(a) General: The storm water system, including storm 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) Integrated practices that address degradation of water resources. 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.
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.
Storm water management practices that meet the criteria in this regulation, and additional criteria required by the Village Engineer shall comply with this regulation.
(2) Practices designed for final use: Storm water management practices shall be designed to achieve the storm water 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 minimal maintenance.
(3) Storm water management for all lots: Areas developed for a subdivision, as defined in Chapter 1111, Design Standards, shall provide storm water management 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 lot.
(4) Storm water facilities in water resources: 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 1197.07 of this regulation, and the activity is in compliance with Chapter 1199, Erosion and Sediment Control, all as determined by the Village Engineer.
(5) Storm water ponds and surface conveyance channels: All storm water 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 storm water 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 1197.09 if it can be shown to the satisfaction of the Village 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, 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 Village Engineer.
(7) Maintenance: All storm water management practices shall be maintained in accordance with Inspection and Maintenance Agreements approved by the Village Engineer as detailed in Section 1197.08.
(8) Ownership: Unless otherwise required by Johnstown, Ohio, storm water management practices 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 Johnstown, Ohio. Storm water management practices 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; and maintaining unconcentrated storm water runoff to and through these areas.
(b) Storm Water Conveyance Design Criteria: All storm 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) Stream relocation or enclosure: The Village 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 1197.07 of this regulation, and the activity is in compliance with Chapter 1199, Erosion and Sediment Control, all as determined by the Village 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 discharges: Off-site storm water runoff that discharges to 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 storm water quality control facilities or, if this is not possible, the storm water quality control facility 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 Village 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 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 Village Engineer.
(4) Open channels: Unless otherwise allowed by the Village Engineer drainage tributary to storm water management practices shall be provided by an open channel with landscaped banks and designed to carry the 10 year, 24 hour storm water runoff from upstream contributory areas.
(5) Open drainage systems: 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 Johnstown, Ohio zoning or where the use of an open drainage system affects public health or safety, all as determined by the Village 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 12 inches. Smaller pipe sizes may be used in private systems, subject to the approval of the Village 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. Full 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 Johnstown, Ohio:
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, 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 (25) percent of the cross-sectional area or a minimum of 1 foot of box culverts and pipe arches must be embedded below the channel bed.
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 headwalls shall be required at all culvert inlets or outlets to and from open channels or lakes.
H. Streams with a drainage area of 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, 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 storm water runoff from the 100 year, 24 hour storm event to an adequate receiving water resource or storm 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 related structures. The peak 100-year water surface 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) 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 Johnstown, Ohio must be compensated by removing an equivalent volume of material. For all areas of development that do not increase the floodplain storage by 1/10th of the floodplain elevation there shall be no compensatory flood storage requirement. 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 Johnstown, Ohio, 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) Storm Water Quality Control:
(1) All stormwater quality control shall be in accordance with the Ohio EPA General Permit for Stormwater Discharges Associated with Construction Activity Under the NPDES.
(d) Storm Water Management on Redevelopment Projects: Comprehensive Storm Water Management Plans for redevelopment projects shall reduce existing site impervious areas by at least 20 percent. Where site conditions prevent the reduction of impervious area, then stormwater management practices shall be implemented to provide storm water quality control facilities for at least 20 percent of the site's impervious area. When a combination of impervious area reduction and storm water quality control facilities is used, the combined area shall equal or exceed 20 percent of the site. Where conditions prevent impervious area reduction or on-site stormwater management for redevelopment projects, practical alternatives as detailed in Section 1197.10 may be approved by the Village Engineer.
(Ord. 13-2019. Passed 4-2-19.)