(a) In general, drainage systems shall be designed according to City of Trotwood’s accepted design practice, sound engineering judgment and conform to the following:
(1) Storm Sewer design shall be sufficient to convey the design storm discharge (Q) with an average recurrence interval of 10 years so that the pipe flowing full condition (hydraulic grade line at top of pipe) is not extended.
(2) Lot grading, in-tract drainage, and street improvements for all subdivisions should be designed so that floods having an average recurrence interval of 100 years or less will not cause inundation or damage to any dwellings. A grading plan for each subdivision will be required to define the lot grading and in-tract drainage. All developments within areas of special flood hazard or delineated on the official National Flood Insurance Program Flood Insurance Rate maps, or as determined by the City, shall comply with FEMA design requirements.
(3) All drainage channels, conduits, and other structures located outside the road right-of-way designed to convey public storm runoff (off-site runoff) should be contained in suitable public easements. Easements for open channels should include sufficient area along the channel banks to permit access for maintenance equipment. Open channels may be fenced along both sides through urban areas fenced along both sides through urban areas where necessary to protect the public as well as preventing encroachment upon needed access area.
(b) Areas adjacent to open drainageways and ponds shall be graded to preclude the entrance of stormwater except at planned locations. Where retention/detention areas are located on the project periphery, the developer may be required to provide additional landscaping or screening to adequately protect abutting properties.
(c) Each development shall provide for the on-site or off-site detention of excess stormwater runoff resulting from that development. For the purpose of this standard, “excess stormwater runoff” shall include all increases in stormwater peak flows and volume resulting from: an increase in the imprevious surface of the site, including all additions of buildings, roads, and parking lots; changes in soil absorption caused by compaction during development; modifications in contours, including the filling or draining of small depressional areas, alterations of drainageways, or regrading of slopes; destruction of forest; alteration of drainageways or installation of collection systems to intercept street flows or to replace swales or other drainageways; or the alteration of subsurface flows, including any groundwater dewatering or diversion practices such as curtain drains, compared with the site in its natural state.
(d) Any foreseeable increase in rates and volume of site surface drainage water runoff caused by site development shall be controlled so the post-development peak rate of runoff does not exceed that of the predevelopment one-year frequency and the critical storm frequency as determined below. A recommended method which may be used to determine changes in rates and volumes of runoff is presented in the U.S. Department of Agriculture, Engineering Division of Soil Conservation Service (SCS), Urban Hydrology for Small Watersheds, Technical Release No. 55, (Washington, DC: USDA, (January, 1975) June, 1986 and the Ohio Supplement (April, 1981). To find the critical storm frequency for which additional control will be needed:
(1) Determine the percent increase in runoff volume for a one-year frequency, 24-hour storm; and
(2) Determine the critical storm frequency for which additional control is needed by using the percent increase in runoff volume, derived in subsection (a)(1) hereof, in the table below:
% Increase in Runoff Volume from a Critical Storm 1-Year Frequency, 24-Hour Storm Frequency Equal to or Less Than Greater Than % % Years --10 1 10 20 2 20 50 5 50 100 10 100 250 25 250 500 50 500 -- 100 |
(e) (The peak rate of runoff from the critical storm occurring over the development shall not exceed the peak rate of runoff from a 1-year frequency storm occurring over the same area under predevelopment conditions.) Storms of less frequent occurrence (longer return period) than the critical storm, shall have peak under predevelopment conditions. As an example, if the total volume is shown to be increased by 35%, the critical storm is a 5-year storm. The peak rate of runoff for all storms up to this intensity shall be controlled so as not to exceed the peak rate of runoff from a 1-year frequency storm under predevelopment conditions in the area. The runoff from a more intense storm need only be controlled so as not to exceed the predevelopment peak rate from the same frequency of storm.
(f) The intent of subsections (d) and (e) hereof will be met if the criteria are fulfilled for the 1-, 2-, 5-, 25-, 50-, and 100-year storm.
(g) Design regulations: All detention facilities and improvements required by this section shall comply with the following regulations.
(1) An access easement shall be provided to all detention facilities from the nearest public road so that these facilities are accessible in case emergency maintenance work is required. The minimum easement width will be 10 feet.
(2) Storage Volumes: Storage may be provided by (set) wet or dry bottom basins or reservoirs (or rooftop storage facilities).
(3) Maximum Depth: The maximum planned depth of stormwater (stored) detained should be five (5) feet unless site conditions lend themselves to greater depths.
