(a) The storm water system, including 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) 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 937 of the Codified Ordinances of the County of Summit.
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 storm water peak flows to less than predevelopment levels.
(2) Areas developed for a subdivision, as defined in Part 11 of the Codified Ordinances of the County of Summit, shall provide water 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 the lots.
(3) 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 Ohio EPA, the U.S. Army Corps, and other applicable federal, state, and local agencies as required in Section 943.07 of this Chapter, and the activity is in compliance with Chapters 937 and 941 of the Codified Ordinances of the County of Summit, all as determined by the County Drainage Engineer.
(4) All storm water pond and storm 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 storm water ponds and conveyance channels the applicant shall include, to the extent practicable, practices to address public safety concerns.
(5) The site where soil-disturbing activities are conducted shall be exempt from the requirements of Section 943.09 of this Chapter if it can be shown to the satisfaction of the County Drainage 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, equal to, or better than, that required herein, 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 County Drainage Engineer.
(6) All storm water management practices shall be maintained in accordance with Inspection and Maintenance Agreements approved by the County Drainage Engineer as detailed in Section 943.08 of this Chapter.
(7) Unless otherwise required by the County, storm water management practices serving multiple lots in subdivisions shall be on a separate lot held, where possible, and maintained by an entity of common ownership. For those subdivisions that are subject to drainage maintenance assessments, as per Section 1109.03(a) of the Codified Ordinances of the County of Summit, maintenance shall be the responsibility of the County Drainage Engineer. Storm water management practices serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.
(8) 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 water runoff to and through these areas.
(9) Concentrated water runoff from BMPs to wetlands shall be converted to diffuse flow before the runoff enters a wetland in order to protect the natural hydrology, hydroperiod, and wetland flora. The flow shall be released such that no erosion occurs down slope. Practices 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 proposing to discharge to natural wetlands, the applicant shall perform a hydrological analysis to demonstrate that the proposed discharge matches the pre-development hydroperiods and hydrodynamics.
(10) The course of flow of storm water discharge from a facility shall be shown to be adequate when such flow reaches a public stream or right-of-way. Where such a course may cross land owned by another, an easement adequate enough for maintenance access shall be provided.
(b) All 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:
(1) The County Drainage Engineer may allow the enclosure or relocation of water resources 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 943.07 of this Chapter, and the activity is in compliance with Chapters 937 and 941 of the Codified Ordinances of the County of Summit, all as determined by the County Drainage 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 runoff that discharges onto, 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 the post construction water quality practices or, if this is not possible, the post construction water quality practices 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 County Drainage Engineer that off-site runoff will be adequately conveyed through, and from, the development site in a manner that does not exacerbate upstream or downstream flooding and erosion.
(3) 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 1.5 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 County Drainage Engineer.
(4) Unless otherwise allowed by the County Drainage Engineer, drainage tributary to storm water management practices shall be provided by an open channel with landscaped banks designed to carry the 10-year, 24-hour storm water runoff from upstream contributory areas.
(5) 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 local zoning requirements, or where the use of an open drainage system affects public health or safety, all as determined by the County Drainage 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 5-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 10-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 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 used in private systems, subject to the approval of the County Drainage 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) The following criteria shall be used to design structures that cross a water resource in the County:
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.
C. 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.
D. 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.
E. 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.
F. Headwalls shall be required at all culvert inlets or outlets to and from open channels or lakes.
G. 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.
H. The minimum inside diameter of pipes to be used for crossings shall be twelve (12) inches.
I. 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 control entrance and outlet velocities.
(7) 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 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 other structures. The peak 100-year, 24-hour storm surface water 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) In order to preserve floodplain storage volumes, and thereby avoid increases in water surface elevations, any filling within floodplains approved by the County 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 County Drainage Engineer, 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.
(9) 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 of the water resource are maintained and protected.
(c) Storm Water Quality Control.
