(a) General.
(1) The hydraulics and hydrology utilized to analyze and design storm water management systems and related appurtenances shall be in accordance with the guidelines and regulations as contained in the most current edition of the ODOT, L&D, Volume II.
(2) A Storm water Design Report shall be prepared and submitted to the Village containing the design calculations and illustrations for the proposed development. This report shall include, but is not limited to, calculations pertaining to the pre and post development watershed basins and sub-basins, hydrologic and hydraulic systems, structures, culverts & pipelines, control structures, erosion & sediment control, water quantity and quality (as applicable) control facilities, and any other information/calculations pertinent to storm water runoff and system design.
(3) The format of the Storm water Design Report shall be at the discretion of the design engineer in so much as it is in a logical, neat and an orderly manner. The Village reserves the right to request additional documentation not included in the design report as submitted and as it deems necessary.
(b) Applicability. This section applies subdivision development regarding the design methodologies to be utilized in computing storm water runoff and flows associated with the existing and proposed surface and subsurface characteristics of the site, and the hydrologic and hydraulic analysis necessary. This section also applies to the storm water design of single lot commercial, industrial, and multi-family residential developments unless otherwise modified by subsequent sections of this Standard.
(c) Drainage Area Properties.
(1) Existing Conditions. The design of storm water management infrastructure including open ditches, channels, storm sewers, culverts, bridges, water quantity/quality control structures, or other facilities begins with examination and identification of the existing physical conditions and properties of the area to be developed and adjoining area which may be tributary to the watershed or a part of the off-site portion of the watershed. This examination and site reconnaissance shall include:
A. Flood Boundaries.
1. The estimated 100-year and 500-year flood plain limits and elevations in or near the watershed including the estimated flood way location, shall be shown on the existing site plan. The designer shall be cognizant of these flood limits in site design and layout.
2. Should the project limits fall within or adjoin any of the flood plains or the floodway as referenced above, a permit shall be obtained from the local Flood Plain Manager for the Village, located in the Mayor’s Office in full compliance to the Flood Plain Regulations.
3. Flood boundary information is to be obtained from the most recent Federal Emergency Management Agency (FEMA), Flood Insurance Rate Map (FIRM) for the subject area.
4. Additionally, contact is to be made with the United States Army Corps of Engineer (USACE), Huntington District, regarding Flood Profiles for the waterway in question for any flood study data which may be available.
B. Topography. A land survey which provides data points and contour lines of like elevations is to be prepared and utilized for delineation of the existing watershed, drainage basins, and subbasins. For preliminary site analysis or off-site drainage areas which may contribute to the storm drainage of the project, aerial photography and topographic mapping is available through the Ohio Geographically Referenced Information Project (OGRIP) http://ogrip.oit.ohio.gov/.
C. Subsurface Conditions. Utilizing a USDA, NRCS Soils Report for Pike County, Ohio the soil types located within the watershed are illustrated to be utilized in development of the existing and proposed storm water runoff flow rates for the site. This information may also be used to aid in design of water quality features utilizing subsurface infiltration or basins dependent upon impermeable soils as a natural bottom liner. Custom soils survey reports and maps may be created from the USDA, NRCS at http://websoilsurvey.nrcs.usda.gov/app/.
D. Natural Features. The location and/or limits of other natural features shall be noted which may impact the site design such as the presents of wetlands, stands of mature trees, or wild life habitat area. Wetlands disturbance shall not be permitted without a wetland delineation by an environmental scientist trained in USACE procedures and permitted. A 401/404 permit from the USACE shall also be required prior to any disturbance. It is strongly recommended that should hydrophilic flora, fauna, and/or boggy depressions are discovered; that the area is entirely avoided by the proposed development and no additional storm water runoff is directed toward this natural feature.
(2) Water Shed Delineation.
A. Existing (Predevelopment). - Given the topographic mapping and other existing physical features, a delineation of the existing watershed shall be made including the drainage basins and sub-basins as appropriate.
B. Proposed (Post Development). - The Post-Development watershed, drainage basins, and sub-basins are developed in a matter similar to the Predevelopment, however, the proposed site grading plan and initial approximations of the storm water infrastructure shall occur.
(d) Hydrologic and Hydraulic Conditions.
