(a) Storm Water Runoff from a developed site such shall be controlled to meet the following criteria:
(i) Runoff Rate: The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations proving no increase in the runoff rates from the two, five, 10, 25, 50, and 100 year storms with a 24 hour duration.
(ii) Runoff Rate During Construction: The peak runoff rate from the development area shall not be greater during construction than it was before development. This includes time of clearing and grading the entire site and periods when lots are stripped for building construction. The applicant shall provide calculations proving no increase in the runoff rates during construction from the two, five, and 10 year storm with a 24 hour duration.
(iii) Runoff Volume: Where there is an inadequate receiving stream, when discharging into the City storm sewer system, or where there is sufficient potential for downstream flooding, as determined by the City Engineer, the site runoff rate may be restricted further based on the runoff volumes. Increases in the runoff volume shall be offset by further restricting runoff rates. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the two year pre-development storm runoff rate. The critical storm shall be calculated as follows:
(1) Determine the total volume of runoff from either the one-year or the two-year frequency, 24-hour storm, occurring on the development area before and after development. (Ord. 35-07. Passed 3-19-07.)
(2) From the volumes in paragraph (1) determine the percent of increase in volume of runoff due to development according to the equation ((Q after minus Q before) divided by Q before) x 100 and, using this percentage, select the critical storm from the following table: (Ord. 45-09. Passed 9-8-09.)
CRITICAL STORM SELECTION
The Percentage Increase in volume of Runoff is:
Equal To Or Greater Than | And Less Than | The 24-Hour "Critical Storm" for Discharge will be |
0 | 20 | 2 years |
20 | 50 | 5 years |
50 | 100 | 10 years |
100 | 250 | 25 years |
250 | 500 | 50 years |
500 | --- | 100 years |
(iv) Detention or Retention Basin Exemption for Redevelopment or for Expansion of Existing Facilities:
(1) For any development regulated by this ordinance, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for a 100 year frequency 24 hour storm increases the existing peak discharge by one cubic foot per second or less using the Soil Conservation Service method of calculation or other method approved by the City Engineer. The City Engineer can waive this requirement if existing storm sewers and drainage structures can safely handle the expected increase in flow.
(2) Only one exemption will be allowed per parcel. Any subsequent expansion must provide for detention or retention and must include the previously exempted area.
(v) The flow of storm water from the developed site must not cause flooding to any proposed buildings within the development or to existing downstream homes, buildings, places of business or other such improvements.
(vi) 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 Engineer.
(b) Two methods for calculating storm water runoff volumes and control measures are approved. They are the Modified Rational Method and the Soil Conservation Service Method. Either may be used for sites up to six acres in size. The Soil Conservation Method must be used for all sites six acres or larger.
(c) Provide a drainage area map (or maps) for the pre-developed and post-developed sites. Perform field surveying as necessary to insure that this drawing is an accurate representation of actual field conditions. The scale of this drawing must be sufficient to clearly present the following information:
(i) Existing and proposed elevation contours at an interval of two feet or less. Contour lines shall be labeled frequently enough to be easily read and interpreted.
(ii) Hydrologic boundaries of watersheds, including areas outside the proposed development that flow into the project area.
(iii) Points of analysis, or the location where the drainage is being evaluated, for each watershed.
(iv) Path used to calculate the time of concentration for each watershed. Indicate types of flow and provide calculations.
(v) Delineate and label the types of land use, surface features, ground cover, soil types, etc. used to determine the runoff coefficients or curve numbers for the project areas.
(vi) Show areas of existing ponds, springs, wetlands, streams, and hydrologically sensitive areas within 1,000 feet of the proposed development.
(vii) Show existing and proposed storm water features such as ditches, swales, inlets, pipes, roadways, etc. within 500 feet of the proposed development, unless the ultimate outfall system is a lesser distance.
(viii) Show the proposed development layout.
(d) Calculations.
(i) Provide routing calculations and their corresponding inflow and outflow hydrographs for the two-year storm up through the 100 year storm. Prepare a summary table of results of the routing calculations. The table shall include, at a minimum, peak inflow, peak outflow, water surface elevations, and storage volume requirements.
(ii) Provide pond volume vs. elevation summaries.
