(a) Stormwater Design General Information. No person shall develop any real property or connect or cause to be connected any building or other structure, either directly or indirectly, with a drain for the removal of surface, roof, ground or other water to be discharged into a ditch, swale, waterway, stream or an existing storm drainage system for such real property, without complying with the performance standards and paying the charges set forth in this chapter.
(b) General: All components of the stormwater system, including SCMs 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 minimize degradation of water resources. 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 SCMs shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project.
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.
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 Ohio Rainwater and Land Development or another design manual acceptable for use by the City and Ohio EPA SCMs that meet the criteria in this regulation, and additional criteria required by the City Engineer, shall comply with this regulation. Owners may propose alternative practices if they demonstrate to the satisfaction of the City Engineer that these practices also meet the above criteria.
(2) SCMs 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 Section 1220.06 (b)(30), 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, 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 shall not be constructed in water resources unless the owner obtains all appropriate permits from the Ohio EPA, the U.S. Army Corps, and other applicable federal, state, and local agencies, and the activity is in compliance with Chapter 1234 Erosion and Sediment control requirements and the City of Brunswick's future riparian setback requirements, after they have been adopted, all as determined by the City Engineer.
(5) Stormwater facilities in the floodplain: Stormwater facilities constructed, manufactured or otherwise, that provide treatment of the water quality volume (see Table 4, Section 1236.09), detention, retention, and/or infiltration, and all related activities, shall not be constructed in any special flood hazard area, as defined in Chapter 1467 Flood Damage Prevention.
(6) Stormwater ponds and surface conveyance channels: All stormwater pond and surface conveyance designs must provide a minimum of one (1) foot freeboard above the projected peak stage within the facility during the 100-year, 24-hour storm. When designing stormwater ponds and conveyance channels, the owner shall consider public safety as a design factor and alternative designs must be implemented where site limitations would preclude a safe design.
(7) Exemption: The site where soil-disturbing activities are conducted shall be exempt from the requirements of Section 1236.09 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 practices defined in a regional or local stormwater management plan approved by the City Engineer.
(8) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City Engineer as detailed in Section 1236.08.
(9) Agreements with Sub-dividers or Developers. A sub-divider or developer shall be required to construct an on-site SCM for the purposes of water quality and water retention approved by the City Engineer. The combination of stormwater quality and quantity requirements for two or more developments may be placed into one detention basin to be located at a strategic site given that a separate agreement with all parties is developed. The City shall enter into an agreement with the sub-divider or developer, to be approved by Council, containing the following conditions:
A. The sub-divider of a major subdivision shall require the formation of a homeowners' association, which shall assume responsibility for all maintenance, upkeep, repair, replacement and management of the SCM. In other developments, the sub-divider or developer shall make provisions acceptable to the City for maintenance of the SCM area as stated in Section 1236.10. Easements shall be granted to the City for access to and maintenance of the stormwater management area.
B. If more than one development is to use a single SCM, a separate association of all members using that SCM shall be formed. This association will be held responsible for all future maintenance and repairs of the SCM as stated in this Chapter.
C. The sub-divider or developer shall be exempt from the application of this Section only if authorized by the City Engineer.
D. Where a sub-divider or developer is exempt from the provisions of this Section, that sub-divider or developer shall comply with and pay fees in accordance with this Chapter, governing subdivisions and development prior to the effective date of this Section.
E. All SCMs will be placed within blocks and/or easements to allow City access (See Section 1236.11).
(10) Preservation of Existing Natural Drainage. SCMs that preserve and/or improve the existing natural drainage shall be used to the maximum extent practicable. Such SCMs 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. Postconstruction 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.
(11) Preservation of Wetland Hydrology: Concentrated stormwater runoff from SCMs to wetlands shall be converted to diffuse flow before the runoff enters a wetland(s) in order to protect the natural hydrology, hydroperiod, and wetland flora. The flow shall be released such that no erosion occurs down slope. SCMs such as level spreaders, vegetative buffers, infiltration basins, conservation of forest covers, and the preservation of intermittent streams, depressions, and drainage corridors may be used to maintain the wetland hydrology.
If the owner proposes to discharge to natural wetlands, a hydrological analysis shall be performed to demonstrate that the proposed discharge matches the pre-development hydroperiods and hydrodynamics.
