(a) General. The storm water system, including storm water management practices for storage, treatment and control, and conveyance facilities, shall be designed to prevent structure flooding during the 100-year, 24-hour storm event; to maintain predevelopment runoff patterns, flows, and volumes; and to meet the following criteria.
(1) Integrated practices that address degradation of water resources. The storm water management practices shall function as an integrated system that controls flooding and minimizes the degradation of the physical, biological and chemical integrity of the water resources receiving storm water discharges from the site. Acceptable practices shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project and complies with Chapter 1336, Riparian Setbacks;
B. Maintain predevelopment hydrology and groundwater recharge on as much of the site as practicable;
C. Only install new impervious surfaces and compact soils where necessary to support the future land use; and
D. Compensate for increased runoff volumes caused by new impervious surfaces and soil compaction by reducing storm water peak flows to less than predevelopment levels. Storm water management practices that meet the criteria in this regulation, and additional criteria required by the City Engineer shall comply with this regulation.
(2) Practices designed for final use. Storm water management practices shall be designed to achieve the storm water management objectives of this regulation, to be compatible with the proposed post-construction use of the site, to protect the public health, safety and welfare, and to function safely with minimal maintenance.
(3) Storm water management for all lots. Areas developed for a subdivision, as defined in Title Threes Subdivision Control, shall provide storm water management for the development of all subdivided lots. This shall include provisions for lot grading and drainage that prevent structure flooding during the 100-year, 24-hour storm; and maintain, to the extent practicable, the pre-development runoff patterns, volumes and peaks from the lot.
(4) Storm water facilities in water resources. Storm water management practices and related activities shall not be constructed in water resources unless the applicant shows proof of compliance with all appropriate permits from the state’s EPA, the U.S. Army Corps and other applicable federal, state and local agencies as required in § 1334.07 of this chapter, and the activity is in compliance with Chapter 1335 Erosion and Sediment Control, and Chapter 1336 Riparian Setbacks, all as determined by the City Engineer.
(5) Storm water ponds and surface conveyance channels. All storm water pond and surface conveyance designs must provide a minimum of one foot freeboard above the projected peak stage within the facility during the 100- year, 24-hour storm. When designing storm water ponds and conveyance channels, the applicant shall consider public safety as a design factor and alternative designs must be implemented where site limitations would preclude a safe design.
(6) Exemption. The site where soil-disturbing activities are conducted shall be exempt from the requirements of § 1334.09 if it can be shown to the satisfaction of the City Engineer that the site is part of a larger common plan of development where the storm water management requirements for the site are provided by an existing storm water management practice, or if the storm water management requirements for the site are provided by practices defined in a regional or local storm water management plan approved by the City Engineer.
(7) Maintenance. All storm water management practices shall be maintained in accordance with inspection and maintenance agreements approved by the City Engineer as detailed in § 1334.08.
(8) Ownership. Unless otherwise required by the city, storm water management practices serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership or, if compensated by the property owners, by the city. Storm water management practices serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.
(9) Preservation of existing natural drainage. Practices that preserve and/or improve the existing natural drainage shall be used to the maximum extent practicable. Such practices may include minimizing site grading and compaction; protecting and/or restoring water resources, riparian areas and existing vegetation; and maintaining unconcentrated storm water runoff to and through these areas.
(b) Storm water conveyance design criteria. All storm water management practices shall be designed to convey storm water to allow for the maximum removal of pollutants and reduction in flow velocities. This shall include, but not be limited to the following.
(1) Stream relocation or enclosure. The City Engineer may allow the enclosure or relocation of water resources only if the applicant shows proof of compliance with all appropriate permits from the state’s EPA, the U.S. Army Corps and other applicable federal, state and local agencies as required in § 1334.07 of this regulation, and the activity is in compliance with Chapter 1335 Erosion and Sediment Control and Chapter 1336 Riparian Setbacks, all as determined by the City Engineer. At a minimum, stream relocation designs must show how the project will minimize changes to the vertical stability, floodplain form, channel form and habitat of upstream and downstream channels on and off the property.
