How does Underlying Geology Affect Coastline Change? An Initial Modeling Investigation

Abstract:

Conceptual geologic framework models predict that areas of the coast underlain by a weak or very fine grained lithology retreat landward faster than adjacent, more resistant or coarser-grained segments. These apparently commonsense predictions seem inconsistent with observations that sandy coastlines underlain by heterogeneous lithology remain fairly smooth. We have developed a numerical model to explore the interaction between shoreface lithology effects and alongshore transport processes on scales greater than years and kilometers. Shoreline indentations, or sections of the coast that are slightly landward of adjacent sections, tend to form in the model where the shoreface weathers more rapidly or is composed of finer-grained material. However, alongshore transport tends to fill in these indentations, slowing the weathering rates there while increasing them in adjacent areas where mobile sediment is preferentially removed. Over the long term this leads to an alongshore uniform retreat rate. Subtle shoreline undulations reflecting the heterogeneous geologic framework exist in the model, but their amplitude reaches a steady state rather than increasing with time. Compositional heterogeneities determine the morphology of the coast while weathering rate heterogeneities control the thickness of sediment on the shoreface. Thus indentations can form in sections composed of harder (less erodible) rocks if they are finer grained.