10 Coastal Response to Tidal Inlets: A Tribute to the Career of Bill Cleary

Thursday, 24 March 2011
1:30 PM - 5:30 PM
Salon A

DELINEATION OF HISTORICAL “NEW INLET”, DEBIDUE ISLAND, SOUTH CAROLINA

Paper number: 10-9
Program: Symposia Sessions
Time: 4:10 PM – 4:30 PM

Authors

Abstract

An historical inlet investigation was performed to settle a land boundary dispute between two contiguous parcels of land along Debidue Island, South Carolina. The property boundaries were historically defined based on the location of “New Inlet” in the early 1800's, which has since closed. Research and fieldwork were designed to determine the location of "New Inlet" prior to its closure. Using historical maps, plats, and aerial photographs, an approximate historical position of the inlet was initially identified using Geographic Information System technology.

Determining the exact position of the paleo-inlet involved detailed field investigations, consisting of geophysical surveying and sediment coring. A total of 3.6-km of ground penetrating radar (GPR) profiles were collected to identify potential inlet fill sequences and to characterize the sub-surface geology. Sixty-five sediment samples were analyzed for grain size and organics to ground-truth the GPR data. The grain size analyses provided the basis for identifying sedimentary units and for distinguishing tidal inlet fill sequences. Four organic samples were radiocarbon dated to prove a chronology for the evolution of the barrier and tidal inlet system. Trees were also age dated to verify the geological data.

The paleo-inlet was identified along the central portion of Debidue Island, 280-m south of the current property boundary. The GPR profile revealed a 3.4-m deep, 60-m wide channel cut-and-fill structure. The grain size data indicate that sediments within the paleo-channel are indicative of channel lag containing moderately sorted, medium to coarse-grained sands, quartz pebbles and shell hash. GPR and sedimentological data suggest that “New Inlet” was about 1-m deep and 40-60 m wide. Geomorphic and hydrodynamic analysis suggest that “New Inlet” was a small, ephemeral, storm-induced inlet connected a “cat-eye-pond” on the barrier, and not a substantial network of backbarrier channels.

A four-stage model was developed to illustrate the evolution of the southern half of Debidue Island. The model includes the initial spit elongation of Debidue Island prior to 1000 ybp, subsequent beach ridge development and “cat-eye-pond” formation, storm breaching and “New Inlet” formation during the 1700’s and finally “New Inlet” closure during the early.