1114.10 POST-CONSTRUCTION FACILITIES.
(a) General: New commercial sites, industrial sites and residential subdivisions as well as existing, downstream properties shall be protected from flood hazard and inundation by storm waters. Existing watercourses either originating in or flowing through a new commercial site, industrial site or residential subdivision shall be received and discharged from the development as nearly as possible at the rate, which existed prior to construction. Runoff from a new commercial site, industrial site or residential subdivision shall be drained to an adequate outlet. This outlet or termini location must be approved by the City of Dover and shall consist of a channel, stream, river, storm sewer, pond, lake or dedicated public drainage easement.
Post-construction storm water management shall be accomplished through the use of Best Management Practices that follow the requirements specified herein and as delineated in the most recent edition of Ohio EPA General Permit Authorization for Storm Water Discharges Associated with Construction Activity.
The hydraulic design and construction of drainage facilities shall be subject to the approval of the City of Dover. Grading within a new subdivision shall provide positive drainage for all lots. Depth of flow or ponding within a new subdivision shall not exceed a level which would cause inundation of foundations or basements of residences.
(b) Control of Storm Water Flow Rate: The flow rate of storm water from a developed site must be controlled in order to meet the following criteria:
(1) The peak flow of storm water from the developed site at an appropriately selected point of analysis shall not exceed the peak flow of storm water from the pre-developed site at the same point of analysis for the same year frequency storm. This requirement applies for all storms with a frequency of one hundred (100) years and less.
(2) The peak flow of storm water from the site during construction at an appropriately selected point of analysis shall not exceed the peak flow of storm water from the pre-developed site and the same point of analysis for the same year frequency storm. This requirement applies for all storms with a frequency of ten (10) years and less.
(3) The flow of storm water from the developed site shall be drained to an adequate outlet. This outlet must be approved by the City of Dover and will consist of a channel, stream, river, storm sewer, pond or lake having sufficient capacity to accommodate flow from the developed site.
(4) The flow of storm water from the developed site must not cause flooding to proposed buildings within the development or to existing downstream homes, buildings, places of business or other structures.
(5) Surface water draining from an existing watershed area cannot be diverted, channeled, piped or otherwise rerouted into another watershed area unless approved by the City of Dover.
(c) Storm Water Control Structures and Methods. Control of storm water runoff from a developed site may be accomplished using a control structure such as a storm water management basin, underground storage tanks or pipes, storage on pavement areas, storage on roof areas or a combination of these types of control measures. The primary purpose of any storm water runoff control structure is to temporarily store water during a storm and release it at a rate that meets the criteria given previously.
(1) The two (2) methods that are permitted for use to determine the peak flow rate of storm water from a site as well as for determining the required storage volume of a runoff control structure are the Modified Rational Method and the Soil Conservation Service Methods.
(2) Precipitation frequency for the developed site shall be determined from the twenty-four (24) hour precipitation duration data published by the National Oceanic and Atmospheric Administration, National Weather Service for Dover, Ohio.
(3) Drainage drawings for both the pre-developed and post-developed sites shall be provided. These drawings shall be to a scale that allows the following information to be neatly presented and clearly read:
A. Existing and proposed elevation contours shall be shown with the contour interval not exceeding a two (2) foot interval for slopes greater than ten percent (10%), and with the contour interval not exceeding a one (1) foot interval for slopes less than ten percent (10%). This requirement may be modified, if drawing clarity demands such.
B. Indicate the discharge location or locations on the drawings to which the pre-developed and post-developed storm waters flow. Also, indicate the location of the hydraulically most distant point or points from which storm waters originate.
C. Delineate in detail the contours and shapes of all storm water holding basins and their control structures and devices at a scale which lends clarity to their construction and installation.
(4) Storm water runoff control structures shall be constructed of reinforced concrete at locations where they can be readily maintained. Flows shall be regulated utilizing fixed weirs. Control structures shall also have overflow openings. Discharges from control structures into streams or other non-piped or rigid outlets shall have the discharge area protected with large aggregate riprap.
