An adequate storm drainage system shall be provided for each subdivision/development. The design of said system shall be prepared using the Rational method and shall be based on a minimum of two year frequency. Rainfall intensity, runoff coefficients, and concentration time used in computing flows and structures sizes shall be in accordance with the tables, charts, and the data established by the Municipal Engineer for such calculations. All areas which contribute storm water to the proposed drainage system must be considered on the determination of the sizes of structures and channels. Detention basins that are properly maintained by their owners or property owners' associations are important to protect the integrity of the public storm sewer system.
(a) (1) A separate grading plan shall be submitted at a scale of 1" = 50' or l" = 100'. The grading plan shall indicate ground elevations with existing and proposed contours shown at intervals of not more than five feet where the slope is greater than 10% and not more than two feet where the slope is less than 10%. Sufficient proposed elevations must be shown such as at all lot corners, etc. in order to explain the proposed grading. First floor elevations of all existing and proposed structures shall be included. Routing of the major storm shall be shown. Sanitary sewer and storm sewer top of castings must be shown on the grading plan.
(2) The minimum building elevation adjacent to the 100-year routing path shall be set a minimum of one foot above the 100-year flood elevation. No basement entrances, windows, or basement level garages shall be permitted adjacent to and below the 100-year routing path.
(3) Swales necessary to carry surface water must have a minimum gradient of 1.0%.
(b) Storm Sewer Requirements.
(1) Minimum cover for storm sewer pipe shall be one foot clear from the bottom of the curb and gutter or from the bottom of the under-drain to the outside top of the pipe except as approved by the Village Engineer. Maintain a minimum of two feet of cover from the finished ground surface to the outside top of the pipe for any storm sewer system located beyond the limits of street right-of-way.
(2) Standard headwalls are to be constructed at the inlet and outfall of all storm sewers, and shall be shown on the plan and profile.
(3) The invert of the first storm sewer appurtenance shall be above the computed floodplain elevation, unless otherwise permitted by the Village Engineer.
(4) Pipe for storm sewers shall not be less than 12 inches in diameter. All storm sewer piping shall be extra strength vitrified clay, reinforced concrete, or ductile iron. Other materials may be approved at the direction of the Village Engineer. All piping shall have sufficient strength to withstand an HS-20 live load.
(5) All drainage calculations, drainage area outlines, and contributing areas used in drainage design shall be furnished on a print of the grading plan.
(6) The inverts of all curb inlets, manholes, yard inlets, and other appurtenances shall be formed to reduce turbulence to a minimum.
(7) Manholes shall be provided at all changes in alignment and grade of storm sewers and at such other locations as necessary to maintain a maximum interval of 500 feet between manholes or storm sewers.
(8) Storm inlet or catch basin grates shall be of a type to permit safe crossing by bicycles as approved by the Village Engineer.
(9) The maximum distance for overland flow shall be 300 feet before entering a surface yard inlet or 400 feet before entering a curb inlet. Except, that the maximum overland drainage area tributary to any yard inlet or curb inlet shall not exceed 1.5 acres. The maximum spacing for curb inlets shall not exceed 400 feet unless approved by the Village Engineer. The maximum spread of flow during a two-year storm shall not exceed eight feet for 30-foot wide streets and nine feet for streets wider than 30 feet. Spread calculations may be required, at the discretion of the Village Engineer.
(10) All storm water runoff shall be conducted through storm drainage systems up to and including the equivalent of a 72-inch diameter pipe.
(11) The flow lines of pipes shall be set such that either the crown, or the 0.8 depth points, at junctions, are at the same elevation. However, the crown of the outlet pipe may be lower.
(12) Where an open watercourse is permitted, an easement shall be provided at least equal to the area required for the 100-year rainfall. Also, the easement shall be shown on the final plat as a "Watercourse and Utility Easement." Restrictions as to the use of this easement shall be shown on the final plat.
(13) No water will be allowed to cross a street intersection unless it is carried in storm sewer.
(c) Storm Sewer Design Criteria.
(1) The method outlined herein will provide a general guide as to the criteria and procedures to be used for storm sewer design.
A. The rational method shall be used for all storm water drainage design for areas up to 200 acres. Storm sewers shall be designed to carry a two-year storm flowing full Q=CIA, in which
Q = Quantity of storm water runoff in cubic feet per second.
