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<br />3. Bank material for Los Gatos Creek. <br /> <br />. Silt and clay - unstable and erodible <br /> <br />4, Bed material at the 1-5 bridges is sand (0.34 to 0.7 mm). Clay hard <br />pan is at unknown depth, Hardpan is not uniform and is erodible. <br /> <br />5, Size analyses are typically completed by a soils laboratory from <br />samples collected by the engineer or designer completing the <br />stability analysis. <br /> <br />6. Important factors to consider in collecting sediment samples include: <br /> <br />a. Size and complexity of study area <br /> <br />b. Number, lengths and areas of tributaries <br /> <br />c, Evidence of, or potential for armoring <br /> <br />d. Structural features that can impact or be significantly <br />impacted by sediment transport <br /> <br />e, Bank failure areas <br /> <br />f. High bank areas <br /> <br />g. Areas exhibiting significant sediment movement (e.g., bars) <br /> <br />7, See HEC-20 (p. 60) for discussion of sampling techniqu,es. <br /> <br />G. Step 4. Evaluate watershed sediment yield, <br /> <br />1. Evaluation of watershed yield, and particularly changes in yield, is <br />important in stream stability assessment. Increases in sediment <br />supply can cause aggradation (siltation) in channels, and decreases <br />in supply can promote degradation (downcutting) and headcutting. <br /> <br />2, Sources of watershed sediment yield include sheet ero::;ion, rilling, <br />gullying, stream channel erosion, and mass movement processes. <br /> <br />3. Los Gatos Creek transports a high concentration of silt, clay, and <br />fine sand. This could increase the transport of bed material and <br />increase contraction scour, but probably would not be a factor in <br />local scour. <br /> <br />4, Overall - not a factor in 1-5 bridge failure. <br /> <br />5.8 <br />