Development and application of a field instrumentation system for the investigation of surf zone hydrodynamics
Citable URI
https://hdl.handle.net/1912/1830Location
Biddeford, MaineDOI
10.1575/1912/1830Keyword
Oceanographic instruments; Ocean currents; Ocean waves; Flow meters; Hydrodynamics; Water current meters; Electromagnetic measurements; Digital countersAbstract
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.
Description
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
Suggested Citation
Thesis: Greer, Matthew Noble, "Development and application of a field instrumentation system for the investigation of surf zone hydrodynamics", 1979-02, DOI:10.1575/1912/1830, https://hdl.handle.net/1912/1830Related items
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