(A) Provided, in addition to the requirements of § 11.06.090.020, that the design is reviewed and approved by an engineer and constructed to substantially resist or withstand the forces associated with hydrodynamic and hydrostatic pressures, including flood depths, velocities, impact, ice, buoyancy and uplift associated with the base flood.
(B) The design must also show compliance with the following applicable criteria (44 C.F.R. § 60.3(a)(3), (d)(3); ARM § 36.15.606):
(1) Levee and floodwall construction or alteration:
(a) Must be designed and constructed with suitable fill and be designed to safely convey a base flood (ARM § 36.15.606(1)(a));
(b) Must be constructed at least three feet higher than the elevation of the base flood, unless the levee or floodwall protects agricultural land only (ARM § 36.15.606(2)(a));
(c) Must meet state and federal levee engineering and construction standards and be publicly-owned and maintained if it protects structures of more than one landowner (ARM § 36.15.505(1)(c)(ii) and (iii)); and
(d) For any increase in the elevation of the base flood, an alteration of the regulated flood hazard area requires approvals pursuant to § 11.06.040.030.
(2) Stream bank stabilization, pier and abutment protection projects:
(a) Must be designed and constructed using methods and materials that are the least environmentally damaging yet practicable and should be designed to withstand a base flood once the project’s vegetative components are mature within a period of up to five years or other time as required by the Floodplain Administrator. Once vegetation is mature and established, it should not require substantial yearly maintenance after the initial period;
(b) Materials for the project may be designed to erode over time, but not fail catastrophically and impact others. Erosion, sedimentation and transport of the materials may be designed to be at least similar in amount and rate of existing stable natural stream banks during the base flood;
(c) Must not increase erosion upstream, downstream, across from or adjacent to the site in excess of the existing stable natural stream bank during the base flood (ARM § 36.15.606(1)(b)); and
(d) Materials for the project may include, but are not limited to, riprap, root wads, brush mattresses, willow wattles, natural woody debris or combinations of analogous materials.
(3) Channelization projects, where the excavation and/or construction of a channel is for the purpose of diverting the entire or a portion of the flow of a stream from its established course, the project must:
(a) Not increase the magnitude, velocity or elevation of the base flood; and
(b) Meet the requirements of division (B)(2) above (ARM §§ 36.15.101(7) and 36.15.606(1)(c)).
(4) Dams:
(a) The design and construction shall be in accordance with the Montana Dam Safety Act, being MCA Title 85, Chapter 15 and applicable safety standards; and
(b) The project shall not increase flood hazards downstream either through operational procedures or improper hydrologic/hydraulic design (ARM § 36.15.606(1)(d)).
(Prior Code, § 11.06.090.130) (Ord. 6, passed 3-16-1981; Ord. 2021-15, passed 12-20-2021)