Modeling cohesive clay coast evolution and response to climate change

A model was used to study the evolution of cohesive clay coasts under different environmental and geological conditions, and the effect of future climate change. The model considered wave impact on the bluff and in the intertidal zone, and bottom generated shear stresses and the protective and abrasive action of beach sediment in the intertidal and subtidal zones. Quasi-equilibrium conditions developed in model runs, with parallel slope retreat in the upper portion of the profiles, and slope decline in the lower portions. Profile gradient below the low tidal level increased with the threshold shear stress for bottom erosion. Therefore, rates of wave attenuation were lower in resistant than in weak materials, and the intertidal zones were wider and more gently sloping. The widest and gentlest intertidal zones developed in runs that had no sediment. The largest waves played little role in platform development, in part because of low frequency but mainly because they expend most of their energy in crossing wide, turbulent surf zones. The model suggested that rising sea level will usually result in accelerated rates of bluff recession, especially if the rise is rapid. Simultaneous increases in the frequency of moderately large waves, as opposed to the largest waves, may also generate marked increases in erosion on bluffs and in intertidal and supratidal zones, although erosion may be reduced at some elevations where there is beach sediment. Positive or negative changes in beach volume can increase or decrease rates of bluff erosion.