(a) General: All components of the storm water system, including SCMs for storage, treatment and control, and conveyance facilities, shall be designed to prevent structure flooding during the 100-year, 24-hour storm event; to maintain predevelopment runoff patterns, flows, and volumes; to meet the requirements of the Construction General Permit, and to meet the following criteria:
(1) Integrated SCMs that address degradation of water resources. The SCMs shall function as an integrated system that controls flooding and minimizes the degradation of the water resources receiving storm water discharges from the site. Acceptable SCMs shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a watercourse restoration project and complies with Chapter 1157
.
B. Maintain predevelopment hydrology and groundwater recharge on as much of the site as practicable.
C. Only install new impervious surfaces 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 predevelopment levels.
E. Be designed according to the methodology included in the most current edition of Rainwater and Land Development or another design manual acceptable for use by the City of Aurora and Ohio EPA.
(2) Practices designed for final use: SCMs 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 routine maintenance.
(3) Storm water management for all lots: Areas developed for a subdivision, as defined in Section 1103.02
(79) A. and B., 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: SCMs and related activities shall not be constructed in water resources per Chapter 1157
.
(5) Storm water ponds and surface conveyance channels: All storm water pond and surface conveyance designs must provide a minimum of one (1) foot freeboard above the projected peak stage within the facility during the 100-year, 24-hour storm. When designing 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 1175.09
if it can be shown to the satisfaction of the City of Aurora 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 SCM, or if the storm water management requirements for the site are provided by SCMs defined in a regional or local storm water management plan approved by the City of Aurora Engineer.
(7) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City of Aurora Engineer as detailed in Section 1175.08.
(8) Ownership: Unless otherwise required by the City of Aurora, SCMs serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership. SCMs serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.
(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 includes minimizing site grading and compaction; protecting and/or restoring water resources, riparian areas, and existing vegetation; and maintaining unconcentrated storm water runoff to and through these areas.
(10) Post-Construction Soil Restoration: Except for areas that will be covered by impervious surface or have been incorporated into an SCM, the soil moisture-holding capacity of areas that have been cleared and graded must be restored to that of the original, undisturbed soil to the maximum extent practicable. Areas that have been compacted or had the topsoil or duff layer removed should be amended using the soil profile restoration design criteria in Rainwater and Land Development.
(b) Storm Water Conveyance Design Criteria: All SCMs 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:
(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 is 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 of Aurora 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 the sheet flow areas. In no case shall the sheet flow length be longer than 100 feet, nor shall a sheet flow area exceed 1.5 acres. Flow shall be directed into an open channel, storm sewer, or other SCM from areas too long and/or too large to maintain sheet flow, all as determined by the City of Aurora Engineer.
(4) Open channels: Unless otherwise allowed by the City of Aurora Engineer, drainage tributary to SCMs 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 the City of Aurora zoning or where the use of an open drainage system affects public health or safety, all as determined by the City of Aurora Engineer. The following criteria shall be used to design storm sewer systems when such systems are determined by the City of Aurora Engineer to be necessary.
A. Storm sewers.
1. Storm drainage piping shall be at least twelve (12) inches diameter.
2. The storm sewer outfall shall be designed to provide adequate protection against downstream erosion and scouring.
3. The design of storm sewers shall be based on the rational method or methodology approved by the City Engineer. Storm sewers shall be designed such that they do not surcharge from runoff caused by a 10-year storm, and that the hydraulic grade line of the storm sewer system stays below the rim elevation of all drainage structures during a 25-year storm. The system shall be designed to meet these requirements when conveying the flows from the contributing drainage area within the proposed development and existing flows from offsite areas that are upstream from the development.
4. The capacity of inlets, catch basins, or other inflow structures draining into storm sewers should be equal to or greater than the peak flow rate from the contributing drainage area for a 25-year storm, such that the inflow structure does not restrict flow. An overland flow path designed for a 100-year storm shall be placed at all low points or sags to further ensure against flooding. Design methodology utilized should be similar to those presented in Section 1103 (Pavement Drainage) of the Ohio Department of Transportation Location and Design Manual Volume Two Drainage Design, latest version.
B. Storm water channels, ditches, swales.
1. Location. Generally acceptable locations of storm water channels in the design of a subdivision may include, but are not limited to, the following:
a. Adjacent to roadways.
b. In a depressed median of a double roadway, road or parkway provided the median is wide enough to permit slopes of one (1) foot drop in six (6) feet horizontal or flatter.
c. Centered on lot lines or entirely within the rear yards of a single row of lots or parcels.
d. In each of the foregoing cases, a drainage easement with sufficient width to facilitate maintenance and design flow shall be provided and shown on the plat.