(12) Soil Preservation and Post-Construction Soil Restoration: To the maximum extent practicable leave native soil undisturbed and protect from compaction during construction. 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 following steps: 1. till subsoil to a depth of 15-18 inches, 2. incorporate compost through top twelve (12) inches, 3. Replace with stockpiled site or imported suitable topsoil to a minimum depth of four (4) inches.
(c) 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) Stream/storm sewer discharge. The stormwater facility (storm sewer main or natural watercourse) that will convey the discharge from the site shall be analyzed to determine if it is capable of conveying the additional storm sewer discharge from the site of a 100-year/24-hour storm. If the designated outlet is not capable of conveying the discharge from the site during the 100-year/24-hour storm, then additional storage must be placed onsite to store the additional volume for a period of forty-eight (48) hours.
(2) Surface water protection: The City Engineer may allow modification to streams, rivers, lakes, wetlands or other surface waters only if the owner 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 and the activity is in compliance with Chapter 1234 Erosion and Sediment Control and any City of Brunswick riparian setback requirements that may be in effect, all as determined by the City Engineer.
(3) Off-site stormwater discharges: Off-site stormwater runoff that discharges to or across the owner'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.
(4) 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. Flow shall be directed into an open channel, storm sewer, or other SCM from areas too long and/or too large to maintain sheet flow, all as determined by the City Engineer.
(5) Open channels: Unless otherwise allowed by the City Engineer, drainage tributary to SCMs shall be provided by an open channel with landscaped banks and designed to carry within these banks the ten (10) year, twenty-four (24) hour stormwater runoff from upstream contributory areas.
(6) Drainage systems: Open drainage systems shall be preferred on all new development sites to convey stormwater where feasible. Storm Sewer systems shall be allowed to augment open drainage systems, such as to limit depth of roadside or conveyance ditches. The following criteria shall be used to design storm sewer systems when necessary.
A. Storm sewer design flow shall be based on the Rational Method. 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 drainage structures outside the roadway, whichever is more restrictive during a 25 - year, 24 - hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributing area within the proposed development and existing flows from offsite areas that are upstream from the development. These calculations will be reviewed and approved by the City Engineer prior to design acceptance. Rainfall data shall be obtained from the latest volume of the NOAA Rainfall ATLAS 14 or per Table 1. Runoff coefficients shall be per Table 2.
Table 1 - Rainfall per Storm Frequency | |
24 Hour Storm (year) | Rainfall (in.) |
2 (50% storm) | 2.44 |
5 (20% storm) | 3.06 |
10 (10% storm) | 3.55 |
25 (4% storm) | 4.35 |
50 (2% storm) | 5.08 |
100 (1% storm) | 5.92 |
Table 2.
Rational Method Runoff Coefficients (C) for City of Brunswick
Runoff Coefficients for Hydrologic Soil Groups | ||||
Cover Description | A | B | C | D |
Cultivated agricultural land | 0.17 | 0.3 | 0.43 | 0.50 |
Pasture or range land continuous grazing | 0.08 | 0.16 | 0.36 | 0.47 |
Meadow protected from grazing | 0.06 | 0.13 | 0.30 | 0.43 |
Runoff Coefficients for Hydrologic Soil Groups | ||||
Cover Description | A | B | C | D |
Woods | 0.05 | 0.10 | 0.29 | 0.41 |
Woods/grass combination (orchard, tree farm, etc.) | 0.07 | 0.14 | 0.33 | 0.45 |
Lawns, parks, golf courses, cemeteries, etc. | 0.08 | 0.16 | 0.36 | 0.47 |
Paved streets, parking lots, roofs, driveways, etc. | 0.96 | 0.96 | 0.96 | 0.96 |
Gravel areas | 0.40 | 0.59 | 0.69 | 0.74 |
Residential Areas | ||||
Average lot size & Average % Impervious Area | ||||
? acre or less 65 | 0.41 | 0.59 | 0.72 | 0.77 |
¼ acre 38 | 0.16 | 0.37 | 0.54 | 0.64 |
? acre 30 | 0.12 | 0.32 | 0.50 | 0.61 |
½ acre 25 | 0.09 | 0.29 | 0.47 | 0.59 |
1 acre 20 | 0.06 | 0.26 | 0.45 | 0.57 |
2 acres 12 | 0.05 | 0.23 | 0.41 | 0.50 |
Dirt or graded areas | 0.41 | 0.61 | 0.74 | 0.83 |
B. Rainfall intensity will be calculated using the equation i = a/(t+b)^c
Where:
i = Rainfall intensity (in./hour)
t = Time of concentration (minutes)
Refer to Ohio Department of Transportation's Location & Design Manual, Volume 2 (or latest edition) Drainage Design, Figure 1101-2 for Rainfall Intensity Constants (a, b & c).The maximum slope allowable shall be a slope that produces no less than 2.5-fps and no more than a 10-fps velocity within the pipe barrel under design flow conditions.
