(A) Introduction
(1) Every Subdivision and land development shall be provided with a Stormwater Management System which is adequate to serve the area and meets the requirements of this chapter and other criteria of the City. Any unsubdivided parcel less than 1 acre in size is exempt from the detention requirement per § 151.04(D) of these regulations.
(2) Regarding stormwater quantity, developers are required to design improvements in accordance with § 151.04(D)(2)(d) of these regulations.
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 City 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 City.
(4) Improvements shall be designed such that, at a minimum, all developed areas are treated with an acceptable post-construction stormwater quality best management practice. Practices chosen must be sized to treat the water quality volume (WQv) and to ensure compliance to the maximum extent practicable with Ohio EPA Water Quality Standards (Ohio Administrative Code Chapter 3745-1) and the Ohio EPA NPDES General Stormwater Permit for Construction Activities (OHC000005 or latest version).
Sites that have been previously developed where no Post-Construction BMP’s were installed are required to provide:
(a) A 20 percent net reduction of the site’s current impervious area, achieved by either the use of pervious pavement or removing the impervious surface.
(b) Treatment of at least 20 percent of the WQv.
(c) A combination of (a) and (b).
The City accepted post-construction stormwater quality best management practices are those that are included in the Ohio EPA NPDES General Stormwater Permit for Construction Activities (OHC000005 or latest version).
(5) Although the submission requirements are specific, they are also the minimum requirements. The City 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) Continued maintenance.
(a) Once a Stormwater 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) An Inspection and Maintenance Agreement shall be made between the property owner and the city ensuring that the Post-Construction Stormwater BMP(s) are inspected and properly maintained. Such agreement shall be in the form of a covenant to run with the land and shall be recorded with the Hamilton County Recorder. A template of such an agreement is available from the city.
(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, and providing adequate post-construction stormwater quality best management practices.
(a) The minor drainage system is for collecting and transporting runoff from frequently occurring storms (both for water quantity and water quality measures). 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 detention/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 development. Plans, profiles, and supporting documentation to verify conformance with this chapter shall be submitted along with the usual plan submissions required in Land Development Rules and Regulations.
(a) Preliminary Plans. In addition to the Land Development Rules and Regulations requirements, a plan showing the total area contributing runoff to the 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 peak flow rates 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. Location of all post-construction stormwater quality best management practices.
5. Any other information required by the City to clarify intent.
(b) Improvement Plans. In addition to the Land Development Rules and Regulations, 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 provide detail);
5. Headwall type (ODOT or provide detail);
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 ODOT or City standard construction drawings;
10. Easements;
11. Detention/Retention facilities;
12. Location of all post-construction stormwater quality best management practices.
13. Maintenance schedule for post-construction stormwater best management practices.
14. Any other information required by the City Engineer to clarify intent or design features.
(c) Drainage and grading plans. In addition to the development plan, a drainage plan shall be submitted. This plan may be the required development plan or a similar plan at a scale not less than l inch = 20 feet showing at least the following additional information:
1. Contours indicating the existing and final grading at vertical increments of no more than 2 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, open channel, and post- constructionstormwater quality best management features, and other locations designated by the City 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 City Engineer;
3. Peak flow rates 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 25 years and 100 years;
6. High and low water horizontal limits and contour elevation of Detention/Retention/Sedimentation/Water Quality 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 City 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 runoff 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. Post-construction stormwater quality best management practice supporting calculations.
9. Sediment and Erosion Control supporting calculations.
10. Any other information required by the City 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 City on construction completion of the project. These shall include all required design features except those waived by the City Engineer. All revisions to the approved plans must be approved by the City prior to construction.
See Building Official for “As-Built Plan Requirements”.
(C) Stormwater Runoff Analysis. See § 150.34 of the Springdale Land Development Rules and Regulations.
(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) Supporting calculations will be required by the City Engineer. Any computer analysis method should be confirmed as acceptable with the City Engineer prior to design commencing. Computer Analysis systems for detention volume may be acceptable at the discretion of the City Engineer. The methodology and calculations used for the design and sizing of detention/retention basins shall be based on the Critical Storm Method as described below. The SCS Method shall be used for the purposes of sizing detention/retention basins.
If the post-development stormwater runoff volume from a site will be greater than the pre- development stormwater runoff from the same site, the peak flow rate from the critical storm and all more frequent storms shall be less than or equal to the peak flow rate from a one-year 24-hour storm occurring on the site under pre-development conditions. The post-development peak flow rate from storms of less frequent occurrence (longer return periods) than the critical storm up to the 100-year 24-hour storm shall be less than or equal to the pre-development peak flow rates from equivalent size storms.
