Development and application of a field instrumentation system for the investigation of surf zone hydrodynamics

The support of the NOAA Sea Grant Program through the MIT SeaGrant Program, along with the MIT/WHOI Joint Research Seed Funds isacknowledged.

Submitted in partial fulfillment of the requirements for the degree of Oceanographic Engineer at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology and for the degree of Master of Science in Ocean Engineering at the Massachusetts Institute of Technology February 1979

The development and application of an autonomous field instrumentationsystem consisting of four current meters and four wave gauges, alongwith a field monitor and digital recorder, is documented.The flow sensors are electromagnetic current meters, which employthe principle of electromagnetic induction to sense an induced electricalpotential from the flow of water through an imposed magnetic field. The10 cm diameter, discus-shaped sensor was tested in the laboratory undera wide variety of conditions, including both steady and oscillatory flowtests. The results of these tests indicate an excellent response interms of linearity and horizontal cosine. The vertical cosine responseis close to ideal in the region of ±30°, but beyond a negative angle ofattack of approximately -30° the response is compromised by the onset ofseparation under dominantly steady flow conditions. The wave gauges aresurface-piercing digital sensors, relying on the presence or absence ofwater at 128 individual sensing electrodes spaced 1.5 cm apart along thefront surface of the wave gauge. On command, the instantaneous watersurface elevation is measured, then telemetered digitally to the shorebasedmonitor and recorder.Field measurements of waves and currents at four stations acrossthe width of the surf zone were made, using this system at a beach alongthe southern coast of Maine. Spilling breakers (approximately 1.0 min height with an angle at breaking of about 8°), translated across the30 m surf zone, generated an observed net longshore current during thefour hour measurement period. The subsequently analyzed data from thisexperiment showed a strong longshore current which varied across thewidth of the surf zone, having a maximum of about 15 cm/ sec just insidethe breaker line. A net offshore current was observed at all fourstations, and averaged approximately 10 cm/sec to 15 cm/sec. Using asimplified force balance model for the generation of longshore currentson a plane, uniform beach, the data was further analyzed to investigatethe validity and parameterization of the momentum flux forces and bottomfriction forces within the surf zone. There was an observed shorewardloss in momentum flux across the width of the surf zone, from about-150,000 dynes/cm outside the breakers to near zero close to the shorewardextent of the surf zone. The computed friction coefficient fromthe balancing longshore current-induced bottom friction was found tobe relatively unstable during periods of changing wave and currentconditions, but was observed to be between 0.10 and 0.15 during morestable conditions.