(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, twenty-four (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 storm water 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. 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.
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 Warren WPC and Ohio EPA.
(2) Practices designed for final use: SCMs shall be designed to achieve the stormwater management objectives of the regulations in this chapter, 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) Storm water management for all lots: Areas developed for a subdivision, as defined in Section 936.02(ff) of this chapter, 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, twnety-four (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 to the full extent of the regulations of this chapter at each phase of the project. Applicant must demonstrate that the performance standards of this chapter and of Chapter 938 Erosion and Sediment Control 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) Storm water 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 936.07 of this chapter, and the activity is in compliance with Chapter 938 Erosion and Sediment Control, all as determined by the City of Warren WPC.
(6) Stormwater ponds and surface conveyance channels: All stormwater pond and surface conveyance designs must provide a minimum of two (2) foot freeboard above the projected peak stage within the facility during the 100-year, twenty-four (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 936.09 if it can be shown to the satisfaction of the City of Warren WPC 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 Warren WPC.
(8) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City of Warren WPC.
(9) Maintenance Responsibility: The installed storm water system shall be properly maintained and operated by the legal entity responsible for maintenance in order to achieve compliance with the conditions outlined in this chapter. All stormwater management plan applications shall contain documentation sufficient to demonstrate that the operation and maintenance entity is the legal entity empowered and obligated to perpetually maintain the stormwater management facilities.
A. The following entities are considered to be acceptable to operate and maintain private stormwater management facilities:
1. Non-profit corporations, including homeowners associations, property owners associations or condominium owners associations, under certain conditions which ensure that the corporation has the financial, legal, and administrative capability to provide for the long-term operation and maintenance of the facilities.
2. The property owner or developer is normally not acceptable as a responsible entity, especially when the property is to be sold to various third parties. However, the property owner or developer may be acceptable under one (1) of the following circumstances provided the maintenance requirements are described in a document that has been submitted to the City:
i. The property is wholly owned by said applicant and the ownership is intended to be retained. This would apply to a farm, corporate office, or single industrial facility, for example.
ii. The ownership of the property is retained by the applicant and is either leased to third parties (such as in some shopping centers), or rented to third parties ( such as in some mobile home parks), for example.
B. The storm water management system shall be maintained by the legal entity. The City of Warren maintains the right to assess costs of labor and materials for such corrective action to the responsible party in accordance with usual and customary costs in place at the time of action.
C. Maintenance of stormwater facilities shall allow the stormwater management system to perform as originally designed and permitted by the City of Warren and as set forth in the written plan.
(10) Ownership: Unless otherwise required by the City of Warren WPC, 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.
(11) 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.
(12) 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 RLDM. Topsoil is the upper layer of natural soil (A horizon), which is typically darker and more fertile than the subsoil due to increased amounts of organic material.
(b) Stormwater Storage Facilities Design Criteria:
(1) Standards and Specifications: Stormwater storage can be classified as surface detention, underground detention, extended dry detention, wet retention, or alternative practices. Some facilities include one or more types of storage. Post-construction stormwater storage facilities shall be designed to meet the standards and specifications in the most current editions of the RLDM and/or the Ohio EPA Construction General Permit, whichever is more stringent, as well as the following standards.
(2) General Design Criteria: Storage facilities shall be designed and constructed with the following characteristics:
A. Water surface depths two (2) feet above the base flood elevation will not damage the storage facility.
B. The storage facilities shall be accessible and easily maintained. All facilities shall have twenty-five (25)-foot ingress/egress easements and other provisions as required for maintenance access.
C. All outlet works shall function without human intervention or outside power and shall operate with minimum maintenance.
D. Outlet works shall have an outlet pipe of minimum twelve (12) inches diameter.
E. Control orifices shall incorporate anti-clogging measures when the orifice measures less than four (4) inches in the shortest direction.
F. Storage facilities shall facilitate sedimentation and catchment of floating material.
G. Storage facilities shall minimize impacts of stormwater runoff on water quality by incorporating best management practices.
H. Storage facilities shall provide an overflow structure and overflow path that can safely pass excess flows through the development site. The minimum design rate shall be the undetained peak flow rate of the upstream watershed for the 100-year design rainfall event.
(3) Safety Features:
A. The primary spillway opening shall not permit access to the public and other non-maintenance personnel.
B. The perimeter of all water pool areas that are deeper than three (3) feet shall be surrounded by benches that meet the following:
1. A safety bench, with a maximum slope of three percent (3%), which extends outward, on dry land, from the shoreline. This bench will be a minimum of twenty-five (25) feet wide to provide for the safety of individuals and maintenance vehicles that are adjacent to the water pool. The safety bench may be landscaped, without the use of structures, to prevent access to the water pool.
2. Side slopes between the bench and the aquatic bench shall not be steeper than 3: 1 (3 feet horizontal for every one (1) foot vertical).
3. An aquatic bench that extends inward from the shoreline far enough to ensure public safety and has a maximum depth of fifteen (15) inches below the normal water surface elevations. The aquatic bench may be landscaped to prevent access to the deeper water pool. The aquatic bench may also be incorporated into the Post-Construction Water Quality Plan.
4. Side slopes beyond the aquatic bench and below the permanent water level shall not be steeper than 2: 1 (2 feet horizontal for every one (1) foot vertical).
5. The contours of the pond will be designed and managed to eliminate drop-offs and other hazards. Side slopes getting to the pond shall not exceed 3: 1 and shall terminate on a safety bench.
