(a) General.
(1) This section applies subdivision development regarding the minimum design Standards to be utilized in designing storm water quantity control systems and related measures. This section also applies to the storm water control measures for single lot commercial, industrial, and multi-family residential developments unless otherwise modified by subsequent sections of this Standard.
(2) The section addresses storm water quantity control measures only as required by these Standards. Storm water quality control is encouraged for each site but is not required. Water quality control measures, if utilized, shall be design in accordance with the ODOT, L&D Manual - Volume II and the ODNR, Manual for Rainwater & Land Development.
(3) The Dam safety laws in the State of Ohio are regulated by the Ohio Department of Natural Resources (ODNR), Soil and Water Conservation Office. Dams are classified in Ohio according to Ohio Administrative Code (OAC) Rule 1501:21-13-01. No earthen embankment or other retention structure may be constructed which qualifies as a Class I, II, or III Dam as defined by the OAC. The designer shall review such regulations to ensure that the proposed detention or retention basin falls outside the category of the ODNR facilities requiring permitting and governance.
(4) It is not the intent of this section or of these Standards to restrict the freedom of the design engineer to the methods listed herein, but these methods are recommended for the purpose of complying with these Standards. Innovation is encouraged when coupled with sound engineer practice.
(5) Proprietary storm water modeling software may be used to perform iterative stage-storage discharge calculations to achieve the required storage volume and allowable release rates.
(b) Quantity Control Design.
(1) Design Frequency and Storage Volume.
A. The Critical Storm Method as described in subsection (b)(4) hereof shall be utilized to determine the storm frequency for the required storage volume and allowable release rates from the quantity control facility. Detention/Retention basins must be designed to limit the critical storm flow out of the basin to the one (1) year pre developed rate and, also have the capacity to store all storm frequencies greater that the critical value up to the 100 year storm under post developed conditions and release the outflows at the pre developed rate for like years.
B. The volume of storage calculated is that needed to reduce the critical storm peak flow rate under post development conditions to equal to or less than the two one (1) year pre development peak flow rate.
C. Storm water quantity control facilities shall be designed to safely pass the 100-year storm frequency.
(2) Requirements.
A. Storm water detention or control measures are not required where the designer can demonstrate that the proposed development will not result in the increase in the storm water runoff volume of greater than 10% to existing storm water conveyance feature receiving the discharge from the site.
B. Storm water detention shall be required when the critical storm is equal to two (2) years or greater as defined by subsection (b)(4) hereof.
C. Each development shall provide for the detention of excess storm water runoff resulting from the development. Excess storm water runoff shall include all additional runoff resulting from increases in the impervious surfaces of the site, including but not limited to, construction of or addition to buildings, construction of roads and parking lots; earthwork, drainage feature modification, elimination of naturalized or farming areas, and other site development related activities.
D. If no detention is required, the storm water runoff velocity leaving the site must be equal to or less than the one (1) year predeveloped storm water runoff velocity.
E. No detention or retention shall be permitted in the public right of way.
F. The designer shall provide a preliminary evaluation regarding the necessity and general magnitude of onsite storm water detention as described in subsection (b)(5) hereof.
G. As-built surveys shall be required and provided by the Owner/Developer/owner in order to demonstrate that the facility was constructed as illustrated on the approved plans or in accordance with field modifications as approved by the Village of Waverly during construction. Such surveys shall be conducted by a registered Professional Surveyor in the State of Ohio. Storm water Control As-built surveys shall be in addition to, and separate from, other required construction surveys and documentation. Any discrepancies revealed in the as-constructed facilities by the as-built survey shall be rectified by the Owner/Developer in accordance with the approved plan.
H. A maintenance plan for the proposed storm water control facility shall be submitted with the design plan for review and approval by the Village. Facility Maintenance requirements are further described the Codified Ordinances of the Village of Waverly, Ohio of this Section.
(3) Runoff Control. The release point is defined as the point at which storm water generated by the subdivision leaves the on-site storm water conveyance or control measures and discharges into an adjoining off-site storm water feature. The subdivision detention/retention basin and/or other storm water management system shall be designed such that the post-development released storm water flow emulates the predeveloped flow volume and characteristics as it is released onto the adjacent property for the 1, 2, 5, 10, 25, 50, and 100-year storm events. No adverse effects to off-site receiving storm water facilities shall occur as a result of the subdivision constructed.
(4) Critical Storm Method.
A. The Critical Storm value for a particular project or development site provides the design engineer with a critical storm frequency that reflects the changes in land surface that occurs to a particular project area after development.
B. Critical storm calculations shall utilize onsite drainage areas, only. The offsite areas are not be used in the calculation of the Critical Storm. The offsite areas are to be used in the determination of the required storage volume of the control basin.
