Modeling the Effect of Weathering on the Evolution and Morphology of Shore Platforms

Abstract

A mathematical wave erosional model was modified to consider the effect of physical and chemical weathering on the development of shore platforms. Severe weathering was represented by a 75 percent reduction in rock strength at the mean high water spring tidal level, and moderate weathering by a 25 percent reduction. Rock strength was reduced at other intertidal levels by amounts proportional to the time of exposure of the platform surface to subaerial conditions. Each of the one hundred model runs was repeated for conditions representing no weathering, moderate weathering, and severe weathering conditions. Platform gradient decreased and width increased, at a declining rate, through the model runs, as the simulated platform profiles trended towards states of static equilibrium. Weathering increased the ability of the waves to continue eroding the cliff base and the upper portion of the platforms, and it therefore required more time for weathered than unweathered platforms to attain equilibrium. Platform width increased with the degree of weathering. The greatest proportional increase occurred on fairly narrow platforms (< 125 m wide) in resistant rocks, where, in some cases, weathering is essential in order for wave erosion to take place, whereas the greatest absolute increases occurred on wide platforms in weak rocks. Most equilibrium profiles in unweathered rocks were slightly concave, but they became more linear with increasing amounts of weathering. There was no relationship between the elevation of the cliff-platform junction and the degree of weathering. Because high waves break in deeper water than lower waves, and therefore dissipate more of their energy in crossing wide surf zones, the relationship between weathered and unweathered platform morphology and exposure to strong wave action is complex. The model suggests that the role of weathering is generally secondary to mechanical wave erosion, although its importance increases with weathering intensity and the resistance of the unweathered rock.