921.17  STORM WATER DRAINAGE REQUIREMENTS.
   (a)   An adequate storm drainage system shall be provided for each subdivision, lot, and development. The design of storm sewer shall be prepared using the rational method and shall be based on a minimum five year storm frequency. Rainfall intensity, runoff coefficients, and time of concentration used in computing flows and structure sizes shall be in accordance with the tables, charts, and the data established by the City Engineer for such calculations. All areas which contribute storm water to the proposed storm drainage system must be considered on the determination of the sizes of structures and channels.
   (b)   The drainage requirements of this section shall apply to all new subdivisions and new construction or developments on any lot, whether development of undeveloped land or redevelopment of previously developed land, but shall not apply to residentially zoned subdivisions or lots including five acres or less and no permits or plans to connect to the storm sewers of the City shall be approved unless and until drainage plans are presented to the Service Director in compliance with the provisions of this section.
      (1)   Drainage plans shall be accompanied by the design computations and a topographic map showing existing and proposed contours and indicating design increments. A hydraulic gradient check shall be made for a 10-year storm and may also be required by the City Engineer for the 25-, 50- and 100-year storms where deemed appropriate.
      (2)   Drainage structure sizes (bridges and culverts) shall be determined by using the current Ohio Department of Transportation design method and shall be based on a 10-year frequency storm for watersheds up to 100 acres, a 25-year frequency storm for watersheds from 100 to 250 acres and a 50-year frequency storm for watersheds greater than 250 acres.
      (3)   Bridges shall be designed in accordance with the current standards of the Ohio Department of Transportation.
      (4)   Detention ponds or basins shall be provided where necessary to control the volume and velocity of storm water leaving the site. Detention ponds and basins shall be designed in accordance with master watershed drainage plans. Where such plans do not exist, storm water retention/detention facilities shall be designed with the following design criteria. The purpose of this is to:
         A.   Permit commercial, industrial, and residential development without increasing the flooding of other lands.
         B.   Limit and/or reduce the adverse impact on receiving streams, storm sewers, and other drainage facilities caused by accelerated runoff due to development.
         C.   Provide a basis for design of storm drainage systems on lands above or below undeveloped areas which will preserve the rights and options of both contributing and receiving property owners and assure the long-term adequacy of storm drainage systems.
   (c)   The design criteria will require that peak discharge flow rates be controlled in proportion to increased runoff volume. If development does not increase runoff volume, peak flows will not be required to be controlled. However, as development increases the runoff volume due to increased area of impervious surfaces, the criteria will be more stringent.
   (d)   The design criteria will apply where necessary and as required by the City Engineer or in accordance with a master watershed drainage plan for:
      (1)   All residential developments containing five acres or more;
      (2)   All commercial developments or sites regardless of size;  and
      (3)   All industrial developments or sites regardless of size.
   (e)   Increased storm water runoff peak flow rates and volumes shall be controlled such that:
      (1)   The peak flow rate or runoff generated from the critical storm and all more frequent occurring storms on the development area does not exceed the peak flow rate or runoff from a one-year frequency, twenty-four hour storm occurring on the same area under pre-development conditions.
      (2)   Storms of less frequent occurrence (longer return periods) than the critical storm up to the 100-year storm have peak flow runoff rates not greater than the peak flow runoff rates from the equivalent size storms under pre-development conditions. Consideration of the 1-, 2-, 5-, 10-, 25-, 50-, and 100-year storms will be considered adequate in designing and developing drainage facilities to meet this standard.
   (f)   The critical storm for a specific development area shall be determined as follows:
      (1)   Determine the total volume of storm water runoff from a one-year frequency, twenty-four hour storm occurring on the development area before and after development. The total runoff volumes shall be determined by the “Peak Discharge Method” of calculating peak rate and total volume of runoff as described in the U.S. Soil Conservation Service's “Urban Hydrology for Small Watersheds”, Technical Release No. 55 and its latest editions, including all applicable supplements. This hydrologic method shall also be utilized for all detention basin inflow design calculations.
      (2)   Based on the volumes calculated in paragraph (1) above, determine the percent increase in volume of runoff due to the development and using this percentage, select the twenty-four hour critical storm from the following table:
 
If the percentage of increase in volume of runoff is
equal to and/or greater than
less than
The critical storm for discharge limitation will be
--
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
 
   (g)   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 structure(s).
      (3)   Detention basin reservoir capacity versus reservoir water surface elevation calculations.
      (4)   Outlet control structure discharge versus 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.
      (8)   Downspouts shall be connected to existing storm sewers.
      (9)   Storm sewers shall be constructed of materials approved for use by the City Engineer based on performance standards as established by ASTM or other testing agencies.
      (10)   Manholes shall be provided at all changes in alignment and grade of storm sewers and at such other locations as necessary to maintain an interval of at least 300 feet and not more than 350 feet between manholes or storm sewers.
      (11)   Storm inlet or catch basin grates shall be of a type designed to permit safe crossing by bicycles as approved by the City Engineer.
      (12)   Easements governing storm drainage facility improvements in the City shall be determined as follows.
   (h)   All storm drainage structures shall be placed on appropriate drainage easements with area-wide facilities placed on City-owned easements and local service facilities placed on homeowner, owner, or homeowner's association-owned easements.
   (i)   An area-wide facility is one which has one or more of the following characteristics or criteria:
      (1)   Dedicated public street drainage is dependent upon the storm sewer or other drainage facility;
      (2)   Storm sewer or ditch receives upstream flows from a defined drainage channel.
      (3)   Storm sewer or ditch serves more than twelve sublots and contains a drainage facility which is more elaborate than a rear yard swale.
      (4)   Any lake, pond or basin shall be designed to detain or retain substantial storm water beyond development in which facility is contained and shall be equipped with aerator facilities where odor problems are likely to occur, which capacity and facilities shall be approved the City Engineer.
      (5)   An addition to or an improvement of an existing area-wide facility.
   (j)   Local service facilities shall include all other facilities not meeting the above area-wide drainage facility criteria.
   (k)   Except for good cause shown, and to the satisfaction of the City, City financial assistance shall only be considered for area-wide drainage facilities. Administrative advice may be provided on local service facilities.
   (l)   Each sublot created by subdivision of property shall have functional access to a rear yard drain connected to the subdivision drainage system. This requirement can be waived when a natural waterway transverses the rear yard and when the rear property is at a slope greater than two percent toward the waterway.
   (m)   The Service Director may waive any requirement of this section to install a retention pond where the Service Director and City Engineer find that extraordinary and unnecessary hardship may result from strict compliance with this regulation as a result of topographic or other physical conditions, provided such waiver does not violate the intent of the regulation and will not be detrimental to the public interest. Extra capacity may be required in specific instances when determined necessary due to limited outfall capacity in the opinion of the Service Director and City Engineer.
(Ord. 10-2001.  Passed 1-16-01.)