Scour around bridge piers is a major concern in the design of a new bridge or the evaluation of the structural stability of an existing bridge. An adequate estimation of potential scour at bridge piers is essential to effective design, construction, and maintenance of hydraulic structures. Reasonably accurate estimates of scour depth are needed for safe, cost-effective bridge design. Although many equations have been developed on the basis of laboratory work that can be used to estimate local scour at piers, site-specific data have not previously been collected to verify the accuracy of these equations or applicability to Missouri streams.

Scour data were collected during floods at 10 sites on streams in Missouri. The recurrence intervals of the floods ranged from less than 2 to more than 50 years. Local-scour holes near bridge piers ranged from 0.5 to 7.1 feet deep. Seven local-pier-scour equations were evaluated as to their usefulness in estimating the measured scour at the sites studied. Scour depths estimated using the Froehlich equatin without a safety factor had an interquartile range nearest in magnitude to the interquartile range of the measured scour depths. None of the medians of scour depths estimated using the seven equations were statistically equal to the median of the measured scour depths at a 0.05 level of significance. However, the Froehlich equation without a safety factor provided the "best fit" estimates of the equations considered.

Contraction scour exceeded local scour by several times in some cases. Total scour of approximately 19 feet, largely becasue of contraction scour, was measured at the Chariton River near Prairie Hill during the flood of July 8, 1993. This large scour was, in part, the result of an accumulation of woody debris de- creasing the bridge-opening area.

A bridge on State Highway 32 near Lebanon over North Cobb Creek (drainage area 52.5 square miles) was destroyed during the flood of May 26, 1990. Bridge scour attributable to contraction of the flood flow caused this loss.