(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. Minimize disturbances to riparian areas.
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 should be considered for 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 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 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 1105 - Definitions, 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.
A. Each residential lot created by subdivision of property shall have functional access to a rear yard drain connected to the subdivision drainage system. This requirement may be waived when a natural waterway traverses the rear yard and when the rear property is at a slope of two percent (2%) or greater toward the waterway.
B. Downspouts for residential lots shall not be directly connected to the City's municipal storm sewer system, except as approved by the City Engineer.
(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 1352.07 of this chapter, and the activity is in compliance with Chapter 1353
- Erosion and Sediment Control and Section 1115.09 (b) - Stream Valley Preservation, all as determined by the City Engineer.
(5) Exemption: The site where soil-disturbing activities are conducted shall be exempt from the requirements of Section 1352.09 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.
(6) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City Engineer.
(7) 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.
(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 1352.07 of this chapter, and the activity is in compliance with Chapter 1353
- Erosion and Sediment Control and Section 1115.09 (b) - Stream Valley Preservation, 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) Open channels: Unless otherwise allowed by the City Engineer, open channels shall be designed with vegetated banks to carry the ten (10)-year, twenty-four (24)-hour stormwater runoff from upstream contributing areas.
(4) Storm sewer systems: 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 two (2) year, twenty-four (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 (10) year, twenty-four (24) hour storm. A hydraulic gradient check may also be required by the City Engineer for the twenty-five (25), fifty (50) and 100-year storms where deemed appropriate. 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 offsite areas that are upstream from the development.
B, The minimum inside diameter of pipe to be used in public storm sewer systems is twelve (12) inches. Smaller pipe sizes may be considered, subject to the approval of the City Engineer.
C. Storm sewers shall be constructed of materials approved for use by the City Engineer based on performance standards as established by ASTM or other testing agencies.
D. 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.
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.
F. 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 Mentor:
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 for watersheds up to 250 acres and a fifty (50)-year, twenty-four (24)-hour storm for watersheds greater than 250 acres.
B. Bridges, open bottom arch or spans are the preferred crossing technique and shall be considered in the planning phase of the development.
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 stream's flow as required in Section 1352.09
(b)(6)A. 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) 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. 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.
4. Use the Precipitation-Frequency Atlas of the United States, NOAA Atlas 14, Vol 2(3) (available online: http://hdsc.nws.noaa.gov/hdsc/pfds/ ) for rainfall depth data for stormwater design.
5. Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than one (1) year. Include lot coverage assumptions used for full build out of the proposed condition.
6. Curve numbers for the pre-development condition shall reflect the average type of land use over the past ten (10) years 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."
7. 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).
8. 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 1352.09
(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 one (1)-year storm is thirty-five percent (35%), the Critical Storm is a five (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 one (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) Stormwater Management for Previously Developed Areas.
(1) SCMs on previously developed sites must meet the criteria in the Construction General Permit.
(Ord. 23-O-001. Passed 1-3-23.)