Fish populations in the Pacific Northwest have declined as spawning grounds have disappeared because of river and stream barriers, often under-road culverts. This research investigated ways to incorporate different kinds of organic material inside a culvert that would both maintain the stream channel shape while also improving the ability of fish to successfully navigate those culverts during low water flow.
“Stream simulation culverts” include a sediment lining of organic material that creates stream features, increases stream bank stability, and provides fish habitat. However, there are no clear guidelines on how organic material should be used inside a culvert, and no systematic research has evaluated the most effective methods to stabilize channels within culverts. Without scientifically based guidelines, testing designs in the field is an expensive trial and error process, and culverts are more likely to become fish barriers.
To provide more detailed and scientifically based design guidance, this project evaluated the effects of different channel designs containing organic material on sediment transport, streambed stability, and culvert hydraulics. The researchers looked at four types of organic material application: deformable grade controls, meander bars, root wad-lined channels, and organic streambed mixtures.
Streambed designs were tested under the stress of 10-, 25-, and 50-year flood events. The researchers recorded sediment transport after each flood event with the use of a handheld 3-D scanner. They then analyzed sediment transport data to determine elevation changes, elevation differences, and the net area change between flood events. Excluding baselines, 15 channels with deformable grade control, 11 with meander bars, three that were root wad-lined, and nine with an organic streambed mixture were tested.
Overall, the research indicated that adding organic material to a streambed reduced sediment transport and maintained the target channel shape.
Deformable grade control is a trench of organic material (logs, branches, and sticks) perpendicular to the stream’s flow. The researchers examined the relationships among dowel size, distribution of dowel size, ratio of dowels to sediment, and sediment transport. The tests showed that a wood-sediment ratio of 75:25 made from a mix of sizes best reduced overall sediment transport at both 2 percent and 3 percent slopes for a straight, U-shaped channel.
Meander bars are trapezoidal and extend into the center of the channel. The researchers investigated the effects of meander bars made of coarse material on channel stabilization. They found that those constructed with coarse material increased stream stability by 15 to 21 percent (at one culvert-width spacing) at a 2 percent slope, by 25 to 36 percent (at three culvert-widths spacing) at a 2 percent slope, and by 52 to 63 percent (at one culvert-width) at a 3 percent slope.
A root wad is a bank stabilization strategy that involves tree roots protruding into the stream. In this project they were simulated with plywood disks and dowels. The researchers investigated the use of a root wad-lined meandering channel and examined the relationship between root wad spacing and channel stabilization. They found that a spacing of one root wad-diameter increased channel stability by 39 to 66 percent, maintaining the target channel shape at a 0.7 percent slope. Increasing the spacing between root wads decreased channel stability.
Finally, the researchers investigated the use of an organic streambed mixture and its effects on channel stabilization. They concluded that a sediment to organic material ratio of 3:1 was best at maintaining channel shape while also not being too difficult to construct. That ratio increased channel stability by 23 to 47 percent for one culvert-width and by 45 to 56 percent for three culvert-widths at a 2 percent slope.
Report: WA-RD 939.1
Authors:
Tyler Fouty
Nicholas Engdahl
Tice Rutan
Sponsor: WSDOT
WSDOT Technical Monitor: Julie Heilman
WSDOT Project Manager: Jon Peterson