SRICOS-EFA Method for Contraction Scour in Fine-Grained Soils

Abstract

Scour at bridges is the number-one cause of bridge collapse in the United States. Much research has been performed to improve the prediction of scour depths in coarse-grained soils, but little has been done for fine-grained soils. Starting in the early 1990s, the scour rate in cohesive soils-erosion function apparatus (SRICOS-EFA) method was developed for fine-grained soils. The first version of the method predicted the scour depth versus time curve for a cylindrical bridge pier in deep water subjected to a multiflood hydrograph and founded in a layered soil. Here, the SRICOS-EFA method is extended to the case of contraction scour, which is the lowering of the river bottom caused the narrowing of the river. Furthermore, the paper addresses clear-water scour only. The input to the new prediction method consists of the soil properties as tested in a special laboratory device called the EFA, the water parameters given by the velocity history over the design period considered, and the geometry of the contraction described by the contraction ratio, the contraction length, the contraction transition angle, and the water depth. This article focuses on the simpler case of a constant water velocity lasting for a limited time. The prediction equations are based on a combination of 14 model scale flume tests, 16 three-dimensional (3D) numerical simulations, and a verification (very limited). The step-by-step procedure is given, and a simple example is presented. The prediction process is automated by the program SRICOS-EFA, which can be downloaded free from http://ceprofs.tamu.edu/briaud/sricos-efa.htm. The advantages of the method are that it is based on site-specific soil testing (EFA), that it introduces the time effect in a simple manner, and that it therefore gives a more realistic prediction of the scour depth in cases where time effects significantly reduce the final scour depth. The drawback is that it requires soil testing.