(a) Introduction.
(1) Every subdivision and land development shall be provided with a stormwater management system which is adequate to serve the area and meet the requirements of this chapter and other criteria of the Village.
(2) Developers are required to design improvements such that in a 100 year storm, the rate of stormwater runoff leaving the project area at strategic points is no more after development than if the project area had remained undeveloped. If necessary, detention/retention facilities shall be constructed to assure that this requirement is met.
Where an existing site is being partially or totally redeveloped all requirements of this chapter will be in full force and effect. If conditions warrant on partially redeveloped sites and the developer can show that the application of all requirements would cause a hardship, he may request partial relief from Planning Commission.
(3) The Planning Commission may waive requirements for an individual detention/retention basin if a common or regional detention/retention basin of adequate design is available or if the Village is reasonably certain one will be constructed and if the major drainage system from the project area to the common or regional detention/retention basin is such that the public health, safety and welfare will not be in jeopardy.
If this option is exercised, the developer must agree in writing to participate in the cost of the common or regional detention/retention basin whether already constructed or planned. The amount of participation and method of collection will be determined by the Village.
(4) It is not the intent of this chapter to hinder innovative and creative solutions to drainage problems, however, in the interest of expediting the processing of plans and construction, use of standard procedures, forms, nomographs, charts, and computer programs is necessary. Deviation from these standards will cause delay in the approval process.
(5) Although the submission requirements are specific, they are also the minimum requirements. The Village Engineer may recommend to Planning Commission a higher degree of protection than specified if the design results do not appear adequate to protect the health, safety, and welfare of the community.
(6) Stormwater management systems shall be designed for the ultimate use of the land.
(7) Subsurface drainage such as underdrains may be required at the discretion of the Village Engineer.
(8) Continued maintenance. Once a Storm Water Management Plan has been approved and constructed it shall be the responsibility of the property owner to maintain the facility as designed and constructed and to ensure its proper operation to meet the intent and requirements of this chapter at all times.
(b) Stormwater Management System.
(1) The development of a comprehensive stormwater management system requires providing two separate and distinct drainage systems, the minor system and the major system.
A. The minor drainage system is for collecting and transporting runoff from frequently occurring storms. It includes open channels, street curbs and gutters, and underground storm sewers, manholes, catch basins, and culverts. This system’s purpose is to lessen or eliminate inconveniences and safety and health hazards associated with frequent storms. Except where indicated otherwise, design criteria and requirements of this chapter are directed to the minor drainage system.
B. The major drainage system is to insure that stormwater runoff which exceeds the capacity of the minor drainage system has a route to follow to the retention basin. It must be recognized that the major drainage system exists even when it is not planned and whether or not physical facilities are intelligently located in respect to it.
(2) Submission requirements for subdivisions. The following plans, profiles and supporting documentation to verify conformance with this chapter shall be submitted to the Planning Commission for approval of any subdivision.
A. Preliminary Plans. In addition to the subdivision requirements, a plan showing the total area contributing runoff to the subdivision or project area being considered shall be submitted with the preliminary plans. This plan shall contain, but is not limited to, the following information:
1. A contour plan showing the outline of all areas outside the project area that contributes runoff to it;
2. Estimated runoff (Q) before and after development for terminal points along natural streams, proposed open channels, and other strategic points such as existing storm sewers or culverts;
3. Location of proposed detention/retention areas;
4. Any other information required by the Village to clarify intent.
B. Improvement Plans. In addition to the subdivision requirements, the improvement plan for the project area shall contain, but is not limited to, the following information:
1. Diameter, length, slope, type pipe, and class of all storm sewers, culverts, and subsurface drainage;
2. Invert elevations on profiles of all pipes at terminal points such as manholes, inlets, catch basins, and headwalls;
3. Top of grate elevations of manholes and grate flowlines of catch basins and inlets;
4. Type of catch basin, inlet and manhole (ODOT or Village designation);
5. Headwall type (ODOT or Village designation);
6. Actual existing and proposed cross sections of open channels showing width of bottom, depth of water, erosion control measures and limits, and side slopes at each point of design along with a profile indicating the longitudinal slope and bottom elevations at the terminal points of design;
7. High and low points indicating the direction of runoff flow along the profile of the roadway;
8. Structural details and design data for detention/retention facilities;
9. Details of construction for all structures not included in the Village standard construction drawings, or other referenced standards;