(2) Off-site storm water discharges. Off-site storm water runoff that discharges to or across the applicant’s development site shall be conveyed through the storm water conveyance system planned for the development site at its existing peak flow rates during each design storm. Off-site flows shall be diverted around storm water quality control facilities or, if this is not possible, the storm water quality control facility shall be sized to treat the off-site flow. Comprehensive storm water management plans will not be approved until it is demonstrated to the satisfaction of the City Engineer that off-site runoff will be adequately conveyed through the development site in a manner that does not exacerbate upstream or downstream flooding and erosion.
(3) Sheet flow. The site shall be graded in a manner that maintains sheet flow over as large an area as possible. The maximum area of sheet flow shall be determined based on the slope, the uniformity of site grading, and the use of easements or other legally-binding mechanisms that prohibit re-grading and/or the placement of structures within sheet flow areas. In no case shall the sheet flow length be longer than 300 feet, nor shall a sheet flow area exceed one and one-half acres. Flow shall be directed into an open channel, storm sewer or other storm water management practice from areas too long and/or too large to maintain sheet flow, all as determined by the City Engineer.
(4) Open channels. Unless otherwise allowed by the City Engineer, drainage tributary to storm water management practices shall be provided by an open channel with landscaped banks and designed to carry the ten-year, 24-hour storm water runoff from upstream contributory areas.
(5) Open drainage systems. Open drainage systems shall be preferred on all new development sites to convey storm water where feasible. Storm sewer systems shall be allowed only when the site cannot be developed at densities allowed under city zoning or where the use of an open drainage system affects public health or safety, all as determined by the City Engineer. The following criteria shall be used to design storm sewer systems when necessary.
A. Storm sewers shall be designed such that they do not surcharge from runoff caused by the five-year, 24-hour storm, and that the hydraulic grade line of the storm sewer stays below the gutter flow line of the overlying roadway, or below the top of drainage structures outside the roadway during a ten-year, 24-hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributory area within the proposed development and existing flows from off-site areas that are upstream from the development.
B. The minimum inside diameter of pipe to be used in public storm sewer systems is 12 inches. Smaller pipe sizes may be used in private systems, subject to the approval of the City Engineer.
C. All storm sewer systems shall be designed, taking into consideration the tail water of the receiving facility or water resource. The tail water elevation used shall be based on the design storm frequency. The hydraulic grade line for the storm sewer system shall be computed with consideration for the energy losses associated with entrance into and exit from the system, friction through the system and turbulence in the individual manholes, catch basins and junctions within the system.
D. The inverts of all curb inlets, manholes, yard inlets and other structures shall be formed and channelized to minimize the incidence of quiescent standing water where mosquitoes may breed.
E. Headwalls shall be required at all storm sewer inlets or outlets to and from open channels or lakes.
(6) Water resource crossings. The following criteria shall be used to design structures that cross a water resource in the city.
A. Water resource crossings other than bridges shall be designed to convey the stream’s flow for the minimum 25-year, 24-hour storm.
B. Bridges, open bottom arch or spans are the preferred crossing technique and shall be considered in the planning phase of the development. Bridges and open spans should be considered for all state scenic rivers, cold-water habitat, exceptional warm water habitat, seasonal salmonid habitat streams and Class III headwater streams. The footers or piers for these bridges and open spans shall not be constructed below the ordinary high water mark.
C. If a culvert or other closed bottom crossing is used, 25% of the cross-sectional area or a minimum of one foot of box culverts and pipe arches must be embedded below the channel bed.
D. The minimum inside diameter of pipes to be used for crossings shall be 12 inches.
E. The maximum slope allowable shall be a slope that produces a ten-fps velocity within the culvert barrel under design flow conditions. Erosion protection and/or energy dissipaters shall be required to properly control entrance and outlet velocities.
F. All culvert installations shall be designed with consideration for the tail water of the receiving facility or water resource. The tail water elevation used shall be based on the design storm frequency.
G. Headwalls shall be required at all culvert inlets or outlets to and from open channels or
lakes.
H. Streams with a drainage area of five square miles or larger shall incorporate floodplain culverts at the bankfull elevation to restrict head loss differences across the crossing so as to cause no rise in the 100-year storm event.
