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(1) A temporary diversion must be constructed to pass safely the peak runoff from a precipitation event with a 10-year, 24-hour recurrence interval, or a larger event as specified by the department.

(2) If channel lining is required to prevent erosion, the channel lining must be designed using standard engineering practices to safely pass design velocities.

(3) Freeboard must be as specified by the department, but no less than 1.0 foot.

(4) Energy dissipators must be installed in streams where exit velocity of the diversion is greater than that of the receiving stream.

(5) Whenever streamflow is allowed to be diverted, the stream channel diversion must be designed, constructed, and removed in accordance with the following requirements:

(a) The longitudinal profile of the stream, the channel, and the floodplain must be designed and constructed to remain stable and to prevent, to the extent possible using the BTCA, additional contributions of suspended solids to streamflow or to runoff outside the permit area. These contributions must not be in excess of requirements of state or federal law. Erosion control structures, such as channel lining structures, basins, and artificial channel roughness structures, may be used in  diversions only when approved by the department as being necessary to control erosion.

(b) The combination of channel, bank, and flood-plain configurations must be adequate to pass safely the peak runoff of a 10-year, 24-hour precipitation event for temporary diversions or larger events specified by the department. However, the capacity of the channel itself must be at least equal to the capacity of the unmodified stream channel immediately upstream and downstream from the diversion.

(6) When no longer needed to achieve the purpose for which it was authorized, a temporary diversion must be removed and the affected land regraded, resoiled, and revegetated, in accordance with subchapters 5 and 7. At the time a diversion is removed, downstream water treatment facilities previously protected by the diversion must be modified or removed to prevent over-topping or failure of the facilities. This requirement does not relieve the operator from responsibility for maintenance of a water treatment facility otherwise required under this subchapter or the permit.

History: 82-4-204, MCA; IMP, 82-4-231, MCA; NEW, 1980 MAR p. 725, Eff. 4/1/80; AMD, 1989 MAR p. 30, Eff. 1/13/89; TRANS, from DSL, 1996 MAR p. 3042; AMD, 2004 MAR p. 2548, Eff. 10/22/04.

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