Section 3708.60 Delineation of the Regulatory Floodway

Section 3708

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

egulatory

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.