Alexander E. Hay, Anthony J. Bowen
Natural Sciences and Engineering Research Council of Canada
Office of Naval Research, Coastal Dynamics Program
The overall objective is to understand the mechanics of the mobilization, suspension and transport of sediments by waves and currents and their modulation due to the active response of the seabed. Specific objectives for DUCK94 are:
a. to determine the vertical and horizontal coherence scales of the sediment concentration field, over distances of cm to m;
b. to determine the 3-dimensional evolution of the local morphology on scales of cm to 10 m;
c. to determine the vertical flux of suspended sediment;
d. to investigate bubble production in relation to wave breaking in the surf zone.
In keeping with pilot nature of DUCK94, these objectives are directed toward characterizing the time and space scales of sediment suspension and bedform evolution at Duck, and toward testing new measurement methods for this purpose.
Rotary fan- and pencil-beam sonars were used to study bedforms, suspension, and bubbles. EM flowmeters and pressure sensors were used to define the hydrodynamics locally. Optical Backscatter Sensors (OBS's) were used as ancillary suspended sediment sensors. The space/time structure of bubble clouds was also studied using ambient noise in the audio band. (p>
The strategy was to deploy two fixed instrument frames, each with a full suite of sensors, one on the seaward face of the bar, and the other in the nearshore trough. Measurements were also made from the pier, using the Sensor-Insertion-System (SIS).
We participated in the October experiment. The primary data set is for the period Oct. 3-15, and together with the SIS measurements constitutes a total data volume of more than 10 Gigabytes. The analysis of these data has three principal foci: (i) the rotary sonar measurements of bedform development and migration; (ii) the rotary sonar images of sediment suspension and bubble clouds; and (iii) the ambient noise and overwater video studies of wave breaking in the surf zone.
The results show that the bedform field during high energy conditions was dominated by 1-5 m horizontal scale, 10-30 cm high, lunate megaripples. These bedforms migrated either alongshore or cross-shore, at rates which at times approached 2 m/h. As incident wave energies decreased, first cross-rippled and then irregular, short-crested ripple patterns were observed. Long-crested vortex ripples either did not appear, or did so only briefly. The ambient noise spectra are dominated by isolated noise bursts, with durations of approximately 1 wave period. Comparison with the overwater video indicates that the events are associated with the passage of breakers over the hydrophone.
A manuscript summarizing the main findings on bedform evolution during the waning stages of the first storm has been submitted to Marine Geology. Some of the ambient noise measurements appear in a paper by Bass and Hay in the Proceedings of the Canadian Coastal Conference. Analysis of other results is in progress.
For a list of addresses of the investigators involved in Duck 94 click here.