An adequate storm drainage system shall be provided for each subdivision. The design of said system shall be prepared using the Rational Method and shall be based on a minimum 2-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 storm water to the proposed drainage system must be considered on the determination of the sizes of structures and channels.
(a) A separate grading plan shall be submitted at a scale of 1" = 50' or 1" = 100'. The grading plan shall indicate ground elevations with existing and proposed contours shown at intervals of not more than 5 feet where the slope is greater than ten percent (10%) and not more than 2 feet where the slope is less than ten percent (10%). Sufficient proposed elevations shall 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.
The minimum building elevation adjacent to the 100-year routing path shall be set a minimum of 1 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.
Swales necessary to carry surface water must have a minimum gradient of 1.0 percent (1%).
(b) Storm Sewer Requirements.
(1) Minimum cover for storm sewer pipe shall be 1 foot clear from the bottom of the curb and gutter or from the bottom of the underdrain to the outside top of the pipe except as approved by the Village Engineer. Maintain a minimum of 2 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 pipe shall be extra strength vitrified clay, reinforced concrete, or smooth lined corrugated polyethylene pipe. Other materials shall require the approval of the Village Engineer. All pipe installations 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 with the 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 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 2-year storm shall not exceed 8 feet for 30-foot wide streets and 9 feet for streets wider than 30 feet. Spread calculations may be required at the discretion of the Village Engineer.
(10) 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.
(11) Where a 72-inch pipe is inadequate for a 2-year storm, an open watercourse may be permitted. When so 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. No buildings, plantings, or other improvements shall be permitted within that easement.
(12) 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 stormwater drainage design for areas up to 200 acres. Storm sewers shall be designed to carry a 2-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 10 minutes to a curb inlet or 15 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" (known as TR-55) dated June 1986 and the "Ohio Supplement to Urban Hydrology for Small Watersheds, Technical Release No. 55" dated April 1981 (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 10 minutes. The minimum time of concentration to the first catch basin shall be 15 minutes. The elevations at the curb line shall be provided to the Village in order to assure that this requirement is met.
(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 3 feet per second (fps) in pipe. The maximum allowable velocity shall be 7 fps.
(5) A hydraulic grade line check based on a 5-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 10-year storm, with a 1-foot freeboard.
(3) The minimum velocity for open channels shall be 2 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 percent (.4%).
(4) The Manning "n" values shall be as follows:
Sod or jute mat lining | 0.05 |
Paved lining | 0.015 |
Rock Protection | 0.08 |
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.
(e) 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 Mannings "n" values shall be as follows:
Box culvert | 0.011 |
Slab top culvert | 0.03 to 0.05 |
Smooth pipe | 0.012 |
(Ord. 06-08. Passed 2-25-08.)
(6) The minimum cover to subgrade shall be 9 inches from top of pipe to bottom of subgrade. The structural design of culverts shall be the same as that required by the Ohio Department of Transportation, Construction and Material Specifications Section 706.05 dated 01-01-2010 for culverts with a span of 12 feet or less. Design calculations for all culverts shall be approved by the Village Engineer. (Ord. 8-12. Passed 6-25-12.)
(f) 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 2-year Volume Runoff | Control Design Frequency (Critical Storm) |
1 - 1.2 Times | 2 year |
1.2 - 1.5 Times | 5 year |
1.5 - 2 Times | 10 year |
2- 3 Times | 25 year |
3- 4 Times | 50 year |
Over 4 Times | 100 year |
The maximum allowable release rate shall be based on the 2-year storm under predeveloped 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 predeveloped rate for the same frequency storm, for storms up to and including the 100-year storm.
Appendix B, "Total Runoff Volume Computations Worksheet," should be used to determine predeveloped and postdeveloped runoff volumes, and runoff volumes for critical storms. Required detention volume calculations shall be based on Chapters 4 and 6 of TR-55.
An exemption may be granted for single family residential development if the downstream storm drainage to the confluence of the Big Darby Creek has sufficient capacity to handle the runoff without stormwater detention.
Detention basins shall have a minimum one percent (1%) transverse slope. A paved ditch, at a minimum 0.5 percent (.5%) slope, shall be provided from the inlet to the outlet of all structures. Such ditches shall be paved with 6-inch minimum thickness concrete paving reinforced with steel mesh to accommodate temperature stresses, of air-entrained Class C concrete, and with synthetic or linseed oil waterproofing treatment. Paved ditches shall be 14 foot minimum total paved width, with 8-foot bottom width, and 3-foot width by 1-foot rise sloped side sections. Minimum depth of paved invert ditch shall be 1 foot.
All stormwater detention facilities shall be owned and maintained by a homeowners' association comprised of properties served by the detention facility. Such homeowners association shall be established and inserted into the chain of title of each lot within such subdivision.
All control facilities shall be designed with overflow provisions to handle the developed 100-year discharge.
(Ord. 06-08. Passed 2-25-08.)