HEC-RAS Hydraulic Reference Manual Estimating Scour at Bridges Computing Contraction Scour Current: Clear-Water Contraction Scour PDF Download PDF Download page Clear-Water Contraction Scour. Current page All pages Clear-Water Contraction Scour The recommended clear-water contraction scour equation by the HEC No. 18 publication is an equation based on research from Laursen (1963): 1) \displaystyle y_2 = \left[ \frac{Q^2_2}{CD^{2/3}_m W^2_2} \right]^{3/7} 2) y_s = y_2 - y_0 SymbolDescriptionUnitsD_mDiameter of the smallest non-transportable particle in the bed material (1.25 D50) in the contracted sectionfeet (meters)D_{50}Median diameter of the bed materialfeet (meters)C130 for English units (40 for metric) Note If the bridge opening has overbank area, then a separate contraction scour computation is made for the main channel and each of the overbanks. ×