The critical storm for a specific development area shall be determined as follows:
1. Determine the total volume of stormwater runoff from a one-year 24-hour storm for both pre-development and post-development conditions.
2. Determine the percent increase in the total volume of stormwater runoff due to development and select the critical storm from the following table:
Stormwater Runoff Volume Increase | Critical Storm | |
Equal to or Greater | And Less Than | |
Stormwater Runoff Volume Increase | Critical Storm | |
Equal to or Greater | 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 |
Hydrographs for the one-year, two-year, five-year, ten-year, 25-year, 50-year, and 100-year storm events shall be developed from routing calculations using the SCS Method to confirm the Critical Storm Method requirements are achieved.
The following items need to be included in a summary sheet of the support information, with references to the applicable page where the corresponding calculation is noted. All information must be submitted in a summary form.
1. Pre-developed Peak flow rates for the storm events listed above.
2. Post-developed Peak flow rates for the storm events listed above.
3. A stage/storage/release table, which also specifically notes actual discharge from outlet structure during the storm events listed above.
(b) Allowable discharge.
1. The peak rate of runoff from an area after full development shall not exceed the peak rate of runoff from the same area before development for each design frequency storm, or as noted in § 151.04(D)(2)(d), whichever is more restrictive.
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 City Engineer may recommend that additional controls be required.
(c) Detention Volume. The detention of stormwater shall be provided according to the Critical Storm Method and the following criteria.
1. If the Detention (Water Quantity Basin) is being utilized as a water quality pond, then an additional initial stage for the Water Quality Volume (WQV) will be required.
2. Outlet flow control devices shall be multistage.
3. Other requirements may be imposed for specific cases.
4. All detention systems must include an emergency overflow to control the post-developed 100-year stormwater flow when maximum storage capacity is surpassed.
5. No on-site storm drainage shall outlet downstream of the main detention/retention facility without providing supplemental retention as per the above criteria.
(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 stormwater 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 City 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.
(F) Inspection of Stormwater Control Facilities and Post Construction Stormwater Quality BMP(s).
(1) Inspection Requirements. All detention/retention and Post Construction BMP(s) shall be routinely inspected as determined by the city.
(a) All underground detention chambers shall be inspected by the property owner to insure that the detention chambers are in compliance with the approved water management and sediment control plan and the city's regulations. Such inspections shall be conducted at a minimum of every three years for underground detention chambers which are 10,000 cubic feet in size or greater, and every four years for underground detention chambers which are less than 10,000 cubic feet. The inspection shall be conducted in compliance with applicable law and regulations concerning confined space entry and shall be videotaped, a copy of which shall be provided to the city Building Department for its review. The property owner of an underground detention chamber shall be responsible to perform any maintenance or repairs which are discovered as a result of the inspection. If the detention facility also functions in whole or in part as a stormwater quality BMP, such facility shall be inspected annually.
(b) Post Construction BMP(s) shall be inspected annually to insure that BMP(s) are in compliance with the approved water management and sediment control plan and to identify and facilitate the removal of any pollutants. Inspection reports shall be submitted to the city no later than 30 days after the inspection. Templates of inspection reports are available from the city.
(2) Correction of Identified Deficiencies. If inspections identify corrections which need to be performed to return the facilities to proper function in accordance with the approved water management and sediment control plan, such corrections shall be made by the owner at his expense within 30 days of discovering such deficiencies. If inspections of Post Construction Stormwater Quality BMP(s) identify pollutants which must be removed, they must be removed and disposed of by the owner at his expense in accordance with city, state and federal guidelines within 30 days.
(3) Failure to Inspect or Correct Deficiencies. If a property owner fails or refuses to conduct an inspection as required by this section, the city shall cause an inspection to be conducted and all costs that the city incurs in performing the inspection shall be the responsibility of the property owner. If the property owner fails or refuses to perform the maintenance and repairs within the time specified by the city, the city shall cause the maintenance or repairs to be performed and the cost shall be the responsibility of the property owner. In addition, the failure or refusal of a property owner to conduct an inspection or make correction/repairs as required by this section shall be deemed to be a misdemeanor and punishable as provided in § 151.06(B). Costs shall be assessed in the same manner as provided for in §§ 155.064 and 155.065.
(Ord. 25-1987, passed 3-18-87; Am. Ord. 77-1996, passed 10-16-96; Am. Ord. 40-2010, passed 12-15-10; Am. Ord. 29-2012, passed 9-5-12; Am. Ord. 21-2023, passed 4-19-23)