§ 54.10  SINKHOLES.
   (A)   General.
      (1)   The use of sinkholes as stormwater management is not permitted, unless there are no other alternatives.
      (2)   The county does not encourage the use of natural sinkholes as outlets for drainage from developed areas and will avoid requests for modifications to sinkhole entrances. The proposed use of sinkholes as outlets for development must be approved by the CCDB and County Engineer.
      (3)   For circumstances that have no other means for drainage, the following criteria shall be implemented.
   (B)   Design. Specific design requirements for the use of sinkholes, when permitted, include, but are not limited to:
      (1)   The sinkhole shall have the volume to store a 100-year, 24-hour NRCS storm with a no outlet condition;
      (2)   Stormwater discharge into a sinkhole shall not be increased over its preexisting rate according to standards as established by the County Drainage Board. Depressions containing sinkholes shall not be utilized for stormwater detention unless no other alternatives exist;
      (3)   Photographic evidence should be submitted to the Board showing the current condition of the sinkhole feature. If recent subsidence is evident, the sinkhole shall not be used for stormwater drainage unless the feature has been evaluated by a geotechnical engineer, and he or she has determined that the feature can be treated so that significant future subsidence is not likely;
      (4)   To confirm the suitability for an existing feature to accept a given runoff volume, the feature must be pump tested using at least 80% of the 100-year design storm for an eight-hour duration. The condition of the sinkhole before and after the pump test should be documented by a licensed professional engineer registered in the state. Any evidence of significant subsidence that occurs during or after the test will be taken as unsuitability of the feature to accept runoff. To confirm that runoff into the sinkhole feature will not affect adversely adjacent properties, fluorescent dye should be injected into the sinkhole during the pump testing. A geotechnical engineer, registered in the state, should be retained to make observations of the fate of the dye in the surrounding area;
      (5)   Protective measures for the sinkhole inlet must be applied prior to the start of construction activities. Surface water runoff from stripped areas should be directed away from the sinkhole until the areas have been developed or ground cover has been installed and has become established;
      (6)   An alternate means of surface water disposal must be provided in the event that the sinkhole ceases to accept runoff or significant subsidence occurs in the feature;
      (7)   Stormwater runoff from paved areas or structures shall not directly enter a sinkhole; drainage plans shall be designed to route runoff through vegetative filters or other filtration measures before it enters a sinkhole. Such filters or filtration methods must be reviewed by the Board;
      (8)   A geotechnical engineer, licensed in the state, must supervise the design and installation of sinkhole treatment measures. The engineer shall also observe installation of treatment measures and shall document that treatment measures comply with approved plans. The engineer shall be responsible for documenting significant subsidence or other changes in the existing sinkhole feature during treatment that may affect the effectiveness or practicality of the approved treatment method; and
      (9)   Any instances of significant subsidence must be fully documented and a geotechnical engineer, licensed in the state, must supervise design of treatment measures, must inspect treatment installation and must document construction of repairs prior to bond release.
(Ord. 2010-11, passed 7-22-2010)