Observation of Sediment Mobilization by an Internal Solibore on the
California Inner Shelf
Abstract
Acoustic backscatter, velocimetry measurements of the nearbed velocity
profiles, and thermistor chain measurements of the temperature
stratification were used to understand the bottom boundary layer flows
and associated sediment transport processes in 35 meters water depth on
the California shelf off of Point Sal where the bottom sediment consist
of fine sand with median grain size diameter of $d_{50}=0.1$ mm.
The observations show that the nearbed flow is dominated by the bore of
a shoaling internal tide whose steepening front generated a series of
internal solitary waves (ISW) with a 15-min period superposed on the
tail of the bore. The bore-induced nearbed flow was strongly asymmetric
with 20 cm/s seaward directed flow under the bore trough that exceeded
the bottom stress threshold for mobilization of the 0.1 mm sand, and
5-10 cm/s onshore flow during the tail of the bore that produced only
subcritical bottom stress. The ISWs induced symmetric 5-10 cm/s nearbed
velocity which however combined with the bore tail to produce onshore
flows under the wave crests with bottom stress that also exceeded the
sediment mobilization threshold.