The following specific information shall be included in the Storm Water Management Plan. The plan shall be submitted to the City Building Commissioner with a scaled site plan for the particular site development.
(a) A Storm Water Management Report providing a narrative of the general project description, including the nature, type and purpose of the earth disturbing activity and the larger common plan of development (if any).
Also include:
(1) Describe the existing condition (pre-development) with the soil type, areas of each type of land use and existing drainage patterns.
(2) Describe the proposed condition (post development) with the area of each type of land use and drainage patterns.
(3) Calculate the "Critical Storm" and use the TR55 methods from the Department of Agriculture, Soil Conservation Service Engineering Division.
(4) Summarize the results of the Storm Water Management Report.
(5) The name, address and telephone number of the plan designer and the owner or person(s) responsible for the development area.
(b) A vicinity sketch locating the larger common plan of development and the development area with respect to major and minor roadways and highways within the City.
(c) A scaled site plan showing the following information:
(1) The larger common plan of development, the development area and all pertinent surrounding features, including water resources, sensitive areas and existing storm water conveyance systems.
(2) Surface water locations, including springs, wetlands, streams, lakes, ponds, etc., on or within 200 feet of the site boundary.
(3) If storm waters will be discharging into a municipal separate storm sewer system or into a storm water management structure such as a detention basin that is off the site, clearly indicate this on the plans.
(4) Existing and planned locations of buildings, roads, utilities and parking facilities which may affect storm water management facilities.
(5) The total area of the site and the area of the site that is expected to undergo excavation, filling, grading or clearing. Include acreage of individual phases if applicable.
(6) Show borrow, spoil and topsoil stockpile areas.
(7) The name and/or location of the immediate receiving stream or surface water, and the subsequent named receiving water and the major river watershed in which it is located.
(8) The plan for protection, enhancement or replacement of riparian zones around any stream, waterway or wetland contained on the site.
(d) A scaled drainage plan with details showing the following information:
(1) Existing and proposed watershed boundary lines, the direction of flow and the existing and proposed watershed acreage.
(2) The types of soils within or affected by the development area and the location of all highly erodible or unstable soils, as determined by the most current edition of the Soil Survey of Cuyahoga County. Other soils information such as permeability, perched water table and location of bedrock may be noted.
(3) The location and description of existing and proposed drainage patterns and facilities, including any related drainage facilities beyond the development area and upstream of the site. Anticipated drainage patterns during major phases of construction shall also be included, as applicable.
(4) The location of areas currently receiving runoff from the development area.
(5) The existing and proposed topography shown in appropriate contour intervals. (1 foot)
(6) Existing and proposed runoff flow paths and lengths.
(7) Drainage facilities and contributing areas.
(8) Typical cross section and pertinent details of proposed detention system.
A. Detention Basin
B. Stormwater Storage Pipes
C. Parking Lot Flooding Storage
D. Details of Infiltration Program
(e) The following methods, procedures and criteria shall be used:
(1) Storm Water Management systems shall be designed for the ultimate use of the land. Phased construction for developments shall provide a Storm Water Management system for the ultimate development of the total development area.
(2) Storm Water Management detention or retention basins and facilities shall be designed so that they will continue to function with minimal maintenance and maximum water quality benefit. Developers and designers shall make all reasonable attempts to avoid wetlands and shall not cut-off the water supply to an identified wetland. Wetland impacts must be coordinated with the USACE and/or OEPA.
(3) The critical storm for a specific development area is determined as follows:
A. Determine the total volume of runoff from a one-year frequency, twenty-four hour storm, occurring on the development area before and after development.
B. From the volumes determined above, determine the percent of increase in volume of runoff due to development.
C. Using this percentage, select the critical storm from Table 1:
TABLE 1 | ||
The Percentage Increase in Volume Runoff is | The 24-Hour Critical Storm for Discharge Will Be (yrs.) | |
Equal to or Greater than | and Less than | |
0 | 10 | 1 |
10 | 20 | 2 |
20 | 50 | 5 |
50 | 100 | 10 |
100 | 250 | 25 |
250 | 500 | 50 |
500 | No Limit | 100 |
(4) Methods for controlling increases in storm water runoff peaks and volumes may include, but are not limited to:
A. Detention or retention basins or subsurface storage areas.
B. Retarding flow velocities by increasing friction.
C. Induced infiltration of increased storm water runoff into soil, (where practical); for example, constructing special infiltration areas where soils are suitable or providing good infiltration areas with proper emergency overflow facilities. Soil limitations shall be determined by the current edition of the Cuyahoga Soil and Water Conservation District survey by the Soil Conservation Service, USDA.