C. 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.
D. All stormwater conveyance 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.
i. Previous studies on file with the City.
ii. FEMA Flood Insurance Rate Maps.
iii. Calculations prepared by a Registered Professional Engineer.
E. Catch basin design spread calculations shall be submitted to the City Engineer for review to determine catch basin spacing and sizing. At a minimum, there shall be at least one total clear lane during a twenty-five (25) year, twenty-four (24) hour storm.
F. 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.
G. Headwalls shall be required at all storm sewer inlets or outlets to and from open channels or lakes.
H. Outlets discharging into an open-water conveyance structure shall have an invert at a minimum of three (3) inches above the average water depth during the snow-melt season.
I. The flood elevation for a 100 year, twenty-four (24) hour storm must be a minimum of ten feet away horizontally from the perimeter of any homes within the new subdivision, nor cause any home flooding to adjacent neighboring properties, and shall be at least two feet below the finished grade elevation of any livable structure.
J. All storm sewer outlets from a subdivision must flow either into a public storm sewer, stream of the State, or a major ditch unless authorized by the City Engineer.
K. The maximum distance for sheet flow shall be 300 feet before entering a storm structure. Except, that the maximum overland drainage area tributary to the storm structure shall be no greater than one and one-half (1.5) acres.
(7) Water Resource Crossings. The following criteria shall be used to design structures that cross a water resource within the City of Brunswick:
A. Water resource crossings other than bridges shall be designed to convey the stream's flow for the minimum 25-year, 24-hour storm or as indicated by the City Engineer. The maximum allowable headwater elevation for such a storm shall be twelve (12) inches below the pavement crown elevation. Water crossings carrying receiving waters located near upstream City borders shall convey no more flow than currently designed to carry unless directed by the City Engineer.
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 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 twelve (12) inches.
E. The maximum slope allowable shall be a slope that produces a ten (10) fps velocity within the culvert barrel. 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 should incorporate floodplain culverts at the bankfull elevation to restrict the rise in headwater elevation upstream of the culvert to no more than one (1) foot during the 100-year, 24-hour storm.
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.
(8) 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.
(9) 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 Brunswick 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 Brunswick, 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.
(10) Velocity dissipation: Velocity dissipation devices shall be placed at discharge locations and along the length of any outfall channel 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.
(d) 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.
(e) 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 postdevelopment 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) Developers or sub-dividers shall include in their preliminary plans a local watershed study to determine the impact from the development or subdivision caused by stormwater onto the lands adjoining or downstream from the area to be developed, to assure that said lands shall not be adversely affected by the proposed development or subdivision.
(3) 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 1, 2, 5, 10, 25, 50, and 100-year storms shall be considered in designing a facility to meet this requirement.
(4) 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 site area only 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, time of concentrations. Time of concentration shall also show the flow path and the separation in flow type.
4. Rainfall data shall be obtained from the latest volume of the NOAA Rainfall ATLAS 14 or per Table 1.
5. Temporal Distribution - 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.
6. Curve numbers for the pre-development condition must reflect the average type of land use over the past 10 years and not only the current land use. Curve Numbers shall conform to the National Engineering Handbook Table 9-1.
i. Post-development Curve Numbers - All areas that are altered by construction practices shall use post-construction Hydraulic Soil Groups from Ohio Rainwater and Land Development.
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 NEH-4 (Spell this out).
8. The volume reduction provided by permeable pavement, bioretention, or other LID SCMs may be subtracted from the post development stormwater volume. Volume reductions for these practices may be demonstrated using methods outlined in Ohio Rainwater and Land Development or a hydrologic model acceptable to the City Engineer.
9. 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 except in instances of engineered permeable pavement systems.
B. From the volume determined in Section 1236.09, 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 need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
(f) Stormwater Management for Previously Developed Areas.
(1) SCMs on previously developed sites must meet the criteria in the Construction General Permit.
(Ord. 81-2022. Passed 11-14-22.)