§ 160.034  EXTENDED DETENTION REQUIREMENT.
   (A)   The requirements of this section will apply only when an existing agricultural land use is downstream of and adjacent to a site runoff storage facility outlet. The runoff from a 0.75-inch rainfall event over the hydraulically connected impervious area of the new development shall be stored below the elevation of the primary gravity outlet (extended detention) of the site runoff storage facility. The facility may be designed to allow for evapotransporation or infiltration of this volume into a subsurface drainage system and shall not be conveyed through a direct positive connection to downstream areas.
   (B)   The hydraulically connected impervious area used in the calculation of required extended detention volume may be reduced by the Administrator if the soils are prepared to maximize infiltration and deep rooted grasses or other plants selected for their ability to promote infiltration or water absorption are planted in areas appropriately dedicated. The reduction in hydraulically connected impervious area used in the calculation shall be equal to the area of the development meeting the above soils/native planting requirement.
   (C)   Subsurface drainage systems may be designed as a component of the extended detention portion of the detention basin to assist in infiltration in accordance with the following criteria:
      (1)   The extended detention volume shall be discharged at a rate no greater than that required to empty the calculated extended detention volume within five days of the storm event.
      (2)   No subsurface drainage pipe shall be located within ten feet of drainage pipes directly connected to the detention basin.
      (3)   For purposes of meeting the maximum subsurface drainage discharge requirements, flow control orifices and weirs may be used.
      (4)   All design extended detention volume shall be provided above the seasonal high ground water table or the invert elevation of the groundwater control system.
      (5)   Farm field tile shall not be considered a subsurface drainage system.
(Ord. 06-0383, passed 3-15-06)