10. Easements;
11. Detention/retention facilities;
12. Any other information required by the Village Engineer to clarify intent or design features.
C. Drainage and Grading Plans. In addition to the improvement plan, a drainage plan shall be submitted. This plan may be the required improvement plan or a similar plan at a scale of one inch = 100 feet or larger showing at least the following additional information:
1. Contours indicating the existing and final grading at vertical increments of no more than two feet;
2. Discharge (Q), coefficient of runoff (c) and drainage area (A) along with the outline of the drainage area for each inlet, catch basin, culvert and open channel point of design and other locations designated by the Village Engineer. Drainage areas that lie partially outside the limits of the drainage and grading plan may be delineated on any contour map acceptable to the Village Engineer;
3. Discharge (Q) before and after development at strategic points within and at extremities of the project area;
4. Delineation of the boundaries and contour elevation, along with the track, of the major drainage system through downstream areas to an adequate outlet even though the outlet may be outside the project area;
5. Delineation of the horizontal limits of ponding areas at low points (sags) in the street profile and low points outside the street right-of-way including, but not limited to, culvert headwater, natural stream water surfaces, and sump type inlets for storms with frequencies of twenty-five years and 100 years;
6. High and low water horizontal limits and contour elevation of detention/retention/sedimentation facilities along with water surface and control weir elevations, outlet structures, etc.;
7. Areas outside of the project area susceptible to sediment deposits or to erosion caused by accelerated runoff;
8. Location of soils that may be limited for the proposed use;
9. All requirements of this chapter;
10. Any other information required by the Village Engineer to clarify intent, specified requirements, or design features.
D. Supporting Data. All data and design information used for the design of drainage facilities and for determining downstream flood information shall be submitted with the drainage and grading plan. To facilitate review and avoid confusion, legends, descriptions, and structure numbering used on design forms or other calculations shall be identical to those used on the improvement plans and the drainage and grading plan. This data shall include but are not limited to:
1. Weighted runoff coefficient calculations for each contributing area;
2. Pavement drainage computations;
3. Storm sewer computations;
4. Culvert design computations;
5. Open channel computations;
6. Detention/retention facilities computations;
7. Inlet capacity computations;
8. Any other information required by the Village Engineer to clarify intent or design features.
E. As-Built Plans. Amended improvement plans specifying the locations, dimensions, elevations and capacities of all facilities as constructed shall be submitted to the Village on construction completion of the project. These shall include all required design features except those waived by the Village Engineer. All revisions to the approved plans must be approved by the Village prior to construction.
(c) Stormwater Runoff Analysis.
(1) General. The design and construction of storm water facilities in the Village is under the jurisdiction of the Village Engineer. Specific design criteria shall be as required by the “Rules and Regulations of the Hamilton County Sanitary Engineering Department for Storm Drainage”, unless specifically superseded herein.
(2) Water management and sediment control. Storm drainage proposals for any development shall reflect through investigation of the measures intended to control sediment and manage storm water as required by this chapter. Water management and sediment control plans are required of all development and redevelopment unless specifically exempted herein.
(3) Stormwater retention/detention. In order to minimize storm runoff damage to downstream properties and overloading of existing drainage courses, the following criteria shall be followed on all development of parcels one acre or larger in size, and on all redevelopment of existing buildings or site usage as determined by Planning Commission.
A. Stage 1 shall allow the discharge of the ten year pre-developed storm flow and provide for the detention of a volume equal to the ten year storm flow, post-development, less the ten year pre- developed discharge.
B. Stage 2 shall allow the discharge of the twenty-five year pre- developed storm flow and provide for the detention of a volume equal to the twenty-five year storm flow, post-development less the twenty-five year pre-developed discharge.
C. Stage 3 shall allow the discharge of the twenty-five year pre- developed storm flow and provide for the detention of a volume equal to the 100 year storm flow, post-development, less the twenty- five year pre-developed discharge. The detention volume shall be determined by multiplying the above difference by twenty-five minutes. The method of retention is subject to approval by the City Engineer.
D. Outlet flow control devices shall be multistage.
E. Other requirements may be imposed for specific cases.
F. All detention systems shall include an emergency overflow to control the storm water flow when maximum storage capacity is surpassed.
G. No on-site storm drainage shall outlet downstream of the main retention facility without providing supplemental retention as per the above criteria.
(4) Storm sewers.
A. Minor Systems. All on-site storm sewers shall be designed based on a ten-year storm curve with the exception of the retention outlet, unless the specific development requires additional capacity. The retention outlet shall be designed based on the runoff calculated in subsection (c)(3) hereof.