(iii) Provide calculations for the inlet/outlet works including weirs, orifices, culverts, spillways, grates, etc. Show their capacities and stage-discharge calculations, including tailwater assumptions.
(iv) Verify capacities of receiving drainage features such as ditches, storm sewers, pond, streams, etc. to verify sufficient capacity to accommodate the runoff of the development. The receiving drainage feature must be approved by the City. Check all storms with a return frequency of 100 years or less. Determine flooding potential for surrounding homes, commercial businesses and industries.
(v) Provide runoff coefficients or curve number calculations for each watershed or sub-area, including impervious calculations.
(vi) Provide time of concentration calculations for each watershed or sub-area.
(vii) Calculate hydraulic grade line for outlet structures with pipes.
(viii) Determine velocities of discharged waters and show details to control erosion.
(ix) Provide calculations for water quality and erosion control measures as required for the Storm Water Pollution Prevention Plan (SWP3) (See Section 1168.09).
(x) Provide narratives, as needed, to describe the methods, assumptions, formulas, and intent of the calculations.
(e) Detention Basins.
(i) The minimum length to width ratio of the basin shall be two-to-one.
(ii) The grading of the detention basin shall be such that it reflects the surrounding topography. The embankment slopes for the detention basin should be four feet horizontal to one foot vertical preferred or a maximum of two feet horizontal to one foot vertical.
(iii) The maximum water depth shall not exceed 10 feet.
(Ord. 35-07. Passed 3-19-07.)
(iv) The minimum top width of the side embankments shall be five feet for non- vehicular traffic and 12 feet for vehicular traffic. The embankment shall be set at an elevation at least 12 inches above the emergency spillway and at least 18 inches above the peak 100 year pond elevation.
(Ord. 45-09. Passed 9-8-09.)
(v) Outlet structures using weir type flow regulators are preferred to reduce the risk of clogging.
(vi) Principal outlet (or outlets in a multi-stage configuration) shall have the capacity to pass the 100 year design storm flow.
(vii) Outlet pipes shall have a minimum diameter of six inches. Orifice plates must be used for restrictions of smaller diameter. Consideration should be given to removable trash racks or other means to prevent clogging.
(viii) Access to entire outlet structure for maintenance and inspection shall be provided and shall follow current OSHA standards.
(ix) An emergency spillway shall be provided and set at an elevation equal to 6 inches above the peak 100 year pond elevation. The spillway length shall be 10 feet at its narrowest point (may be less for very small ponds, as approved by the City). The location of the emergency spillway shall be such that its overflow can be directed to an acceptable location. Permanent erosion control measures at the emergency spillway may be required by the City based on potential erosion at the site.
(x) Detention structures shall be graded to drain to the outlet structure. The minimum grade in the pond shall be two percent. No paved gutters shall be used as the low flow channel.
(xi) A permanent easement shall be provided a minimum of 20 feet beyond the detention basin’s outside perimeter. See Section 1168.08.
(xii) A 20 foot permanent easement shall be provided from the pond to the closest public street or alley. Provide a gravel access drive, or gravel base topped with topsoil, per City standards, for the length and width of the easement. See Section 1168.08.
(xiii) All pipes through the embankment shall have anti-seep collars.
(xiv) Soils borings and testing shall be performed by an approved soils testing laboratory. Submit a report certifying suitability of the soils on-site for embankment and basin construction. If soils are highly permeable, a six inch clay layer, or other measures may be required.
(xv) Where the detention basin outlet flows onto private property or onto property owned by a government other than the City (such as the State of Ohio right-of-way), a discharge easement agreement (or Government permit) with that property owner must be prepared and executed. Proof of such easement shall be presented prior to the City’s approval of the drainage.
(Ord. 35-07. Passed 3-19-07.)
(xvi) Show dimensions of the detention basin on the plans that meet or exceed calculated pond values.
(Ord. 45-09. Passed 9-8-09.)
(f) Retention Basins. All of the criteria for the Detention Basins shall apply, with the following additional requirements:
(i) The grading of the Retention Basin shall be such that it reflects the surrounding topography. The embankment slopes for the Retention Basin shall be four feet horizontal to one foot vertical leading to the pond. It shall terminate at an aquatic safety bench.
