Project Description

The existing 165 ft (50 m) long box culvert under Highway 101 was a depth and velocity barrier for all steelhead during low and moderate migration flows. In addition, the culvert outlet was perched 4 ft (1.2 m), with flow cascading over riprap. Offset baffles (sometimes referred to as Washington baffles) were installed within the box culvert and outlet apron to reduce water velocities at mid-range flows and increase water depth at low flows. A separator wall down the culvert and a cut-off wall across the inlet directs and concentrates low flows into the baffled section.

The riprap at the outlet was removed and replaced with three step pools constructed from 1.5 – 3 ton grouted rock. Drops between pools were approximately 1 ft (0.3 m). Poor soil conditions and constant backwater by the downstream channel limited the construction area for the step pools to 30 feet (9 m) below the outlet apron. Flow exiting the unbaffled culvert section does not enter the step pools, but flows over an existing riprap apron.

Lessons Learned

Observational monitoring indicates the structure functions well at low flows. Fish passage conditions are suitable at very high flows, when the culvert becomes backwatered by San Luis Obispo Creek, 300 ft (91 m) downstream. However, during mid-range flows the top pool becomes extremely turbulent due to a lack of volume and depth needed to dissipate the energy associated with the plunging flow. Excessive turbulence is known to create barriers to fish passage.

Turbulence in step pools is a function pool volume, flow rate entering the pool, and the drop height. Using pool-and-weir fishway design guidelines, the step pools could have been sized for an adequate volume to dissipate energy and prevent the excessive velocity and turbulence during migration flows. Additionally, the low flow cutoff wall at the inlet could have been set lower to allow more water to flow around the step pools.

Currently, increasing the pool volume is not considered practical, and other modifications have been developed. The low-flow notch at the culvert outlet was enlarged, but this failed to improve conditions. Future modifications include cutting out a section of the separator wall to allow more flow into the unbaffled section of culvert and around the step pools during moderate and high flows.

In retrospect, the project manager would have made three changes: (1) install a baffle system that is less prone to debris accumulation, (2) divert less water into the baffles at moderate flows, and; (3) make the top step pool much larger and deeper to reduce turbulence.

Published 06/08/2006