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(a) The purpose of this regulation is to establish technically feasible and economically reasonable storm water management standards to achieve a level of storm water quality and quantity control that will minimize damage to property and degradation of water resources and will promote and maintain the health, safety, and welfare of the citizens of the City of Parma;
(b) This regulation requires owners who develop or redevelop their property within the City to:
(1) Control storm water runoff from their property and ensure that all storm water management practices are properly designed, constructed, and maintained.
(2) Reduce water quality impacts to receiving water resources that may be caused by new development or redevelopment activities.
(3) Control the volume, rate, and quality of storm water runoff originating from their property so that surface water and ground water are protected and flooding and erosion potential are not increased.
(4) Minimize the need to construct, repair, and replace subsurface storm drain systems.
(5) Preserve natural infiltration and ground water recharge, and maintain subsurface flow that replenishes water resources, except in slippage-prone soils.
(6) Incorporate storm water quality and quantity controls into site planning and design at the earliest possible stage in the development process.
(7) Reduce the expense of remedial projects needed to address problems caused by inadequate storm water management.
(8) Maximize use of storm water management practices that serve multiple purposes, including, but not limited to, flood control, erosion control, fire protection, water quality protection, recreation, and habitat preservation.
(9) Design sites to minimize the number of stream crossings and the width of associated disturbance in order to minimize the City future expenses related to the maintenance and repair of stream crossings.
(10) Maintain, promote, and re-establish conditions necessary for naturally occurring stream processes that assimilate pollutants, attenuate flood flows, and provide a healthy water resource.
(c) This regulation shall apply to all parcels used or being developed, either wholly or partially, for new, redevelopment, or relocated projects involving highways and roads; subdivisions or larger common plans of development; industrial, commercial, institutional, or residential projects; building activities on farms; redevelopment activities; grading; and all other uses that are not specifically exempted in this Section 2301.01.
(d) Public entities, including the State of Ohio, Cuyahoga County, and the City of Parma shall comply with this regulation for roadway projects initiated after the effective date of this chapter and, to the maximum extent practicable, for projects initiated before that time.
(e) This regulation does not apply to activities regulated by, and in compliance with, the Ohio Agricultural Sediment Pollution Abatement Rules.
(f) This regulation does not require a Comprehensive Storm Water Management Plan for linear construction projects, such as pipeline or utility line installation, that do not result in the installation of impervious surface as determined by the City Engineer. Such projects must be designed to minimize the number of stream crossings and the width of disturbance. Linear construction projects must comply with the requirements of Chapter 2303 Erosion and Sediment Control.
(Ord. 160-11. Passed 6-20-11.)
For the purpose of this regulation, the following terms shall have the meaning herein indicated:
(a) “Acre.” A measurement of area equaling 43,560 square feet.
(b) “As-built survey.” A survey shown on a plan or drawing prepared by a registered surveyor indicating the actual dimensions, elevations, and locations of any structures, underground utilities, swales, detention facilities, and sewage treatment facilities after construction has been completed.
(c) “Best management practices (BMPs).” Schedule of activities, prohibitions of practices, operation and maintenance procedures, treatment requirements, and other practices to reduce the pollution of water resources and to control storm water volume and rate.
(d) “Certified Professional in Erosion and Sediment Control (CPESC).” A person that has subscribed to the code of ethics and have met the requirements established by the CPESC Council of Certified Professional in Erosion and Sediment Control, Inc. to be a Certified Professional in Erosion and Sediment Control.
(e) “City Engineer.” Refers to the City Engineer or his or her designee. The City Engineer or his or her designate shall be the Storm Water Administrator for the City of Parma.
(f) “Clean Water Act.” Pub. L. 92-500, as amended, Pub. L. 95-217, Pub. L. 95-576, Pub. L. 96-483, Pub. L. 97-117, and Pub. L. 100-4,33 U.S.C. 1251 et. seq. Referred to as the Federal Water Pollution Control Act or the Federal Water Pollution Control Act Amendments of 1972.
(g) “Community.” The City of Parma, its designated representatives, boards, or commissions.
(h) “Comprehensive Storm Water Management Plan.” The written document and plans meeting the requirements of this regulation that sets forth the plans and practices to minimize storm water runoff from a development area, to safely convey or temporarily store and release post-development runoff at an allowable rate to minimize flooding and stream bank erosion, and to protect or improve storm water quality and stream channels.
(i) “Construction limits.” The delineation of the limits of all construction activities, including all disturbed areas, and any area encompassing the field yard, stockpile, and staging areas of a project site.
(j) “Critical storm.” A storm that is calculated by means of the percentage increase in volume of runoff by a proposed development area. The critical storm is used to calculate the maximum allowable storm water discharge rate from a developed site.
(k) “Detention facility.” A basin, pond, oversized pipe, or other structure that reduces the peak flow rate of storm water leaving the facility by temporarily storing a portion of the storm water entering the facility.
(l) “Development area.” A parcel or contiguous parcels owned by one person or persons, or operated as one development unit, and used or being developed for commercial, industrial, residential, institutional, or other construction or alteration that changes runoff characteristics.
