(A) For appropriate uses other than bridge or culvert crossings, on-stream structures or dams, all effective regulatory floodway conveyance lost due to the project will be replaced for all flood events up to and including the 100-year frequency flood.
(B) In calculating effective regulatory floodway conveyance, the following factors shall be taken into consideration:
(1) Regu1atory floodway conveyance.
K = [1.4863 A RH 2/3] / n
Where:
n = Manning’s roughness factor.
A = The effective area of the cross-section.
RH = The hydraulic radius (ratio of the wetted area to the wetted perimeter).
(Note: See Open Channel Hydraulics, Ven Te Chow, 1959, McGraw-Hill Book Company, New York)
(2) The same Manning’s “n” value shall be used for both existing and proposed conditions unless a recorded maintenance agreement with a federal, state, or local unit of government can assure the proposed conditions will be maintained or the land cover is changing from a vegetative to a non-vegetative land cover or vice versa.
(3) Transition sections shall be provided and used in calculations of effective regulatory conveyance. The following expansion and contraction ratios shall be used unless an applicant’s engineer can prove to IDNR-OWR through engineering calculations or model tests that more abrupt transitions may be used with the same efficiency:
(a) When water is flowing from a narrow section to a wider section, the water should be assumed to expand no faster than at a rate of one foot horizontal for every four feet (4:1 expansion) of the flooded stream’s length.
(b) When water is flowing from a wide section to a narrow section, the water should be assumed to contract no faster than at a rate of one foot horizontal for every one foot (1: 1 contraction) of the flooded stream’s length.
(c) When expanding or contracting flows in a vertical direction, a minimum of one foot vertical transition for every ten feet (10: 1 vertical transition) of stream length shall be used.
(d) Transition sections shall be provided between cross-sections with rapid expansions and contractions and when meeting the regulatory floodway delineation on adjacent properties. Transition sections must be used to determine the effective conveyance areas on adjacent properties. Cross-sections shall extend at least 100 feet upstream and downstream of the project limits.
(e) All cross-sections used in the calculations shall be located perpendicular to flood flows.
(Ord. 3187, passed 1-22-19)