(a) General: The stormwater conveyance system, including stormwater 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 stormwater 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 stormwater discharges from the site. Acceptable practices shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project and is approved by the City Engineer.
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 equal or less than predevelopment levels. Stormwater management practices that meet the criteria in this regulation, and additional criteria required by the City Engineer, shall comply with this regulation.
(2) Practices designed for final use: Stormwater management practices 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 minimal maintenance.
(3) Stormwater management for all lots: Areas developed for a subdivision, as defined in the City's 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, 24-hour storm; and maintain, to the extent practicable, the pre-development runoff patterns, volumes, and peaks from the lot.
(4) Stormwater facilities in water resources: Stormwater 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 and the activity is in compliance with all other City requirements 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 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 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 these requirements 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 stormwater management practice, 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.
(7) Maintenance: All stormwater management practices shall be maintained in accordance with Inspection and Maintenance Plan and Agreements approved by the City Engineer pursuant to Section 920.21(c).
(8) Ownership: Unless otherwise required by the City, stormwater 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 the City of Mount Vernon. Stormwater 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 vegetative buffer strips; phasing of construction operations in order to minimize the amount of disturbed land at any one time, and maintaining unconcentrated stormwater runoff to and through these areas. Post-construction 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.
(b) Stormwater Conveyance Design Criteria: All stormwater management practices shall be designed to convey stormwater to allow for the maximum practical 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 the 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, and the activity is in compliance with all other City requirements, including this Chapter 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, if this is not possible, the stormwater quality control facility shall be sized to treat the offsite flow. 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. 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 stormwater management practice from areas too long and/or too large to maintain sheet flow, as determined by the City Engineer.
(4) Open channels: Unless otherwise allowed by the City Engineer, drainage tributary to stormwater management practices shall be provided by an open channel with landscaped banks and designed to carry the 10-year, 24-hour stormwater runoff from upstream contributory areas.
(5) Open drainage systems: Open drainage systems are preferred on all new development sites to convey stormwater where feasible. Storm sewer systems are allowed when the site cannot be developed at densities allowed under City of Mount Vernon zoning or where the use of an open drainage system affects public health or safety, all as determined by the City Engineer.
(6) Storm sewer systems, culverts, bridges, and velocity dissipation measures within the City of Mount Vernon are to be designed and constructed per the requirements of the current version of the Ohio Departments of Transportation's "Location and Design Manual, Volume Two - Drainage Design."
(7) 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 stormwater 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 the City Engineer must be compensated by providing an equivalent 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, 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.
(c) Stormwater Quality Control:
(1) Direct runoff to a BMP: The site shall be designed to direct runoff to one or more of the stormwater management practices listed in Table 2 of the Ohio EPA CGP, designed to meet the requirements of the Ohio EPA CGP, and the guidance provided in the most recent online edition of the Ohio EPA Rainwater and Land Development Manual.
A. For sites less than five (5) acres, but greater than one (1) acre and not part of a common plan of development, where five (5) or less acres are disturbed, the City Engineer may approve other BMPs if the applicant demonstrates to the City Engineer's satisfaction that these BMPs meet the objectives of this regulation.
B. For sites 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 City Engineer may approve other BMPs if the applicant demonstrates to the City Engineer's satisfaction that these BMPs meet the objectives of this regulation, and has prior written approval from the Ohio EPA.
C. For the construction of new roads and roadway improvement projects by public entities (i.e. the state, counties, townships, cities, or villages), the City Engineer may approve BMPs not included in Table 2 of the Ohio EPA CGP, but must show compliance with the current version of the Ohio Departments of Transportation's "Location and Design Manual, Volume Two Drainage Design."
(2) Criteria applying to all stormwater management practices. Practices chosen must be sized to treat the water quality volume (WQv) per the requirements of the Ohio EPA CGP, and the guidance provided in the most recent online edition of the Ohio Rainwater and Land Development Manual, in compliance with Ohio water quality standards (OAC Chapter 3745-1).
(3) Criteria for the Acceptance of Alternative post-construction BMP's: The applicant may request approval from the City Engineer for the use of alternative structural post construction BMP's if the applicant shows to the satisfaction of the City Engineer that these BMP's are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in the Ohio CGP, prior approval from the Ohio EPA is mandatory.
(Ord. 2019-35. Passed 11-12-19.)