(m) “Development drainage area.” A combination of each hydraulically unique watershed with individual outlet points on the development area.
(n) “Disturbed area.” An area of land subject to erosion due to the removal of vegetative cover and/or soil-disturbing activities.
(o) “Drainage.” The removal of excess surface water or ground water from land by surface or subsurface drains.
(p) “Erosion.” The process by which the land surface is worn away by the action of wind, water, ice, gravity, or any combination of those forces.
(q) “Extended conveyance.” A storm water management practice that replaces and/or enhances traditional open or closed storm drainage conduits by retarding flow, promoting percolation of runoff into the soil, and filtering pollutants during the storm water quality event.
(r) “Extended detention.” A storm water management practice that replaces and/or enhances traditional detention facilities by releasing the runoff collected during the storm water quality event over at least 24 to 48 hours, retarding flow and allowing pollutants to settle within the facility.
(s) “Final stabilization.” All soil-disturbing activities at the site have been completed and a uniform perennial vegetative cover with a density of at least 80% coverage for the area has been established or equivalent stabilization practices, such as the use of mulches or geotextiles, have been employed.
(t) “Freeboard.”
(1) The vertical distance between the level of the water surface usually corresponding to the design flow and a point of interest such as a bridge beam, levee top or specific location on the roadway grade.
(2) The distance between the normal operation level and the top of the sides of an open conduit; the crest of a dam, etc., designed to allow for wave action, floating debris, or any other condition or emergency, without overtopping the structure.
(u) “Grading.” The process in which the topography of the land is altered to a new slope.
(v) “Hydrologic Unit Code.” A cataloging system developed by the United States Geological Survey and the Natural Resource Conservation Service to identify watersheds in the United States.
(w) “Impervious cover.” Any surface that cannot effectively absorb or infiltrate water. This may include roads, streets, parking lots, rooftops, sidewalks, and other areas not covered by vegetation.
(x) “Infiltration.” A storm water management practice that does not discharge to a water resource during the storm water quality event, requiring collected runoff to either infiltrate into the ground water and/or be consumed by evapotranspiration, thereby retaining storm water pollutants in the facility.
(y) “Larger common plan of development.” A contiguous area where multiple separate and distinct construction activities may be taking place at different times on different schedules under one plan.
(z) “Maximum extent practicable.” The level of pollutant reduction that operators of small municipal separate storm sewer systems regulated under 40 C.F.R. Parts 9, 122, 123, and 124, referred to as NPDES Storm Water Phase II, must meet.
(aa) “NPDES.” National Pollutant Discharge Elimination System. A regulatory program in the Federal Clean Water Act that prohibits the discharge of pollutants into surface waters of the United States without a permit.
(bb) “Nonstructural storm water management practice.” Storm water runoff control and treatment techniques that use natural practices to control runoff and/or reduce pollution levels.
(cc) “Post-development.” The conditions that exist following the completion of soil-disturbing activity in terms of topography, vegetation, land use, and the rate, volume, quality, or direction of storm water runoff.
(dd) “Pre-construction meeting.” Meeting prior to construction between all parties associated with the construction of the project, including government agencies, contractors and owners to review agency requirements and plans as approved and submitted.
(ee) “Pre-development.” The conditions that exist prior to the initiation of soil-disturbing activity in terms of topography, vegetation, land use, and the rate, volume, quality, or direction of storm water runoff.
(ff) “Professional engineer.” A professional engineer registered in the State of Ohio with specific education and experience in water resources engineering, acting in conformance with the Code of Ethics of the Ohio State Board of Registration for Engineers and Surveyors.
(gg) “Redevelopment.” A construction project on land where impervious cover has previously been developed and where the new land use will not increase the runoff coefficient. If the new land use will increase the runoff coefficient, then the project is considered to be a new development project rather than a redevelopment project. (Refer to Table 1 in Section 2301.09)
(hh) “Riparian and wetland setback.” The real property adjacent to a water resource on which soil- disturbing activities are limited, all as defined by Chapter 2305, Riparian and Wetland Regulations.
(ii) “Riparian area.” Land adjacent to any brook, creek, river, or stream having a defined bed and bank that, if appropriately sized, helps to stabilize streambanks, limit erosion, reduce flood size flows, and/or filter and settle out runoff pollutants, or performs other functions consistent with the purposes of this regulation.
(jj) “Runoff.” The portion of rainfall, melted snow, or irrigation water that flows across the ground surface and is eventually returned to water resources.
(kk) “Sediment.” The soils or other surface materials that can be transported or deposited by the action of wind, water, ice, or gravity as a product of erosion.
(ll) “Sedimentation.” The deposition of sediment in water resources.
(mm) “Site owner/operator.” Any individual, corporation, firm, trust, commission, board, public or private partnership, joint venture, agency, unincorporated association, municipal corporation, County or State agency, the Federal government, other legal entity, or an agent thereof that is responsible for the overall construction site.
