Understanding differences between DELFT3D and empirical predictions of alongshore sediment transport gradients

Predictions of alongshore transport gradients are critical for forecasting shoreline change. At
the previous ICCE conference, it was demonstrated that alongshore transport gradients
predicted by the empirical CERC equation can differ substantially from predictions made by
the hydrodynamics-based model Delft3D in the case of a simulated borrow pit on the
shoreface. Here we use the Delft3D momentum balance to examine the reason for this
difference. Alongshore advective flow accelerations in our Delft3D simulation are mainly
driven by pressure gradients resulting from alongshore variations in wave height and setup,
and Delft3D transport gradients are controlled by these flow accelerations. The CERC
equation does not take this process into account, and for this reason a second empirical
transport term is sometimes added when alongshore gradients in wave height are thought to be
significant. However, our test case indicates that this second term does not properly predict
alongshore transport gradients.