(A)   Detention basins shall be designed in accordance with the following criteria.
   (B)   Detention basins are typically designed to remain empty during dry weather and to backup or detain excessive runoff generated during a storm.
   (C)   Fencing must be provided if deemed necessary by the Board. The Board must approve design and locations.
   (D)   Basin volume design.
      (1)   A minimum basin volume shall be the difference in runoff volume discharged from the project area to the basin site between the six-hour NRCS pre-development and post development 100-year storm. In cases where the volume requirement governs, the design calculations must not only show that the required volume has been created, but that the basin functions to detain the volume difference.
      (2)   In many areas of the county the increased runoff volumes can be as critical, if not more critical, than the rate of discharge. CCDB will address this issue on a site-specific basis. All development submittals will be evaluated for the impacts of increased runoff and volume control. Satisfying the volume requirement may be met onsite, at approved off-site locations, or by purchase of volume in a Flood Compensation Bank if one is available in the watershed.
      (3)   Maximum basin side slopes shall be three to one, unless paved.
      (4)   Low flow channels shall be grass if the channel grade is greater than 1%.
      (5)   Basin design must include maintenance accessibility and responsibility.
      (6)   The professional engineer shall address provisions for anti-seep collars, extended detention basins, wet ponds, soil bioengineering, baffles, outlet protection and length to width ratios.
      (7)   Detention basins must be completely within a recorded permanent detention basin easement.
      (8)   Maps depicting the NRCS hydrologic soil groups, existing land use and projected land use for each watershed shall be evaluated to determine the appropriate surface condition factors for use in runoff calculations.
   (E)   Basin discharge shall be designed with the following criteria.
      (1)   Discharge control structures shall be multi-stage and capable of limiting two-, ten- and 100-year post-development discharges to the respective pre-development peak discharge rates or downstream system capacity and shall be constructed of concrete or approved alternate.
      (2)   The emergency spillway shall be sized to accommodate a flow equal to the design overflow of the 100-year storm post-development discharge without overtopping the dam. Erosion protection must be provided for the spillway and receiving stream.
      (3)   The dam elevation shall not be less than one foot above the 100-year storm storage and overflow elevation.
      (4)   Appropriate downstream channel protection must be installed.
      (5)   Storage, discharge and routing calculations for the two-, ten- and 100-year discharges must be submitted for review.
      (6)   Spillways shall be protected from erosion and shall employ energy dissipation, if necessary.
      (7)   Detention basins shall be fully discharged within 36 hours after the storm event unless specifically approved by the County Drainage Board.
      (8)   The detention basin shall be the first item of construction prior to any other earth moving or land disturbing activities and must be designed to function as a sediment basin through the construction period. The Basin design must be checked for capacity due to additional runoff generated by disturbed site conditions.
   (F)   Paved parking lots may be designed to provide temporary detention storage of stormwater directly from the parking area, but are not appropriate for storing large volumes. Ponding should, in general, be confined to those positions of the parking lots farthest from the area served. Ponding areas must not conflict with disabled parking and access routes. Such ponding areas should be exposed to sunlight in winter months to minimize icing. Storage depth must be limited so as not to conflict with parking lot use. Any detention facility utilizing a parking lot must take resurfacing and other parking lot maintenance activities into consideration during design.
   (G)   Facility maintenance responsibility: maintenance of drainage facilities during construction must be the responsibility of the land developer. Maintenance responsibilities must be documented by appropriate restrictive covenants to property deeds prior to final drainage plan approval.
      (1)   Routine maintenance is the developer’s responsibility for a minimum of five years after completion of the drainage facility.
      (2)   After that time, upon the approval of the County Engineer and the County Plan Commission, the county may accept responsibility for routine maintenance of the drainage facility.
      (3)   The permanent pool of a wet bottom basin is the responsibility of the developer or homeowners’ association. Routine maintenance must, at a minimum, assure that the drainage facility performs the functions for which it was designed and constructed.
      (4)   Unless specifically accepted by the county, all drainage facilities must be privately owned and funded.
      (5)   The detention basin must be mowed a minimum of once a year and be kept free of trees and shrubs.
(Ord. 2010-11, passed 7-22-2010)