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Measurements of the wave, current and sea level dynamics of an exposed coastal site

Black, KP; Pattiarachi, C; Rosenberg, M; Simonds, G; Simons, RR; (1992) Measurements of the wave, current and sea level dynamics of an exposed coastal site. In: (Proceedings) International Biennial Conference on Physics of Estuaries and Coastal Seas. (pp. 29 - 58). Americal Geophysical Union

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Abstract

Field studies were undertaken at a coastal location (Bass Strait) to investigate the interaction of waves and currents and to establish relationships between nearshore and offshore wave parameters, including friction in mixed wave/current environments. The area is characterised by moderate tidal currents (maximum spring flow of about 0.18 ms−1) and wave energy (H5 and T5 of about 2 m and 10 s respectively). Wave-recording electromagnetic current meters were deployed along a 13 km shore- normal transect at 3 sites of depth 20, 40 and 50 m. Six week deployments during winter and summer were made with current meters at up to four levels above the bed at each site. At the inshore site, suspended sediment monitors and a video camera were deployed on a frame lowered to the sea bed. The underwater video camera was used to specify the sediment threshold and the changing nature of the bedforms. In addition, to determine the bed roughness along the cross-shelf transect, the sea bed was observed using ship-borne video at 23 sites 400–800 m apart. Bed characteristics were also observed at an additional 103 sites in the surrounding regions. The fall velocity and grain size of the bed sediments from these sites were determined using settling tube analyses. The interaction of waves and currents and the frequency attenuation of the wave spectrum with distance below the surface is considered to be responsible for a belt of coarser sands and larger bedforms identified at 30–45 m depth. The sediments were finer and the bedform heights were smaller both shorewards and offshore of this region. A thermocline was common in summer and up to 4 °C temperature differences were observed between the bed and near the surface in 40 m depth. Using multiple regression, 68% of the temperature gradient could be explained by 3 variables; air temperature, wind stress and wave orbital amplitude. Near the bed, 98% of the temperature variation over the 6-week measurement period could be explained by temperatures at a site to the west plus the longshore, cross-shore and vertical temperature gradients. The vertical gradient was the most important. Wind-induced stirring eliminated the thermocline and caused the bottom layer temperature to rise sharply as upper level water mixed with the lower layer. The dynamics of the coastal environment and the changing magnitude with depth of the various current components is considered. Wave refraction diagrams and velocity profile measurements are used to examine the interaction of the waves and currents, including their joint effect on the development of bed roughness and nearbed shear stress during storms.

Type:Proceedings paper
Title:Measurements of the wave, current and sea level dynamics of an exposed coastal site
Event:International Biennial Conference on Physics of Estuaries and Coastal Seas
Location:Margaret River, W.Australia
Dates:1992-12-08 - 1992-12-10
ISBN:0-87590-264-2
Keywords:waves, wave-current interaction
UCL classification:UCL > School of BEAMS > Faculty of Engineering Science > Civil, Environmental and Geomatic Engineering

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