As described in the previous section, if a soil layer has less than 20 percent (or a user specified percentage) cohesive material, BSTEM will apply a transport function to compute toe scour (below). The USDA-ARS BSTEM model uses an NSED transport function library to compute transport. This library includes six transport functions, including:
Engelund-Hansen
Parker (1990)
Wilcock and Crowe (2000)
Meyer-Peter Muler
Wu (2000)
Select and apply transport functions with extreme caution recognizing the intent and range of applicability of each. Transport functions are notoriously uncertain, computing transports that commonly differ by at least one order of magnitude. Engelund and Hansen (1967) and Yang (1996) work best for sand. Meyer-Peter and Müller (MPM) (1948) will probably perform best for coarse materials. Parker (1990) and Wilcock and Crowe (2003) are both surface based methods, intended for heterogeneous soil mixtures with sand and gravel components. These two methods account for mixing, hiding and armoring implicitly, which tends to moderate transport and sometimes makes the methods more appropriate for toe scour in heterogeneous materials.
Also, it should be noted that most of these transport functions were derived for one-dimensional alluvial transport at the cross section scale. BSTEM applies these transport functions to bank scour at the node scale. This makes transport functions, already uncertain in their intended setting, loose process analogies in toe scour. The transport functions often over predict scour substantially and results should be interpreted carefully.