(5) All storm water holding basins shall have emergency overflows incorporated into their design in case the runoff control structures become blocked or their capacity is exceeded. The emergency overflow spillway shall be designed such that the flow of storm water from the developed site does not cause flooding to building or structures within the development or to downstream buildings or structures.
(6) The City of Dover reserves the right to select the type or types of storm water runoff control methods and structures that are to be designed for a definitive site. Only detention, retention or infiltration basins are acceptable runoff control structures for new residential subdivisions or other types of residential complexes, new large commercial developments, or new large industrial developments.
(d) Water Quality Volume: For large construction activities that are disturbing two (2) acres or more of soils, or that are part of a total common plan of development which will collectively disturb two (2) acres or more of soils, the post construction facilities must be sized to treat a volume of storm water (known as WQv) equal to the volume of storm water runoff from a nine-tenth (.90) of an inch rainfall over the developed site.
The water quality volume (WQv) shall be determined using the following equations:
WQv = ((Rv) (P) (A))/12 (Equation 1)
where,
WQv = Water quality volume in acre-feet
Rv = the volumetric runoff coefficient calculated using equation 2
P = 0.90 inch precipitation depth
A = acres draining into BMP in acres
Rv = 0.05 + 0.9i (Equation 2)
where i = fraction of post construction impervious surface
An additional volume of storm water runoff equal to twenty percent (20%) of the WQv shall be incorporated into the post construction facility for sediment storage and outlet control.
(1) The Storm water quantity volume (the volume of water stored to control the storm water runoff flow rate) must be stacked on top of the storm water quality volume for any storm water facility that will also serve as the post-construction storm water facility. In other words, use the top of the WQv as the base elevation for the storm water runoff control volume.
(2) The initial settling pool or forebay volume for sediment storage shall be equal to one half of the additional twenty percent (20%) volume required for sediment storage. To accommodate relatively frequent sediment cleanout, easy equipment access should be provided to the forebay.
(3) The other half of the additional twenty percent (20%) volume required for sediment storage shall be a micropool that filters and controls the rate of flow of the WQv. The flow control orifice cannot be less than two and one-half (2) inches in diameter and shall be designed to be non-clogging.
(e) Detention Basins:
(1) Detention basins shall provide the control of storm water flow rates and the water quality volume (water quality volume when applicable) prescribed in the preceding subsections (b) and (d). The construction of the basins shall conform to the design requirements required herein and those presented in the current edition of the previously referenced Ohio's "Rainwater and Land Development" manual.
(2) For safety considerations, the interior slopes of a detention basin shall not exceed a 4: 1 slope unless the perimeter of the basin is fenced with at least one gate large enough to allow maintenance equipment, such as trucks and backhoes, to enter into the basin for maintenance purposes. Large basins shall have additional gates for maintenance as required and approved by the City of Dover.
(3) An emergency spillway shall be provided as heretofore stated. The bottom elevation of the spillway shall be above the water elevation in the basin for the design runoff from all storms with a frequency of one hundred (100) years or less, and the required water quality volume.
(4) For basins that are to be dedicated to the City of Dover as part of its storm sewer system, the minimum top width of the side embankments shall be six (6) feet for non-vehicular traffic. Access easements from public streets or other public easements to the basins for vehicular maintenance equipment shall be a minimum of fourteen (14) feet in width and be dedicated to the City with the basin. The top width of side embankments for private basins not being dedicated to the City of Dover may vary from the preceding stated requirements; however, such widths shall be adequate enough, as determined by the City, for the basins to be properly maintained. All basins dedicated to the City for maintenance shall have their perimeter adequately fenced with six (6) feet high aluminized chain link fence and gates, arranged as per the maintenance needs of the City.
(f) Retention Basins:
(1) Retention basins shall conform to all of the construction and maintenance requirements stated for detention basins in addition to the following requirements.
A. Provisions shall be installed to prevent the water in the retention basin from becoming stagnant through the use of a properly designed aeration system.
