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The Tennessee Department of Transportation has constructed a new bridge and approaches on State Route 30 over Cane Creek near Spencer, Tennessee. The old bridge and its approaches were fairly low, permitting considerable flow over the road during floods. The new bridge is longer and its approaches are considerably higher, causing different flow conditions at the site. Analysis of the effects of the new bridge, as compared to the old bridge, on a flood of the magnitude of the May 27, 1973 flood is presented. The May 27, 1973 flood was greater than a 100-year flood. Analyses of the 50and 100-year floods for the new bridge are also presented. Results of the study indicate that the new construction will increase the water-surface elevation by approximately 1 foot upstream from bridge for a flood equal to that of May 27, 1973. INTRODUCTION The Tennessee Department of Transportation has built a new bridge with new approaches over Cane Creek just upstream from the old bridge about 4 miles east of Spencer, Tennessee (fig. 1). The old bridge is to be removed. The new bridge, 17 feet longer than the old bridge, slopes upward from left to right (facing downstream) and the left (low) end is about 5 feet higher than the old bridge. The vehicular approaches to the new bridge consequently are higher than those to the old bridge (fig. 2). These changes in the physical conditions of the crossing alter the floodflow characteristics at the site. The changes cause a larger percentage of a given discharge to flow through the bridge opening and a smaller percentage to flow over the road as compared to conditions existing prior to construction of the new bridge. For example, for a flood equal to the May 27, 1973 flood, 4 percent of the peak discharge would flow over the new road compared to 13 percent over the old road. The purpose of this report is to evaluate the relative hydraulic performance of the two bridges with respect to the flood of May 27, 1973, which was a major flood in the area. Computed data for the 50-year and 100-year floods at the site of the new bridge are also presented. The 50-year and 100-year floods are defined as the peak discharges which will be exceeded once, on the average, in 50 and 100 years, respectively, or stated another way, the peak discharge which has a 2 or a 1 percent chance, respectively, of being exceeded in any year.

Cane Creek flood-flow characteristics at State Route 30 near Spencer, Tennessee