Sec. 26.20.21.03. Diversions and Conveyance of Overland Flow and Shallow Ground Water Flow, and Ephemeral Streams

A. Overland flow, including flow through litter, and shallow ground water flow from undisturbed areas, and flow in ephemeral streams, may be diverted away from disturbed areas by means of temporary or permanent diversions, if required or approved by the Bureau as necessary to minimize erosion, to reduce the volume of water to be treated, and to prevent or remove water from contact with acid-forming or toxic-forming materials.

B. The following requirements shall be met for all diversions and for all collection drains that are used to transport water into water treatment facilities and for all diversions of overland and shallow ground water flow and ephemeral streams:

(1) Temporary diversions shall be constructed to pass safely the peak runoff from a 2-year, 24-hour precipitation event with sufficient freeboard to pass a 10-year, 24-hour precipitation event or a larger event as specified by the Bureau.

(2) To protect fills and property and to avoid danger to public health and safety, permanent diversion shall be constructed to pass safely the peak runoff from a 10-year, 24-hour precipitation event, or a larger event as specified by the Bureau. Permanent diversions shall be constructed with gently sloping banks that are stabilized by vegetation. Asphalt, concrete, or other similar linings shall be used only when approved by the Bureau to prevent seepage or to provide stability.

(3) Diversions shall be designed, constructed, and maintained in a manner which prevents additional contributions of suspended solids to streamflow and to runoff outside the permit area, to the extent possible using the best technology currently available. Appropriate sediment control measures for the diversions may include, but not be limited to, maintenance of appropriate gradients, channel lining, revegetation, roughness structures, and detention basins.

(4) A diversion may not be located so as to increase the potential for landslides. A diversion may not be constructed on existing landslides, unless approved by the Bureau.

(5) When no longer needed, each temporary diversion shall be removed and the affected land regraded, topsoiled, and revegetated.

(6) Diversions shall be designed to minimize adverse impacts to the hydrologic balance within the permit and adjacent areas, to prevent material damage outside the permit area, and to assure the safety of the public.

C. Diversion design shall incorporate the following:

(1) Channel lining shall be designed using standard engineering practices to pass safely the design velocities.

(2) Freeboard shall be not less than 1 foot. Protection shall be provided for transition of flows and for critical areas such as swales and curves. Where the area protected is a critical area as determined by the Bureau, the design freeboard may be increased.

(3) Energy dissipators shall be installed when necessary at discharge points, where diversions intersect with natural streams and exit velocity of the diversion ditch flow is greater than that of the receiving stream.

(4) Excess excavated material not necessary for diversion channel geometry or regarding of the channel shall be disposed of.

(5) Topsoil shall be handled in accordance with the Regulatory Program.

(6) Diversions may not be constructed or operated to divert water into underground mines without the approval of the Bureau.