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SECTION VII COMPARISON OF FLOODPLAIN STUDIES the river due to erosion and due to bridge measurement errors. The erosion has lowered the thalweg in the vicinity of the bridge. the WWE 1984 Study thalweg in Figure 3 is about 0.5 to 1.0 feet lower than those of the previous studies. The bridge low chord and top of road elevations used in the HEC-2 models by the Flood Insurance Study and Centennial Downs are lower than the elevations surveyed by Delta Aerial Surveys and confirmed by Wright Water Engineers. As a result, the 100-year flood water surface elevation of the previous studies is above the Crestline Avenue Bridge low chord. The WWE 1984 Study shows that the low chord is above the 100-year water surface elevation for the 12,000 cfs flow. The two previous flood studies (FIA 1978 and Centennial Downs 1983) are compared to the WWE 1984 study in this section of the report. The reader is referred to the map and profiles presented in this section of the report, along with the three clear overlays in the packet. These are: The 12,000 cfs discharge floodplain analyses have shown that the 1978 Flood Insurance Study in the vicinity of Bowles Avenue has resulted in a lOa-year flood water surface profile which is too high. It results in a floodplain and floodway which are too wide for the 12,000 cfs discharge used. The analyses have confirmed that the Centennial Downs 1983 floodplain study is representative of 12,000 cfs discharge conditions as expressed in terms of the HEC-2 camputations and the computed water surface profiles. Therefore, the initial effort of the WWE 1984 study, to analyze the 12,000 cfs flood discharge for comparison with the previous flood studies, was successful in identifying certain problems as well as confirming the reliability of the 1984 work. The comparison of the water surface profiles for the various studies is presented in Figure 3. The profile shows that both the Centennial Downs study and the WWE 1984 Study resulted in similar profiles. In the area upstream of Crestline Avenue the two studies are less than 0.5 feet apart. Both the Centennial Downs water surface profiles and the Wright Water Engineers prOfiles are one to two feet lower than the Flood Insurance Study upstream of Crestline Bridge. The Flood Insurance Study shows a 5 to 6 foot rise in the water surface eleva- tion upstream and downstream of the Crestline Avenue Bridge. The Centennial Downs stUdy shows a similar but lower rise in this area. The explanation for this difference lies partially in the physical changes which have occurred in