(nn) “Soil-disturbing activity.” Clearing, grading, excavating, filling, or other alteration of the earth's surface where natural or human-made ground cover is destroyed and that may result in, or contribute to, increased storm water quantity and/or decreased storm water quality.
(oo) “Stabilization.” The use of best management practices that reduce or prevent soil erosion by storm water runoff, trench dewatering, wind, ice, gravity, or a combination thereof.
(pp) “Structural storm water management practice.” Any constructed facility, structure, or device that provides storage, conveyance, and/or treatment of storm water runoff.
(qq) “Surface waters of the State.” All streams, lakes, reservoirs, marshes, wetlands, or other waterways situated wholly or partly within the boundaries of the State, except those private waters which do not combine or affect a junction with surface water. Waters defined as sewerage systems, treatment works or disposal systems in Ohio R.C. 6111.01 are not included.
(rr) “Total maximum daily load (TMDL).” The sum of the existing and/or projected point source, non-point source, and background loads for a pollutant to a specified watershed, water body, or water body segment. A TMDL sets and allocates the maximum amount of a pollutant that may be introduced into the water and still ensures attainment and maintenance of water quality standards.
(ss) “Water quality volume.” The volume of runoff from a contributing watershed that must be captured and treated, equivalent to the maximized capture volume as defined in the American Society of Civil Engineers (ASCE) Manual and Report on Engineering Practice No. 87 and Water Environment Federation Manual of Practice No. 23 titled Urban Runoff Quality Management.
(tt) “Water resource.” Any public or private body of water; including wetlands; the area within the ordinary high water level of lakes and ponds; as well as the area within the ordinary high water level of any brook, creek, river, or stream having a defined bed and bank (either natural or artificial) which confines and conducts continuous or intermittent flow.
(uu) “Water resource crossing.” Any bridge, box, arch, culvert, truss, or other type of structure intended to convey people, animals, vehicles, or materials from one side of a watercourse to another. This does not include private, noncommercial footbridges or pole-mounted aerial electric or telecommunication lines, nor does it include below-grade utility lines.
(vv) “Watershed.” The total drainage area contributing storm water runoff to a single point.
(ww) “Wetland.” Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions, including swamps, marshes, bogs, and similar areas (40 C.F.R. 232, as amended).
(Ord. 160-11. Passed 6-20-11.)
(a) Compliance with the provisions of this regulation shall not relieve any person from responsibility for damage to any person otherwise imposed by law. The provisions of this regulation are promulgated to promote the health, safety, and welfare of the public and are not designed for the benefit of any individual or any particular parcel of property.
(b) By approving a Comprehensive Storm Water Management Plan under this regulation, the City does not accept responsibility for the design, installation, and operation and maintenance of storm water management practices.
(Ord. 160-11. Passed 6-20-11.)
(a) Where this regulation is in conflict with other provisions of law or ordinance, the most restrictive provisions, as determined by the City Engineer, shall prevail.
(b) If any clause, section, or provision of this regulation is declared invalid or unconstitutional by a court of competent jurisdiction, the validity of the remainder shall not be affected thereby.
(c) This regulation shall not be construed as authorizing any person to maintain a nuisance on their property, and compliance with the provisions of this regulation shall not be a defense in any action to abate such a nuisance.
(d) Failure of the City to observe or recognize hazardous or unsightly conditions or to recommend corrective measures shall not relieve the site owner from the responsibility for the condition or damage resulting therefrom, and shall not result in the City, its officers, employees, or agents being responsible for any condition or damage resulting therefrom.
(Ord. 160-11. Passed 6-20-11.)
(a) This regulation requires that a Comprehensive Storm Water Management Plan be developed and implemented for soil-disturbing activities disturbing one or more acres of total land, or less than one acre if part of a larger common plan of development or sale disturbing one or more acres of total land, and on which any regulated activity of Section 2301.01(c) is proposed.
(b) The City shall administer this regulation, shall be responsible for determination of compliance with this regulation, and shall issue notices and orders as may be necessary. The City may consult with the Cuyahoga SWCD, private engineers, storm water districts, or other technical experts in reviewing the Comprehensive Storm Water Management Plan.
(Ord. 160-11. Passed 6-20-11.)
(a) Pre-Application Meeting. The applicant shall attend a pre-application meeting with the City Engineer to discuss the proposed project, review the requirements of this regulation, identify unique aspects of the project that must be addressed during the review process, and establish a preliminary review and approval schedule.
(b) Preliminary Comprehensive Storm Water Management Plan. The applicant shall submit three sets of a Preliminary Comprehensive Storm Water Management Plan (Preliminary Plan) and the applicable fees to the City Engineer. The Preliminary Plan shall show the proposed property boundaries, setbacks, dedicated open space, public roads, water resources, storm water control facilities, and easements in sufficient detail and engineering analysis to allow the City Engineer to determine if the site is laid out in a manner that meets the intent of this regulation and if the proposed storm water management practices are capable of controlling runoff from the site in compliance with this regulation. The applicant shall submit three sets of the Preliminary Plan and applicable fees as follows:
(1) For subdivisions. In conjunction with the submission of the preliminary subdivision plan and with submittal of the improvement plans.