(1) Description. Many different methods are available to calculate storm water flows. These methods range from simple calculations to complex computer modeling. However, all methods are based upon the application of metrological, hydrologic, and hydraulic principles to compute the estimated storm water discharge rate for the catchment being studied.
The methods listed below are acceptable unless approved otherwise by the Village. The Owner/Developer has the option to utilize either of the methods within the parameters of sound engineering practice and based upon the specific application. The Village reserves the right to request calculations utilizing the method not selected as a check of the results calculated, as they deem appropriate.
A. Hydrologic Methods
Rational
SCS, TR-55
Other (as approved)
(2) Rational Method.
A. Land Type. This method is recommended for small, primarily existing urban areas, with a relatively high percentage of impervious surfaces. The total water shed area, including offsite drainage, shall be less than 200 acres. Under such conditions, the Rational Method typically produces relatively accurate storm water runoff results.
This method determines the peak rate of runoff for all design frequencies selected. The Rational Method calculates flow as follows:
Q=CiA; Where:
Q = Peak rate of runoff, (cfs)
C = Coefficient of runoff
I = Intensity of rainfall for the selected storm frequency, duration
(in/hr)
A = Drainage area, (acres)
B. Coefficient of Runoff, C. This is a dimension less decimal value that estimates the percentage of rainfall that becomes runoff. The C value can be considered as a lump sum parameter that accounts for abstractions, antecedent moisture conditions, and other variables affecting the runoff rate.
See the Table 1 for a range of values for the various contributing surfaces which are acceptable.
Table 1. Common Runoff Coefficients, C
Type of Drainage Area | Runoff Coefficient, C |
Business/Commercial: | |
Downtown areas | 0.70 - 0.95 |
Neighborhood areas | 0.50 - 0.70 |
Residential: | |
Single Family areas | 0.30 - 0.50 |
Multi-units, detached | 0.40 - 0.60 |
Multi-units, attached | 0.60 - 0.75 |
Residential (suburban) | 0.25 - 0.40 |
Apartment Dwellings | 0.50 - 0.70 |
Industrial: | |
Light areas | 0.50 - 0.80 |
Heavy areas | 0.60 - 0.90 |
Park, cemeteries | 0.10 - 0.25 |
Playgrounds | 0.20 - 0.35 |
Railroad-yard areas | 0.20 - 0.40 |
Unimproved areas | 0.10 - 0.30 |
Streets: | |
Asphaltic | 0.70 - 0.95 |
Concrete | 0.80 - 0.85 |
Brick | 0.70 - 0.85 |
Roofs, Drives and Walks | 0.75 - 0.85 |
Lawns: sandy soil | |
Flat, 2% slope | 0.05 - 0.10 |
Average, 2-7% slope | 0.10 - 0.15 |
Steep, 7% slope | 0.15 - 0.20 |
Lawns, heavy soil | |
Slat 2% slope | 0.13 -0.17 |
Average, 2-7% slope | 0.18 - 0.22 |
Steep, 7% slope | 0.25 - 0.35 |
Adapted from the Standard Handbook for Civil Engineer - Fourth Edition, 1996
When a single catchment area consists of several areas with different C coefficients, a weighed coefficient shall be computed and represented as follows:
Cw = 
(CA) / Area (TOTAL)
(CA) / Area (TOTAL)Since the rational formula assumes as constant uniform rainfall for the time of concentration over the entire area, the area (A) to which the Runoff Coefficient (C) is applied is to be selected as precisely as possible.
C. Time of Concentration. The time of concentration is the estimated time that it takes runoff to travel from the hydraulically most distant point of the watershed to a point of concentration such as a culvert, catch basin, or ditch checkpoint. The time of concentration is computed by the summation of the runoff time of travel across the various components of the watershed. Runoff is delivered to the impoundment or discharge area by one of three methods: overland flow, shallow concentrated flow, and open channel flow or a combination of these three.
D. Overland Flow. Overland or sheet flow is flow over a plan surface. It usually occurs in the headwater of streams and is generally restricted to no greater than 100-feet. Sheet flow travel time is computed by the following Manning’s kinematic equation.
TO = 1.8(1.1-C)(L) 0.5) / (S 0.333)
Where:
TO = travel time of overland flow, (minutes)
C = Coefficient of Runoff
L = Distance to most remote location in drainage area, (ft) (300-ft Max)
S = Overland slope (%)
Note that these methods shall not be used to determine time of travel for gutter, swale, or ditch flow.