C. Critical Storm Determination. The critical storm for a specific development area is determined as follows.
1. Calculate the total volume of runoff from a 1-year, 24 hour storm event, occurring on the development area for the predevelopment and post-development site conditions utilizing the Rational Method or SCS-TR55 as detailed previously. The rainfall depth utilized to calculate the 1-year, 24-hour storm as well as the 5, 10, 25, 50 and 100 year storm events is based upon information provided by the NOAA, Atlas 14, Volume 2, Version 3.0 - V 2004, June 10, 2012 located at http://dipper.nws.noaa.gov/hdsc/pfds/ for the Village of Waverly coordinates (39.1025 N, 82.9719 W), elevation 567-feet is as follows:
Rainfall Depths (inches)
• 1-year, 24 hour storm = 2.25 inches
• 2-year, 24 hour storm = 2.69 inches
• 5-year, 24 hour storm = 3.30 inches
• 10-year, 24 hour storm = 3.79 inches
• 25-year, 24 hour storm = 4.47 inches
• 50-year, 24 hour storm = 5.02 inches
• 100-year, 24 hour storm = 5.59 inches
2. The runoff volume calculated for use in determining the Critical Storm shall pertain to on-site drainage areas only. However, any off-site drainage areas that are tributary to the site shall be used in designing the storm water conveyance, control systems, and other drainage improvements for the subdivision or land development site.
3. Calculate the percent of increase in runoff volume due to development.
(Post V -Pre V)/(Pre V)* 100 = % Increase in Runoff Volume
4. Using this percentage, select the critical storm from Table 4 below:
Table 4. Critical Storm Determination
Percent Increase in Runoff Volume | Critical Storm for Peak Rate Control | |
Equal to or Greater Than | And Less Than | |
0 | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | -- | 100 year |
Adapted from the ODNR, Soil and Water Conservation Office - Rainwater and Land Development Manual, Third Edition 2006.
D. Critical Storm Controls. The Critical Storm Frequency as determined above shall be utilized to control the peak runoff rates from storm water control facilities.
1. The peak rate of runoff from the Critical Storm and all more frequent storms (less return period) occurring on the development or project area shall not exceed the peak rate of runoff from a 1-year, 24-hour storm event that would occur under predevelopment conditions. Storms of less frequent occurrence (greater return periods) than the critical storm up to the 100- year, 24-hour storm event shall have peak runoff rates no greater than the peak runoff rates from equivalent size storms under predevelopment conditions.
For example, if the critical storm was calculated to be a 10-year, 24-hour storm, the peak rate of storm water runoff that would occur from the 2, 5, and 10 year postdevelopment storms could not exceed the peak rate of runoff that which would occur from 1-year frequency storm under predevelopment conditions.
2. Storms of less frequency occurrence than the critical storm up to the 100 year, 24 hour storm shall have peak rates of runoff equal to or less than the peak rates of runoff for the same frequency of storms under predevelopment conditions.
3. A 1-year, 24 hour critical storm value does not require detention. However, the post development outlet velocities of the storm water system must be equal to or less than the 1-year, 24 hour predevelopment outlet velocities.
(5) Preliminary Storage Estimate.
A. An estimate of the preliminary detention area shall be provided and shown on preliminary plans submitted for review. This estimate shall also aid in determination of the most appropriate storm water control means and allocating an area on the site.
B. An estimate of storm water storage required may be obtained utilizing SCS-TR55 graphic methods. An estimation of the storage volume required may also be obtained by calculating the area on the inflow hydrograph from the point where significant runoff begins to occur, to the peak flow rate, and then to the target discharge control rate from the control structure. The area contained within the limits of the inflow hydrograph as generally defined above, provides an estimate of the storage volume required.
C. Subsurface soil investigations shall occur in the proposed basin location(s) to determine if the site is suitable and to select the most appropriate means of storm water control for the site and overall project conditions.
D. Additionally, careful consideration shall be given to the site selection for the control basin to provide the best combination of storage requirements, site landscape and aesthetics, access to outlet and embankment slopes for maintenance, and to minimize earth moving among other site specific considerations.
(6) Control Structure Design.
A. Types. Multiple options are provided for achieving the required storm water quantity control. The types listed below are acceptable control facilities. The selection of the storm water control facility type shall be dependent upon the existing site conditions present. Each site varies in its design constraints and, as with other site design elements, the storm water control facility shall be selected and designed accordingly.