(2) For other construction projects. In conjunction with the application for a building permit and/or storm water management permit.
(3) For general clearing projects. In conjunction with the application for a tree removal permit.
(c) Final Comprehensive Storm Water Management Plan. The applicant shall submit three sets of a Final Comprehensive Storm Water Management Plan (Final Plan) and the applicable fees to the City Engineer in conjunction with the submittal of the final plat, improvement plans, or application for a building or storm water management permit for the site. The Final Plan shall meet the requirements of Section 2301.08 and shall be approved by the City Engineer prior to approval of the final plat and/or before issuance of a storm water management permit by the City Engineer or building permit by the Building Commissioner.
(d) Review and Comment. The City Engineer shall review the Preliminary and Final Plans submitted, and shall approve or return for revisions with comments and recommendations for revisions. A Preliminary or Final Plan rejected because of deficiencies shall receive a correction letter stating specific problems and the procedures for filing a revised Preliminary or Final Plan.
(e) Approval Necessary. Land clearing or tree removal and/or soil-disturbing activities shall not begin and storm water management and/or building permits shall not be issued without an approved Comprehensive Storm Water Management Plan.
(f) Pre-Construction Meeting. A pre-construction meeting shall be held, with City representative(s) in attendance, prior to the commencement of any work.
(g) Valid for One Year. Approvals issued in accordance with this regulation shall remain valid for one year from the date of approval. The applicant may request an extension for time by submitting a written request to the City Engineer.
(Ord. 160-11. Passed 6-20-11.)
Approvals issued in accordance with this regulation do not relieve the applicant of responsibility for obtaining all other necessary permits and/or approvals from other Federal, State, and/or County agencies. If requirements vary, the most restrictive shall prevail. These permits may include, but are not limited to, those listed below. Applicants are required to show proof of compliance with these regulations before the City will issue a building or storm water management permit.
(a) Ohio EPA NPDES Permits Authorizing Storm Water Discharges Associated with Construction Activity or the Most Current Version Thereof. Proof of compliance with these requirements shall be the applicant's Notice of Intent (NOI) number from Ohio EPA, a copy of the Ohio EPA Director's Authorization Letter for the NPDES Permit, or a letter from the site owner's qualified professional certifying and explaining why the NPDES Permit is not applicable.
(b) Section 401 of the Clean Water Act. Proof of compliance shall be a copy of the Ohio EPA Water Quality Certification application tracking number, public notice, project approval, or a letter from the site owner certifying that a qualified professional has surveyed the site and determined that Section 401 of the Clean Water Act is not applicable. Wetlands, and other waters of the United States, shall be delineated by protocols accepted by the U.S. Army Corps of Engineers at the time of application of this regulation.
(c) Ohio EPA Isolated Wetland Permit. Proof of compliance shall be a copy of Ohio EPA's Isolated Wetland Permit application tracking number, public notice, project approval, or a letter from the site owner certifying that a qualified professional has surveyed the site and determined that Ohio EPA's Isolated Wetlands Permit is not applicable. Isolated wetlands shall be delineated by protocols accepted by the U.S. Army Corps of Engineers at the time of application of this regulation,
(d) Section 404 of the Clean Water Act. Proof of compliance shall be a copy of the U.S. Army Corps of Engineers Individual Permit application, public notice, or project approval, if an Individual Permit is required for the development project. If an Individual Permit is not required, the site owner shall submit proof of compliance with the U.S. Army Corps of Engineer's Nationwide Permit Program. This shall include one of the following:
(1) A letter from the site owner certifying that a qualified professional has surveyed the site and determined that Section 404 of the Clean Water Act is not applicable.
(2) A site plan showing that any proposed fill of waters of the United States conforms to the general and special conditions specified in the applicable Nationwide Permit. Wetlands, and other waters of the United States, shall be delineated by protocols accepted by the U.S. Army Corps of Engineers at the time of application of this regulation.
(e) Ohio Dam Safety Law. Proof of compliance shall be a copy of the ODNR Division of Water permit application tracking number, a copy of the project approval letter from the ODNR Division of Water, or a letter from the site owner certifying and explaining why the Ohio Dam Safety Law is not applicable.
(Ord. 160-11. Passed 6-20-11.)
(a) Comprehensive Storm Water Management Plan Required. The applicant shall develop a Comprehensive Storm Water Management Plan describing how the quantity and quality of storm water will be managed after construction is complete for every discharge from the site and/or into a water resource. The Plan will illustrate the type, location, and dimensions of every structural and non-structural storm water management practice incorporated into the site design, and the rationale for their selection. The rationale must address how these storm water management practices will address flooding within the site as well as flooding that may be caused by the development upstream and downstream of the site. The rationale will also describe how the storm water management practices minimize impacts to the physical, chemical, and biological characteristics of on-site and downstream water resources and, if necessary, correct current degradation of water resources that is occurring or take measures to prevent predictable degradation of water resources.
