Project Summary

The existing barrier is a concrete and rock grade control apron located downstream of the Highway 1 bridge.  The apron spans the channel, is roughly 40 ft (12.2 m) in length, and maintains a change in grade of about 5 ft (1.5 m).  At flows under 20 cfs (0.57 cms) water cascades across the apron and creates a barrier for adult and juvenile steelhead.  At flows above 20 cfs (0.57 cms) adult steelhead are able to leap over the apron, but risk injury from the abrasive nature of the concrete.  Passage conditions continue to improve with increasing flow.

The project objective was to improve low-flow passage conditions for adult steelhead by creating as series of three small step-pools cut into the existing apron.  Reinforced concrete was used to build the pools in the lower portion of the apron.  A 30 ft (9.1 m) long concrete sidewall was constructed to direct low-flows into the step pools.  Pools ranged in length from 4 ft to 10 ft (1.2 m to 3 m).  They were sized to fit into the limited width of the apron and not based on volume needed to dissipate energy associated with the 1.5 ft (0.45 m) drops between pools.

Fish Passage Monitoring

Upstream and downstream migrant trapping has been conducted at the exit of the upper step-pool since it was constructed in 2002.  The trapping data suggest the step-pools provide good adult passage over the apron at flows up to 4 to 5 cfs (0.11 to 0.14 cms).  At higher flows the pools likely become too turbulent, prohibiting passage for many of the fish.  Once flows reach about 20 cfs the adult steelhead can leap over the apron along the right bank rather than use the step-pools.

The upstream migrant trapping also found a limited number of juvenile steelhead and resident rainbow trout area able to ascend the step-pools at flows below 3 cfs (0.08 cms).  However, the drop height between pools is much greater than the CDFG drop criteria for juvenile trout of 0.5 ft (0.15 m), and there are likely a number of juveniles unable to ascend the step-pools.

Post Project Observations and Lessons Learned

The sidewall was built with 90 degree angles.  The California Department of Fish and Game (CDFG) was concerned that fish could be injured if they landed on the wall while jumping between pools.  The solution was to grind the top edge of the walls to a 45 degree angle.

The series of pools run diagonal to the overall streamflow. Because of its orientation, water begins to flow across the apron and directly into the lower two pools as streamflow increases. This has the potential of creating undesirable hydraulics for fish passage and could have been avoided by adding an additional sidewall.

The volume of each step-pool is relatively small, creating excessive turbulence at relatively low flows.  Assessing turbulence during the design process involves identifying the highest flow for passage through the step-pools and then sizing the pools to dissipate the energy associated with that flow. Turbulence in step-pools is assessed using the Energy Dissipation Factor (EDF).  If the EDF is excessive at the high passage design flow, either the pool volume should be increased, the drop height reduced, or increase the proportion of streamflow that bypasses the pools.  In a subsequent passage project on Salsipuedes Creek, upstream at the Jalama Road Bridge, pool volume, turbulence, and operational flows were successfully considered in the design.  For more information on assessing turbulence in step-pools, view the online presentation by Kozmo Bates.

Published 04/04/07