(1) The site shall be designed to direct runoff to one or more of the following water quality practices which shall be designed to comply with the current version of the Summit County Engineer Storm Water Drainage Manual:
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. The County Drainage Engineer may require a soil engineering report to be prepared for the site to demonstrate that any proposed infiltration facilities meet these performance standards.
D. For sites less than five (5) acres, but greater than one (1) acre and not part of a common plan of development, where (1) or more acres are disturbed, the County Drainage Engineer may approve other BMPs if the applicant demonstrates to the County Drainage Engineer's satisfaction that these BMPs meet the objectives of this Chapter.
E. For sites equal to or greater than five (5) acres, or less than five (5) acres but part of a larger common plan of development or sale which will disturb five (5) or more acres, the County Drainage Engineer may allow alternative BMPs if the applicant demonstrates that these BMPs meet the objectives of this Chapter and has prior written approval from the Ohio EPA.
F. For the construction of new roads and roadway improvement projects by public entities (i.e. the state, counties, townships, cities, or villages), the County Drainage Engineer may approve BMPs not included in this Chapter, but they must show compliance with the current Ohio Department of Transportation standards.
(2) Each BMP 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. All BMPs must be sized to treat the water quality volume (WQv), and to ensure compliance with Ohio Water Quality Standards (OAC Chapter 3745-1) and the latest revision of the NPDES Construction Storm Water General Permit for Ohio.
(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, if the seasonal high water table is at least three (3) feet below the final grade elevation, and any underlying bedrock is at least six (6) feet below the final grade elevation.
B. All runoff directed into an infiltration basin must first flow through a pretreatment practice such as a grass channel or filter strip to remove 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 criteria shall:
1. Not be located in areas where the depth to bedrock and/or seasonal high water table is less than three (3) feet below the final grade elevation.
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 five-tenths (2.5) foot deep layer of soil amendment with a permeability equivalent to a HSG A or B soil, and an under drain system is provided.
C. Facilities designed according to the flow through design drain time criteria shall:
1. Only be allowed on sites where:
a. The total area disturbed is five (5) acres or less.
b. The discharge rate from the BMP will have negligible hydrologic impacts to received waters as described in the most current version of the Ohio EPA's General Permit for Storm Water Discharge from Small and Large Construction Activities.
c. Prior written approval is given by the County Drainage Engineer; and
d. For sites greater than five (5) acres or less than five (5) acres but part of a larger common plan of development or sale that will disturb five (5) or more acres, prior written approval has been given by the Ohio EPA.
2. Be designed to slow and filter runoff flowing through the turf grasses with a maximum depth of flow no greater than three (3) inches.
3. Be designed to have a minimum hydraulic residence time of five (5) minutes.
D. Concentrated runoff shall be converted to sheet flow, or a diffuse flow, using a plunge pool, flow diffuser, or level spreader, before entering an extended conveyance facility designed according to the flow through drain time.
(5) Additional criteria for extended detention facilities. Additional criteria for extended detention facilities shall comply with the latest revisions of the Ohio Department of Natural Resources Division of Soil and Water Resources manual, Rainwater and Land Development, and the Summit County Engineer Storm Water Drainage Manual.
(d) The Comprehensive Storm Water Management Plan shall describe how the proposed 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 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 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.
(3) The Critical Storm for each specific development drainage area shall be determined as follows:
A. Calculate, using a curve number-based hydrologic method that generates hydrographs, or other hydrologic methods approved by the County Drainage 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. Curve numbers for the pre-development condition must reflect the average type of land use over the past ten (10) years and not only the current land use.
4. 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 Section 943.09 (d)(3)A, determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from the latest revision of the Summit County Engineer Storm Water Drainage Manual.
(e) Comprehensive Storm Water Management Plans for redevelopment projects shall comply with the requirements of the most current version of the Ohio EPA's permit, "General Construction Permit Authorization for Storm Water Discharges associated with Construction Activity under the National Pollutant Discharge Elimination System".
(Ord. 2013-364. Adopted 8-26-13.)
(Ord. 2013-364. Adopted 8-26-13.)