I. Bridges shall be designed such that the hydraulic profile through a bridge shall be below the bottom chord of the bridge for either the 100-year, 24-hour storm, or the 100-year flood elevation as determined by FBMA, whichever is more restrictive.
(7) Overland flooding. Overland flood routing paths shall be used to convey storm water runoff from the 100-year, 24-hour storm event to an adequate receiving water resource or storm water management practice such that the runoff is contained within the drainage, easement for the flood routing path and does not cause flooding of buildings or related structures. The peak 100-year water surface elevation along flood routing paths shall be at least one foot below the finished grade elevation at the structure. When designing the flood routing paths, the conveyance capacity of the site’s storm sewers shall be taken into consideration.
(8) Compensatory flood storage mitigation. In order to preserve floodplain storage volumes and thereby avoid increases in water surface elevations, any filling within floodplains approved by the city must be compensated by removing an equivalent volume of material. First consideration for the location(s) of compensatory floodplain volumes should be given to areas where the stream channel will have immediate access to the new floodplain within the limits of the development site. Consideration will also be given to enlarging existing or proposed retention basins to compensate for floodplain fill if justified by a hydraulic analysis of the contributing watershed. Unless otherwise permitted by the city, reductions in volume due to floodplain fills must be mitigated within the legal boundaries of the development. Embankment slopes used in compensatory storage areas must reasonably conform to the natural slopes adjacent to the disturbed area. The use of vertical retaining structures is specifically prohibited.
(9) Velocity dissipation. Velocity dissipation devices shall be placed at discharge locations and along the length of any outfall to provide non-erosive flow velocity from the structure to a water resource so that the natural physical and biological characteristics and functions of the water resource are maintained and protected.
(c) Storm water quality control.
(1) Direct runoff to a BMP. The site shall be designed to direct runoff to one or more of the following storm water management practices. These practices are listed in Table 2 of this regulation and shall be designed to meet the following general performance standards:
A. Extended conveyance facilities that slow the rate of storm water runoff; filter and biodegrade pollutants in storm water; promote infiltration and evapotranspiration of storm water; and discharge the controlled runoff to a water resource;
B. Extended detention facilities that detain storm water; settle or filter particulate pollutants; and release the controlled storm water to a water resource;
C. Infiltration facilities that retain storm water; promote settling, filtering and biodegradation of pollutants; and infiltrate captured storm water into the ground. The City Engineer may require a soil engineering report to be prepared for the site to demonstrate that any proposed infiltration facilities meet these performance standards; and
D. The City Engineer may approve other BMPs if the applicant demonstrates to the City Engineer satisfaction that these BMPs meet the objectives of this regulation as stated in division (c)6) below.
(2) Criteria applying to all storm water management practices. Practices chosen must be sized to treat the water quality volume (WQv) and to ensure compliance with Ohio Water Quality Standards (OAC Chapter 3745-1).
A. The WQv shall be equal to the volume of runoff from a 0.75-inch rainfall event and shall be determined according to one of the following methods:
1. Through a site hydrologic study approved by the City Engineer that uses continuous hydrologic simulation; site-specific hydrologic parameters, including impervious area, soil infiltration characteristics, slope and surface routing characteristics; proposed best management practices controlling the amount and/or timing of runoff from the site; and local long-term hourly records; or
2. Using the following equation:
WQv = C*P*A/12
a. With the following meanings:
WQv = Water quality volume in acre-feet
C = Runoff coefficient appropriate for storms less than one inch
P = 0.75 inch precipitation depth
A = Area draining into the storm water practice, in acres.
b. Runoff coefficients required by the Ohio Environmental Protection Agency (Ohio EPA) for use in determining the water quality volume are listed in Table 1. Alternatively, the City Engineer may consider use of the following equation to calculate the runoff coefficient if the applicant can demonstrate that appropriate controls are in place to limit the proposed impervious area of the development:
C = 0.858i3 - 0.78i2 + 0.774i + 0.04, where: area that is impervious
Table 1: Runoff Coefficients Based on the Type of Land Use | |
Land Use | Runoff Coefficient |
Table 1: Runoff Coefficients Based on the Type of Land Use | |
Land Use | Runoff Coefficient |
Industrial and 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 |
Where land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the storm water treatment structure is low density residential, 30% is high density residential, and 10% is open space, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0ri)(0.2) = (0.35) | |
B. An additional volume equal to 20% of the WQv shall be incorporated into the storm water practice for sediment storage.
