(a) Drainage Outlet. Storm water runoff from a development shall be drained through an adequate drainage outlet as approved by Planning Commission and/or City Engineer. The outlet may be a ditch, stream, storm sewer, or approved basin, which has sufficient capacity and structural integrity to accommodate the runoff in an approved manner. If the proposed outlet has current problems downstream (as identified by the City), the City may either revise the proposed drainage criteria specified to alleviate the downstream problems or require that a storm drainage system be provided to an adequate outlet.
(b) Major Flood Path. The intent of planning a major flood path is to ensure that the storm water runoff that may exceed the capacity of the open channel or storm sewer system has a route to follow that will not cause a loss of property or life. Depending on topography, the site proposed for development may have multiple major flood paths or a single major flood path that could be comprised of the street and sewer system and/or swales or channels located outside of the street right-of-way.
The combination of the major flood path and sewer/channel system shall have the capacity to carry runoff from a 100-year frequency storm. Where a street is designated as a portion or all of the major flood path, the depth of flow shall not exceed 8" at the face of the curb. When a portion of the major flood path is located outside a street right-of-way, drainage easements shall be provided and a grading plan is to be submitted with detailed elevations showing the flood being contained in this area.
The minimum basement and first floor opening elevations of garages, doors, windows, etc. along all calculated one-hundred (100) year major flood paths are to be set by the design engineer. These elevations are to be a minimum of one (1) foot above the calculated one hundred (100) year water surface elevation. These elevations are to be clearly labeled on the plans and the subdivision plat (if applicable). In addition, all lots along the major flood path shall have the building "envelope" (that area where structures are allowed to be built) established by the design engineer.
Since streets may be used as a routing path, the major flood path system must be taken into account in the initial design of the development. If detention or retention is required, the major flood path shall be designed in such a manner so to direct the storm water into the detention or retention area. Calculations indicating the capacity of the major flood path and the excess storm water runoff from the sewer system and/or swales or channels shall be provided with the final drainage plan.
(c) Detention/Retention Facilities.
(1) Detention and/or retention basins or other structures shall be constructed as necessary to provide temporary storage of storm water runoff for newly developed and redeveloped areas where the proposed improvements increase the peak runoff rate from the site.
(2) Design criteria. The peak rates of runoff and volumes shall be controlled using the "Critical Storm Method". This method is used to determine the design frequencies utilized in the design of the detention/retention structures for the development.
A. "Critical Storm Method".
1. The peak rate of runoff from the Critical Storm (as determined below) and all more frequent storms occurring on the development area shall not exceed the peak rate of runoff from a 2-year frequency storm (of 24-hour duration) at the outlet location under pre-development conditions. The City may require the use of a 1-year pre-developed storm.
2. Storms of less frequent occurrence than the Critical Storm, up to the 100-year storm, shall have peak runoff rates no greater than peak runoff rates from equivalent size storms under pre- development conditions at the outlet location.
The Critical Storm for a specific development area is determined as follows:
a. Determine, by appropriate hydrologic methods (described below), the total volume of runoff from a 2-year frequency, 24-hour storm occurring over the development area before and after development.
b. From the volumes determined above, calculate the percentage increase in volume of runoff due to development, and using this percentage, select the 24 hour Critical Storm from the following table:
% Increase in Volume of Runoff “Critical Storm” Discharge Limitation | ||
(at least) | (but less than) | Year |
0 | 20 | 2 |
20 | 50 | 5 |
50 | 100 | 10 |
100 | 250 | 25 |
250 | 500 | 50 |
500 | or more | 100 |
B. For detention/retention structures located "off-line" (not within a channel draining upstream off-site flows or otherwise receiving off- site flows), storage volume does not have to be provided for runoff from off-site upstream areas. Upstream runoff should be conveyed through the site in accordance with the current runoff conditions (with respect to flow rate and location).
For all detention structures located "in-line" (receiving flows from upstream off-site areas), the upstream peak flow and volume shall also be detained. The "in-line" basin volume shall be designed to function according to the above design criteria assuming the upstream watershed is fully developed in accordance with the current zoning.
C. The requirements of this section for runoff rates and volumes shall be satisfied at each location where runoff leaves the development area.
D. For areas less than 5 acres, the Modified Rational Method shall be used to develop the inflow hydrographs. For areas between 5 and 20 acres, the Modified Rational and Technical Release 55 (TR-55) Methods are acceptable. Any development over 20 acres shall use the TR-55 Method or USGS Regression Equations as approved by the City.
Pre-developed and developed hydrographs shall be completed for a 24-hour duration when using the Soil Conservation Technical Release 55 (SCS TR-55) and the applicable durations when using other methods, as approved by the City.
E. Routing calculations and their corresponding inflow and outflow hydrographs shall be provided for the Critical Storm and all less frequent storms up through the 100-year storm, the results of which shall be tabulated and included in the drainage plan. The table shall include, but is not limited to: peak inflow, peak outflow, storage volumes, and maximum water surface elevation.
