Modelling the effect of rising sea level on beaches with resistant foundations

A mathematical model was used to study the formation of beaches with resistant foundations (shore platforms) and their response to rising sea level. Beach morphology and occurrence depended on the amount and grain size of the sediment, prevailing wave conditions, the gradient of the foundation, and the equilibrium gradient of the beachface. Beach gradient varied between model runs according to the grain size and within model runs owing to changing wave conditions. Most runs used one of three grain sizes, representing fine sand, coarse sand, and pebble, although some additional runs employed two of the three grain sizes to represent more heterogeneous materials. There were five types of wave, representing calm to stormy conditions. The resistant foundation had a linear, convex, or concave profile with gradients ranging from 1-4°. Rising sea level in model runs with linear foundations caused lateral compression of the beaches as they migrated up the intertidal zone, with a corresponding decline in beach width. Commensurate elevation of the berm accommodated sediment transferred from the beachfoot, but continued rise in sea level lowered the back of the beach resulting in its immersion during high tide. In the field, where rising sea level may be accompanied by increased storminess and reduced beach gradients, scour generated by wave reflectance from the cliff face could cause the complete loss of sediment at the beachfoot. Sea level induced beach compression, immersion, and sediment loss also occurred over concave and convex foundations in the model runs, but beach response was more varied than on linear surfaces because of changes in local gradient that exacerbated or ameliorated the loss of sediment.