C. Storm water quality management practices shall be designed such that the drain time is long enough to provide treatment and protect against downstream bank erosion, but short enough to provide storage available for successive rainfall events as defined in Table 2.
Table 2: Draw Down Times for Storm Water Management Practices | |
Best Management Practice | Drain Time of WQv |
Table 2: Draw Down Times for Storm Water Management Practices | |
Best Management Practice | Drain Time of WQv |
Constructed wetlands (above permanent pool) | 24 hours |
Extended conveyance facilities (vegetated swales, filter strips) extended conveyance detention design through design | 24 hours |
Best Management Practice | Drain Time of WQv |
Extended detention facilities extended dry detention basins | 48 hours |
Infiltration facilities | 24 - 48 hours |
Media filtration, bioretention | 40 hours |
Wet detention basins ** | 24 hours |
* Size to pass a hydrograph with a volume equal to the WQv, a duration of two hours, and peak rainfall intensity of one inch/hour at a depth of no more than three inches. The use of this criterion is limited to sites where the total area disturbed is five acres or less. **Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQv. | |
D. Each practice shall be designed to facilitate sediment removal, vegetation management, debris control and other maintenance activities defined in the inspection and maintenance agreement for the site.
(3) Additional criteria applying to infiltration facilities.
A. Infiltration facilities shall only be allowed if the soils of the facility fall within hydrologic soil groups A or B, and if the seasonal high water table and any underlying bedrock are at least six feet below the final grade elevation.
B. All runoff directed into an infiltration basin must first flow through an extended conveyance facility to remove coarser sediments that could cause a loss of infiltration capacity.
C. During construction, all runoff from disturbed areas of the site shall be diverted away from the proposed infiltration basin site. No construction equipment shall be allowed within the infiltration basin site to avoid soil compaction.
(4) Additional criteria applying to extended conveyance facilities.
A. Facilities shall be lined with fine turf-forming, flood-tolerant grasses.
B. Facilities designed according to the extended conveyance detention design drain time shall:
1. Not be located in areas where the depth to bedrock and/or seasonal high water table is less than three feet below the final grade elevation; and
2. Only be allowed where the underlying soil consists of hydrologic soil group (HSG) A or B, unless the underlying soil is replaced by at least a two and one-half foot deep layer of soil amendment with a permeability equivalent to a HSG A or B soil and an underdrain system is provided.
C. Facilities designed according to the flow through design drain time shall:
1. Only be allowed on sites where the total area disturbed is five acres or less; and
2. Be designed to slow and filter runoff flowing through the turf grasses with a maximum depth of flow no greater-than three inches.
D. Concentrated runoff shall be converted to sheet flow before entering an extended conveyance facility designed according to the flow through drain time.
(5) Additional criteria for extended detention facilities.
A. The outlet shall be designed to release the bottom 50% of the water quality volume in no less than two-thirds of the drain time. A valve shall be provided to drain any permanent pool volume for removal of accumulated sediments. The outlet shall be designed to minimize clogging, vandalism and maintenance.
B. The basin design shall incorporate the following features to maximize multiple uses, aesthetics, safety and maintainability.
1. Basin side slopes above the permanent pool shall have a run to rise ratio of 4:1 or flatter.
2. The perimeter of all permanent pool areas deeper than four feet shall be surrounded by an aquatic bench that extends at least eight feet and no more than 15 feet outward from the normal water edge. The eight feet wide portion of the aquatic bench closest to the shoreline shall have an average depth of six inches below the permanent pool to promote the growth of aquatic vegetation. The remainder of the aquatic bench shall be no more than 15 inches below the permanent pool to minimize drowning risk to individuals who accidentally or intentionally enter the basin, and to limit growth of dense vegetation in a manner that allows waves and mosquito predators to pass through the vegetation. The maximum slope of the aquatic bench shall be ten (H) to one (V). The aquatic bench shall be planted with hearty plants comparable to wetland vegetation that are able to withstand prolonged inundation.
