To protect property from flood damage and channel erosion, and to protect water resources from degradation resulting from accelerated storm water flows, all development areas shall be designated and constructed according to these regulations. If the construction site is subject to regulations of the Ohio EPA’s NPDES permit, the post construction storm water management best management practices must satisfy any additional requirements of Part III.G.2.e. of the NPDES permit, or the regulations set out in subsections (a) and (b) hereof, whichever is more restrictive.
(Ord. 05-89. Passed 11-14-05.)
(Ord. 05-89. Passed 11-14-05.)
(a) Requirements.
(1) Storm water management systems shall be designed for the ultimate use of the land. Areas developed for a subdivision shall provide a storm water management system for the ultimate development of all the subdivided lots.
(2) Storm water management facilities shall be designed and constructed so that they will continually function with minimal maintenance.
(3) Storm water management facilities shall be designed and constructed as a multi-use facility wherever practical.
(4) Storm water management facilities shall be designed and constructed with specific regard to safety.
(5) The design criteria shall be applied to each watershed within the development area. The City of Amherst does not allow post-development drainage to cross pre-development drainage divides, therefore all pre- and post-development runoff rates and volumes shall be calculated using their respective drainage divides.
(b) Storm Water Management Design Criteria.
(1) The runoff rate from the development area shall not be greater after development than it was before development.
(2) Increases in the runoff volume shall be offset by further restricting run off 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, post-development, shall be restricted to the one year pre-development storm runoff rate. The term critical storm is defined in Section 1348.10 (b).
The critical storm shall be calculated as follows:
A. Determine by appropriate hydrologic methods 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 volume determined in subsection (b)(2)A. hereof, determine the percent increase in the runoff volume due to development.
C. Using this percentage, select the critical storm from the following table:
If the percent of increase in runoff volume is: | ||
Equal to or greater than | And less than | The critical storm runoff rate will be limited to: |
0 | 10 | 1 year |
10 | 20 | 2 years |
20 | 50 | 5 years |
50 | 100 | 10 years |
100 | 250 | 25 years |
250 | 500 | 50 years |
500 | – | 100 years |
Source: Model Regulations Water Management and Sediment Control
(c) Calculations Required for Submittal.
(1) Submitted calculations should show that the peak rate of runoff from the critical storm and all more frequent storms occurring on the development area does not exceed the peak rate of runoff from a one year frequency, twenty-four hour storm occurring on the same area under pre-development conditions.
(2) Submitted calculations should also show that storms of less frequent occurrence (longer return periods) than the critical storm up to the 100 year storm, have peak runoff rates no greater than the peak runoff rates from equivalent storms under predevelopment conditions. Consideration of the one, two, five, ten, twenty-five, fifty, and 100 year storms will be considered in design and construction to meet this standard.
(d) Calculation Methods. The selection of a calculation method should be based upon the size of the drainage area and the output information required. The engineer selecting the calculation method and/or performing the calculation shall do so with full knowledge of the method's limitations, applicable conditions and degree of accuracy. The City of Amherst reserves the right to reject any calculation method it deems inappropriate for the given situation. The "rational method" shall not be considered an appropriate calculation method in determining the critical storm.
(e) Methods for Controlling Runoff. Methods for controlling increases in the rate and volume of storm water runoff may include, but are not limited to, the following:
(1) Retarding flow velocities by increasing friction;
(2) Grading and construction of terraces or diversions to slow runoff by diffusion, or use of grade control structures;
(3) Induced infiltration of increased storm water runoff into the soil where practical;
(4) Provisions for detention of storm water.
Specific methods for controlling storm water runoff are contained in the latest edition of “Rainwater and Land Development” and other sources.
(f) Storm Water Facility Maintenance. The responsible party shall maintain all storm water management facilities for eighteen months following final inspection and acceptance. All storm water management facilities shall be maintained to meet the design standards and the provisions of these regulations. Failure to maintain the improvement during the eighteen month maintenance period may result in action against the owner and/or developer according to Section 1348.14.
Following the eighteen month period described above, the City of Amherst or the designated homeowners' association shall be responsible for permanent maintenance of storm water management facilities and other facilities designed to manage storm water runoff when the land served involves two or more property owners. An inspection and maintenance agreement binding on all subsequent owners of land draining into the storm water facilities shall be developed as part of the design criteria. Such agreements shall provide access to the facilities, at reasonable times, for regular inspection by the City of Amherst to ensure that the facility is maintained to meet design standards and the provisions of these regulations. Such agreements or restrictions shall be recorded on the deed for the property, and reference thereon be made to the entity or individual(s) to be responsible for maintenance. The location, dimensions and bearing of storm water facilities and easements shall be recorded on the deed, or when in an approved subdivision, on the final plat, and reference thereon be made to the entity or individual(s) responsible for maintenance.
(g) Easements and Rights of Way. Permanent access and access easements required to perform inspection and maintenance of storm water control structures, storm water conveyance systems, permanent vegetative plantings and other conservation practices shall be provided as follows:
(1) Access to flood control storm drainage ditches, channels, and storage facilities, shall be by means of easements. Such easements shall not be less than six meters (twenty feet) in width, exclusive of the width of the ditch, channel, or other facility it is to serve, and an easement of this type shall be provided at least on one side of the flood control or storm drainage ditch, channel, or similar type facility.
(2) Access along the storm sewers shall be by means of easements. Such easements shall not be less than six meters (twenty feet) in width, with a minimum width of two meters (five feet) on each side of the center line.
(3) Access around storm water storage facilities shall be a eight meter (twenty- five foot) easement in the case of dry bottom detention basins, and a eight meter (twenty-five foot) easement with a eight meter (twenty-five foot) level beach in the case of wet bottom detention basins, measured from the top of bank and shall include the storage facility itself.
(4) Easements for the emergency flow ways shall be a minimum of six meters (twenty feet) in width or larger if required.
(5) Flood control or storm drainage easement containing underground facilities, shall have a minimum width of six meters (twenty feet).
(6) Those lots crossed by an easement are restricted against the planting within said easement of trees, shrubbery or planting with woody growth characteristics, and against the construction therein of buildings, fences, walls or any other obstructions to the free flow of storm water and the passage of inspectors and maintenance equipment, and, also restricted against the changing of final grade from that described by the grading plan.
(Ord. 98-13. Passed 2-23-98.)