Methods for controlling increases in storm water runoff peaks and volumes may include, but are not limited to permanent retention ponds and lakes, dry detention basins, and subsurface detention systems.
(a) Storm Water Basins.
(1) Pool geometry. The recommended length-to-width ratio for the pond is 3:1 (the length will be three (3) times the width).
(2) Riser in embankment. The riser shall be located within the embankment for purposes of maintenance access. Access to the riser will be by manholes.
(3) Water drains. Each retention basin shall have a drainpipe that can completely drain the pond. The drain shall have an elbow within the pond to prevent sediment deposits from plugging the drain.
(4) Principal spillway. Each principal spillway shall be designed in accordance with the NRCS standards and specifications for the office serving the county. Each principal spillway shall have the capacity to pass the 100-year design storm flow. The inlet or riser size for the pipe drops shall be designed so that the flow through the structure goes from weir flow control to pipe flow control without going into orifice control in the riser. The crest elevation of the primary spillway shall be no less than one foot below the emergency spillway crest. Premium joint pipe is required and a removable trash rack shall be installed at each location. Anti-seep collars shall be provided for all pipe conduits through an embankment.
(6) Emergency spillway. An emergency spillway shall be provided on each storm water management basin. Emergency spillways shall convey flood flows safely past the embankment, and shall be designed in accordance with NRCS standards and specifications for the office serving the local county. Emergency spillways shall have a 100-year design storm capacity unless exempted in writing by the City Engineer.
(b) Embankments. Each dam embankment shall be designed in accordance with the standards and specifications in the current edition of the Ohio Rainwater and Land Development manual, the NRCS Field Office Technical Guide for Sandusky County, and the Ohio EPA standards, whichever are more stringent. Anti-seep collars shall be proved for all pipe conduits through an embankment.
(1) Safety features.
A. The primary spillway opening shall not permit access to the public and other non-maintenance personnel.
B. The perimeter of all water pool areas that are deeper than three (3) feet shall be surrounded by benches that meet the following:
1. A safety bench, with a maximum slope of three percent (3%), which extends outward, on dry land, from the shoreline. This bench will be a minimum of twenty-five (25) feet wide to provide for the safety of individuals and maintenance vehicles that are adjacent to the water pool. The safety bench may be landscaped, without the use of structures, to prevent access to the water pool.
2. Side slopes between the safety bench and the aquatic bench shall not be steeper than 3:1 (3 feet horizontal for every 1 foot vertical).
3. An aquatic bench that extends inward from the shoreline far enough to ensure public safety and has a maximum depth of fifteen (15) inches below the normal water surface elevations. The aquatic bench may be landscaped to prevent access to the deeper water pool.
4. Side slopes beyond the aquatic bench and below the permanent water level shall not be steeper than 2:1 (2 feet horizontal for every 1 foot vertical).
5. The contours of the pond will be designed and managed to eliminate drop-offs and other hazards.
6. Side slopes getting to the pond shall not exceed 3:1 and shall terminate on a safety bench.
(c) Runoff Rate. The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations demonstrating no increases in the runoff rates from the one (1), two (2), five (5), ten (10), twenty-five (25), fifty (50) and one hundred (100) year storms.
(d) Runoff Volume. Increases in the runoff volume shall be offset by further restricting runoff rates. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the one (1) year pre-development storm runoff rate. The critical storm shall be calculated as follows:
(1) Determine the total volume of runoff from a one-year frequency, twenty-four hour storm, occurring on the development area before and after development.
(2) From the volumes in paragraph (1) determine the percent of increase in volume of runoff due to development (2) using this percentage, select the critical storm from this table:
Table 4: Critical Storm Selection | ||
The Percentage Increase In volume of Runoff is: | ||
Equal To or Greater Than | And Less Than | The 24-Hour “Critical Storm” For Discharge Will Be |
0 | 10 | 1 Year |
10 | 20 | 2 Years |
20 | 50 | 5 Years |
50 | 100 | 10 Years |
100 | 250 | 25 Years |
250 | 500 | 50 Years |
500 | --- | 100 Years |
(e) Water Quality. To meet the post-construction requirements of this Chapter, the Storm Water Management and Erosion and Sediment Control Plan must contain a description of the Post-Construction Best Management Practices (BMPs) that will be installed during construction for the site and the rationale for their selection. The rationale must address the anticipated impacts on the channel and floodplain morphology, hydrology, and water quality.
(1) Structural BMPs. Structural (designed) Post-Construction storm water treatment practices shall be incorporated into the permanent drainage system for the site.