(ii) The aquatic safety bench shall be a minimum of 10 feet wide with a maximum slope of three percent. The aquatic safety bench shall have a maximum water depth of one foot. Side slopes beyond the aquatic safety bench shall not be steeper than two feet horizontal to one foot vertical under the water.
(iii) Additionally, the developer or designer may add a second dry safety bench meeting the same width and slope requirements. Having a dry safety bench does not eliminate the requirement of an aquatic safety bench.
(iv) Retention basins must have a minimum of 10 acres draining to pond, or as approved by the City.
(v) Provide aeration equipment of adequate size for the pond to prevent the water from becoming stagnant.
(vi) Provide piping and valves necessary to completely drain the pond periodically for maintenance.
(g) Bio-Retention basins may be required whenever the proposed detention/retention facility discharges into an existing wetland, a creek or as required by the City. The design of the Bio-Retention basin is to follow the guidelines shown in Design of Stormwater Wetland Systems, prepared by Thomas R. Schueler. The manual can be obtained from the Metropolitan Washington Council of Governments. The allowable side slopes shall follow the Detention Basin requirements of these standards. The plant materials shall be suitable for the environment created.
(h) In certain applications, alternative detention facilities may be permitted and encouraged. These methods may be especially useful on small sites or where the increase in runoff is small. They may also be useful in conjunction with these methods described above. Requests for alternative detention facilities shall be submitted to the City within the appropriate design standards.
(i) Infiltration basin, dry wells, and infiltration ditches.
(1) Soils borings and testing shall be performed by an approved soils testing laboratory. Submit a report certifying the suitability of the soils on-site for infiltration.
(2) Infiltration measures must make provisions for overflow to a suitable area.
(ii) Underground Detention Tanks or Pipes
(1) May be used for commercial or industrial sites only.
(2) Must provide access for inspection and maintenance.
(3) Must have sufficient load bearing capacity.
(4) Must have a minimum bottom slope of 0.5 percent.
(iii) For very small sites (under two acres) detention in parking lots using catch basins.
(1) A maximum water depth of eight inches in parking areas may be used. Beyond such depth, an overflow or outlet must be provided.
(2) Maximum water depth shall not exceed the finished floor elevations of existing or proposed structures.
(3) Slopes for parking lots used for this purpose shall be a minimum of one percent and a maximum of 10 percent.
(iv) If none of these practices are applicable, the City reserves the right to waive storm water management requirements for developments under two acres. This may only be done after the designer has shown that all other options are not feasible.
(i) Maintenance of Storm Water Management Practices.
(i) An as-built survey must be completed by qualified registered engineer or surveyor and submitted to the City showing the location, detention volume (include depth and capacity) of all stormwater practices.
(ii) The City Engineer and/or Safety-Service Director shall approve an inspection and maintenance agreement binding on all subsequent owners of land served by the planned storm water management practices before the City accepts the final plat of the proposed project.
(iii) All inspection and maintenance agreements shall do the following:
(1) Designate the party responsible for maintenance of structural and nonstructural storm water management practices including mowing and ensuring outlet structures are clear and in good repair. Unless otherwise approved by the City, this shall be an entity of common ownership within the proposed subdivision (such as an Association) or the owner on an industrial or commercial site.
(2) Prohibit unauthorized alterations of structural and nonstructural storm water management practices.
(3) Provide access to storm water management practices for inspection by the City to document the condition of the practices. Authorize the City to make corrections to the facility, if deemed necessary, and assess the affected property owners. See Sec. 1168.07(j) below.
(iv) The location, dimensions, and bearing (including the depth and capacity) of all storm water management practices shall be incorporated on the final plat, prior to approval by the City, and reference thereon shall be made to the entity or individual(s) responsible for maintenance.
(j) Inspection of Storm Water Management Practices.
(i) The City may inspect storm water management practices periodically.
(ii) Upon finding a malfunction or other need for maintenance, the City shall notify the responsible party of the need for maintenance.
(iii) Upon notification, the responsible party shall have 15 working days, or other time stipulated by the City, to make repairs.
(iv) Should repairs not be made within this time, or a plan approved by the City for these repairs not be in place, the City may undertake necessary repairs and assess the responsible party.