An engineering geological evaluation of a steep-sided, narrow canyon along Big Chico Creek was performed for input to a planned fish ladder repair and construction project. The primary geologic constraint involves potential rock falls from the canyon wall above the ladder. Based on geological observations, together with limited kinematic and static\pseudostatic limit equilibrium analyses, blocks exposed in the canyon wall were concluded to be generally stable under existing static conditions. However, seven potentially unstable rock blocks were identified, with one judged to pose a greater potential risk to the project due to its relatively low stability characteristics and unfavorable position above the ladder. Stability of rock fall debris within the creek channel and supporting the weirs was also considered as a potential project constraint. Although anecdotal reports suggest some movement of the rock debris has occurred since the original ladder was constructed, field observations and short-term monitoring of surveyed benchmarks indicate that key foundation blocks are stable. Risks identified through engineering geologic and rock mechanics analysis methods were evaluated within the framework of cost-benefit analysis for a non-critical structure, and were addressed in the design plans and specifications.
1. INTRODUCTION
An engineering geological evaluation of a steep-sided, narrow canyon in the northern Sierra Nevada foothills was performed to identify and assess conditions that could adversely impact the proposed repair, construction, and long-term operations and maintenance of a fish ladder along Big Chico Creek. The primary geologic issues include stability of a 43-meter-high basalt cliff above the ladder and stability of rock fall debris along the creek channel adjacent to or supporting the ladder. This paper illustrates some of the challenges encountered in evaluating long-term rock slope stability impacts within a qualitative cost-benefit hazard analysis for the construction of a non-critical structure.
2. BACKGROUND
The fish ladder is located on Big Chico Creek, a tributary of the Sacramento River, where it flows through Iron Canyon in Upper Bidwell Park, northeast of Chico, California (Figure 1). The canyon is approximately 50 m deep and 150 m wide. Within the canyon, the creek channel is filled with rock fall debris derived from the canyon walls. In the area of the fish ladder, the rock fall debris forms an interlocking mass of randomly orientated, various sized blocks up to 25 m in greatest dimension. Big Chico Creek flows through and beneath this accumulated mass of blocks (Figure 2).
Fig. 1. Site location map. Not to scale. Photo source: U.S. Geological Survey.(available in full paper)
2The ladder, formally referred to as the Iron Canyon fish ladder, was constructed in 1958 by the California Department of Fish and Game (DFG) to bypass an approximately 4-meter-high channel obstruction reportedly deposited as rock fall debris in the early 1900s [1]. Anecdotal reports suggest that the upstream reach of Big Chico Creek supported a healthy steelhead trout and Chinook salmon population prior to the rock fall event, and the ladder was constructed to provide a passage route for fish during low flows [1].