3. A forebay designed to allow larger sediment particles to settle shall be placed at basin inlets. The forebay volume shall be equal to at least 10% of the water quality volume (WQv).
(6) Additional criteria applying to extended conveyance facilities.
A. Facilities shall be lined with fine turf-forming, flood-tolerant grasses.
B. Facilities designed according to the extended detention design drain time shall:
1. Not be located in areas where the depth to bedrock and/or seasonal high water table is less than three feet below the final grade elevation; and
2. Only be allowed where the underlying soil consists of hydrologic soil group (HSG) A or B, unless the underlying soil is replaced by at least a two and one-half foot deep layer of soil amendment with a permeability equivalent to a HSG A or B soil and an underdrain system is provided.
C. Swales and filter strips designed according to the flow through drain time shall:
1. Only be allowed on sites where the total area disturbed is five acres or less; and
2. Be designed to slow and filter runoff flowing through the turf grasses with a maximum depth of flow no greater than three inches.
D. Concentrated runoff shall be converted to sheet flow before entering an extended conveyance facility designed according to the flow through drain time.
(7) Alterative post-construction BMPs. The applicant may request approval from the City Engineer for the use of alternative structural post-construction BMPs if the applicant shows, to the satisfaction of the City Engineer and with prior written approval from the state’s EPA, that these BMPs are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in Table 2.
(d) Storm water quantity control. The comprehensive storm water management plan shall describe how the proposed storm water management practices are designed to meet the following requirements for storm water quantity control for each watershed in the development.
(1) The peak discharge rate of runoff from the critical storm and all more frequent storms occurring under post-development conditions shall not exceed the peak discharge rate of runoff from a one-year, 24-hour storm occurring on the same development drainage area under pre-development conditions.
(2) Storms of less frequent occurrence (longer return periods) than the critical storm, up to the 100-year, 24-hour storm shall have peak runoff discharge rates no greater than the peak runoff rates from equivalent size storms under pre-development conditions. The one-, two-, five-, ten-, 25-, 50- and 100-year storms shall be considered in designing a facility to meet this requirement.
(3) The critical storm for each specific development drainage area shall be determined as follows.
A. Determine, using a curve number-based hydrologic method that generates hydrographs, or other hydrologic method approved by the City Engineer, the total volume (acre-feet) of runoff from a one-year, 24-hour storm occurring on the development drainage area before and after development. These calculations shall meet the following standards:
1. Calculations shall include the lot coverage assumptions used for full build-out as proposed;
2. Calculations shall be based on the entire contributing watershed to the development area;
3. Curve numbers for the pre-development condition must reflect the average type of land use over the past ten years and not only the current land use; and
4. To account for future post-construction improvements to the site, calculations shall assume an impervious surface such as asphalt or concrete for all parking areas and driveways, regardless of the surface proposed in the site description.
B. From the volume determined in (d)(3)A. above, determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour critical storm from Table 3.
Table 3: 24-Hour Critical Storm | ||
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than:
| |
Table 3: 24-Hour Critical Storm | ||
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than:
| |
10 | 1 year | |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
250 | 500 | 50 year |
500 | — | 100 year |
For example, if the percent increase between the pre- and post-development runoff volume for a 1-year storm is 35%, the critical storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
(e) Storm water management on redevelopment projects. Comprehensive storm water management plans for redevelopment projects shall reduce existing site impervious areas by at least 20%. Where site conditions prevent the reduction of impervious area, then storm water management practices shall be implemented to provide storm water quality control facilities for at least 20% of the site’s impervious area. When a combination of impervious area reduction and storm water quality control facilities is used, the combined area shall equal or exceed 20% of the site. Where conditions prevent impervious area reduction or on-site storm water management for redevelopment projects, practical alternatives as detailed in § 1334.10 may be approved by the City Engineer.
(Ord. 05-17, passed 3-30-2017)