A. Properly Sized BMPs. The BMP(s) chosen must be sized to treat the water quality volume (WQv) and ensure compliance with Ohio's Water Quality Standards in OAC Chapter 3745-1. The WQv shall be equivalent to the volume of runoff from a 0.75-inch rainfall and shall be determined according to one of the two following methods:
1. Through a site hydrologic study approved by the City Engineer authority that uses continuous hydrologic simulation and local long-term hourly precipitation records; or
2. Using the following equation:
WQv = C * P * A/12 where:
WQv = water quality volume in acre-feet
C = runoff coefficient appropriate for storms less than
1 inch (see Table 5)
P = 0.75 inch precipitation depth
A = area draining into the BMP in acres.
Table 5: Runoff Coefficients Based on the Type of Land Use | |
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Residential (>8 dwellings/acre) | 0.5 |
Medium Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
B. Where the land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if sixty percent (60%) of the contributing drainage area to the storm water treatment structure is Low Density Residential, thirty percent (30%) is High Density Residential, and ten percent (10%) is Open Space, the runoff coefficient is calculated as follows (0.6)(0.3) + (0.3)(0.5) + (0.1)(0.2) = 0.35.
C. An additional volume equal to twenty percent (20%) of the WQv shall be incorporated into the BMP for sediment storage and/or reduced infiltration capacity. The BMPs will be designed according to the methodology included in the Ohio Rainwater and Land Development manual, ODOT Post-Construction storm water standards, or other manual that is acceptable to Ohio EPA.
D. BMPs shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage available for successive rainfall events as described in Table 6 below:
Table 6: Target Draw Down (Drain) Times for Structural Post-Construction Treatment Control Practices | |
Best Management Practice | Drain Time of WQv |
Infiltration | 24 - 48 Hours |
Vegetated Swale and Filter Strip | 24 Hours |
Extended Detention Basin (Dry Basins) | 48 Hours |
Retention Basins (West Basins)* | 24 Hours |
Constructed Wetlands (above permanent pool) | 24 Hours |
Media Filtration, Bioretention | 40 Hours |
*Provide both a permanent pool and an extended detention volume above the permanent pool, each sized at 0.75 * WQv
E. The owner may request approval from the City Engineer to utilize alternative structural Post-Construction BMPs if the owner can demonstrate, in a way that is consistent with Ohio EPA rules and regulations that the alternative BMPs are equivalent in effectiveness to those listed in Table 6 above. The use of alternative or vendor supplied Post-Construction BMPs should be limited to redevelopment projects where justification is provided that the traditional BMPs in Table 6 are technically and economically unfeasible.
F. Construction activities shall be exempt from this condition if it can be demonstrated that the WQv is provided within an existing structural Post-Construction BMP which is part of a larger common plan of development or sale or if structural Post-Construction BMPs are addressed in a regional or local storm water management plan.
G. For redevelopment projects (namely, developments on previously developed property), Post-Construction practices shall either ensure a twenty percent (20%) net reduction of the site impervious area, provide for treatment of at least twenty percent (20%) of the WQv, or a combination of the two.
(2) Water quality basin. If a Water Quality Basin is needed and cannot be incorporated into an existing or planned Detention or Retention Basin then a separate Water Quality Basin will need to be planned, designed, constructed and maintained into perpetuity.
(f) The requirements shall be satisfied at each location where runoff leaves the development area. The runoff rates and volumes shall be considered for both the conditions before and after development at these locations.
(g) To ensure the continued functioning of storm water control structures, the following information shall be identified to the satisfaction of the City Engineer.
(1) The person or entity responsible for continued maintenance of the storm water control structure shall be identified to the satisfaction of the City Engineer.
(2) Maintenance requirements and schedules;
(3) Permanent access easements required for those conducting maintenance to perform inspection and maintenance of storm water control structures and storm water conveyance systems. The minimum size of an easement shall be approved by the City Engineer. Within such easements, no person shall plant trees, shrubbery, plantings, or construct buildings, fences, or walls that obstruct free flow of storm water or obstruct the movement of construction equipment.
(4) An owner or owners of two (2) or more existing or proposed lots may apply for approval of a SWM plan serving more than one lot. Such plan shall comply with applicable provisions of this chapter and all other relevant laws and ordinances. No subdivision of lands or building permit shall be approved under any multiple lot plan until owners of all affected lots have entered into a mutual agreement or formed an association for joint maintenance of the SWM facility and until all easements needed to effectuate the plan have been granted. The mutual agreement or association and the easements must be signed, acknowledged, delivered and recorded in the manner for documents relating to real estate titles.
(h) Detention or retention basin exemption for development or for expansion of existing facilities:
(1) For any development regulated by this chapter, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for the one (1) year through 100 year frequency twenty-four (24) hour storm causes no increase to the existing peak discharge using the TR-55 method of calculation or other method approved by the City Engineer. Even if the determination is made that a detention or retention basin is not required, the development still must comply with BMPs to address pre and post development storm water quality control such as installation of mechanical cleaning devices to capture sediments as approved by the City Engineer.
(Ord. 2021-4065. Passed 5-6-21.)