Hydraulics of River Flow Under Arch Bridges

IN T R O D U C T IO N Bridges are an integral part of any highway system. The design of a particular bridge depends on many variables. A major considera­ tion, often necessary, is the length of the bridge. In certain circum­ stances and locations the length may be dictated by factors other than cost. But, where necessary the designer must decide, “How far can the bridge approaches extend onto the flood plain?”. The shorter and therefore less expensive the bridge, the less waterway area generally provided. Waterway area is an important problem. Bridge approaches extending far onto the flood plain decrease waterway area and produce, during high water, a large constriction causing excessive backwater and possible damage to the structure as well as unnecessary flooding of upstream areas. In some cases the state may be held liable for damage to property caused by bridge backwater. In the past, studies pertaining to backwater caused by constrictions have considered shapes of opening such as that provided by a straight deck bridge. The Bureau of Public Roads has prepared, in coopera­ tion with the Colorado State University, a report, entitled “Computa­ tion of Backwater Caused by Bridges.”1 This report in particular considered openings such as are provided by a straight deck bridge. Fig. 1 is a definition sketch for a normal crossing of the type used in the Colorado experiments. The typical water surface profile is shown as a solid line in view A. The maximum backwater supereleva­ tion is designated as hi* and the depression of the water surface below