By employing the sand-volume conservation equation, calculated depth changes indicate where hard bottom may restrict the transport and profile change. The equation is conveniently solved on a staggered grid where the elevations are defined in the middle of a calculation cell and the transport rate at the boundaries. The elevation of the hard bottom must be known at all grid points across shore. An algorithm for correcting the transport rate must not only identify points where hard bottom constrains the transport but, because the conditions at neighboring bottom grid points are coupled, restrictions imposed by hard bottom lying updrift must also be checked. Corrections should be made in the direction of transport.
Significant scour can appear downdrift of an exposed hard-bottom area.
For constant waves and water level, it is expected that scour would only continue until some equilibrium depth is attained, after which there would be no further local erosion. However, the model might not properly describe this situation and could overestimate the local scour depending on the hard-bottom configuration. A simple means of limiting local scour downdrift of hard bottom was introduced. It is assumed that the transport rate increases exponentially with distance downdrift of the hard bottom to the potential value, where the spatial rate of increase is determined by an empirical parameter called the scour attenuation coefficient. Based on sensitivity testing, values of the scour attenuation coefficient are expected to lie in the range from 0.1 to 1.0 m -1 .