On a high-energy coast, with heterogeneous nearshore bathymetry and sediment dynamics (New Plymouth, New Zealand), two colours of artificial fluorescent tracer were used to i) investigate the sediment entrapment/bypassing of a port entrance, and ii) to monitor sediment movement from an experimental nearshore dredged-sand dump mound (47,000 m3). Two simultaneous releases (one for each colour) were made in 6 to 10 m water depths, near the port entrance and on the dump mound, 1400 m apart. The tracer was tracked over 10 months by collecting and analysing 1179 surficial sediment samples, with concomitant wave/current recording at up to 13 nearshore sites.
The results demonstrate trapping in the breakwater tip-shoal of the part, by-passing and movement through the complex nearshore reef system. The dominant transport was alongshore. Tracer released near the port entrance depicted a natural sediment “pathway” past the harbour shipping entrance and over the rocky reefs to the beaches 4.5 km away. Very little tracer was detected within the harbour, indicating that the main transport from the release depths (6–10 m) by-passed the harbour entrance and tip-shoal. The tracer shows that only sediments very close to the harbour breakwater (i.e. within 100 m) are trapped at the harbour entrance. From the dump mound, tracer was observed to migrate mostly longshore away from the port, although a minor “updrift” migration was observed.
The tracer results are in accordance with measured and numerically modeled currents, which show a dominant longshore flow away from the port and weak reversing flows in the quiescent lee of the harbour at the dump mound. The tracer data also suggest that the raised shore-normal reefs do not present a significant impediment to longshore transport, but rather that the nearshore circulation patterns are the predominant influence on the sediment transport vectors. Rates of tracer spreading were found to range from 0.095 to 0.288 m2s−1, which is consistent with a high-energy coastal environment. Diffusion and advection of suspended sediments is the primary mechanism of sediment transport, and tracer was found on beaches 4.5 km distant from the release point within 13 days of injection.