(b) Preparation by Certified Professional. The Comprehensive Storm Water Management Plan shall be prepared by a registered professional engineer, a Certified Professional Erosion and Sediment Control, or Landscape Architect and include supporting calculations, plan sheets, and design details. To the extent necessary, as determined by the City Engineer, a site survey shall be performed by a registered professional surveyor to establish boundary lines, measurements, or land surfaces.
(c) Community Procedures. The City Engineer shall prepare and maintain procedures providing specific criteria and guidance to be followed when designing the storm water management system for the site. These procedures may be updated from time to time, at the discretion of the City Engineer based on improvements in engineering, science, monitoring, and local maintenance experience. The City Engineer shall make the final determination of whether the practices proposed in the Comprehensive Storm Water Management Plan meet the requirements of this regulation. The City Engineer may also maintain a list of acceptable best management practices that meet the criteria of this regulation to be used in the City.
(d) Contents of Comprehensive Storm Water Management Plan. The Comprehensive Storm Water Management Plan shall contain an application, narrative report, construction site plan sheets, a long-term inspection and maintenance agreement, and a site description with the following information provided:
(1) Site description:
A. A description of the nature and type of the construction activity (e.g. residential, shopping mall, highway, etc.).
B. Total area of the site and the area of the site that is expected to be disturbed (i.e. grubbing, clearing, excavation, filling or grading, including off-site borrow areas).
C. A description of prior land uses at the site.
D. An estimate of the impervious area and percent of imperviousness created by the soil-disturbing activity at the beginning and at the conclusion of the project.
E. Existing data describing the soils throughout the site, including the soil series and association, hydrologic soil group, porosity, infiltration characteristics, depth to ground water, depth to bedrock, and any impermeable layers.
F. If available, the quality of any known pollutant discharge from the site such as that which may result from previous contamination caused by prior land uses.
G. The location and name of the immediate water resource(s) and the first subsequent water resource(s).
H. The aerial (plan view) extent and description of water resources at or near the site that will be disturbed or will receive discharges from the project.
I. Describe the current condition of water resources including the vertical stability of stream channels and indications of channel incision that may be responsible for current or future sources of high sediment loading or loss of channel stability.
(2) Site map showing:
A. Limits of soil-disturbing activity on the site.
B. Soils types for the entire site, including locations of unstable or highly erodible soils.
C. Existing and proposed one-foot contours. This must include a delineation of drainage watersheds expected before, during, and after major grading activities as well as the size of each drainage watershed in acres.
D. Water resource locations, including springs, wetlands, streams, lakes, water wells, and associated setbacks on or within 200 feet of the site, including the boundaries of wetlands or streams and first subsequent named receiving water(s) the applicant intends to fill or relocate for which the applicant is seeking approval from the U.S. Army Corps of Engineers and/or Ohio EPA.
E. Existing and planned locations of buildings, roads, parking facilities, and utilities.
F. The location of any in-stream activities, including stream crossings.
(3) Contact information: Company name and contact information as well as contact name, addresses, and phone numbers for the following:
A. The certified professional who prepared the Comprehensive Storm Water Management Plan.
B. The site owner.
(4) Phase, if applicable, of the overall development plan.
(5) List of sublot numbers if project is a subdivision.
(6) Ohio EPA NPDES Permit number and other applicable State and Federal permit numbers, if available, or status of various permitting requirements if final approvals have not been received.
(7) Location, including complete site address and sublot number if applicable.
(8) Location of any easements or other restrictions placed on the use of the property.
(9) A site plan sheet showing:
A. The location of each proposed post-construction storm water management practice.
B. The geographic coordinates of the site and each proposed practice in North American Datum Ohio State Plan North.
It is necessary that the entire site be shown on one plan sheet to allow a complete view of the site during plan review. If a smaller scale is used to accomplish this, separate sheets providing an enlarged view of areas on individual sheets shall also be provided.
(10) An inspection and maintenance agreement. The inspection and maintenance agreement required for storm water management practices under this regulation shall be a stand-alone document between the City and the applicant and shall contain the following information and provisions:
A. The location of each storm water management practice, including those practices permitted to be located in, or within 50 feet of, water resources, and identification of the drainage area served by each storm water management practice.
B. A schedule for regular maintenance for each aspect of the storm water management system and description of routine and non-routine maintenance tasks to ensure continued performance of the system as is detailed in the approved Comprehensive Storm Water Management Plan. This schedule may include additional standards, as required by the City Engineer, to ensure continued performance of storm water management practices permitted to be located in, or within 50 feet of, water resources.
C. The location and documentation of all access and maintenance easements on the property.
D. Identification of the landowner(s), organization, or municipality that responsible for long-term maintenance, including repairs, of the storm water management practices.
E. The landowner(s), organization, or municipality shall maintain storm water management practices in accordance with this regulation.
F. The City has the authority to enter upon the property to conduct inspections as necessary to verify that the storm water management practices are being maintained and operated in accordance with this regulation.
G. The City shall maintain public records of the results of site inspections, shall inform the landowner(s), organization, or municipality responsible for maintenance of the inspection results, and shall specifically indicate any corrective actions required to bring the storm water practices into proper working condition.
