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<br />E. Step 2. Evaluate land use changes. <br /> <br />1. Land use was mostly grazing when the bridge was built. <br /> <br />2. Land use is now agriculture downstream of the bridge and for 3.2 to <br />4.8 km (2 to 3 miles) upstream. <br /> <br />3, At the time of construction of the bridge, Chino Creek disappeared <br />into an alluvial fan before its confluence with Los Gatos Creek. To <br />intercept any flow that might not infiltrate into Chino Creek's alluvial <br />fan, a channel was constructed along and parallel to 1-5. This <br />channel entered Los Gatos Creek at the bridge; howE,ver, these <br />flows would be small. With the advent of farming, Chino Creek was <br />channelized so that now its flow no longer disappears into its alluvial <br />fan, but enters Los Gatos Creek approximately 0.4 km (one-fourth <br />mile) upstream of the bridge. <br /> <br />F. Step 3. Assess overall stability. <br /> <br />1. The stream is in an area of subsidence which has resultE~d in about <br />3 m (10 feet) difference in ground elevation between the Los Gatos <br />gaging station upstream of the bridge and the termination of the <br />creek near the California Aqueduct, a distance of approximately 14.5 <br />km (9 miles), Pumping for agriculture and seismic activity (May <br />1983, Coalinga earthquake) have contributed to subsidence, <br /> <br />2. After construction of the bridge, the overall stability of the stream at <br />the bridge was decreased considerably by the change in the <br />confluence of Chino Creek with Los Gatos Creek and the increase <br />in slope from subsidence, <br /> <br />3, There is considerable debris available which may have played a role <br />in the bridge failure. <br /> <br />4. Several general techniques to assess overall stability are illustrated <br />in HEC-20, Chapter 4 (pp, 53-55). <br /> <br />4.12 <br />