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(1) The board may alter a designated floodplain or designated floodway, after a public hearing, when sufficient data becomes available.

(a) When scientific or technical flood data shows that the base flood elevation was erroneously established and the designation of the floodplain boundary was therefore incorrect.

(b) When property has been raised to a level above the base flood elevation by suitable fill provided that:

(i) the filled area is contiguous to areas naturally above the base flood elevation and not within the designated floodplain;

(ii) the filled area is a minimum of 2 feet above the base flood elevation;

(iii) the fill is suitable material according to definition in ARM 36.15.101 and is not subject to settlement and has been compacted to 95 percent of the maximum density obtainable with the standard proctor test method of the American society for testing and materials (ASTM Standard D-698) or equivalent;

(iv) no portion of the fill is within the floodway;

(v) the fill slope must not be steeper than 1 1/2 horizontal to 1 vertical unless substantiating data justifying a steeper slope is provided and adequate erosion protection is provided for fill slopes exposed to floodwaters. The erosion protection for fill slopes exposed to velocities of four feet per second and less may consist of vegetative cover consisting of grasses or similar undergrowth as approved by the permit issuing authority. Slopes exposed to velocities greater than four feet per second shall be protected by armoring with stone or rock slope protection;

(vi) compaction of earthen fill and erosion protection measures must be certified as meeting these criteria by a registered professional engineer;

(vii) the fill does not increase the elevation of the base flood in areas of existing development.

(c) when areas have been protected by a properly engineered flood protection project provided that:

(i) dams are designed and operated for flood control purposes and constructed in accordance with acceptable safety standards and the Montana Dam Safety Act;

(ii) levees and floodwalls comply with ARM 36.15.606 and are publicly owned and maintained. Minimum freeboard above the base flood elevation shall be three feet with an additional foot 100 feet either side of a structure such as bridges. An additional 1/2 foot above the minimum is required at the upstream end of the levee, tapering to the minimum at the downstream end. The levee must be designed and constructed to offer base flood protection without human supplementation. Human intervention is only acceptable for the operation of closure structures such as gates or stop logs. A levee system designed for human operation of closures are acceptable provided that:

(A) adequate warning time exists for the operation of closures before floodwaters reach the base of the closure,

(B) the closure is an integral part of the system during operation,

(C) operation and maintenance of closure structures are responsibilities mandated by local regulation with periodic operation performed for testing and training purposes,

(D) a formal operation plan is available and capable of being implemented.

(iii) dams and levees must be designed by a registered professional engineer and flood protection certified as adequate to provide protection from the base flood;

(iv) floodway channels designed by a registered professional engineer may carry less than the discharge of the base flood provided that they do not increase the extent of flooding.

(2) No alteration of a designated floodplain is required when property located within the flood fringe is naturally above the base flood elevation as proven by a certified elevation survey provided by a registered professional engineer or licensed land surveyor.

History: Sec. 76-5-204, MCA; IMP, Sec. 76-5-204, MCA; NEW, 1989 MAR p. 1665, Eff. 10/27/89.

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