BEFORE AND AFTER: IMPACTS OF HURRICANE-INDUCED FLOODING ON SEDIMENTS OF THE NEUSE RIVER ESTUARY AND PAMLICO SOUND, NORTH CAROLINA

During September-October 1999 heavy rainfall from three hurricanes brought record flooding to eastern North Carolina. Satellite imagery and ground observation showed that the floods moved massive quantities of suspended sediment. We have used modern sediment chronology (excess 210Pb, fallout 137Cs as tracers) to assess flood-induced erosion and deposition in the Neuse River estuary and western Pamlico Sound.

Divers collected sediment cores during November 1999, July 2000, and October 2001. Most sampling locations were re-occupations of sites at which pre-flooding sediment chronologies were available: lacking tracers specific to flood discharge, we can best assess flood impacts by comparing pre-flood and post-flood chronologies. Several lines of evidence suggest that we can reliably distinguish spatial variability from temporal change; these include time-series chronology (Neuse estuary, one station), general spatial uniformity in pre-flood tracer profiles (Pamlico Sound), and close correspondence between pre-flood and post-flood profiles below the depths of flood-induced disturbance.

Despite visual evidence of massive sediment redistribution during the flood event, we detected only minor post-flood changes. Acoustic surveys revealed no evidence of major erosion or deposition. Coarse sediment deposition in our study area was limited to isolated bedforms. Correspondingly, pre-flood and post-flood porosity profiles were nearly identical at our stations. Where most reliably estimated, net erosion or deposition was < 7 cm, with no clear geographic pattern among our nine stations. However, one site in the lower Neuse estuary may have received ca 20 cm net deposition. At two additional sites substantial early erosion (ca 12 cm , >20 cm) may have been followed by compensating deposition, yielding low net change.