Subdivisions and Commercial
(a) Detention Basin
(b) Retention Basin
Primarily Commercial Applications
(c) Parking Lot Storage
(d) Underground Storage
Alternative Methods
(e) Infiltration Methods
(f) Extended Detention Basin
(g) Bioswale
(h) Bioretention Basin
(i) Other as proposed and supported by the Design Engineer and as approved by the Village
B. Design Parameters. The general design parameters associated with each type are provided herein. The intent of this information is not to be exhaustive nor a step by step procedure to design of these facilities. Rather, key design parameters or guidelines are provided to aid the designer.
1. Detention Basin - Specific to Detention Basins Only
a. An earthen dry storage depression created by typical excavated embankment construction methods with no normal pool level. Detention basins serve to capture and temporarily store the surface water runoffs which result from urban development. This temporary storage allows for the release of the storm runoff at discharge rates which are acceptable to the receiving waterway.
b. Detention basins are designed to limit the critical storm flow out of the basin to the 1 year, 24 hour predeveloped rate and to have the capacity to store all storm frequencies greater that the critical value up to the 100 year, 24 hour storm (where required) under post developed conditions and release the outflows at the predeveloped rate for like years.
c. The bottom of the basin should be constructed with slopes equal to or greater than 1.0% to facilitate interior drainage.
d. All inlets to the basin shall be connected to the outlet by a concrete low flow channel. The minimum slope on this channel shall be 0.5 percent.
2. Retention Basin - Specific to Retention Basins Only
a. Retention Basins are permanent ponds where additional storage capacity is provided above the normal water level and special features for controlled release are included. Mechanical aeration devices may be used and are encouraged for aesthetic purposes and/or to help prevent water stagnation and algae growth.
b. Carefully examine the tributary drainage basin to ensure that adequate storm water runoff will exist to sustain the normal pool level of the basin throughout the year. A drainage area to basin water surface area of approximately 6:1 should be provided.
c. Retention basin bottom slope profile shall be graded to provide the following:
- Minimum berm 3-feet wide at 2% slope between the right -of-way and the top of the basin.
- A 3:1 slope down to the commencement of a safety bench.
- A safety bench 5-feet wide minimum at no greater than 2% slope to the commencement of the edge of the impoundment area.
- The side slope of the storage area shall be least 3H: 1V minimum.
- The average pond depth in the deep pool shall be a minimum of 4-feet and a maximum depth of 8 feet.
- A minimum of 1-feet of freeboard shall be provided from the maximum water surface elevation and the top edge of the storage area.
d. Rock channel protection, type D, or turf reinforcement mat (TRM) as approved by the Village shall be placed at the normal water elevation, around the entire perimeter of the basin, five feet wide, centered on the normal water elevation.
3. Detention and Retention Basins - Applicable to both Detention and Retention Basins.
a. The length to width ratio of the basin shall generally be 3:1.
b. The inlet piping to the basin shall generally be located on the opposite side from the discharge structure to prevent short circuiting of inflow.
c. Levee embankments shall be no steeper than 3H: 1V slopes including both sides of the levee.
d. When conduits are used for the outlet of the basin they shall be protected by bar screens or other suitable provisions to prevent blockage of the outlet.
e. The minimum outlet conduit shall be 1-feet in diameter.
f. Safety measures shall be provided for any pipe or opening to prevent children or large animals from crawling into structures. For safety, a suggested maximum opening is six inches for the safety grate or other safety measure proposed.
g. Danger signs should be mounted at appropriate locations to warn of deep water, possible flood conditions that exist during storm periods and other dangers that exist.
h. Grass or other suitable vegetative cover shall be established on all slopes of the basin.
i. An emergency spillway shall be provided. The spillway shall be capable of handling the peak discharge and peak velocity from the 100-year, 24-hour or larger storm event under post-development conditions. There shall be a minimum of 1-feet of freeboard between the 100 year storm elevations and the top of the levee or embankment containing the basin.
j. Provide spillway crest elevations. The spillway area (plan view), cross section detail, and other spillway details shall be located on the site plan or accompanying construction drawings including water surface elevations.
k. All outlet (release) structure details must be shown on the site plan or accompanying construction drawings. Include the following, but not limited to, pipe and orifice size, invert elevation, weir length, and elevation; Provide window sizes, elevations, and locations.
l. Anti-seep collars shall be installed on all pipe outlets which pass through the basin embankment.
m. Since these Standards require that the outflow rate from control basin is held to a 1 year, 24-hour predevelopment rate and the control basin must also be designed to detain the expected runoff from a 100 year, 24 hour post development condition, a two stage orifice control may be required on most basins.
4. Parking Lot Detention.
a. Parking Lot Detention is primarily applicable to single lot commercial, industrial and multi-family developments. This type of storm water control means for subdivision development must be approved by the Village for proper application.
b. See Section 1178.05(c)(2) for details and requirements regarding this type of storm water control measure.
