Quantification of Historical Variations in Longshore Sediment Transport Rates: A Tool for Estimating Future Coastal Change

Abstract:

Estimates of future changes of sandy shores are often unreliable in regions with highly variable historical change rates, as is frequently the case along coasts dominated by Pleistocene glacial deposits. In an effort to improve estimates of future coastal change for the outer section of the Provincelands at the distal end of Cape Cod, Massachusetts, we applied a simple one-dimensional numerical model to calculate net longshore sediment transport rates for three time periods: 1833-1889, 1889-1933 and 1933-2010. Since the entire 9 km study area was within a single littoral cell and included its terminus (at Long Point), we back-calculated net transport rates from that point of no transport to estimate the transport at updrift locations throughout the study area. The results for all three time periods indicated a net alongshore transport rate on the order of 100,000 cubic meters per year entering the study area (at Race Point). However, the transport rate patterns for the three periods varied markedly, albeit consistently, throughout the study area. Regional decreases and increases in transport rates, indicating zones of net accretion and erosion, were pronounced in the earliest period, less so in the second, and almost absent in the third. We interpret these results as indicating that the study area has been evolving toward an increasingly stable morphology over the past two centuries.