70 / 2025-04-24 12:55:35

A New Scour Tool: NextScour Shear Stress Decay Method

The Federal Highway Administration (FHWA) is advancing NextScour, an innovative research initiative aimed at enhancing scour evaluation and delivering more accurate estimates of scour depths for the design of bridge foundations. NextScour addresses the two principal elements of scour: the hydraulic forces at play and the ability of soils to resist erosion. To calculate scour depths near bridges more accurately, shear decay functions for contraction, pier, and abutment scour were developed. These functions are the result of comprehensive research that integrates flume scour testing, computational fluid dynamics (CFD) simulations, and probabilistic analysis. When compared against a soil's erosion resistance, especially across different layers, the shear decay functions enable a more accurate determination of scour depths.

This paper describes the detailed process of applying FHWA’s new shear stress decay method for accurately estimating the depths of contraction, pier, and abutment scour. The shear stress decay method treats contraction scour distinctly from local, i.e., pier or abutment, scour. It employs grain size roughness and flow conditions to determine the shear stress in the bridge opening specifically for clear-water contraction scenarios. Additionally, adjustments were made for shear decay across different soil layers by considering changes in roughness at each layer. For clear-water contraction, the scour depth is considered to reach an equilibrium when the shear stress becomes equal to or less than the critical shear stress of the soil. Furthermore, the method used the velocity ratio to calculate shear for live-bed contraction scour at the scour depth determined during live-bed conditions.

For local scours, corresponding bed shear stresses are subsequently amplified to estimate the maximum shear stress at the piers and abutments. For pier and abutment scour, shear stress decay functions were utilized to identify the diminished shear stress at incremental scour depths. The equilibrium depth for pier or abutment scour is determined at the point where the shear decay function intersects with the resistance offered by the soil layers.