be used.
C. The time of concentration of the design will vary with slope, surface cover and the length of path of the surface flow. Other variables, including anticipated rainfall intensity and infiltration capacity of the soil and surface cover, will also affect the time of concentration. An initial inlet time of concentration of fifteen minutes shall be used for multiple and commercial/industrial land used, while twenty minutes shall be used for single-family residential land use. The time of concentration for unimproved lands (in minutes) should be derived from U.S. Department of Agriculture or F.A.A. methods.
(2) Detention. Detention facilities will be required if the limitation of discharge into the County drain or sewer is deemed necessary by the County Agency.
(e) Sewers.
(1) The minimum size of storm sewer and culvert pipe shall be twelve inches in diameter.
(2) The following general pipe materials shall be used for storm sewer and culvert pipe.
A. Reinforced Concrete Pipe - ASTM C76
B. Reinforced Concrete Elliptical Pipe - ASTM C507
C. Non-Reinforced Concrete Pipe - ASTM C14
D. Other materials which comply with applicable County standards may be used upon acceptance by the County Agency.
(3) Bituminous type joint sealer shall be used, except that wherever the design hydraulic gradient is above the top of the pipe or at locations determined by the County Agency, internal rubber type gaskets shall be used.
(4) Concrete or riprap headwalls with erosion protection shall be placed at the inlet and outlet ends of all sewer outlets and culverts in accordance with prevailing County Agency standards.
(5) A minimum of three vertical feet of clearance from the top of the pipe to the invert of a drain being crossed shall be maintained unless otherwise approved by the County Agency.
(f) Construction Requirements.
(1) Proper construction practices shall be employed to protect water courses from siltation and obstruction to flow.
(2) Construction materials and methods shall comply with the current requirements of the County Agency.
(Ord. 712. Passed 2-14-01.)