E. Shallow Concentrated Flow. After a maximum of 100-ft of flow, overland flow generally becomes shallow concentrated flow. The velocity of shallow concentrated flow can be estimated using the following relationship:
TS = L / 60V
Where:
TS = travel time for shallow concentrated flow, (minutes)
L = Length of Flow (ft)
V = Velocity (ft/s) - (from equation below)
The velocity for shallow concentrated flow is computed by:
V = 3.281ks(0.5)
Where:
V = Velocity (fps)
k = Intercept coefficient - See Table 2 below
s = overland slope (%)
Table 2. Intercept Coefficients, C
Type of Surface | Intercept Coefficient, k |
Forest with heavy ground litter | 0.076 |
Minimum tillage cultivated, woodland | 0.152 |
Short grass pasture | 0.213 |
Cultivated straight row | 0.274 |
Poor grass, untilled | 0.305 |
Grassed waterways | 0.457 |
Unpaved area; bare soil | 0.491 |
Paved area | 0.619 |
Adapted from the ODOT, L&D Manual, Volume II - Table 1101-1
Shallow concentrated flow generally terminates at a defined carrier such as a ditch, channel, or pipe system.
F. Channelized / Piped Flow. The velocity in open channel flow or piping system is computed by the Manning’s equation as:
V = (1.486 r 0.67 s 0.5)/n
Where:
V = average velocity (ft/s)
n = Manning’s roughness coefficient for open channel flow or pipe
r = hydraulic radius (ft)
s = slope of hydraulic grade line or channel slope (ft/ft)
Manning roughness coefficient, n, represents frictional losses due to the physical character of the channel bottom or conduit material type. Manning’s n value is obtained from various technical resources.
Channelized/Pipe Flow Travel Time
Td = L / 60V
Where:
Td = travel time for channel/pipe flow, (minutes)
L = Length of Flow (ft)
V = Velocity (ft/s) - V (from equation above)
The time of concentration is the summation of the travel times for the various land surfaces described. This process is reiterated for each sub-basin within the catchment to determine the time of concentration for the catchment. The time of concentration is also equivalent to the duration of the storm.
G. Rainfall Intensity. Knowing the time of concentration (duration) of the runoff, the intensity of the rainfall is determined utilizing Rainfall Intensity-Duration-Frequency (IDF) curves. See Appendix C for IDF Curves from the ODOT, L&D, Volume II. Utilizing the duration for the various year storm frequency, the intensity (in/hour) is determined.
(3) Soil Conservation Service, Technical Release No. 55.
A. “Urban Hydrology for Small Watersheds”, the SCS-TR55 program is intended for examination of drainage areas which are more rural in character but urbanizing. The methods of SCS TR-55 shall be used to determine the peak rate of runoff in such areas. Additionally, when utilized for design for drainage systems, including offsite drainage, greater than 200 acres the SCS-TR55 method generally produces more accurate results than the Rational Method.
B. Given the advent of computerized models, the SCS-TR55 Method is available in a windows based computer model, WinTR-55 for calculation of runoff and flow rate given the input parameters associated with the location of the site and the hydrologic properties of the site and properties regarding the components used to calculate the time of concentration.
For the purposes of SCS TR-55 or WinTR-55, the following information shall be prepared and ready for input:
* Units = English
* Area = Square acres
* Land use category - site dependent
* Soil Hydrologic Properties - (From USDA, NRCS Soil
* Survey)
* Enter flow path dimensions:
- Sheet flow = 100-ft (maximum)
- Shallow concentrated flow
- Channelized flow
- Structure Type & Size (as applicable)
- Outlet
* Rainfall shall be based upon SCS Type II Rainfall Distribution
C. Utilizing this program, the time of concentration (Tc) is calculated for the sub-basins indicated in place of the manual calculations as indicated for the Rational Method. The Tc shall not be less than 10-minutes. This program shall be executed for both the pre and post development conditions.
(4) Alternate Hydrologic Methods. Alternate hydrologic methods, utilizing one or more of the principles described above may be used in conjunction with proprietary computer models/programs to determine peak flows with the approval of the Village. The design engineer shall submit all documentation necessary for the review and approval of alternate methods as requested.
(Ord. 65-2015. Passed 11-3-15.)
(Ord. 65-2015. Passed 11-3-15.)