Quantifying the influence of shoreface shape on coastal responses to sea level rise

Abstract

The coastal zone is a very dynamic region, constantly seeking to reach a state of equilibrium due to variations in the external forces. The shallow zone along the inner continental shelf known as the shoreface, can be divided into the upper and lower shoreface. While the former responds to processes operating across time spans ranging from that of a single event, such as storm, through to seasonal variations in environmental conditions, the latter evolution's occurs in geological scale, more in response to mean trends in environmental conditions (decades to millennia). These zones are connected via sedimentary exchanges, by a transition zone that adapts to changes imposed from behaviors and displacements of the upper and lower shoreface, such as upward profile displacement with sea level rise, aeolian transport, alongshore transport gradient, cross-shore transport. To analyze the influence of shoreface morphology variations on coastal response to mean sea level rise (MSLR), computer simulations were performed using synthetic shoreface profiles with variable shoreface shape parameter (‘m’) values. The random shoreface translation model (RanSTM) was used to simulate coastal response through variations in the shape of shoreface profiles. The ‘m’ value was changed in the upper and lower shoreface separately, to check the sensitivity of coastal response (in this case, shoreline recession distance) as a function of morphological change in each of the two zones. In addition, the variations of ‘m’ were analyzed with a variable and a fixed transition zone length (in order to isolate effects of a variable ‘m’). The simulations indicated that changes in ‘m’ on the upper shoreface have minimal influence on shoreline recession distance, especially when the transition zone remained constant. Conversely, profiles with higher ‘m’ values in the lower shoreface exhibited greater shoreline recession. It was also observed that, a longer transition zone produced a higher shoreline recession. Profiles adjusted with ‘m’ values of 0.66 and 0.4 in both shoreface zones had the highest and lowest shoreline recession distances (141.4 and 203.7 m), respectively. Sensitivity tests have shown that an increase of only 0.02 units in ‘m’ in the lower shoreface had a strong positive correlation (r = 0.97, p < 0.01) with increased shoreline recession. The results obtained in this study demonstrate the control that lower shoreface shape variations have on coastal response during MSLR. Critically, even small variations in shoreface shape parameter, such as those that occur along adjacent coastal sectors, and due to climate change effects may have implications for management and adaptation on a regional scale.