5. Other Alternative Methods. Other methods of providing storm water quantity control inherently also provide water quality control. Should these methods be selected for consideration, coordination shall be made with the Village regarding the applicability to the proposed site. Generally, design Standards for these methods shall conform to the ODOT, L&D - Volume II and the ODNR Manual for Rainwater and Land Development, Second Edition.
(c) Commercial Site Storm System Design.
(1) General.
A. The system shall be designed to convey the flows from the tributary area within the proposed development and existing flows from offsite areas that are upstream from the development which may be interrupted by the proposed development.
B. The parking lots shall be drained by means of catch basins and storm sewers to an adequate outlet or quantity control structure.
C. The parking lot shall be graded such that the surface water runs off to the catch basin or basins in a low area to permit detention of the runoff and the use of parking lot storage where applicable.
D. All roof top areas and/or building storm water drainage systems shall drain to the parking lot system for controlling storm water rate of runoff prior to its release to downstream properties.
E. The catch basins must pick up the water on the paved surface. Sheet flow will not be permitted into the public right-of-way for collection by street storm water conveyance systems.
F. The desired maximum distance for overland flow should be approximately 250 to 300 feet before entering a storm structure.
G. The desired maximum overland drainage area tributary to the storm structure should be no greater than 1.5 acres as permitted by storm sewer system hydraulics.
(2) Design Criteria.
A. Storm Sewers. Private storm sewers; such as in single lot multi-family residential, commercial, or industrial development, shall be designed and constructed with the capacity to adequately handle the 2-year, 24 hour storm event at full capacity and nonsurcharging. A 5-year, 24 hour storm event shall be used to develop the hydraulic gradient which shall not exceed the window or grate elevation of the storm structure or exceed the gutter/curb line of the roadway or parking area.
B. Open Channels. Ditches shall be designed and constructed with the capacity to adequately handle the discharge expected from a 10-year, 24 hour storm event. Shear stress protection shall be constructed with respect to the velocity from a 2-year frequency (post development).
C. Quantity Control. If the site (or a portion of the site) post-development runoff volume is calculated to be greater than the predevelopment runoff volume from the same area; the runoff volume, and thereby flow rate, shall be offset by employing the Critical Storm Method as described previously. The detention or ponding area shall be designed as per Section 1178.05 of the Standards.
1. Types. Multiple options are provided for achieving the required storm water quantity control. Refer to Section 1178.05(b)(6), for a complete description and design details associated with Detention or Retention Basins. See also this same section for the other alternate methods as listed and their respective design reference documents.
Acceptable Commercial
a. Detention Basin
b. Retention Basin
c. Parking Lot Storage
d. Underground Storage
Alternative Methods - Infiltration Trench
a. Infiltration Basin
b. Extended Detention Basin
The selection of the storm water control facility type shall be dependent upon the existing site conditions present. Each site varies in its design constraints and, as with other site design elements, the storm water control facility shall be selected and designed accordingly.
E. Parking Lot Detention.
1. Surface storage is achieved through shallow ponding designed to flood specific graded areas of the parking lot.
2. Controlled release features are incorporated into the surface drainage system of the parking lot via proper design discharge pipe size and slope in the receiving inlet and increased curb heights. Other control features not listed may be employed as approved by the Village.
3. Special attention must be paid to the geometric design, surface slope, and inlet elevation of the parking lot for Parking Lot Detention to be properly employed.
4. Ponding areas in parking or traffic areas shall be designed for a maximum potential depth of 8-inches with flood/overflow routing to an appropriate receiving storm water conveyance system after the maximum storage depth is reached.
5. This method is intended to control the runoff directly from the parking area, and therefore, typically not large enough to serve as a primary detention system or for storing large runoff volumes.
6. Clogging of the flow control device and icy conditions during cold weather must be maintained for proper operation of this system.
F. Underground Detention/Retention.
1. Underground retention/detention achieves the capture and temporary storage of storm water collected from the tributary drainage area. Curb inlets or surface drains lead storm water to underground vaults or systems of large diameter interconnected storage pipes. The storm water is then released directly through an outlet pipe back into a storm water drainage system. The outlet system is designed to meet the quantity control requirements.
2. Underground retention/detention may be useful for developments where land availability and land costs predicate against the development of surface storm water control measures and in retrofit and redevelopment settings.
3. Pretreatment is strongly recommended for minimizing maintenance of the storage unit and should be designed to remove sediment, floatables, and oils if prevalent in the drainage area.
4. Where an opening is provided that could allow the entry of personnel, the opening shall be marked, “DANGER CONFINED SPACE”.
(Ord. 65-2015. Passed 11-3-15.)