F. The peak 100-year water surface elevation of the detention basin and/or retention basins or other structures shall be 1 foot lower than the lowest grade of the adjacent parcel of land or as directed by the City to prevent property damage.
G. Detention and/or retention basins or other structures and appurtenances shall be designed to avoid hazardous conditions and prevent stored runoff from seeping through or undermining the facility.
(d) Detention Basin.
(1) Allowable side slopes – The grading of the detention basin shall be such that it reflects the surrounding topography. The embankment slopes for the detention basin should be 4'horizontal to 1' vertical (4:1) preferred or at a maximum of 2' horizontal to 1' vertical (2:1).
(2) Outlet/overflow structure(s) – The outlet structure(s) shall be designed to maximize the available storage area. For non-water quality purposes, the outlet structure(s) should be a minimum of 6" in diameter. Access to the entire outlet structure for maintenance and inspection purposes shall be provided. Any orifices or other outflow devices needed for water quality purposes shall not be considered in the detention routing analysis. The total allowable discharge from the basin shall be subject to approval by the City Engineer.
(3) Emergency spillway – An Emergency Spillway shall be provided and set at an elevation equal to 0.5' (minimum) above the 100-year water surface elevation in the basin. The design of the spillway shall be as follows:
A. For outlets having a primary orifice diameter less than or equal to 15" – The emergency spillway shall be designed to pass the 100-year storm minus the flow through any secondary outlets. The depth of water in the spillway shall be no greater than 1 foot.
B. For outlets having a primary orifice diameter greater than 15" – The emergency spillway length shall be 10 feet and set at an elevation 1 foot below the top of the embankment.
Permanent erosion control measures at the emergency spillway may be required by the City based on potential erosion at the site.
(4) Low flow channel – Detention facilities shall be graded to drain to the outlet structure. The minimum grade in the pond shall be 3%. Paved gutters shall not be used as the low flow channel.
(e) Retention Basin.
(1) Allowable side slopes – The grading of the retention basin shall be such that it reflects the surrounding topography. The embankment slopes for the retention basin shall be 4'horizontal to 1' vertical (4:1) leading to the permanent pond. All ponds shall have a 10-foot wide bench around the entire pond. The bench shall be set at an elevation 1 foot below the normal water surface elevation of the permanent pond.
(2) Outlet/overflow structure(s) – The outlet structure(s) shall be designed following the guidelines in subsection (d)(2) hereof except for the following:
A. The invert of the primary structure shall be set at an elevation such that the depth of the pond shall be a minimum of 3 feet.
B. Anti-seep collars shall be provided for all outlet pipes through embankments.
(3) Emergency spillway – Refer to subsection (d)(3) hereof.
(4) Miscellaneous – Retention basins shall have a minimum of 10 acres draining to the pond or as approved by the City.
Soil borings may be required by the City to verify the suitability of the soils at the site. If soils are highly permeable, a 6" clay liner may be required.
(f) Bio-retention Basin. The detailed design of bio-retention basins 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. General guidance for the design of bio-retention basins is also available in the Ohio Department of Transportation Location and Design Manual, Volume II – Drainage Design, latest edition.
The allowable side slopes shall follow subsection (e)(1) of these standards. The plant materials shall be suitable for the environment created.
(g) Alternative Detention Facilities. In certain applications, alternative detention facilities may be permitted on a case-by-case basis. Alternative detention facilities could consist of (but are not limited to) the following:
(1) Infiltration Basin
(2) Underground Detention (Limited to sites with single owners having the ability to maintain the facility)
All requests for alternative detention facilities shall be submitted to the City with appropriate design standards. After review of the request, if approval is granted, soil borings shall be required for an infiltration basin.
(h) Detention Facilities on Small Sites. Small developments (less than 2 acres) may have difficulty providing the storm water management required, as described above. Maintenance problems arise when the outlet for common detention structures becomes small (less than 6"). Where this applies, the following options shall be considered to decrease the peak rate of runoff.
(1) Provide detention in parking areas utilizing catch basins with the orifice exiting from the bottom of the basins to the sewer.
(2) Provide a reduction in peak flows with the use of vegetative best management practices (BMPs). These include the use of grass swales and filter strips. To utilize these BMPs, the proposed parking areas and roof drains should discharge into these structures, which will detain the peak flow by increasing the flow time across the site. Detailed information on these structures is contained in "CONTROLLING URBAN RUNOFF: A Practical Manual for Planning and Designing Urban BMPs", which can be obtained by contacting the Metropolitan Washington Council of Governments. General guidance for the design of vegetative BMPs is also available in the Ohio Department of Transportation Location and Design Manual, Volume II – Drainage Design, latest edition.
(3) Provide detention using a drywell or other infiltration device with an overflow as approved by the City of Louisville.
If none of these practices are applicable, the City reserves the right to waive the storm water management requirements for developments under 2 acres. This may only be done after the designer has shown that all other options are not feasible.