(a) General. The stormwater system, including SCMs for storage, treatment and control, and conveyance facilities, shall be designed to minimize structure flooding during the 100-year, 24-hour storm event; to maintain predevelopment runoff patterns, flows and volumes; meet or exceed the city's stormwater design standards for stormwater management; and to meet the following criteria:
(1) Integrated practices that address degradation of water resources.
A. The SCMs 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 stormwater discharges from the site. Acceptable practices shall:
1. Maintain predevelopment hydrology and groundwater recharge on as much of the site as practicable;
2. Only install new impervious surfaces and compact soils where necessary to support the future land use;
3. Compensate for increased runoff volumes caused by new impervious surfaces and soil compaction by reducing stormwater peak flows to less than predevelopment levels; and
4. Be designed according to the methodology included in the most current edition of Rainwater and Land Development or another design manual acceptable for use by the city.
B. SCMs 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. SCMs shall be designed to achieve the stormwater 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 routine maintenance.
(3) Stormwater management for all lots. Areas developed for a subdivision, as defined in Chapter 1242, shall provide stormwater management for the development of all subdivided lots. This shall include provisions for lot grading and drainage that minimize structure flooding during the 100-year, 24-hour storm; and maintain, to the extent practicable, the pre-development runoff patterns, volumes and peaks from each lot.
(4) Stormwater facilities in water resources. SCMs and related activities shall not be constructed in water resources unless the applicant shows proof of compliance with all appropriate permits from the Ohio EPA, the United States Army Corps, and other applicable federal, state and local agencies as required in § 1257.07, and the activity is in compliance with Chapter 1254, all as determined by the City Engineer.
(5) Stormwater ponds, surface conveyance channels and underground detention. All stormwater pond and surface conveyance designs must provide a minimum of one and one-half foot freeboard above the projected peak stage within the facility during the 100-year, 24-hour storm. Underground detention systems, at a minimum, shall provide 1.1 times the required 100-year detention volume. When designing stormwater ponds, conveyance channels and underground detention, 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 this section 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 stormwater management requirements for the site are provided by an existing SCM, or if the stormwater management requirements for the site are provided by practices defined in a regional or local stormwater management plan approved by the city.
(8) Ownership. Unless otherwise required by the city, SCMs serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership. SCMs 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; vegetative buffer strips; phasing of construction operations in order to minimize the amount of disturbed land at any one time, and designation of tree preservation areas or other protective clearing and grubbing practices; and maintaining unconcentrated stormwater runoff to and through these areas. Post-construction stormwater practices shall provide perpetual management of runoff quality and quantity so that a receiving stream's physical, chemical and biological characteristics are protected and ecological functions are maintained.
(b) Stormwater conveyance design criteria. All SCMs shall be designed to convey stormwater to allow for the maximum removal of pollutants and reduction inflow velocities. This shall include, but not be limited to, the following:
(1) Surface water protection. The City Engineer may allow modification to streams, rivers, lakes, wetlands or other surface waters only if the applicant shows proof of compliance with all appropriate permits from the Ohio EPA, the United States Army Corps and other applicable federal, state and local agencies as required in § 1257.07, and the activity is in compliance with Chapter 1254, 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 stormwater discharges. Off-site stormwater runoff that discharges to or across the applicant's development site shall be conveyed through the stormwater conveyance system planned for the development site at its existing peak flow rates during each design storm. Off-site flows shall be diverted around stormwater quality control facilities or, if this is not possible, the stormwater quality control facility shall be sized to treat the off-site flow. Comprehensive Stormwater 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 SCMs from areas too long and/or too large to maintain sheet flow, all as determined by the City Engineer.
(4) Storm sewer systems. Open drainage systems to convey stormwater overland to SCMs may be allowed with approval of the City Engineer. The following criteria shall be used to design storm sewer systems.
A. Storm sewers shall be designed such that they do not surcharge from runoff caused by the 10-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 the drainage structures outside the roadway during a 25-year, 24-hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributing drainage 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 shall be 12 inches.
C. All storm sewer systems shall be designed taking into consideration the tailwater of the receiving facility or water resource. The tailwater 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. Full height headwalls shall be required at all storm sewer inlets or outlets to and from open channels, ponds or lakes.
E. 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.
(5) 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. The water surface elevation shall be no less than one foot from the finished edge of pavement or top of curb.
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. The conduit or conveyance must to be sized to carry the 25-year storm under these conditions.
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 10-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 tailwater of the receiving facility or water resource. The tailwater elevation used shall be based on the design storm frequency.
G. Full height headwalls and integral concrete outfall pads 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 FEMA, whichever is more restrictive.
(6) Overland flooding. Overland flood routing paths shall be used to convey stormwater runoff from the 100-year, 24-hour storm event to an adequate receiving water resource or SCM 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 of all structures. When designing the flood routing paths, the conveyance capacity of the site's storm sewers shall be taken into consideration.
(7) 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) Stormwater quality control. The site shall be designed to direct runoff to one or more SCMs that meet or exceed the criteria in the most current Ohio EPA Construction General Permit.
(d) Stormwater quantity control. The Comprehensive Stormwater Management Plan shall describe how the proposed SCMs are designed to meet the following requirements for stormwater 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 1-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 rate no greater than the peak runoff rate from a 5-year, 24-hour storm under pre-development conditions. The 1-, 2-, 5-,10-, 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 or other hydrologic method approved by the City Engineer, the total volume (acre-feet) of runoff from a 1-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. Model pervious, directly connected impervious and disconnected impervious areas as separate subwatersheds;
4. Drainage area maps shall include area, curve number, time of concentrations. Time of concentration shall also show the flow path and the separation in flow type;
5. Rainfall depth - Use the Huff & Angel 1992 "Rainfall Frequency Atlas of the Midwest";
6. Temporal distribution - Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than one year. Include lot coverage assumptions used for full build out of the proposed condition;
7. Curve numbers for the pre-development condition must reflect the current land use or as determined soley by the city; and
8. Time of concentration - Use velocity based methods from (TR-55 NRCS USDA Urban Hydrology in Small Watersheds, 1986) to estimate travel time (Tt) for overland (sheet) flow, shallow concentrated flow and channel flow.
a. Maximum sheet flow length is 300 feet.
b. Use the appropriate "unpaved" velocity equation for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Section 4 - Hydrology (NEH-4).
B. 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. From the volume determined in division (d)(3)A. hereof, 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: | |
— | 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 |
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 will be controlled to meet pre-development peak discharge rates for a 5 year storm. | ||
(e) Stormwater management on redevelopment projects. SCM's on previously developed sites shall meet or exceed the criteria in the most current Ohio EPA Construction General Permit.
(Ord. 2016-208A, passed 12-19-2016; Ord. 2022-66, passed 3-7-2022)