COASTAL HAZARDS ALONG THE U.S. ATLANTIC SEABOARD: ASSESSING VULNERABILITY TO ISOSTATIC AND TECTONIC MOVEMENT AND RELATIVE SEA-LEVEL RISE

An important societal issue for coastal geology is predicting responses of coastal landforms to sea level change. Global sea level has risen about 18cm over the past century and modeling suggests an additional 48cm rise by 2100 is likely. In the long term sea-level is critical in determining shoreline position and stability. The Coastal Vulnerability Index being used in a national assessment is a numerical function combining six attributes that reflect the relative vulnerabilty of coasts to sea-level rise. Two attributes emphasize vulnerability: land elevation and relative sea level (RSL) rise. Clearly low-relief landforms located where RSL rise exceeds the global average are highlighted. The rate of RSL rise along the Atlantic coast is uneven and likely reflects differential vertical movement of Coastal Plain sediments. Tide gages, for example, show erratic RSL rise rates between FL and ME, however rates are consistently higher along the mid Atlantic coast. Assuming that the current rate of eustatic sea level rise is ~2 mm/yr, then the FL coast, Cape Fear, NC region, and MA and ME coasts are experiencing “normal” RSL rise. The RSL rates for the mid Atlantic coast are 1-2 mm/yr greater than the global rate. Not surprisingly the zones of anomalously high RSL rates correspond to those coasts identified as most vulnerable by the CVI assessment. These same zones are underlain by thick Coastal Plain sediments and are identified as residual subsidence anomalies related to isostatic “forebulge” collapse due to postglacial readjustment to ice loading in Hudson Bay region. Isostatic subsidence rates of ~1.3 mm/yr are assumed based on basal peat-dates along the coast. The Cape Fear region represents a condition where tectonic uplift of ~1 mm/yr offsets isostatic subsidence. A comparison between high risk areas from CVI and the subsidence and uplift anomalies clearly shows their influence on the coasts. Still higher rates of RSL rise from tide gage records suggest local conditions that may relate to fluid withdrawal or local fault movement.