(a) General: The stormwater system, including SCMs for storage, treatment and control, and conveyance facilities, shall be designed to prevent structure flooding during the 100-year, twenty-four (24)-hour storm event; to maintain predevelopment runoff patterns, flows, and volumes; to meet the requirements of the Construction General Permit; and to meet the following criteria:
(1) Integrated SCMs that address degradation of water resources. The SCMs shall function as an integrated system that controls flooding and minimizes the degradation of the water resources receiving stormwater discharges from the site. Acceptable SCMs shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project and complies with Chapter 926 Riparian Setbacks and Wetland Setbacks.
B. Maintain predevelopment hydrology and groundwater recharge on as much of the site as practicable. Where feasible, bioretention, permeable pavement with infiltration, underground storage with infiltration, infiltration trenches, infiltration basins and/or rainwater harvesting must be the water quality SCMs used. Separate SCMs may be used for peak discharge control and water quality treatment.
C. Only install new impervious surfaces and compact soils where necessary to support the future land use.
D. Compensate for increased runoff volumes caused by new impervious surfaces and soil compaction by reducing stormwater peak flows to less than predevelopment levels.
E. 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 of North Olmsted and Ohio EPA.
(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 1101 Subdivision Regulations, shall provide stormwater management and water quality controls for the development of all subdivided lots. This shall include provisions for lot grading and drainage that prevent structure flooding during the 100-year, twenty-four (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 U.S. Army Corps, and other applicable federal, state, and local agencies as required in Section 928.07 of this regulation, and the activity is in compliance with Chapter 927 Erosion and Sediment Control and Chapter 926 Riparian Setbacks and Wetland Setbacks, all as determined by the City Engineer.
(5) Stormwater ponds and surface conveyance channels: All stormwater pond and surface conveyance designs must provide a minimum of two (2) foot freeboard above the projected peak stage within the facility during the 100-year, twenty-four (24)-hour storm. When designing stormwater 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 Section 928.09 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 stormwater management requirements for the site are provided by an existing SCM, or if the stormwater management requirements for the site are provided by SCMs defined in a regional or local stormwater management plan approved by the City Engineer.
(7) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City Engineer.
(8) Ownership: Unless otherwise required by the City of North Olmsted, SCMs 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 of North Olmsted. 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 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 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.
(10) Post-Construction Soil Restoration: Except for areas that will be covered by impervious surface or have been incorporated into an SCM, the soil moisture-holding capacity of areas that have been cleared and graded must be restored to that of the original, undisturbed soil to the maximum extent practicable. Areas that have been compacted or had the topsoil or duff layer removed should be amended using the soil profile restoration design criteria in Rainwater and Land Development.
(b) Stormwater Conveyance Design Criteria: All SCMs shall be designed to convey stormwater to allow for the maximum removal of pollutants and reduction in flow velocities. This shall include but not be limited to:
(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 U.S. Army Corps, and other applicable federal, state, and local agencies as required in Section 928.07 of this regulation, and the activity is in compliance with Section 927 Erosion and Sediment Control and Section 926 Riparian Setbacks and Wetland 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 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 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. The sheet flow length shall not exceed seventy-five (75) feet from impervious area or 150 feet from pervious areas. 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) Open channels: Unless otherwise allowed by the City Engineer, drainage tributary to SCMs shall be provided by an open channel with vegetated banks and designed to carry the ten (10) year, twenty-four (24) hour stormwater runoff from upstream contributory areas.
(5) Open drainage systems: Open drainage systems shall be preferred on all new development sites to convey stormwater where feasible. Storm sewer systems shall be allowed only when the site cannot be developed at densities allowed under City of North Olmsted 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 capacity and hydraulic grade line shall be designed in accordance with the current version of UNIFORM STANDARDS FOR SEWARGE IMPROVEMENTS unless otherwise approved by the City Engineer.
B. The minimum inside diameter of pipe to be used in public storm sewer systems is twelve (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 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. 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 of North Olmsted:
A. Water resource crossings other than bridges shall be designed to convey the stream's flow for the minimum twenty-five (25) year, twenty-four (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, coldwater habitat, exceptional warmwater 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, twenty-five percent (25%) of the cross-sectional area or a minimum of one (1) foot of box culverts and pipe arches must be embedded below the channel bed. The conduit or conveyance must be sized to carry the twenty-five (25)-year storm under these conditions.
D. The minimum inside diameter of pipes to be used for crossings shall be twelve (12) inches.
E. The maximum slope allowable shall be a slope that produces a ten (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. 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 (5) 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, twenty-four (24) hour storm or the 100 year flood elevation as determined by FEMA, whichever is more restrictive.
(7) Overland flooding: Overland flood routing paths shall be used to convey stormwater runoff from the 100 year, twenty-four (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 two feet 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.
(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 of North Olmsted must be compensated by providing an equivalent storage volume. 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 of North Olmsted, 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) Stormwater Quality Control: The site shall be designed to direct runoff to one or more SCMs that meet or exceed the criteria in the 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 one (1) -year, twenty-four (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, twenty-four (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 (1), two (2), five (5), ten (10), twenty-five (25), fifty (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 one (1)-year, twenty-four (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, and time of concentrations. Time of concentration shall also show the flow path and the separation in flow type.
5. Use the Precipitation-Frequency Atlas of the United States, NOAA Atlas 14, Vol 2(3). [http://hdsc.nws.noaa.gov/hdsc/pfds/] for rainfall depth data for stormwater design.
6. Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than 1 year. Include lot coverage assumptions used for full build out of the proposed condition.
7. Curve numbers for the pre-development condition shall reflect the average type of land use prior to any development and not only the current land use.
i. Pre-development Curve Numbers - For wooded or brushy areas, use listed values from TR-55 NRCS USDA Urban Hydrology for Small Watersheds, 1986 in good hydrologic condition. For meadows, use listed values. For all other areas (including all types of agriculture), use pasture, grassland, or range in good hydrologic condition.
ii. Post-development Curve Numbers - Open space areas shall use post-construction hydrologic soil groups from Rainwater and Land Development unless the soil is amended using the soil profile restoration design criteria in Rainwater and Land Development. All undisturbed areas or open space with amended soils shall be treated as "open space in good condition."
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.
i. Maximum sheet flow length is 100 feet.
ii. Use the appropriate "unpaved" velocity equation for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Section 4 - Hydrology (NEH-4).
9. The volume reduction provided by runoff reduction SCMs may be subtracted from the post-development stormwater volume. Volume reductions for these SCMs may be demonstrated using methods outlined in Rainwater and Land Development or a hydrologic model acceptable to the City Engineer.
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 except in instances of engineered permeable pavement systems. From the volume determined in Section 928.09(d)(3)A., determine the percent increase in volume of runoff due to development. Using the percentage, select the twenty-four (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 need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
(4) The minimum storage volume for the stormwater quantity control system shall be calculated following the requirements of subsections (d)(1) through (3) above. However, in no case shall the required storage volume be less than the total proposed hard surface area times two inches unless approved by the City Engineer. Hard surface areas shall include roof areas, concrete areas, asphalt areas, brick areas, stone pavers, and other similar areas as determined by the City Engineer.
(5) An access manhole or inspection port as approved by the City Engineer shall be provided at each end of an underground storage facility for sufficient inspection, maintenance and discovery of the system from the surface.
(e) Stormwater Management for Previously Developed Areas.
(1) SCMs on previously developed sites must meet the criteria in the Construction General Permit.
(Ord. 2021-93. Passed 1-18-22.)