2. Roadside Storm Water Channels. In some instances, roadside ditches shall be permitted subject to approval by the City. The size, slope and cross-section of roadside ditches shall be designed for the calculated capacity in accordance with the applicable ODOT design standards. Erosion protection, such as seeding, sodding, jute matting rip-rap, concrete or rock channel protection, shall be provided as necessary for the calculated flow velocity.
3. Channels along lot lines. Whenever the plans call for the passage and/or storage of storm water runoff along lot lines, the grading of all such lots shall be prescribed and established for the passage and/or storage of waters, and no structure or vegetation which would obstruct the flow of storm water shall be allowed, nor shall any change be made to the prescribed grades and contours of the specified storm water channels.
C. Roof Drains. All roof drainage, and footer drainage if required, shall be collected by downspouts and directed into the storm sewer drainage system where storm sewers are available. The applicant may request approval from the City of Aurora Engineer for the use of an alternative system for managing roof and footer drainage. The City of Aurora Engineer shall grant the relief requested if the alternative system for managing roof drainage uses BMPs which will not cause excessive erosion or water damage. Where storm sewers are not available, a system for managing roof drainage shall be designed and installed as approved by the City of Aurora Engineer. Under no circumstances, however, shall any such system violate the design criteria in the current applicable building code including requirements to keep roof and footer drainage a sufficient distance from the structure so as to prevent damage to that structure.
D. Driveway culverts shall be a minimum twelve inch (12") diameter pipe with a minimum slope of one percent (1%). Pipe material shall be as specified by the City of Aurora Engineer.
E. Roadway culverts shall be designed in accordance with Section 1105 of the Ohio Department of Transportation Location and Design Manual Volume Two Drainage Design, latest version. Roadway culverts shall be designed such that the full-flow capacity is equal to or greater than the peak flow rate from the contributing drainage area for a 25-year storm event. In addition, the roadway culvert shall be sized to prevent flooding of the adjacent roadway for a 100-year storm event.
Replacement of existing roadway culverts shall attempt to meet the criteria above provided that increased flow rates do not adversely impact downstream properties or contribute to downstream flooding. At a minimum, roadway culverts that replace existing culverts shall maintain an equivalent full-flow capacity as the existing culverts.
(6) Water Resource Crossings. The criteria in Chapter 1157
shall be used to design structures that cross a water resource in the City of Aurora. Bridges shall be designed to comply with ODOT and AASHTO standards.
(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) 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: The site shall be designed to direct runoff to one or more SCMs that meet or exceed the criteria in the Construction General Permit.
(d) Storm Water Quantity Control: The Comprehensive Storm Water Management Plan shall describe how the proposed SCMs 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. Additionally, to protect against stream erosion or hydromodification, the post-developed peak flows from the 1-year and 2-year 24-hour storms should be released at a flow rate less than or equal to 30% of the pre-developed 2-year 24-hour peak flow.
(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 of Aurora 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 prescribed in the City of Aurora's Stormw Water Management Report Requirements document.:
B. From the volume determined in subsection (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 1:
Table 1: 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 | ||
(4) When a site consists of more than one drainage area and runoff will be discharged from the property in various directions under post-development conditions, the post-development peak rate of discharge for each of the drainage areas shall not exceed the allowable discharge rates in subsections (d)(1) and (d)(2) as based on the pre-development discharge from each of the drainage areas. The Critical Storm for each of the drainage areas shall be calculated in accordance with subsection (d)(3) and runoff volumes shall be calculated based on the actual size of each drainage area for both pre-development and post-development conditions.
(e) Storm Water Quality Management on Redevelopment Projects. SCMs on previously developed sites must meet the criteria in the Construction General Permit.
(f) Spillway. Emergency spillways shall be provided to permit the safe passage of runoff generated above the 100-year basin elevation. The spillway elevation shall be one half (0.5) feet above the 100-year basin elevation.
(g) Freeboard. Detention and retention facilities shall have adequate capacity to contain the storage volume of tributary storm water runoff with at least one (1) foot of freeboard above the water surface of flow in the emergency spillway, as required by State law.
(Ord. 2022-080. Passed 5-23-22.)