| Summary: | This study is concerned principally with obtaining direct measurements of nearshore sediment transport at Timaru using artificial tracers, with finding relationships between the measured transport rates and wave or wave-dependent parameters, and with investigating the practical application of the artificial tracing technique in nearshore sediment transport studies. All field work for the study was undertaken within an area extending approximately 5 km north and 3.6 km south of Timaru Harbour to a distance of 5 km offshore. Water depths in this area do not generally exceed 15 m. The nearshore environment was investigated by examining the bathymetry, surface sediments, hydraulic conditions and depth of disturbance of sediment in the study area. It was found that there was considerable potential for wave-induced sediment transport at the 7 m isobath. Sediment transport is expected to increase shoreward and decrease seaward of this line. Four sediment tracing experiments were undertaken. Experiment I used fluorescent tracer to obtain transport rates for sediments ranging in size from very fine sand to pebbles. A magnetic tracer, ironsand, was also used but was not successful. Experiment 2 used fluorescent tracer to determine transport rates for very fine sand. Results from these two experiments were used to look for relationships between transport rates and wave and wave-dependent parameters. The relationships that were found were used to calculate long-term sediment transport rates, for three water depths, from one full year of recorded wave data. Ignoring the medium and coarse sand size classes (where representative long-term rates were not available) the calculated long-term transport rates ranged from 0.014 m ³ m⁻¹ day⁻¹ (granules) to 0.028 m ³ m⁻¹ day-l (very fine and very coarse sand) for the 7.6 m water depth, 0.009 m ³ m⁻¹ day⁻¹ (granules) to 0.016 m ³ m⁻¹ day⁻¹ (very coarse sand) for the 10.0 m water depth and 0.005 m ³ m⁻¹ day⁻¹ (fine sand) to 0.008 m ³ m⁻¹ day⁻¹ (very fine and very coarse sand) for the 12.2 m water depth. The calculated rate for very fine sand in the 7.6 m water depth was considerably lower than had previously been calculated for the area. Experiment 3 used fluorescent tracer to assess whether coarse sediments could move from the nearshore seabed to the foreshore of a mixed sand and gravel beach. The results showed that waves with an average significant height slightly greater than 1.0 m, and an average wave period ranging from 9 - 11 s, are capable of moving pebbles up to 28.4 mm median diameter from the seafloor onto the beach. The beaches can therefore receive nourishment from the seafloor. Investigations were undertaken to find an alternative to fluorescent tracer for tracing fine nearshore sands in New Zealand. A chemical tracer, europium labelled sand, was tested in the laboratory using atomic emission for detection. It was then used in a field test, experiment 4, but was found to lack sufficient sensitivity. Sensitivity would have to be improved if this tracer type is to be used successfully for future experiments. Results from experiments I and 2 showed that there was a wave-induced net landward movement of all grain sizes tested, with an increasing tendency for landward movement with increasing grain size. It is proposed that coarse sediments will eventually end up on the beaches but that the onshore movement of finer sediments is balanced by an increasing offshore gravitational component as the coast is approached. This pattern of predominant on-offshore movement is contrary to the pattern of dominant longshore movement suggested for the area by other workers. It is also proposed that the processes operating on the beaches, in the nearshore, and on the shelf proper are quite different and therefore it is not satisfactory to represent movement over the entire area by a single transport rate. It is thought that a slightly modified version of the null point theory best explains the observed patterns of movement in the study area. In the study area the gradient of the seafloor decreases in a seaward direction so that a particle seaward of its neutral line will move shoreward and a particle shoreward of its neutral line will move seaward. Sedimentation in Timaru Harbour entrance channel and the dispersion of dredge spoil are considered briefly in the light of the results from this study. A number of suggestions are made concerning the practical application of the artificial tracing technique in nearshore sediment transport studies. The assumptions required for the spatial integration method can be satisfied in the nearshore. However, there is some uncertainty regarding the importance of tracer accounting procedures.
|
|---|
