COASTAL HYDRODYNAMICS AND NATURAL GEOLOGICAL EVOLUTION (CHANGE) PART 2: PROCESSES AND DRIVERS OF LARGE-SCALE COASTAL REORGANIZATION

The NSF-funded CHaNGE project is evaluating large-scale coastal morphodynamic changes in response to various forcing factors. The project utilizes geological data within North Carolina estuarine sediments to define and model major episodes of increased marine influence and hydrodynamic changes. Geological data (seismic, physical property, sedimentologic, isotopic, elemental and organic geochemical) are used to define and characterize the temporal and spatial effects of system changes. Significant departures from the modern system (Estuary Anomalies, or EA) occurred at ~4300 to 3700 Cal BP (EA1), ~2500 Cal BP (EA2), and ~1200 to 400 Cal BP (EA3). EA3 may be subdivided into EA3a (the Medieval Climate Anomaly or MCA) and EA3b (the Little Ice Age or LIA). The main characteristic that defines an EA is an increase in marine influence in the estuary as indicated by geological or historical data. Possible drivers of past and future changes include accelerated rates of sea-level rise (SLR), century-scale oscillations in storm frequency and intensity, and changes in astronomical tidal forcing. An increase in the rate of relative SLR is documented for the EA3a event, and was accompanied by likely increases in hurricane activity during the MCA. The EA1, EA2 and EA3b events coincide with peaks in astronomical tidal forcing resulting from the 1800-year period lunar declination cycle, as well as North Atlantic Ice Rafted Debris events, and rapid climate change events documented by others. The EA3b event (LIA) is specifically characterized as a time of greater overwash activity along the northeast U.S. coast based upon geological data, and greater numbers of inlets along the North Carolina barrier islands from geologic data and historic maps. These have been attributed to greater storm activity (hurricane or nor’easter). However, we hypothesize that the increase in tidal forcing during the LIA maintained a greater number of inlets, and contributed to more overwash events along the U.S. east coast. These observations suggest that attention should be given to understanding astronomically-induced tidal variations (which may be enhanced by morphodynamic feedbacks) when creating hindcast models of coastal evolution, hydrodynamics and sea-level change.