950.10 PERFORMANCE STANDARDS.

   (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, 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.   Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project and complies with Chapter 1305, Flood Damage Prevention.

         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 must be the water quality SCMs used. Separate SCMs may be used for peak discharge control and water quality treatment. The stormwater system must also meet any additional green infrastructure regulations found on the City of Youngstown stormwater website.

         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 Rainwater and Land Development or another design manual acceptable for use by the City of Youngstown 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 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 for phased projects: For multi-phase projects, there must be a plan for overall phasing, and stormwater must be controlled at each phase of the project. Applicant must demonstrate that the performance standards are met via the installed SCMs for each phase and provide plans to demonstrate full conversion from construction BMPs to post-construction SCMs for each completed phase of construction.

      (5)   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 950.07 of this regulation, and the activity is in compliance with Chapter 952, Erosion and Sediment Control, and Chapter 1305, Flood Damage Protection, all as determined by the City of Youngstown Public Works Department.

      (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 applicant 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 950.09 if it can be shown to the satisfaction of the City of Youngstown Public Works Department 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 of Youngstown Public Works Department.

      (8)   Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City of Youngstown Public Works Department.

      (9)   Ownership: Unless otherwise required by the City of Youngstown, SCMs serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership. SCMs serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.

      (10)   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. These practices must be used wherever feasible at the discretion of the City of Youngstown.

      (11)   Post-Construction Soil Restoration: 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 soil profile restoration design criteria in Rainwater and Land Development.

   (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 of Youngstown Public Works Department 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 950.07 of this regulation, and the activity is in compliance with Section 952, Erosion and Sediment Control, and Section 1305, Flood Damage Protection, all as determined by the City of Youngstown Public Works Department. 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 of Youngstown Public Works Department 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 regrading and/or the placement of structures within sheet flow areas. The sheet flow length shall not exceed 75 feet from impervious area or 150 feet from pervious areas. Flow shall be directed into an open channel, storm sewer, or other SCMs from areas too long and/or too large to maintain sheet flow, all as determined by the City of Youngstown Public Works Department.

      (4)   Storm drainage systems: The following criteria shall be used to design storm sewer systems:

         A.   Storm sewers shall be designed to meet the requirements presented in latest revision of the Mahoning County Drainage Criteria and Storm Water Manual.

         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 City of Youngstown Public Works Department.

         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.

      (5)   Water Resource Crossings. The following criteria shall be used to design structures that cross a water resource in the City of Youngstown:

         A.   Water resource crossings other than bridges shall be designed to meet the requirements presented in latest revision of the Mahoning County Drainage Criteria and Stormwater Manual.

         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 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 twenty-five (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 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.

      (6)   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 two feet 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.

      (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 except the one (1) year storm occurring under post-development conditions shall not exceed the peak discharge rate of runoff from a two (2) year, twenty-four (24) hour storm occurring on the same development drainage area under pre-development conditions. The one (1) year storm must not exceed the peak discharge rate of runoff from a one (1) year, twenty-four (24) hour storm in 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 ten (10) year, twenty-four (24) hour storm 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.   Determine, using a curve number-based hydrologic method or other hydrologic method approved by the City of Youngstown Public Works Department, 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.   Model pervious, directly connected impervious and disconnected impervious areas as separate subwatersheds.

            4.   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.

            5.   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.

            6.   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.

            7.   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."

            8.   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 ft.

               ii.   Use the appropriate "unpaved" velocity equation for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Section 4 - Hydrology (NEH-4).

            9.   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 of Youngstown Public Works Department.

         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 950.09(D)(3)(a), determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from 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 2-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 2-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms shall be controlled so as not to exceed the peak discharge rate from the 10-year frequency storm under pre-development conditions in the development drainage area.

   (e)   Stormwater Management for Previously Developed Areas.

      (1)   SCMs on previously developed sites must meet the criteria in the Construction General Permit.

         (Ord. 22-344. Passed 9-21-22.)