(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) Surface water protection: The City of Warren WPC 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 936.07 of this chapter, and the activity is in compliance with Chapter 938 Erosion and Sediment Control, as determined by the City of Warren WPC. 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 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 stormwater quality control facilities or the stormwater quality control facility shall be sized to treat the off-site flow. Post-Construction Stormwater Control Plans shall not be approved until it is demonstrated to the satisfaction of the City of Warren WPC 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 cross slope 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. While sheet flow area shall be maximized, the distance that sheet flow shall travel on a site before entering a catch basin or other storm water infrastructure shall not exceed 100 feet from impervious areas or 150 feet from pervious areas. Flow beyond this distance, as determined by the City of Warren WPC, shall be directed into an open channel, storm sewer, or other SCM.
(4) Storm drainage systems: Storm sewer systems shall be preferred over storm ditches on all new development sites to convey stormwater where feasible. During the design process for all storm sewer systems, the City of Warren Sewer Systems Superintendent must be consulted. The following criteria shall be used to design storm sewer systems when applicable:
A. New storm sewer installations shall be designed such that they do not surcharge from runoff caused by the ten (10)-year, twenty-four (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 twenty-five (25)-year, twenty-four (24)-hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributing drainage area within the proposed development and existing run-on 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. Ditch pipe must be minimum of eight (8) inches, or must match size of upstream and downstream ditch pipe such that it does not create a hydraulic bottleneck, whichever is greater. The invert elevations of the new ditch pipe must match the invert elevations of the upstream and downstream pipes. Smaller pipe sizes may be used in private systems, subject to the approval of the City of Warren WPC. Installation of ditch pipe shall be coordinated with the City of Warren Sewer Systems Superintendent.
C. All storm sewer systems shall be designed taking into consideration the tail water 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 by the project's design engineer to design structures that cross a water resource in the City of Warren:
A. Water resource crossings other than bridges shall be designed to convey the stream's flow for the minimum twenty-five (25)-year, twenty-four (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 ID 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 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 ten (10) feet per second 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 tail water of the receiving facility or water resource. The tail water 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 the Federal Emergency Management Agency (FEMA), whichever is more restrictive.
(6) Overland flooding: Overland flood routing paths shall be used to convey stormwater runoff from the 100-year, twenty-four (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 (2) 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) 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 Warren 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 Warren, 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.
(8) 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.
(d) Stormwater Quality Control: For new development and redevelopment projects, the site shall be designed to direct runoff to one (1) or more SCMs that meet or exceed the criteria in the Construction General Permit.
(e) Stormwater Quantity Control: For all proposed development and redevelopment sites, the Post-Construction Stormwater Control 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 (shorter return period) 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 peak discharge rate of runoff from the one (1)-year storm post-development conditions shall not exceed the one (1)-year storm pre-development conditions.
(2) Storms of less frequent occurrence (longer return periods) than the Critical Storm, up to and including the 100-year, twenty-four (24)-hour storm, shall have peak runoff discharge rates no greater than the peak runoff rates from the ten (10)-year storm under predevelopment conditions. The 1, 2, 5, 10, 25, 50, and 100-year storms shall be considered in designing a facility to meet this requirement.
(3) In addition to the above regulations, in no case shall the post-development peak discharge runoff rate exceed the pre-development peak discharge runoff rate for an equivalent storm event.
(4) For sites which are currently developed and are scheduled to be redeveloped, the pre-developed condition shall be defined to be one hundred percent (100%) of the site as grassland in good condition for all critical storm and volume storage calculations.
(5) 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 Warren WPC, the total volume (acre-feet) of runoff from a two (2)-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 drainage area, as defined in Section 936.02(h) of this chapter.
3. Model pervious, directly connected impervious and disconnected impervious areas as separate sub-watersheds.
4. Drainage area maps shall include area presumed, curve number, and time of concentrations. Calculated 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. Determine Curve Numbers for pre-development and post-development conditions.
i. 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.
ii. Pre-development Curve Numbers - Consult TR-55 NRCS USDA Urban Hydrology for Small Watersheds, 1986. For wooded or brushy areas, use listed values 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.
iii. Pre-development Curve Numbers for Currently Developed Sites: For sites which are currently developed and are scheduled to be re-developed, use listed values for grassland in good condition. This corresponds to a curve number of eighty (80) for Hydrologic Soil Group D, a curve number of seventy-four (74) for Hydrologic Soil Group C, 61 for Hydrologic Soil Group B, and 39 for Hydrologic Soil Group A.
iv. Post-development Curve Numbers - Open space areas shall use post-construction hydrologic soil groups from RLDM, unless the soil is amended using the soil profile restoration design criteria in RLDM. 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 as segments present in the Tc flow path.
i. Maximum sheet flow length is 100 ft.
ii. Use the appropriate velocity equation (paved or unpaved) for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Part 630 Hydrology, Chapter 15.
iii. Use a minimum time of concentration of six (6) minutes.
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 Warren WPC.
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 936.09(e)(5)A., determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from Table 2:
Table 2:24-Hour Critical Storm
If the Percentage of Increase in Volume of runoff is: | The Critical Storm shall 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 two (2)-year storm is thirty-five percent (35%), the Critical Storm is a five (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 two (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 ten (10)-year frequency storm under pre-development conditions in the development drainage area.
C. In no case shall the post developed runoff exceed the pre-developed runoff condition for an equivalent storm event.
(f) Storm water Management for Previously Developed Areas.
(2) SCMs on previously developed sites that are being re-developed must meet all applicable criteria set forth in the Construction General Permit.
(Ord. 13157/2022. Passed 9-28-22.)