Past and future drivers of increased erosion risk in the northern Gulf of Mexico

Abstract:

We use hourly observations of water levels from two tide gauges and wave data from three buoys to assess their relative contribution to past and potential future changes in the erosion risk for Dauphin Island, a barrier island located off the coastline of Alabama. Topographic information (i.e. beach slopes and dune toe and crest heights) is obtained from the most recent lidar survey conducted in the area in July 2013. Water levels and wave parameters (i.e. significant wave height and peak period) from the two tide gauges and three wave buoys are merged into single records spanning the period from 1981 to 2013. The Stockdon et al. (2006) run-up model is used to estimate the 2% exceedance values of wave run-up maxima, which are then combined with the observed water levels at the representative tide gauge site to obtain total water levels (TWLs). With this information we assess the relative contribution of geocentric sea level rise, vertical land-movement, and long-term changes in the wave parameters to the observed increase in erosion risk. The latter is approximated using the concept of impact hours per year (IHPY; Ruggiero 2013) at dune toe and dune crest elevation thresholds derived from the lidar data.

Wahl et al. (2014) recently discovered a significant increase in the amplitude of the seasonal sea level cycle in the Gulf of Mexico. Here, we explore the potential of these changes, and similar developments in the seasonal cycle of the wave data and corresponding IHPY, to affect coastal erosion. Such intra-annual signals with longer-term variations have not been included in most earlier studies in favour of analysing the effects of annually averaged long-term trends. Finally, scenarios of potential future changes of all relevant parameters are used to explore their relative contribution to further increase in the coastal erosion risk over the next few decades.