PREDICTING MORPHODYNAMIC LONG-TERM CHANGES ALONG THE SOUTHERN RHODE ISLAND SHORELINE IN THE NEXT DECADES

Abstract

Shoreline recession presents a real threat to coastal communities. Both the Napatree spit and the Ninigret-Trustom Ponds (NTP) barrier system in Rhode Island are at risk of erosion from physical morphodynamic processes and increases in sea level. The purpose of this thesis is to assess the ability of a new numerical model, ShorelineS, to predict the long-term shoreline position along the southern Rhode Island coast over the next 50 years. The 1-D shoreline model, ShorelineS, was calibrated and validated at these sites based on historical observations. Calibration at Napatree explicitly displayed the shoreline to be relatively static over the past decades (1985- 2014), with the observed shoreline changes, modeled shoreline changes without sea level rise, and modeled shoreline changes with sea level rise varying at 0.30 m/yr, 0.16 m/yr, and 0.12 m/yr, respectively. These observations were driven by the longshore current to the degree of 53 %, while only 47 % of the coastline changes were due to SLR. However, at the Ninigret-Trustom Ponds site, the method could not be directly applied because it displayed different morphodynamic behavior than Napatree. Historically, this site retreats an average of -0.53 m/yr along the barrier beaches. To be able to reproduce this retreat and accurately model future shoreline positions, modification to Bruun’s law in the governing ShorelineS equation was introduced. This approach included other processes, in particular the loss of sediment due to overwash and inundation caused by cross-shore transport, summarized into an empirical parameter under the assumption that these processes are linearly related to sea level rise. Without this approach, the modeled long-term shoreline changes resulted to be only 7-9 % and 8- 10% due to longshore transport and SLR, respectively. The introduction of the modification contributed to 82-85 % of the shoreline retreat at this site, and significantly reduced the root mean square error between the observed and modeled shorelines. Therefore, ShorelineS adequately reproduced the observed historical shoreline changes and in turn, was used to predict the shoreline changes over the next 50 years. Long-term projections of the shoreline at Napatree predicted a shrinking in the width of the barrier, while projections of shoreline changes at NTP for 2035, 2050, and 2070 showed significant regression of the shoreline through the coastal ponds.