(a) Design Storms:
(1) The initial drainage system is that part of the storm drainage system which is used regularly for collecting, transporting, and disposing of storm runoff from frequent and low magnitude storm events, snowmelt, and miscellaneous minor flows. The capacity of the initial drainage system should be equal to the maximum rate of runoff expected from a design storm of established frequency (i.e., Initial Storm). For purposes of design, the initial drainage system portion of the overall storm drainage system shall be designed to contain the runoff from a storm with a return period of not less than five-years.
(2) The major drainage system is that part of the storm drainage system which carries the runoff which exceeds the capacity of the initial drainage system. The major drainage system shall have the capacity to carry runoff from a storm with a return period of not less than 100-years (i.e., Major Storm) without posing significant threat to property or public safety.
(b) Initial Storm - Physical Design Criteria for On-Site Improvements:
(1) Depth of flow in natural channels shall not exceed bank full stage with backwater effects considered.
(2) Depth of flow in man-made channels shall not exceed 0.8 bank full stage. Velocity of flow shall be determined in accordance with the design criteria for open channels and shall not exceed 5 feet per second, or a rate determined by the Municipality’s Engineer to be detrimental to the watercourse. Where flows exceed this rate, special channel lining and erosion protection shall be provided. See Section 935.08(d) for specific design criteria for open watercourses.
(3) Depth of flow in road-side ditch swales shall not exceed one foot or be of such depth that flow would extend out of the right-of-way if the side ditch is less than one foot in depth. Velocity at this depth shall not exceed six feet per second for grass swales or ten feet per second for paved ditches.
(4) Depth of flow in streets with curb and gutter shall not exceed the curb height. Velocity of flow in the gutter at design depth shall not exceed ten feet per second. In addition to the above, the following are maximum encroachments of the minimum five-year initial design storm onto the pavement. See Section 935.08(c) for specific design criteria for curb inlet design.
A. For minor streets carrying traffic from the individual residence to collector and secondary streets, the flow may spread to the crown of the street.
B. For collector and secondary streets, one lane shall be free from water.
C. For primary streets, one lane in each direction shall be free from water.
D. For freeways, no encroachment is allowed on traffic lanes.
(5) In design of conduit, the conduit may be designed on the basis of flowing full with surcharge to gutter line. Backwater effects shall be considered.
(c) Major Storm - Physical Design Criteria for On-Site Improvements:
(1) The major storm floodway and floodway fringe for natural streams shall be as defined by the Federal Emergency Management Agency (FEMA), U.S. Army Corps of Engineers, the Ohio Department of Natural Resources, or where such determinations have not been made by these agencies, the major storm floodway and floodway fringe for natural streams may be estimated through a technical analysis by a registered Professional Engineer in the State of Ohio, in a manner found acceptable by the Engineer.
(2) Many of the drainage ways associated with the major storm system are in areas beyond those designated as floodway or floodway fringe. For these areas, the major storm flood limits shall be determined by the U.S. Corps of Engineers’ HEC-2 method or other accepted methods of determining water profiles using the major design storm runoff. One-half foot elevation shall be added to the flood profile as freeboard for protection in the event of future encroachments into the floodway fringe or in the drainage way.
(3) In order to protect the integrity of the non-street drainage rights-of-way, the design engineer is encouraged to design routing paths for multi-purpose functions. Pedestrian and bicycle paths lend themselves naturally to this application. Linear parks aligned along the major drainage corridor are also very effective, but usually require greater width than would normally be necessary for drainage purposes.
(4) Where the street is designed as the major drainage system, the depth of flow shall not exceed 12-inches at gutter line for minor, collector and secondary streets, and shall not exceed 6-inches depth at crown for primary streets and freeways. The same maximum depth criteria will apply where a major drainage way crosses the street. Where a major drainage way is located outside a street, right-of-way easements will be provided.
(5) In determining the required capacity of surface channels and other drainage ways provided for the major storm runoff, the street storm inlets and conduit provided for the initial design storm may be assumed to carry a portion of the total runoff volume, if appropriate. The following equation shall be used to determine the required capacity of surface channels and drainage ways in their design, when a portion of the runoff is conveyed within the initial piped system:
Q100 = C I10 A + 0.96 (I100 – I10) A |
and |
Q flood routing path = Q100 – Q pipe |
Where:
Q flood routing path = Design flow, major storm runoff (cfs)
Q pipe = Peak flow within piped system (i.e., 5-year event) (cfs)
Q100 = Peak flow for 100-year event (cfs)
C = Rational runoff coefficient, site developed condition
I10 = rainfall intensity for 10-year storm event (inches/hour)
I100 = rainfall intensity for 100-year storm event (inches/hour)
A = Drainage area contributory to design point (acres)
(d) Retention and Storage: Areas designed for storage of stormwater by retention should be incorporated into the natural features of the general area, when possible. Cooperative planning and joint owner construction of detention or retention facilities and use of natural land contours is encouraged. No such facilities will be permitted which may be or become aesthetically unpleasing, or which may result in construction, or maintenance problems. The Municipality encourages that detention or retention facilities be designed as multipurpose spaces such as open space, recreation and/or scenic areas. The Municipality encourages use of fountains for aeration and reserves the right to require such an appurtenance as a condition to plan approval.
(e) Methods of Calculation: The methods of calculation as listed in Table E (located at the end of this chapter) shall be used unless otherwise approved by the Municipality’s Engineer:
(1) Rainfall volumes shall be in accordance with data for Central Ohio provided in “Bulletin 71: Rainfall Frequency Atlas of the Midwest”, 1992 and any subsequent updates thereto.
(2) Rainfall distribution for stormwater management systems is to be in accordance with SCS Type II Rainfall Distribution.
(3) The appropriate Runoff Curve number (i.e., “RCN” factor) may be determined by using Technical Release No. 55 (S.C.S.) or its Ohio Supplement.
(f) Drainage Area Determination: The drainage area shall be determined from any of the following sources, which are listed in order of priority preference:
(1) Actual field investigation.
(2) Franklin County Auditor, topographic maps;
(3) U.S. Geological Survey quadrangle (7.5 minute series) contour maps; or
(4) Soil Survey of Franklin County, Ohio, U.S.D.A.
(Ord. 08-021. Passed 4-14-08.)