Influence of Underlying Geology on Shoreface Bathymetry and Sediment Distribution at Duck NC
McNinch, J. E., and Drake, T.G., 1999, Influence of underlying geology on shoreface bathymetry and sediment distribution at Duck NC: EOS Trans. AGU, 80(46), Fall Meeting Suppl., F548.
"Hotspots" exhibiting high erosion during storms and subsequent rapid
post-storm recovery have been identified by shoreline-mapping studies on
beaches around the world. High-resolution surveys of bathymetry and
sediment characteristics across the shallow shoreface (depths of 2 to 10 m)
were conducted at a 500 m by 500 m site located 2 km north of the pier at
the US Army Corps of Engineers Field Research Facility at Duck NC. An
amphibious vessel was used to conduct monthly and post-storm surveys of the
site, which is an anomalous, isolated region having complex bathymetry and
sediment distribution. A 1-m-high, 20-m-wide ridge extends across shore
from near the shoreline to a depth of 7 m. A 25-m wide, 1-m deep trough
runs roughly parallel to the ridge; both features have persisted through
the 6-month study. Sediment samples reveal an organic-rich mud exposed
along the surface of the trough and a well-sorted fine sand covering the
surface of the ridge and surrounding shoreface. The persistence of these
high-relief features suggests that complex morphology is maintained across
a high-energy shoreface despite the unconsolidated nature of the surfacial
and underlying sediment. This persistence probably results from
differential erosion of older sedimentary units, for example, relict stream
channels and lagoon deposits that are now exposed across the shoreface.
Other studies of the subsurface geology of the NC inner-shelf demonstrate
strong correlation between long-term shoreline erosion and underlying
geology, but physical mechanisms responsible for coupling shoreline changes
with shoreface geology are poorly known. The shoreline may respond to
variations in wave climate and wave-driven currents induced by
geologically-influenced bathymetry, or to differences in volume, grain-size
characteristics and supply of sediment contributed to the nearshore
sediment budget, or to other presently unrecognized large-scale nearshore
processes.
Supported by the Terrestrial Sciences Program of the Army Research Office