B. Provisions for draining the retention basin for maintenance shall be an integral part of the basin design, and such shall be described on the submitted approval drawings for the basis.
(g) Underground Infiltration Basin, Dry Well or Infiltration Trench:
(1) Control of stormwater runoff can be achieved with the use of an underground infiltration basin, dry well or infiltration trench when such facilities are constructed in glacial soils and the use of such is approved by the City of Dover. Glacial soils in which these facilities are constructed shall have infiltration rates greater than 0.30 of an inch per hour. Buried underground infiltration basins shall be designed in accordance with the manufacturer's design requirements for its basins.
A. Infiltration rates used in the design of underground infiltration basins, dry wells and infiltration trenches shall be determined by soil tests conducted by a firm or individual who are normally engaged in providing soil testing. The results of such tests shall be certified in writing.
B. Only buried infiltration basins or structures shall be permitted. Open basins, open wells or open trenches will not be permitted.
(h) Pavement Storage:
(1) Storm water runoff control can be achieved by temporarily ponding water in parking areas. The maximum depth of water at its deepest point shall not exceed eight (8) inches, and in no case shall be greater than the finished floor elevation of any adjacent building.
A. The minimum slope of a parking lot surface shall be one percent (1%) and the maximum slope shall be ten percent (10%).
B. Outlet control structures shall be designed using the design criteria presented in subsection (b) hereof.
C. Provisions to prevent flooding of any adjacent building shall be made for the cases when the outlet control structure becomes clogged or with storms exceed the one hundred (100) year frequency.
(i) Roof Top Storage: Storage of storm water runoff can also be provided by temporarily ponding water on flat roofs with parapets that contain water. The building structural system shall be designed to accommodate the amount of water that is being stored and meet all of the structural requirements of the Ohio Building Code.
(1) Roofs shall have a minimum slope of twenty-five hundredths (0.25) of a percent.
(2) Outlet and overflow control drains shall be designed using the design criteria presented in subsection (b) hereof.
(j) For small construction activities, construction activities authorized under this permit which result in a disturbance of less than two (2) acres, a post-construction practice shall be used to treat storm water runoff for pollutants and reduce adverse impacts on receiving waters as per the most recent edition of Ohio EPA General Permit Authorization for Storm Water Discharges Associated with Construction Activity, Provisions of this permit which require approval from the regulated MS4 will be adhered to. As such, the use of non-standard BMP' s that are required because of a site’s physical constraints will be authorized by the City of Dover on a case by case basis.
(k) For redevelopment of previously developed areas, a reduction of the water quality requirement is permissible as outlined in the most recent edition of Ohio EPA General Permit Authorization for Storm Water Discharges Associated with Construction Activity, provided the conditions outlined in the permit are achieved.
(l) Maintenance Plans: Detail drawings and maintenance plans shall be provided for all post-construction BMPs in the SWPPP. Maintenance plans shall be provided by the permittee to the post-construction operator of the site (including home owner associations) upon completion of construction activities (prior to termination of permit coverage). Maintenance plans shall insure that pollutants collected within structural post-construction practices are disposed of in accordance with local, state and federal regulations. To ensure that storm water management systems function be designed and constructed, the post-construction operation and maintenance plan shall be a standalone document which contains the following information:
(1) There shall be a designated entity for storm water inspection and maintenance responsibilities.
(2) The plan shall include the routine and non-routine maintenance tasks to be undertaken.
(3) There shall be a schedule for inspection and maintenance.
(4) The plan shall include any necessary legally binding maintenance easements and agreements.
(5) The construction drawings or excerpts showing the plan view, profile and details of the outlet(s) shall be included.
(6) There shall be a map showing all access, and maintenance easements.
(7) The plan shall provide relevant elevations and associated volumes that dictate when removal of accumulated sediments must occur.
Plans shall address future changes in ownership to ensure that long-term operations and maintenance criteria continue to be met.
(m) Any desired modifications to post-construction water runoff controls that occur after the initial SWPPP approval must be approved the City of Dover.
(Ord. 48-21. Passed 12-6-21.)