B. Major System.
1. A designated routing shall be designed to convey storm water runoff which exceeds the capacity of the minor drainage system, i.e. storm sewer without causing loss of property or any loss of life.
2. Surface runoff for the major drainage system shall be determined using a storm frequency of 100 years. The runoff which the major storm routing path shall convey shall be equal to the peak flow minus the flow in the minor drainage system.
(5) Drainage calculations. A professional engineer licensed to practice in Ohio shall submit detailed sewer calculations and drainage maps of sufficient scale and contour interval to verify the proposed hydraulic design.
(6) Surface run-off. To determine the quantity of surface runoff for areas up to 640 acres, use the “Rational Method”. (Q=CIA)
(7) Intensity of precipitation. The “point” values of average precipitation intensity, ten inches per hour, at Cincinnati is shown below in subsection (c)(8) hereof, taken from page 35 of U.S. Department of Commerce, Weather Bureau, Technical Paper No. 25, “Rainfall Intensity-Duration- Frequency Curves”. For any given storm duration (concentration time of runoff), the curves show the average precipitation intensity of storms having 2-, 5-, 10-, 25-, 50- and 100-year frequencies. These values may be used for drainage tributary areas of 300 acres or less. For acreage above 300, see subsection (c)(9) hereof.
(8) Rainfall intensity duration-frequency curves.
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(9) Duration of storm in minutes (tc).
Area in Acres | 10 | 20 | 30 | 60 | 90 | 120 |
300 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
400 | 0.993 | .993 | .993 | .995 | .996 | .99 |
500 | .986 | .986 | .987 | .990 | .993 | .99 |
1,000 | .997 | .997 | .978 | .981 | .985 | .98 |
1,500 | .968 | .970 | .972 | .974 | .978 | .98 |
2,000 | .958 | .963 | .966 | .968 | .975 | .98 |
2,500 | .948 | .953 | .957 | .962 | .970 | .97 |
3,000 | .938 | .944 | .949 | .957 | .965 | .97 |
3,500 | .928 | .935 | .941 | .951 | .960 | .96 |
4,000 | .918 | .927 | .933 | .945 | .956 | .96 |
4,500 | .908 | .918 | .925 | .939 | .952 | .96 |
5,000 | .898 | .910 | .918 | .935 | .948 | .95 |
5,500 | .888 | .901 | .911 | .930 | .945 | .95 |
6,000 | .880 | .894 | .905 | .927 | .942 | .95 |
(10) Inlet time. At the head of the system the inlet time may vary from ten to fifteen minutes, depending upon the size of the area and factors affecting rapid runoff.
(11) Runoff coefficient. Compute a weighted value of the drainage areas, using nine-tenths for roof areas and hard-surfaced paved area, nine-tenths for sidewalk and macadam driveways, and four-tenths for unpaved areas, yards, and lawns.
(12) Size of sewer. The size of the sewer shall be determined on the basis of the hydraulic gradient to provide adequate capacity for the computed runoff, using n = .015 for vitrified, concrete pipe, bituminous paved corrugated metal, and brick sewers, and n = .013 for monolithic concrete sewers. In no case shall the size be less than twelve inches in diameter.
(13) Depth. The minimum depth for storm sewers shall be planned to provide clearance for all utilities and to permit inlet leads to be laid on not less than two percent (2%) slope, with the invert of the inlet pipe at the manhole, no lower than the top of the bench wall.
(14) Minimum and maximum velocities. Velocities in storm sewer pipe, when flowing full at average peak flows, shall be no less than two and five-tenths feet per second nor more than twelve feet per second, special provisions shall be made to protect the sewer pipe against erosion and against displacement by shock, or for checking the flow velocity.
(15) Gradients of pipe. The sewer pipe shall be laid on such gradients so that the full flow velocities shall be kept within the foregoing stated minimum and maximum. The pipe sizes should be so selected as to avoid large differences in velocities between consecutive reaches.
(16) Manholes. Manholes shall be placed at intersections and termini of sewers; at all changes in size, alignment, and slope of sewer; in pipe sizes under thirty inches in diameter; at or near conversion chambers and curves on sewers thirty inches in diameter or larger at points where inlets are to be connected; and at intermediate intervals as required for maintenance.
(17) House connections. House connections shall be made to “Y” junctions or to slanted junctions on sewers, and not to the manholes, unless unavoidable.
(18) Inlets.