H. If the City notifies the landowner(s), organization, or municipality responsible for maintenance of the maintenance problems that require correction, the specific corrective actions shall be taken within a reasonable time frame as determined by the City.
I. The City is authorized to enter upon the property and to perform the corrective actions identified in the inspection report if the landowner(s), organization, or municipality responsible for maintenance does not make the required corrections in the specified time period. The City shall be reimbursed by the landowner(s), organization, or municipality responsible for maintenance for all expenses incurred within ten days of receipt of invoice from the City.
J. The method of funding long-term maintenance and inspections of all storm water management practices.
K. A release of the City from all damages, accidents, casualties, occurrences, or claims that might arise or be asserted against the City from the construction, presence, existence, or maintenance of the storm water management practices.
Alteration or termination of these stipulations is prohibited. The applicant must provide a draft of this inspection and maintenance agreement as part of the Comprehensive Storm Water Management Plan submittal. Once a draft is approved, a recorded copy of the agreement must be submitted to the City to receive final inspection approval of the site.
(11) Calculations required. The applicant shall submit calculations for projected storm water runoff flows, volumes, and timing into and through all storm water management practices for flood control, channel protection, water quality, and the condition of the habitat, stability, and incision of each water resource and its floodplain, as required in Section 2301.09 of this regulation. These submittals shall be completed for both pre- and post-development land use conditions and shall include the underlying assumptions and hydrologic and hydraulic methods and parameters used for these calculations. The applicant shall also include critical storm determination and demonstrate that the runoff from upper watershed areas have been considered in the calculations.
(12) List of all contractors and subcontractors before construction: Prior to construction or before the pre-construction meeting, provide the list of all contractors and subcontractors names, addresses, and phones involved with the implementation of the Comprehensive Storm Water Management Plan including a written document containing signatures of all parties as proof of acknowledgment that they have reviewed and understand the requirements and responsibilities of the Comprehensive Storm Water Management Plan. The list of contractors and subcontractors shall be updated to keep the list current and before any such contractors or subcontractors perform work on the site.
(13) Existing and proposed drainage patterns: The location and description of existing and proposed drainage patterns and storm water management practices, including any related storm water management practices beyond the development area and the larger common development area.
(14) For each storm water management practice to be employed on the development area, include the following:
A. Location and size, including detail drawings, maintenance requirements during and after construction, and design calculations, all where applicable.
B. Final site conditions including storm water inlets and permanent nonstructural and structural storm water management practices. Details of storm water management practices shall be drawn to scale and shall show volumes and sizes of contributing drainage areas.
C. Any other structural and/or non-structural storm water management practices necessary to meet the design criteria in this regulation and any supplemental information requested by the City Engineer.
(Ord. 160-11. Passed 6-20-11.)
(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 pre-development 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 or wetland areas, unless the disturbance is intended to support a watercourse restoration project and complies with Chapter 2305, Riparian and Wetland Requirements.
B. Maintain pre-development hydrology and ground water 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 pre-development levels.
Storm water 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. 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 single family subdivision, single-family cluster development, or two-family development shall provide storm 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 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 of Engineers, and other applicable Federal, State, and local agencies as required in Section 2301.07 of this regulation, and the activity is in compliance with Chapter 2303, Erosion and Sediment Control and Chapter 2305, Riparian and Wetland Requirements, all as determined by the City Engineer.
(5) Storm water ponds and surface conveyance channels. All storm water pond and surface conveyance designs must provide a minimum of one 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 2301.09 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 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 City Engineer.
(7) Maintenance. All storm water management practices shall be maintained in accordance with inspection and maintenance agreements approved by the City Engineer as detailed in Section 2301.08.
(8) Ownership. Unless otherwise required by the City, 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 the City. 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 and/or wetland areas, and existing vegetation; and maintaining unconcentrated storm water runoff to and through these areas.
(10) Preservation of wetland hydrology.
A. Concentrated storm 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.
B. If the applicant proposes to discharge to natural wetlands, a hydrological analysis shall be preformed to demonstrate that the proposed discharge matches the pre-development hydroperiods and hydrodynamics.
(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 City 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 of Engineers, and other applicable Federal, State, and local agencies as required in this Chapter 2301, and the activity is in compliance with Chapter 2303, Erosion and Sediment Control and City Riparian and Wetland Requirements, all 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 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 in 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 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 storm water management practice from areas too long and/or too large to maintain sheet flow, all as determined by the City Engineer.
(4) Open channels. Unless otherwise allowed by the City 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 City zoning or where the use of an open drainage system affects public health or safety, all as determined by the City 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 off-site 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 City 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 or other measure approved by the City Engineer 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 the City:
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. The footers or piers for these bridges and open spans shall not be constructed below the ordinary high water mark unless documentation is approved by the City Engineer demonstrating no adverse impact.
C. If a culvert or other closed-bottom crossing is used, 25% of the cross-sectional area or a minimum of one 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. Headwalls or other measure approved by the City Engineer shall be required at all culvert inlets or outlets to and from open channels or lakes.
