Shoreline change (accretion or erosion of the beach) occurs when the amount of sand brought to the beach does not match the amount of sand removed from the beach. This sand can be transported along the coast or it can be transported perpendicular to the coast in onshore or offshore direction. Waves and currents predominantly affect the movement of sand within the littoral cell. The process-based shoreline model used in the Study, UNIBEST, only considers shoreline change due to alongshore sediment transport. UNIBEST uses a variety of input parameters, including: waves, tidal currents, sediment transport rates at the lateral boundaries of the area of interest, an elevation profile of the beach, initial shoreline positions and sediment sizes. The waves are measured at a wave buoy near the Grays Harbor inlet. The (alongshore) currents are generated by the tides, and their role is less important than the role of waves along the coast of Washington.
The application of the model consists of two phases. In the first phase (calibration phase) it is attempted to simulate the historical measured behaviour as good as possible, i.e. the modeled shoreline positions should accurately match the historical shoreline positions. This model phase involves making small adjustments of model variables within reasonable limits. When the model is able to simulate past shoreline behaviour, future predictions can be made. In general, the calibration phase is for a period from the 1950s to 1995. Shoreline change predictions are being estimated through 2020 using this modeling technique.
Shoreline modeling is used together with the sediment budget analysis. The sediment budget determines the sediment transport rates at the boundaries of the model area, and likewise, the shoreline model and the sediment budget are complementary. They both improve the understanding of the natural system.