Effect of Coastal Erosion on Storm Surge: A Case Study in the Southern Coast of Rhode Island

Abstract

The southern coast of Rhode Island consists of many coastal lagoons and barriers; this coast is eroding at an significant rate due to the combined effects of coastal storms and sea level rise (SLR). The propagation of waves and storm surge in the nearshore can potentially be affected by the coastal erosion and is neglected in many studies. The objective of this work was to assess the effect of shoreline retreat and dune erosion on coastal flooding in a case study located on the southern coast of Rhode Island. An ADCIRC model was developed to simulate the propagation of storm surge in the coastal ponds along the southern coast (Ninigret Pond, Trustom Pond, and Point Judith Pond). The model was validated with data that was collected in the coastal ponds from September 10th, 2010 to September 13th, 2011. Tides as well as a storm surge case of Hurricane Irene were captured in this data and were used to validate the ADCIRC model. Storm surge scenarios, such as the 100-yr storm and Hurricane Bob were then considered along with multiple beach erosion scenarios, including shoreline retreat in 25 years and the failure of dunes. A simplified methodology based on the historical trend of the shoreline retreat in this area was incorporated in the model to represent coastal erosion. Further, a geological assessment of dune erosion profiles after significant storm events was implemented to include the eroded dune profiles in the Digital Elevation Model (DEM). The results showed that for extreme storms (e.g. 100-yr event), where coastal dunes are overtopped and low-lying areas are flooded, the flooding extent is not significantly sensitive to coastal erosion. However, failure of the dunes leads to a significant increase of the flooding extent for smaller storms, which is useful for risk assessment. Substantial dampening of the storm surge elevation in coastal lagoons for moderate and small storm surges, thereby limiting the flooding extent, can be explained by using tidal inlet theory. When dunes are overtopped or breached by surge and wave actions, the storm surge was no longer limited by the effect of inlets. It was shown that the dune erosion has a considerable impact on the flooding extent of smaller more frequent storm events, with an increase of 207%. Larger events didn't show increased flooding due to dune erosion. The shoreline change did not significantly affect the extent of flooding.