C = Coefficient of runoff (0.4 for single-family residential areas).
I = Average rainfall intensity in inches per hour for the period of concentration to the point under consideration. The minimum length of time of concentration is ten minutes to a curb inlet or l5 minutes to a ditch catch basin.
A = Drainage area in acres tributary to the point of concentration.
B. For drainage areas over 200 acres, the method explained in "Urban Hydrology for Small Watershed, Technical Release No. 55" (can be obtained from the Soil Conservation Service, U.S. Department of Agriculture) shall be used.
(2) The minimum time of concentration to the first curb inlet shall be ten minutes. The minimum time of concentration to the first catch basin shall be l5 minutes.
(3) Storm sewer pipe sizes are to be determined by using Manning's Formula with a minimum coefficient of roughness of n = .013.
(4) The minimum allowable velocity shall be three feet per second (fps) in pipe. The maximum allowable velocity shall be seven fps.
(5) A hydraulic grade line check based on a five-year storm may be required, at the discretion of the Village Engineer.
(d) Open Watercourses.
(1) Access to storm drainage ditches and channels shall be assured by means of maintenance easements. Such maintenance easements shall be not less than 20 feet in width, measured horizontally from the top of the bank, exclusive of the width of the ditch, or channel, and shall be provided on each side of the ditch. Maintenance easements are to be kept free of obstructions. Detailed provisions regarding the entities to be responsible for maintenance of the facility shall be submitted in text form with the subdivision plat. As applicable, notes regarding maintenance shall be made on the plat.
(2) Design storm frequency for open channels shall be based on bank full for ten-year storm, with a one foot freeboard.
(3) A. The minimum velocity for open channels shall be two feet per second. The maximum velocity is dependent on the type of channel protection provided. The desirable minimum grade for open channel is 1.0%.
B. The Manning "n" values shall be as follows:
Sod or jute mat lining 0.05
Paving lining 0.015
Rock Protection 0.08
C. The Municipal Engineer on a case-by-case basis will review Manning "n" values for other lining materials.
D. The minimum side slope shall be 3:1 for unprotected slopes and 2:l for slopes protected with riprap. Flatter slopes are more desirable. Steeper slopes for materials other than riprap will be reviewed on a case-by-case basis.
(4) If the proposed improvements are located in a floodplain area as identified by the Federal Emergency Management Agency, the limits of this floodway and floodway fringes must be shown on the improvement plans.
(e) Storm Water Detention.
(1) If the post-development runoff volume is greater than the predevelopment runoff volume, storm water detention must be provided in accordance with the following table:
Increase in 2-year Volume of Runoff | Control Design Frequency (Critical Storm) |
1 - 2 Times | 10-year |
2 - 3 Times | 25-year |
3 - 4 Times | 50-year |
Over 4 Times | 100-Year |
(2) The maximum allowable release rate shall be based on the two-year storm under pre-developed site conditions, for all rainfall events up to and including the critical storm. The maximum allowable release rate for storms greater than the critical storm shall not exceed the pre-developed rate for the same frequency storm, for storms up to and including the 100-year storm.
(3) "Total Runoff Volume Computations Worksheet," in the Village of Commercial Point Specification, Materials, and Standard Drawings Manual should be used to determine predeveloped and post developed runoff volumes, runoff volumes for critical storm, and required detention volume calculations.
(4) Detention basins shall be lined with riprap or other bank protection as approved by the Village Engineer, extending at least three feet horizontally and two feet vertically from the established water table line. Soils information shall be provided to demonstrate that the retention basis will be impervious.
(5) All wet basins shall have fountains approved by Council. Dry basins must have concrete channels to handle low flows.
(6) Detention basins shall have a minimum 1% slope.
(7) All control facilities shall be designed with overflow provisions to handle the developed 100-year discharge.
(8) The maintenance of detention basins and all outlets thereto shall be the responsibility of the property owner or property owners' association, as applicable and not the Village of Commercial Point. Such maintenance shall ensure that water in each detention basin through the detention basin outlet does not restrict the flow of the public storm sewer system.
(Ord. 2021-28, passed 8-16-2021)