A. Capacity. The capacity of the inlet should not be less than the quantity of flow tributary to the inlet. Inlets at low points or grade pockets should have extra capacity as a safeguard for flooding from flows in excess of design flows. Calculations shall be submitted verifying the capacity of each inlet. Special inlets may be required for streets with steep gradients to provide the extra capacity such situations require.
B. Type. The desirable minimum depth of a terminal inlet shall be three feet from the inlet top to the invert of the outlet pipe. Any inlets having a depth in excess of five feet shall be provided with manhole-type steps. Wingwall and ditch-type inlets shall be used where required to drain storm water from watercourses and drainage channels.
C. Location. The inlets shall be so located that they shall pick up no more than five cubic feet per second, and this only when street grades do not exceed six percent (6%) on either side of the inlet, and when the maximum concentration of water on each side of the inlet does not exceed two and five-tenths cubic feet per second. Inlets shall be located upstream of all sidewalks at street intersections, crosswalks, driveways, and roadways; in all pockets in streets, where a street grade flattens, and at intervals not greater than 350 feet in gutters.
(19) Outfalls.
A. When a storm sewer system outfalls into a flood plain of any major watercourse, the outfall must not be subject to frequent floods or backwaters. Standard wingwalls with erosion control shall be constructed for all outfalls. Suitable baffles or other energy dissipaters shall be provided if determined to be necessary by the Village Engineer.
B. The invert of the first storm sewer appurtenance upstream of the outfall structure shall be above the elevations of the flood plain.
(20) Specifications for construction and materials. In all other respects, the materials and construction shall be as specified in Sections 603, 604, 706, and 707, State of Ohio Department of Transportation “Construction and Material Specifications”.
(d) Detention/Retention Basins.
(1) Introduction. Detention/retention of stormwater refers to storage of excess runoff on the site of a development area or redeveloped area and gradual release of the stored runoff at an acceptable rate. The detention facility may be a dry surface structure, a pond or lake with additional freeboard or underground structure. The parking lot may not be used to provide for any of the detention requirements.
(2) Design.
A. In some instances, as determined by the Village Engineer, a hydrograph method may be necessary to predict the length of time of high discharge from a detention/retention basin. This determination will be made at the preliminary plan state. It is the developer’s responsibility to obtain this determination prior to beginning design procedures.
B. Allowable Discharge.
1. The volume and peak rate of runoff from an area after full development shall not exceed the volume and peak rate of runoff from the same area before development for twenty- four hour storms with frequencies of twenty-five years.
2. For those areas where a study of the downstream area indicates the extended time of high discharge or velocity due to restricted release rate and storage may cause flooding or excessive erosion, the Village Engineer in cooperation with the Hamilton County Soil and Water Conservation District may require additional controls.
C. Detention Volume. The detention of storm water shall occur in three stages.
1. Stage 1 shall allow the discharge of the ten year pre- developed storm flow and provide for the detention of a volume equal to the ten year storm flow, post-development, less the ten year pre-developed discharge.
2. Stage 2 shall allow the discharge of the twenty-five year pre- developed storm flow and provide for the detention of a volume equal to the twenty-five year storm flow, post- development less the twenty-five year pre-developed discharge.
3. Stage 3 shall allow the discharge of the twenty-five year pre- developed storm flow and provide for the detention of a volume equal to the 100 year storm flow, post-development, less the twenty-five year pre-developed discharge. The detention volume shall be determined by multiplying the above difference by twenty-five minutes.
4. Outlet flow control devices shall be multistage.
5. Other requirements may be imposed for specific cases.
6. All detention systems must include an emergency overflow to control the storm water flow when maximum storage capacity is surpassed.
(e) Major Storms - Water Control.
(1) Introduction. Planning for the major storm is to insure that stormwater runoff which exceeds the capacity of the drainage system has a route to follow that will not cause loss of property or any loss of life. This system exists whether or not it is planned.
(2) Criteria.
A. Storm Frequencies. Surface runoff for the major drainage system shall be determined using a storm with a frequency of 100 years.
B. Total Runoff. The peak discharge of water will be determined as previously outlined in this chapter. The peak discharge may be reduced by an amount equal to the flow in the minor storm system as designed.
(3) Points of consideration.
A. All open channels, street cross sections, low points, and culvert entrances will be considered as possible flood areas due to the 100- year storm and will be included as part of the major storm investigation. The investigation may include downstream facilities to a point designated by the Village Engineer whether or not these facilities are contained within the project area or controlled by the land developer requesting approvals.
B. All calculations will be submitted with the drainage plan.
(Ord. 91-10. Passed 8-12-91.)