(a) Submittal Procedures. For the Village of Franklin's approval of storm sewer systems, the applicant shall furnish to the Village of Franklin a detailed estimate of cost and three sets of plans and specifications. Detailed storm sewer design computations, retention/detention computations, and the drainage area map indicating the various areas going into the points of inlet, shall be provided. The Village Engineer shall review the estimate and the plans for conformity to standards set forth herein and certify that they are consistent with the overall utility plans of the Village of Franklin, after which the Village Engineer will return one of the three sets with appropriate comments. After making any changes requested on the set of plans returned to him or her, the applicant shall then submit three sets of revised plans to the Village of Franklin for final approval. The Village Engineer shall then review these revised plans for conformity to the comments mentioned heretofore, and, if the changes have been properly made, will retain two copies for the Village of Franklin's records and return one approved copy to the applicant. The applicant shall obtain approval of the Oakland County Road Commission and the Oakland County Drain Commissioner, where applicable.
(b) Design Standards for Storm Sewers.
(1) Storm drainage systems shall be designed for a ten-year storm. The rational method for arriving at storm water runoff shall be used.
The formula for rainfall intensity shall be equivalent to the following:
I = 175/T + 25
in which T is the time of concentration. The consulting engineer shall use judgment in arriving at proper impervious factors.
The consulting engineer shall submit a map outlining the various areas, including off-site upstream areas, which drain to the points of inlet used for design, together with the storm sewer design computations. In general, sufficient capacity shall be provided in the storm sewer system to take fully developed (fully paved and sewered) upstream drainage into the system.
(2) Where the hydraulic gradient is above the top of the sewer pipe, the design elevation of the hydraulic gradient shall be indicated in the profile view of the proposed storm sewer.
(3) Manhole spacing for storm sewers shall be as follows:
Diameter of Maximum
Sewer (in.) Manhole Spacing (ft.)
66 and larger 600
NOTE: Height of Lo-Head pipe shall be used as the criteria for manhole spacing. Catch basins shall not be constructed over a sewer line to replace manholes.
(4) The following information shall be indicated on the storm sewer profile:
A. Length of run between manholes;
B. Size and slope of sewer between manholes;
C. Class of bedding, where concrete is required;
D. Top elevation of all manholes;
E. Type of pipe material; and
F. Backfill material.
Where possible, the slope of the sewers shall provide a minimum velocity of 2.5 feet per second.
(5) A note or detail shall show the type of bedding upon which the sewer pipe shall be installed.
(6) Where possible, a minimum of three feet of cover from the top of the curb (or road centerline) to the top of any storm sewer shall be provided. If the depth of cover to the top of the pipe is less than one foot, C-76 - CL-5 pipe shall be used. If the cover is between one foot and three feet from the top of the curb to the top of the pipe, C-76 - CL-4 pipe shall be used. All catch basin leads within a street shall be C-76 - CL-4 pipe, including the leads to the manholes.
(7) In general, catch basins shall be located as follows:
A. At the radius return of street intersections. A 150- foot maximum distance along the street, between a high point and a corner catch basin, is allowed when drainage is required to go around the corner.
B. At all low points in streets.
C. At intermediate points along the street such that there is a maximum of 600 feet of drainage draining from a high point to a catch basin or from a previous intercepting catch basin to this catch basin.
A maximum of 900 feet of drainage is allowed from two directions.
(8) Field catch basins shall be provided at all low points in easements. Intercepting field catch basins shall be located such that not more than 600 feet of drainage runs into any one catch basin other than at a low point of such catch basin. One thousand, two hundred feet of drainage is allowed to run into a low point of a catch basin. Field catch basins shall be located in rear lot easement swales when such swales changes direction by more than forty-five degrees.
(9) Finished easement grades shall be indicated on the plans.
(10) Improved open drains may be permitted under special circumstances, in accordance with Section 1228.11.
(11) Headwalls and inlet structures shall be placed as required. Bar screens are to be provided on all end sections and pipe ends where the pipe size exceeds fifteen inches in diameter.
(12) Allowable types of sewer pipe and joints shall conform to current Village of Franklin standards and shall be covered on the plan by a note or on the profile, where applicable.
(13) For new subdivisions, lateral storm sewers shall be constructed to provide an outlet for the footing drain discharge house leads. Extensions of storm sewer laterals to provide for footing drain discharge only shall be a minimum of eight inches in diameter on a minimum 0.4 percent grade with manholes spaced a maximum of 300 feet apart.
(14) Wherever sufficient grade is available, storm sewers shall be constructed at a depth adequate to allow for gravity drainage of the building footing drains. Where grading is not available to allow for gravity drainage of the footing drains, a sump pump shall be provided with the sump pump discharge connected to the enclosed storm sewer system. The plans shall clearly state which units require pumped sump outlets.
(15) A minimum four inches diameter storm sewer lead shall be constructed from the lateral storm sewer to the property line to provide an outlet for the footing drain discharge. The lead shall be constructed at a minimum depth of three and one-half feet on a minimum of 1.0 percent grade. The materials and methods of construction shall be in accordance with current Village of Franklin standards.
(c) Design Standards for Retention and Detention Basins. Any new development or addition to an existing development must retain/detain the increased runoff on-site unless otherwise directed by the Village Engineer. Acceptable means of retention/detention can be achieved through standing water in parking areas (to a maximum depth of six inches) or a separate retention/detention basin.
The County method of retention basin design, as available from the County Drain Commissioner's Office, shall be utilized in determining the volume of retention required. Basins with orifice outlets will be required to hold the volume of a ten-year storm, while basins with no outlets will be required to hold two consecutive 100-year storms. Discharge through an orifice outlet shall not exceed 0.2 cfs/acre.
(1) Minimum specifications.
A. All man-made open storm water retention/detention basins shall have side slopes no steeper than one vertical to five horizontal.
B. The basin must be constructed to drain entirely, unless it is designed to retain a permanent water level that conforms to the aesthetics of a landscape plan relating to the surrounding landscape.
C. The bottom of the basin must have stable vegetative cover or some other approved method of stabilization.
D. A minimum of twelve inches of freeboard must be maintained with a positive, nonerodible overflow capable of handling the capacity of a 100-year storm.
E. The minimum grade on the bottom of the basin shall be 1.0 percent, where feasible.
(2) Maintenance agreement. A maintenance agreement shall be required for all storm water retention/detention basins.
(Ord. 96-87. Passed 5-13-96.)