Administrative Rules of Montana › Department 36 - NATURAL RESOURCES AND CONSERVATION › Chapter 36.15 - FLOODPLAIN MANAGEMENT › Subchapter 36.15.5 - Floodplain and Floodway Delineations › Rule 36.15.505 - ALTERATION OF FLOODPLAINS AND FLOODWAYS

Current through Register Vol. 13, July 5, 2024
(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.