H. Streams with a drainage area of five 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 the City 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 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.
A. The requirement for compensatory floodplain storage is only in effect when the riparian setback does not include the entire 100-year floodplain, when the City grants a variance that allows filling in the floodplain due to site constraints, or when the City Engineer determines that stream or floodplain restoration is needed to meet the objectives of this regulation.
(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) Direct runoff to a BMP. The site shall be designed to direct runoff to one or more of the following storm water management practices. These practices are listed in Table 2 of this regulation and shall be designed to meet the following general performance standards:
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; and infiltrate captured storm water into the ground. The City 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 acres, but greater than one acre and not part of a common plan of development, where one or more 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 as stated in Section 2301.09(c)(6).
E. For sites greater than five acres, or less than five acres but part of a larger common plan of development or sale which will disturb five 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 as stated in Section 2301.09(c)(6), 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 City Engineer may approve BMPs not included in Table 2 of this regulation, but plans 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 storm water management practices. Practices chosen must be sized to treat the water quality volume (WQV) and to ensure compliance with Ohio Water Quality Standards (O.A.C. Chapter 3745-1). The size of the water quality volume (WQv) orifice shall be limited to 2.5 inches for extended detention ponds when drainage areas are too small at allow a practical WQV orifice size.
A. The WQV shall be equal to the volume of runoff from a 0.75-inch rainfall event and shall be determined according to one of the following methods:
1. Through a site hydrologic study approved by the City Engineer that uses continuous hydrologic simulation; site-specific hydrologic parameters, including impervious area, soil infiltration characteristics, slope, and surface routing characteristics; proposed best management practices controlling the amount and/or timing of runoff from the site; and local long-term hourly records; or
2. Using the following equation:
WQV = C*P*A/12
where terms have the following meanings:
WQV = water quality volume in acre-feet
C = runoff coefficient appropriate for storms less than one inch
P = 0.75 inch precipitation depth
A = area draining into the storm water practice, in acres.
Runoff coefficients required by the Ohio Environmental Protection Agency (Ohio EPA) for use in determining the water quality volume can be determined using the list in Table 1 or using the following equation to calculate the runoff coefficient, if the applicant can demonstrate that appropriate controls are in place to limit the proposed impervious area of the development:
C = 0.858i3- 0.78i2 + 0.774i+0.04, where:
i = fraction of the drainage area that is impervious
Table 1: Runoff Coefficients Based on the Type of Land Use
Land Use
| Runoff Coefficient
|
Industrial and Commercial | 0.8 |
High-Density Residential (>8 dwellings/acre) | 0.5 |
Medium-Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low-Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
Where land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the storm water treatment structure is Low- Density Residential, 30% is High-Density Residential, and 10% is Open Space, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) = (0.35) | |
B. An additional volume equal to 20% of the WQV shall be incorporated into the storm water practice for sediment storage. This volume shall be incorporated into the sections of storm water practices where, pollutants will accumulate.
C. Storm water quality management practices shall be designed such that the drain time is long enough to provide treatment and protect against downstream bank erosion, but short enough to provide storage available for successive rainfall events as defined in Table 2.
Table 2: Draw Down Times for Storm Water Management Practices
Best Management Practice | Drain Time of WQV |
Infiltration Facilities* | 24 - 48 hours |
Extended Conveyance Facilities (Vegetated Swales, Filter Strips) | |
Vegetated Filter Strip with Berm | 24 hours |
Enhanced Water Quality Swale | 24 hours |
Flow-Through Design | ** |
Extended Detention Facilities | |
Extended Dry Detention Basins*** | 48 hours |
Wet Detention Basins+ | 24 hours |
Pocket Wetland^ | 24 hours |
Constructed Wetlands (above permanent pool) | 24 hours |
Bioretention* | 40 hours |
Sand and other Media Filtration | 40 hours |
* The WQV shall completely infiltrate within 48 hours so there is no standing or residual water pool. ** Sized to pass a hydrograph with a volume equal to the WQV, a duration of two hours, peak rainfall intensity of one inch/hour at a depth of no more than three inches and have a minimum hydraulic residence time of five minutes. The use of this criterion is limited to sites where the total area disturbed is five acres or less. Prior approval from the City Engineer is necessary to use this practice. For sites greater than five acres or less than five acres but part of a larger common plan of development or sale which will disturb five or more acres, prior written approval is required from the Ohio EPA. *** The use of a forebay and micropool is required on all extended dry detention basins. Each is to be sized at a minimum 10% of the WQV. + Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQV ^ Pocket wetland must have a wet pool equal to the WQV, with 25% of the WQV in a pool and 75% in marshes. The EDV above the permanent pool must be equal to the WQV. | |
D. Each practice 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.
(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 feet below the final grade elevation, and any underlying bedrock is at least six 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 coarser 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 design drain time shall:
1. Not be located in areas where the depth to bedrock and/or seasonal high water table is less than three 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 2.5 foot deep layer of soil amendment with a permeability equivalent to a HSG A or B soil and an underdrain system is provided.
