TITLE: Nearshore Sediment Dynamics: Suspension, Bedforms, and Bubbles



Personnel Information

Alexander E. Hay

Department of Oceanography

Dalhousie University

Halifax, NS B3H 4J1

Canada

Phone: 902-494-6657

FAX: 902-494-3877

INTERNET: alex.hay@dal.ca

Anthony J. Bowen

Department of Oceanography

Dalhousie University

Halifax, NS B3H 4J1

Canada

Phone: 902-494-7082

FAX: 902-494-2885

INTERNET:tony.bowen@dal.ca







SCIENCTIFIC OBJECTIVE



We are planning three inter-related studies of nearshore sediment

dynamics, with emphasis on the processes active within the surf zone:

(i) sediment suspension mechanisms;

(ii) bedform scales, migration velocities, and development rates; and

(iii) ambient noise generation and propagation in the surf zone.

The projects will address sediment dynamics issues mainly within the surf zone itself: specifically, in the nearshore trough and in the immediate vicinity of the bar crest.



Central objectives are:

(1) To investigate the processes contributing to sediment suspension under different bedform and fluid forcing conditions; and to test our understanding of the net local suspended sediment flux balance;

(2) To determine the time and space scales of bedform development and migration as a function of wave and current forcing parameters, at different points within and immediately outside the surf zone, simultaneously;

(3) To investigate the roles of long waves and low frequency currents in nearshore sediment dynamics;

(4) To investigate wave breaking in the surf zone; and the effects of wave breaking intensity and bubble plume penetration on sediment suspension, bedform development, and sound propagation;

(5) To test models of bedform growth and bedload transport;

(6) To estimate the relative importance of bedform and suspended load transport in the natural surf zone.

EXPERIMENT PLAN



The approach involves a 6-node, L-shaped array, with 20- to 50-m horizontal spacing between adjacent nodes. At each node, rotary pencil-beam and fan-beam sonars will image the bedform field. PUV sensors will monitor the local hydrodynamics. Fixed upward- and downward-looking sonars will monitor sediment suspension and bubble cloud penetration.

The central node in the array will have additional sensors, including a newly developed 3-component coherent Doppler profiler, for velocity and suspension measurements. Near four of the nodes, broadband hydrophones will be deployed on separate single pipes, for the ambient noise measurements.

The instruments to be deployed are: rotary fan and pencil beam sonars (5 and 6, respectively), 3-component coherent Doppler profiler (1), fixed up- and down-looking sonars (4), broadband hydrophones (4), narrowband (50 kHz) projector (1),Marsh-McBirney flowmeters (12), pressure and temperature sensors (6), dual axis tilt sensors (6). The location of the instruments is shown in the figure below.

>>CLIFF - insert link to SNDBEACH.JPG here.<<





Much of this work will be collaborative. Dr. Len Zedel (Memorial University), will be participating in the coherent Doppler profiler work. Dr. Diane Foster (Dalhousie University), will be working with us on bottom boundary layer models. Planned collaborations with other SandyDuck investigators include: R. Beach (Oregon State), low frequency variations in sediment suspension; J. Haines and G. Gelfenbaum (USGS), vertical structure and bottom roughness; D. Hanes (Florida), sediment suspension processes; T. Lippmann (Scripps), wave breaking statistics; and T. Stanton and E. Thornton (NPS), sediment suspension mechanisms and bedforms, and bubble injection by breaking waves. Other potential collaborations include: S. Elgar (Washington) and R. Guza (Scripps), on surf zone bathymetry and bottom roughness; and R. Heitmeyer (NRL), on surf zone ambient noise.

PROPOSED SCHEDULE



a) July 21-28: Deploy 3 innermost cross-shore nodes

b) Aug. 21-31: Deploy 2 longshore nodes

c) Sept. 1-14: Deploy outermost cross-shore node.

d) Nov. 1-15: Pullout

FUNDING STATUS

Partially funded by:

Natural Sciences and Engineering Research Council of Canada.

U.S. Office of Naval Research, Coastal Sciences Program.