Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Courtney K. Harris
Carl T. Friedrichs
The Waiapu River drains a small mountainous basin, characterized by steep terrain, heavy rainfall, and unconsolidated soft Tertiary mudstone and siltstone. These factors, combined with heavy deforestation over the past 100 years have created one of the world's highest sediment yields. Water discharge of the Waiapu River is very episodic over both inter- and intra-annual timescales, and almost all of the discharge is associated with floods brought by cyclonic storms. The Waiapu River drains an active margin that has a narrow shelf and steep slope. Marine conditions on the Waiapu continental shelf are very energetic, with strong waves as well as shelf currents. This special river-ocean system makes the Waiapu area an ideal site to study gravity-driven flows. Instrumented tripods deployed at water depths of 40 m and 60 m on the Waiapu shelf directly offshore of the river mouth recorded data on waves, currents, and sediment fluxes from May through August, 2004. The tripod data showed direct field evidence of current-supported gravity flows on the Waiapu shelf. Data analysis indicated that the Waiapu River floods were characterized by two distinct phases: a flood phase and a resuspension phase. The flood phase was characterized by large sediment input, coupled with moderate to strong waves but weak currents. Strong near-bed sediment signals, however, were not recorded by the tripods until the post-flood resuspension phases, during which seaward near-bed currents started to intensify. A one-dimensional boundary layer model provided the inference that those strong seaward near-bottom turbid flows during the resuspension phase were dynamically similar to wave-supported gravity flows observed on Eel and Po Shelves, except that both waves and currents were important for sediment resuspension. In contrast to thin and dense wave-supported gravity flows, current-supported gravity flows on the Waiapu shelf were significantly thicker and more dilute. Another two-dimensional model for wave- and current-supported sediment gravity flows was used to estimate sediment deposition on the Waiapu shelf from September 2003 to August 2004. The time period for the model calculations was divided into two segments: a low-energy (September to May) and high-energy portion (May to August). Model results showed that sediment delivered by the Waiapu River were trapped between the 20- and 80-m isobaths during the low-energy period, but then redistributed obliquely across the shelf between the 60- and 120-m isobaths during the high-energy period. Depositional locations estimated for the low- and high-energy portions, respectively, matched well with short- and long-term observed accumulation patterns based on 7 Be and 210Pb activity as reported by Kniskern (2007; Kniskern et al. 2008). Sensitivity analysis indicated that the gravity-driven flows on the Waiapu shelf were mainly wave-supported landward of the 40-m isobath, but became increasingly current-supported as wave orbital decayed in deeper water. This dissertation provided the first documentation of current-supported gravity flows, and hence contributed greatly to the study of sediment transport on continental shelves.
© The Author
Ma, Yanxia, "Continental shelf sediment transport and depositional processes on an energetic, active margin: The Waiapu River Shelf, New Zealand" (2009). Dissertations, Theses, and Masters Projects. Paper 1539616757.