Project Summary

The 4 ft (1.2 m) high water diversion dam built in the 1950’s blocked upstream movement for all fish. Over time, the reservoir filled with fine sediment, forming an impounded high-value wetland.  The stream flowed over the dam’s spillway, which consisted of a concrete box culvert.  The spillway created a 4 ft (1.2 m) drop into a shallow plunge pool. 

The project objective was to preserve the upstream impounded wetland for juvenile rearing habitat while providing fish passage over the dam. The preferred alternative involved removal of the concrete spillway and construction of a roughened rock channel designed to (1) maintain the existing upstream grade, (2) avoid release of stored sediments, and (3) provide upstream and downstream passage for all native fish and other aquatic organisms. 

The roughened channel is 100 ft (30.5 m) long, with an average slope of 5%.  The shape and features of the roughened channel are intended to create a hydraulic environment similar to a natural channel of similar slope.  Since the upstream channel material is mostly fine grain sands and silts, the larger rock in a roughened channel will not be replenished if it is transported downstream.  Therefore, the D84 sized rock used in the roughened channel was designed to be stable up to the 100-year design flow. Because the dam crest also serves as an access road, a 40 ft (12.1 m) long prefabricated steel bridge was placed over the roughened channel at the location of the removed spillway.

Channel Design

Design of the roughened channel involved a bed stability analysis to determine the minimum rock size necessary to maintain a stable channel bed during the 100-year peak flow of 290 cfs (8.2 cms).  The fish passage analysis examined water depth, velocity and turbulence during fish migration flows.  By design, a roughened channel provides a wide distribution of water velocities, with many areas of slower water.

This analysis required an iterative process involving the interdependent variables of particle size, particle stability, channel roughness, and channel geometry.  Two methods were used: the Unit-Discharge Bed Equation as defined by Bathurst (1978) for incipient motion of the D₈₄ particle, (84% of the particles have a smaller diameter than the D₈₄) and the US Army Corps of Engineers Steep Slope Riprap Design for the D₃₀ particle (ACOE, 1994 in WDFW, 2003).  A particle distribution was then developed following methods outlined in (WDFW, 2003) for the Engineered Streambed Material within the channel.

Using a maximum roughened channel slope of 5% as a “rule-of-thumb”, the final design converged on an active channel base width of 7 ft (2.1 m), bankfull width of 12 ft (3.7 m), and bankfull depth of about 2 ft (0.6 m).  To concentrate low flows, ensure adequate water depth for adult fish, and provide slower edge-water for smaller fish, the channel bottom includes a side slope of 10% towards the center.  The banks were constructed of large rock to create a rigid and confined channel, characteristic of steep stream channels.

A series of rock structures constructed of 2 layers of 1 ton rock were built across the channel and backfilled with the Engineered Streambed Material.  Rock structures were designed as rigid bed controls and to create small drops and complex flow patterns.  The top of the rock structures were placed flush with the finished channel grade and maximum spacing between structures was limited to 20 ft (6 m).  By design, higher streamflows were expected to move and sort the smaller rock, exposing the larger rock and create an intricate series of small steps, pools, and flow constrictions.  This complex hydraulic environment creates suitable migration pathways for fish over a wide flow range, similar to those found in a naturally steep channel reach.

Lessons Learned

In general, construction of a roughened channel requires skilled equipment operators, a large quantity of imported rock and aggregate, and on-site construction guidance from persons familiar with this type of design.  Due to a lack of thorough construction oversight,  the upper section of the channel was built with a width far wider than designed.  Additionally, the slope of the upper channel section was less than designed, requiring steepening the channel slope under the bridge to approximately 8%.  These deviations from the design have the potential to create insufficient depth at lower migration flows, possibly hindering fish passage.

The rock used to construct the channel banks was donated to the project, and larger than called for in the design.  This resulted in large voids within the bank rock that should have been chinked with smaller material to prevent water from flowing behind the rocks and scouring the native material.

The horizontal transition apron constructed at the downstream end appears to be functioning well. The transition effectively dissipates energy and has prevented scour of the downstream natural channel.

Two years after construction the channel appears to be stable and functioning properly.

References

Bathurst, J.C. 1978. Flow Resistance of Large-Scale Roughness. Journal of the Hydraulics Division, AM. Soc. Civil Engr., Vol. 104, No. HY12, pp. 1587-1603.

Washington Department of Fish and Wildlife Environmental Engineering Division. 2003. Fish passage design at road culverts: a design manual for fish passage at road crossings. May 2003. http://www.wdfw.wa.gov/hab/engineer/cm/.

Published 04/04/07