Minimum Water Level . The water level in the culvert should not be less 

 than 20 cm during the open-water season unless fish passage is not required. 



Structure Design 



Shape . 



A. If suitable timber is available, native log stringer or rough-sawed 

 timber bridges and log culverts are the most desirable temporary 

 structures for the passage of fish. They maintain the natural stream 

 bed and gradient and are easy to remove. 



B. Arch culverts with an open bottom are preferred culverts for permanent 

 roads. These culverts retain natural bed material. Closed arch cul- 

 verts are second in preference. 



C. Horizontal ellipse culverts can maintain stream flow width and natural 

 bed material if the culvert invert is placed below the stream bed 



e levat ion. 



D. Circular culverts are impractical for fish passage unless installed 

 as described by U. S. Department of Agriculture (1979), summarized in 

 the following section. 



Installation and Design . 



A. Culvert inverts should be laid a minimum of 15 cm below normal stream 

 bed elevation. The Alaska State Pipeline Coordinator's Office often 

 recommends a burial depth of 20% of culvert diameter. 



B. Inverts should be designed to prevent hydrostatic uplift at the down- 

 stream or upstream end. 



C. The culvert gradient should be kept as close to 0% gradient as pos- 

 sible so that upstream or downstream velocity barriers are not 

 created. 



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