C. Facilities designed according to the flow through design drain time shall:
1. Only be allowed on sites where:
a. The total area disturbed is five acres or less;
b. The discharge rate from the BMP will have negligible hydrologic impacts to received waters as described in Section 2301.09(c)(6)B.;
c. Prior written approval is given by the City Engineer; and
d. For sites greater than five acres or less than five acres but part of a larger common plan of development or sale which will disturb five 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 inches.
3. Be designed to have a minimum hydraulic residence time of five 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.
A. The outlet shall be designed to not release more than the first half of the water quality volume in less than one-third of the drain time. A valve shall be provided to drain any permanent pool volume for removal of accumulated sediments. The outlet shall be designed to minimize clogging, vandalism, maintenance, and promote the capture of floatable pollutants.
B. The basin design shall incorporate the following features to maximize multiple uses, aesthetics, safety, and maintainability:
1. Basin side slopes above the permanent pool shall have a run-to-rise ratio of 4:1 or flatter.
2. The perimeter of all permanent pool areas deeper than four feet shall be surrounded by an aquatic bench that extends at least eight feet and no more than 15 feet outward from the normal water edge. The eight-foot wide portion of the aquatic bench closest to the shoreline shall have an average depth of six inches below the permanent pool to promote the growth of aquatic vegetation. The remainder of the aquatic bench shall be no more than 15 inches below the permanent pool to minimize drowning risk to individuals who accidentally or intentionally enter the basin, and to limit growth of dense vegetation in a manner that allows waves and mosquito predators to pass through the vegetation. The maximum slope of the aquatic bench shall be ten horizontal for every one vertical. The aquatic bench shall be planted with hearty plants comparable to wetland vegetation that are able to withstand prolonged inundation.
3. A forebay designed to allow larger sediment particles to settle shall be placed at basin inlets. The forebay and micropool volume shall be equal to at least 10% of the water quality volume (WQV).
4. When post-construction detention/water quality basin are to be used as temporary sediment trapping BMPs, a skimmer discharge device consistent with the Ohio Rainwater Manual shall be utilized during construction phase and until the site is deemed permanently stabilized by the Administrator as required by Chapter 2303, Erosion and Sediment Control. More information on skimmers can be found in Chapter 6 of the current version of the Ohio Rainwater and Land Development Manual: http://vww.dnr.state.oh.us/tabid/9186/Default.aspx
(6) Criteria for the acceptance of alternative post-construction BMPs. The applicant may request approval from the City Engineer for the use of alternative structural post- construction BMPs if the applicant shows to the satisfaction of the City Engineer that these BMPs are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in Table 2. If the site is greater than five acres, or less than five acres but part of a larger common plan of development or sale which will disturb five or more acres, prior approval from the Ohio EPA is necessary. To demonstrate the equivalency, the applicant must show:
A. The alternative BMP has a minimum total suspended solid (TSS) removal efficiency of 80%, using the Level II Technology Acceptance Reciprocity Partnership (TARP) testing protocol.
B. The water quality volume discharge rate from the selected BMP is reduced to prevent stream bed erosion, unless there will be negligible hydrologic impact to the receiving surface water of the State. The discharge rate from the BMP will have negligible impacts if the applicant can demonstrate one of the following conditions;
1. The entire water quality volume is recharged to ground water.
2. The development will create less than one acre of impervious surface.
3. The development project is a redevelopment project with an ultra-urban setting, such as a downtown area, or on a site where 100% of the project area is already impervious surface and the storm water discharge is directed into an existing storm sewer system.
4. The storm water drainage system of the development discharges directly into a large river of fourth order or greater or to a lake, and where the development area is less than 5% of the water area upstream of the development site, unless a Total Maximum Daily Load (TMDL) has identify water quality problems in the receiving surface water of the State.
(d) Storm Water Quantity Control. The Comprehensive Storm Water Management Plan shall describe how the proposed storm 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. Determine, using a curve number-based hydrologic method that generates hydrographs, or other hydrologic method approved by the City 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 years and not only the current land use.
4. 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, regardless of the surface proposed in the site description.
B. From the volume determined in Section 2301.09(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 1-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 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
(e) Storm Water Management on Redevelopment Projects. Comprehensive Storm Water Management Plans for redevelopment projects shall reduce existing site impervious areas by at least 20%. A one-for-one credit towards the 20% net reduction of impervious area can be obtained through the use of pervious pavement and/or green roofs.
(1) Where site conditions prevent the reduction of impervious area, storm water management practices shall be implemented to provide storm water quality control facilities for at least 20% of the site’s impervious area.
(2) When a combination of impervious area reduction and storm water quality control facilities are used, the combined area shall equal or exceed 20% of the site.
(3) Where projects are a combination of new development and redevelopment, the total water quality volume that must be treated shall be calculated by a weighted average based on acreage, with the new development at 100% water quality volume and redevelopment at 20%.
(4) Where conditions prevent impervious area reduction or on-site storm water management for redevelopment projects, practical alternatives as detailed in Section 2301.10 may be approved by the City Engineer.
(Ord. 160-11. Passed 6-20-11.)
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