(4) Outlet Control Structures: Outlet Control Structures shall be designed as simply as possible and shall operate automatically. They will be designed to limit discharges into existing or planned downstream channels or conduits so as not to exceed the existing flow from the site in its natural condition or as further controlled in subsection (d) hereof.
(5) Spillway: Emergency overflow facilities shall be provided unless inflow is controlled to divert flows when the basin is at capacity. The spillway shall safely discharge the detention basin overflow without damage to the detention basin. A minimum of one (1) foot freeboard shall be provided for the 100-year 24-hour storm event.
(6) Dry Bottom Basin: For basins designed without permanent pools:
A. Interior Drainage: Provisions must be made to facilitate interior drainage, to include the provision of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, or the installation of subsurface drains.
B. Multipurpose Features: These may be designed to serve secondary purposes for recreation, open space, or other types of use which will not be adversely affected by occasional or intermittent flooding.
C. Cleaning: The basins shall be designed for periodic cleaning and removal of sediments, which shall be removed from the site or otherwise disposed of in an appropriate manner.
(7) Wet Basins: For basins designed with permanent pools:
A. Depth for Fish: If fish are used to help keep the basin clean, at least one-quarter (25%) of the area of the permanent pool must have a minimum depth of ten (10) feet.
B. Facilities for Emptying: For emergency purposes, cleaning, or shoreline maintenance, facilities shall be provided or plans prepared for the use of auxiliary equipment to permit emptying and drainage.
C. Pollution Abatement: Aeration facilities may be required when the quality of the influent and detention time would result in a lowering of dissolved oxygen content in the basin.
D. Slopes: Approach slopes shall be at least 6:1 but not more than 3:1 and shall be at least four (4) to six (6) feet wide and slope gently toward the basin. The side slopes shall be of nonerosive material with a slope of 1:1 or flatter. The ledge shall be four (4) to six (6) feet wide and slope gently toward the shore to prevent people or objects from sliding into deep water. There shall be a freeboard of twelve (12) to eighteen (18) inches above the high-water elevation on all retention basins. Alternate designs for side slopes may be considered under special circumstances where good engineering practice is demonstrated.
E. Cleaning: The basins shall be designed to include sediment traps in all inlets. Sediment traps shall be designed to permit periodic cleaning and maintenance. A basin maintenance plan shall be developed to insure that the design depths of the basin will remain over time.
(8) Building Regulations:
A. Rooftop Storage: The depth and volume of storage, design of outlet devices and downdrains, elevations of overflow scuppers, design loadings for the roof structure, and emergency overflow provisions. (Rooftop storage shall not be permitted to drain directly into sanitary sewers or streets.)
B. Parking Lot Storage: Paved parking lots may be designed to provide temporary detention storage of stormwater on a portion of their surfaces. Outlets shall be designed to empty the stored waters slowly, and depths of storage must be limited so as to prevent damage to parked vehicles, and storage areas shall be posted with warning signs.
C. Detention Storage: All or a portion of the detention storage may also be provided in underground detention facilities, provided they meet the design regulations of subsection (g)(2), (3) and (7)C. hereof.
D. Off-Site easements for stormwater management facilities will be required when either of the following conditions exist:
1. The discharge is into any man-made facility for which the City does not have either a drainage easement or right of way.
2. The discharge is into a natural system such that the rate or character (i.e., sheetflow vs. concentrated flow) of the flow at the property line has been changed. The easement will be required to a point at which natural conditions are duplicated.
E. Design Criteria for Off-Site Drainage: Off-Site areas which drain to or across a site proposed for development must be accommodated in the stormwater management plans for the development. The stormwater management system for the development must be capable of transporting existing off-site flows through or around the development without increasing stages or flows upstream or downstream of the development. The estimation of the off-site flows must be done separately from the estimation of on-site pre- and post- development flows (i.e., separate off-site and on-site hydrographs must be computed due to the typically significant differences in land use characteristics).
F. For sites containing less than ten thousand (10,000) square feet of total impervious surface, alternatives to detention or retention ponds that utilize landscaped buffers and swale drainage as a means of attenuating rate of runoff are encouraged, provided it can be shown that applicable design and performance criteria are met.
G. Measures shall be taken to protect the embankment of detention and retention facilities from erosion and provide a definitive protective flow path downstream of the facility.
(Ord. 26-03. Passed 11-17-03.)