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I I I I I I I I I I I I I I I I I I I basically of three types: (1) cantilever failures; (2) general collapses including wedge and block falls; and (3) rotational slump failures. Low bank failures were generally cantilever failures. These occur as a result of a general undermining of the upper bank which is held together by vegetation. Cantilever failures are typically small-scale failures and are usually non- destructive. General collapses and rotational slumps are considerably more serious than cantilever failures and are of larger magnitude. They occur when the shear stress to which a soil is subjected exceeds the soil shear strength, and they often result from excessive bank height. 5.3.1 Dominant Processes Major bank instability problems in the study area occur where the banks reach a height such that general collapses and rotational slumps can occur. Cantilever failures are generally too small to warrant major consideration. Excess bank height develops as a result of bed degradation. Bed degradation, as opposed to bank cutting, dominates in the study area as a result of the cohesive banks offering more resistance to scour than does the bed. AS the bank height increases, a threshold is reached where the bank becomes unstable and a failure occurs. Failures generally occur during a storm when the soil is saturated and soil shear strength is at a minimum. Bank instability problems may be further aggravated by desiccation cracks which occur during the drying process between storms. These desiccation cracks provide planes of weakness which make for likely failure surfaces when the soil shear strength is next reduced during a storm. For most of the Fossil Creek study area the bank heigLt threshold where the bank becomes unstable at approximately 13 feet from field investigations. When the height approaches 13 feet general collapses are evident, usually con- sisting of block and wedge falls, and occasionally rotational slumps. An exception to the 13-foot threshold occurs just upstream of Portner Reservoir where Fossil Creek has eroded a 20- to 25-foot-high bank into the hillside. 5.3.2 Underlying Cause As described above, the major bank instability problems in the Fossil Creek study area are produced by general degradation of the streambed. The increased degradation of stream beds in the study area is also tied to urbani- zation effects due to increased runoff and decrease in sediment supply. Also, 42 an underlying cause of the problem seems to be the alteration of the natural stream hydraulics induced by the use of culverts to convey the flow beneath street and railroad crossings. Current engineering practice often neglects the fact that stream flow involves a two-phase (water and sediment) fluid. Culverts may constrict a natural stream and produce ponding upstream of the culvert. AS ponding occurs, the velocity upstream of the culverts can decrease significantly below its normal value. Since sediment transport is approximately proportional to velocity to the fourth power, a significant drop in velocity can cause the stream to drop most of its sediment load. AS a result, relatively clear, sediment-free water exits from the culvert. The clear water will scour its bed and banks until it is carrying sediment at capacity. Where the banks are cohesive and erosion resistant, bed degradation will predominate. Examples of scour downstream of culverts are found throughout the study area. Along the reach between Taft Hill Road and Shields Street, the stream banks are relatively stable and there is little evidence of bed cutting or bank failures. In this reach instability problems are confined to local areas downstream of culverts. On the other hand, where the stream hydraulics are heavily influenced by culvert flow and ponding, such as on Fossil Creek near its confluence with Stanton Creek, more extensive erosion problems are evi- dent. In this area, bank instabilities extend well beyond the local areas downstream of culverts. There are cases in the study area where the culverts perform well and convey both water and sediment with no lnajor erosion and sedimentation probl.ems. For example, although Lang Gulch flows through several culverts between Shields Street and its confluence with Smith Creek, banks are very stable in this reach. This results from several factors. Most important, however, is the fact that the low gradient and grassy channel in this area tend to produce a low normal velocity with a small sediment transport capa- city. Since velocities are already low, the culverts are capable of conveying the flow with little or no impact on the magnitude of the velocity. Futhermore, since sediment transport capacities are also low, any ponding due to culvert transition losses has less of an impact in terms of sediment settling out.