Current guidance in Hydraulic Engineering Circular No. 18 (HEC-18), "Evaluating Scour at Bridges," (Arneson et al. 2012) provides equations for estimating contraction scour. Existing equations are based on sediment transport theory using approaches developed by Laursen, 1960 (live-bed contraction scour) and Laursen, 1963 (clear-water contraction scour). Both equations assume that the scour is due solely to the contraction effect and that local effects are negligible (i.e., that the contraction is hydraulically "long"). Depending on the ratio of the length of contraction L to the approach channel width b1, (L/b1) channel contractions are designated as long or short. In a short contraction, local scour occurs throughout the contracted section as a result of large-scale turbulent flow structures created at the entrance to the contraction, and the total scour is the result of both the contraction and local effects. Analysis of existing laboratory data sets conducted under NCHRP Project 24-34, "Risk-Based Approach for Bridge Scour Prediction" revealed that the clear-water contraction scour equation does not envelope the observed data as a design equation. Rather, it is a predictive equation which is seen to under predict observed scour relatively frequently compared to pier and abutment scour equations. No laboratory data sets of live-bed contraction scour were identified during NCHRP Project 24-34. NCHRP Project 24-34 found that all of the previous studies suffered from a flaw in the experimental design, as none actually measured the depth of flow y0 in the contracted section before scour began to occur. Therefore, this value had to be estimated in order to determine the depth of scour. In addition, a number of laboratory studies did not directly measure the depth of scour using bed elevation measurements. Instead, the assumption was made that y0 was equal to y1 (the depth of flow in the approach section upstream of the contraction). This assumption ignores the hydraulic drawdown effect in the contraction which occurs during subcritical flow (particularly in a bridge reach). There is a need to develop a contraction scour design approach that can be used for various bed materials. Separating hydraulic erosion forces from the soil erosion resistance allows developing bed material independent equations and is the future in bridge scour research. Also, considering the decay of the hydraulic scour forces and how they interact with the associated bed layer will lead to a more realistic scour depth.
The objective of this research is to develop live-bed and clear-water contraction scour equations suitable for use in risk-based bridge design for a wide range of hydraulic conditions, contraction ratios, and bed material (cohesive, non-cohesive and rock–see Special Note H) and gradation uniformities.