Year

1990

Degree Name

Doctor of Philosophy

Department

Department of Geography

Abstract

ring suspended sediment concentration in the surf zone is a difficult task because of complicated and adverse conditions therein; however, such measurement is very important in determining suspended sediment transport under both wave and current interactions. In this thesis I attempt to measure suspended sediment concentration directly in the surf zone and define some of the underlying relationships regarding suspended sediment characteristics and movement under different hydrodynamic and morphologic conditions. A modified version of Nielsen's suspended sediment sampler was used to measure concentration in a water column on three beaches on the south coast of New South Wales, Australia. These beaches morphologically range from transitional to reflective types. Thirty-six concentration measurements were taken under low to high wave energy conditions and at various locations in the surf zone that included shoals, bars, troughs, rips and the breaker line. Current and wave data, bed material samples were collected and nearshore topographic profiles were also surveyed. These surveys have provided a data set containing hydraulic, sedimentary and morphologic information from which some very fundamental but simple rules on the nature of suspended sediment transport over a range of surf zone morphologies have been drawn by using various statistical analyses.

Principal component analysis of the initial data set shows a clear pattern in which hydraulic, sedimentary and morphologic variables are highly correlated to each other. Drawing upon these results, regression analysis indicates that offshore wave period is the best predictor of inshore current velocity components. This relationship yields an effective and efficient method to estimate mean and maximum current velocities in the surf zone. Grain sizes of both suspended sediment and bed material are significantly related to maximum current velocity components. Trend surface analysis also shows a well defined pattern of grain size distribution in the water column with variations in current velocities.

Suspended sediment concentrations are non-linearly distributed in the water column and decrease logarithmically with increasing elevation above the sea bed. The lapse rate of the profile varies proportionally with the level of sediment concentration. However, this pattern changes with increasing current velocity. When the mean current velocity component exceeds 0.2 m/sec, a two-layer concentration distribution appears. There is a low concentration layer in the middle of the water column (0.2- 0.4 m from the bed). Both above and below this layer sediment concentration increases rapidly. Suspended sediment concentration distribution in the surf zone also varies dramatically between different beach types. On low energy reflective beaches, sediment concentration is very low offshore but high within the narrow breaker zone. Onshore sediment transport is dominant in the breaker zone. On transitional beaches, sediment concentration and transport are closely controlled by the nearshore circulation system. Sediment concentration is generally higher over bars or shoals and in rips.

The modification of Nielsen's suspended sediment sampler facilitated the collection of both current velocity data and suspended sediment samples simultaneously and at virtually the same location. This is vitally necessary in examining and understanding the process of interactions between waves, currents and suspended sediment. The sampling technique is reliable, effective and inexpensive requiring limited man power and effort. It can be used under various conditions in the surf zone including higher wave energies produced by breaker heights in excess of 1.5 m and under strong currents such as those found in rip channels.

Download

Share

COinS
 

Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.