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(A) Subdivision drainage systems. An adequate storm drainage system shall be provided for each subdivision. The design of the system shall be prepared using the rational method and shall be based on a minimum two-year frequency. Rainfall intensity, runoff coefficients, and concentration time used in computing flows and structure sizes shall be in accordance with the tables, charts, and the data established by the Village Engineer for such calculations. All areas which contribute stormwater to the proposed drainage system must be considered on the determination of the sizes of structures and channels.
(B) Grading and elevation requirements.
(1) A separate grading plan shall be submitted at a scale of one inch = 50 feet or one inch = 100 feet. 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 shall be shown such as at all lot corners and the like 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%.
(C) 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. 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 head walls 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 pipe shall be extra strength vitrified clay, reinforced concrete, or ODOT Item 603 PVC. Other materials may be approved at the direction of the Village Engineer. All pipes 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 425 feet before entering a curb inlet, except that the maximum overland drainage area tributary to any yard inlet or curb inlet shall not exceed one and one-half acres. The maximum spacing for curb inlets shall not exceed 400 feet unless approved by the village. 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 stormwater runoff shall be conducted through storm drainage systems up to and including the equivalent of a 72-inch inside 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.
(D) Storm sewer design criteria.
(1) The method outlines 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 stormwater 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 stormwater 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 15 minutes to a ditch catch basin; and
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 15 minutes.
(3) Storm sewer pipe sizes are to be determined by using Manning’s formula with a minimum coefficient of roughness of n = .01.
(4) The minimum allowable velocity shall be three feet per second (fps) in pipe. The maximum allowable velocity shall be seven fps.
(5) The hydraulic profile must be submitted as part of the storm drainage calculation required in this section of the code. Storm sewer should be designed such that hydraulic grade line does not extend above the top of any inlet grate or manhole for a ten-year design storm.
(E) 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 25-year storm, with a one-foot freeboard.
(3) 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 0.4%.
(4) (a) The Manning “n” values shall be as follows:
Sod or jute mat lining
(b) Manning “n” values for other lining materials will be reviewed by the Village Engineer on a case-by-case basis.
(5) The minimum side slope shall be 3:1 for unprotected slopes and 2:1 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.
(6) 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 fringe must be shown on the improvement plans.
(F) Roadway culverts.
(1) Single span culverts, including concrete box or slab top, should always be considered in lieu of multiple cell pipe culverts in order to meet the physical requirements introduced by rigid headwater controls.
(2) The plan for each culvert shall indicate the drainage area in acres and the design discharge in cubic feet per second.
(3) The culvert invert elevation should be set such that it will be deep enough to provide an adequate outlet for future storm sewer improvements upstream.
(4) The design storm frequency for culverts is 25 years, or as otherwise approved as required, with a maximum allowable headwater of 12 inches below edge of pavement, with consideration for effects on upstream property.
(5) The Manning’s “n” values shall be as follows:
Slab top culvert
0.03 to 0.05
(6) The minimum cover to sub-grade shall be nine inches from top of pipe to bottom of sub- grade. The structural design of culverts shall be the same as that required by the state’s Department of Transportation.
(G) Stormwater detention.
(1) If the post-development runoff volume is greater than the predevelopment runoff volume, stormwater detention must be provided in accordance with the following table:
Increase in Two-Year Volume of Runoff
Control Design Frequency (Critical Storm)
1 - 2 times
2 - 3 times
3 - 4 times
Over 4 times
(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) Appendix B, “Total Runoff Volume Computations Worksheet,” attached to Ord. 96-106, should be used to determine pre-developed and post-developed runoff volumes, runoff volumes for critical storm, and required detention volume calculations.
(4) Retention 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 basin will be impervious.
(a) Detention basins shall have a minimum 1% slope.
(b) All control facilities shall be designed with overflow provisions to handle the developed 100-year discharge.
(Ord. 96-106, passed 3-20-1996)
Flood hazard regulations, see Ch. 151