Project Summary

A concrete grade control structure across a bedrock channel 70 ft (21 m) downstream of the Jalama Road Bridge created a 4 ft (1.2 m) drop and leap barrier to adult and juvenile steelhead/rainbow trout.  The structure protects the bridge from scour and served as the control for a long-term streamflow gage.  To improve fish passage, a pool-and-weir fish ladder recessed approximately 3 ft (0.9 m) into the existing mudstone bedrock was constructed.

Fish ladders are generally not the preferred alternative for retrofitting a stream crossing due to their susceptibility to debris plugging and sedimentation, and the potential for not providing the desired level of passage. However, a fish ladder was found to be the most suitable solution for this due to site and cost constraints.

Fish Ladder Design

The ladder is comprised of four 6 ft (1.8 m) long weirs, and 3 plunge pools. The drop between each weir is 0.9 ft (27 cm), which satisfies adult steelhead passage criteria (Federal and California State) and is within the leaping abilities of many adult resident trout and some larger juveniles.  The compound weir shape contains a 90-degree V-notch to concentrate low flows.  The V-notch is sized to contain lower-flows, when juvenile salmonids are expected to ascend the ladder.  Once the V-notch is full, at 5.7 cfs (0.16 cms), a portion of the streamflow begins to flow over the concrete grade control structure, with the remaining flow conveyed through the fish ladder. 

Fish Attraction
The proportion of flow in the fish ladder is referred to as attraction flow, and must be sufficient for fish to find the entrance (downstream end) of the ladder.  This ladder is designed to convey at least 25% of the total flow at streamflows up to the high passage design flow of 60 cfs (1.7 cms), the 3% exceedence flow. 

The fish ladder entrance weir is also oriented perpendicular to the channel near the grade control to increase attraction.  Fish attempting to leap over the grade control structure will sense the flow jet exiting the ladder and investigate, allowing them to find the entrance.

Pool Sizing
The upper weir was positioned vertically so at 60 cfs (1.7 cms) the ladder conveys 25% of the total streamflow, or 15 cfs (0.4 cms). The pools between weirs are sized to provide sufficient pool volume to adequately dissipate energy associated with the plunging water and avoid creating a turbulence barrier.  The measure of turbulence used was the Energy Dissipation Factor (EDF), and the applied turbulence threshold for adult salmon and steelhead was EDF > 4 ft-lb/s/ft³.

EDF = Qγh/V

where
Q is flow in the ladder; 
γ is the unit weight of water;
h is the drop height into the pool; and
V is the volume of the pool.

Based on this calculation, the pools need to contain at least 210 ft³ (6 m³) of water at the high passage design flow to provide adequate energy dissipation.  The middle pool is longer to accommodate the 90 degree bend.

Plunging and Streaming Flow Regimes
Pool and weir ladders are designed to function in plunging flow regime. When water level rises, the weirs become backwatered by the downstream pool. When the weirs become significantly submerged flow transitions to a streaming or skimming regime, which can create a velocity barrier. The ladder was designed so only a small portion of the V-notch is submerged by the downstream pool at the high passage design flow, thus minimizing the occurrence of streaming flow.

Post Project Observations and Effectiveness Monitoring

During post project monitoring, hydraulic conditions were evaluated by surveying water surface profiles at three flows: 1.8 cfs (0.05 cms), 7.3 cfs (0.21 cms), and 54-63 cfs (1.53–1.81 cms).   Depth over the weirs, drop height between pools, drop over the grade control structure, and qualitative descriptions of hydraulic conditions and sedimentation within the pools were recorded.  Field measurements show that hydraulic performance matched expectations, as documented in the monitoring report.

During the monitoring the project engineer observed flows in the ladder begin to transition from plunging to streaming at approximately 60 cfs (1.7 cms).

Weirs with gradual side-slopes create a thin sheet of plunging water along the edges. The hydraulics of this thin sheet of water in the receiving pool creates good leaping conditions for smaller fish. Here, the 90 degree angle combined with the bevel on the downstream edge of the V-notch weir creates less than ideal conditions for leaping by smaller fish. Instead, the tight angle of the V-notch concentrates the plunging flow and the beveled edge prevents the formation of an aerated nappe, which can also aid in creating better leaping conditions. Placing the bevel on the upstream side may also improve debris passage.

Published 04/04/07