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

Abstract

The development and application of an autonomous field instrumentation system consisting of four current meters and four wave gauges, along with a field monitor and digital recorder, is documented. The flow sensors are electromagnetic current meters, which employ the principle of electromagnetic induction to sense an induced electrical potential from the flow of water through an imposed magnetic field. The 10 cm diameter, discus-shaped sensor was tested in the laboratory under a wide variety of conditions, including both steady and oscillatory flow tests. The results of these tests indicate an excellent response in terms of linearity and horizontal cosine. The vertical cosine response is close to ideal in the region of ±30°, but beyond a negative angle of attack of approximately -30° the response is compromised by the onset of separation under dominantly steady flow conditions. The wave gauges are surface-piercing digital sensors, relying on the presence or absence of water at 128 individual sensing electrodes spaced 1.5 cm apart along the front surface of the wave gauge. On command, the instantaneous water surface elevation is measured, then telemetered digitally to the shorebased monitor and recorder. Field measurements of waves and currents at four stations across the width of the surf zone were made, using this system at a beach along the southern coast of Maine. Spilling breakers (approximately 1.0 m in height with an angle at breaking of about 8°), translated across the 30 m surf zone, generated an observed net longshore current during the four hour measurement period. The subsequently analyzed data from this experiment showed a strong longshore current which varied across the width of the surf zone, having a maximum of about 15 cm/ sec just inside the breaker line. A net offshore current was observed at all four stations, and averaged approximately 10 cm/sec to 15 cm/sec. Using a simplified force balance model for the generation of longshore currents on a plane, uniform beach, the data was further analyzed to investigate the validity and parameterization of the momentum flux forces and bottom friction forces within the surf zone. There was an observed shoreward loss 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 shoreward extent of the surf zone. The computed friction coefficient from the balancing longshore current-induced bottom friction was found to be relatively unstable during periods of changing wave and current conditions, but was observed to be between 0.10 and 0.15 during more stable conditions.