(5) The runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations to the City Building Commissioner, to be approved by the City Engineer, proving no increase in the runoff rates from the two, five, ten, twenty-five, fifty, and 100 year frequency storms.
(6) 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 and all shorter duration storms shall be restricted to the one (1) year pre-development storm runoff rates.
(7) The design criteria shall be applied to each watershed within the development area.
(8) A paved channel for low water flow is required across all detention basins for each point source into the basin.
(9) Velocity dissipation shall be placed at the outfall structures of all detention or retention basins and along the length of any outfall channel as necessary to provide a non-erosive flow velocity from the structure to a water course.
(10) Storm Water Management Detention Basins and facilities may be designed to serve the dual purpose of a sediment trap during construction and post construction.
(11) Storm sewer design for a ten year storm using the Rational Method to estimate the storm runoff rate "Q". Use Cleveland Regional Rainfall Intensity Frequency Duration Curves for this region.
(12) For sites which are currently developed and are scheduled to be re- developed, the increase between pre-developed and post-developed runoff shall be used for critical storm and volume storage calculations.
(f) Complete flood routing calculations will be required to confirm that the proposed detention basin facility meets the above criteria. As a minimum, the flood routing calculations shall include the following information:
(1) Overall Drainage Plan of the site showing existing and proposed grading, existing and proposed runoff flow paths and drainage facilities, contributing areas draining to the drainage system and predominant soil types.
(2) Plan View, typical cross-sections and pertinent details of the proposed detention basin and its primary and emergency outlet control structures.
(3) Detention Basin Reservoir Capacity vs. Reservoir Water Surface Elevation Calculations.
(4) Outlet Control Structure Discharge vs. Reservoir Water Surface Elevation Hydraulic Calculations, including the assumptions or calculations made to determine the existing drainage system hydraulic gradient or water surface elevation at the outlet control structure discharge point.
(5) Inflow-Outflow Hydrographs, both in tabular and graphical form for the one year storm, the CRITICAL STORM and all storms exceeding the CRITICAL STORM. The required inflow hydrographs shall include both pre development and post-development conditions.
(6) The assumptions and/or calculations utilized in determining the inflow hydrographs in accordance with the accepted hydrologic method.
(7) Hydraulic Gradient Calculations for the proposed storm drainage system emptying into the detention basin based on the CRITICAL STORM design year and other storm design year frequencies as may be required by the City Engineer.
(g) There are additional Post Construction Management requirements for construction areas of over five (5) acres of land as follows:
(1) Area smaller then five (5) acres shall be included when they are part of a larger plan. The additional requirements shall be incorporated into the permanent drainage system for the site. The detention basin must be sized to treat Water Quality Volume (WQV) and comply with Ohio Water Quality Standards in Ohio Administrative Code (OAC) Chapter 3745-1.
(2) The Water Quality Volume shall be equivalent to the volume of runoff from a 0.75 inch rainfall and shall be determined using the following equation:
WQV =CxPxA/12
Where: WQV = water quality volume in acre-feet
C = runoff coefficient appropriate for storms less than 1 inch (see Table 2)
P = 0.75 inch precipitation depth
A = area draining into the basin in acres
TABLE 2 | |
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Resident (>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 the 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 coefficients calculated as follows (0.6) (0.3) + (0.1) (0.2) = 0.35.
(3) An additional volume equal to 20% of the WQV shall be incorporated into dry basin size for sediment storage. Additional methods to assure long term water quality will be considered on a case by case basis.
(4) Whatever method is used the system shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage available for successive rainfall events as described in Table 3 below.
TABLE 3 Target Draw Down (Drain) Times for Structural Post-Construction Treatment Control Practices | |
Detention System | Drain Time |
Infiltration | 24-48 hours |
Vegetated Swale and Filter Strip | 24 hours |
Extended Detention Basin (Dry Basins) | 48 hours |
Retention Basins (Wet Basins)* | 24 hours |
Constructed Wetlands (above permanent pool) | 24 hours |
Media Filtration, Bioretention | 40 hours |
*Provide both a permanent pool and an extended detention volume above the permanent pool, each sized at 0.75 * WQV.
(5) For redevelopment projects (i.e., developments on previously developed property), post-construction practices shall either ensure a 20 percent net reduction of the site impervious area, provide for treatment of at least 20 percent of the WQV, or a combination of the two.
(Ord. 7664-04. Passed 1-17-05.)