Section 3708.60  Delineation of the Regulatory Floodway

a)         The regulatory floodway is defined based on a flood event that has a one percent annual chance of exceedence or an expected 100-year recurrence interval. The Department encourages, but does not require, that the 100-year frequency flood event be determined  based on anticipated future land use in the watershed.  When utilized,  anticipated future land use should be based on adopted local or regional land use plans.

b)         The boundary of the regulatory floodway is portrayed on Department regulatory floodway maps.  To locate the regulatory floodway boundary on any site, the regulatory floodway boundary should be scaled off the regulatory floodway map and located on a site plan, using reference marks common to both maps.  Where interpretation is needed to determine the exact location of the regulatory floodway boundary, the Office should be contacted for the interpretation.

c)         The regulatory floodway boundaries are determined by hydraulic and hydrologic analyses, which calculate that portion of the flood plain which must be preserved to store and discharge floodwaters without causing damaging or potentially damaging increases in flood stage and flood velocities or loss of flood storage which would result singularly or cumulatively in more than a 0.1 foot increase in flood stage or a 10% increase in velocity.

d)         The need to preserve storage when defining the regulatory floodway will be waived by the Department if all the municipalities and counties along a hydraulically significant portion of the watershed require effective compensatory storage for all construction and fill in the 100-year frequency flood plain.  Effective compensatory storage requires flood plain storage volumes be replaced at the same flood frequency event as previously existed.  Additionally, legal assurances such as easements must be provided so that the compensatory storage site will remain open to the stream system in order to allow flood waters to reach it.

e)         Determination of the flood elevation at any point along the stream shall be made from the flood profile.

(Source:  Amended at 38 Ill. Reg. 983, effective December 27, 2013)