Date Awarded

2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Courtney Harris

Committee Member

Steve Kuehl

Abstract

The Waiapu River, located on the North Island of New Zealand, drains a small catchment and has one of the highest sediment yields in the world. The river delivers most of its annual sediment load during floods into energetic coastal waters. These conditions are favorable for producing multiple sediment transport mechanisms, including transport in positively and negatively buoyant freshwater plumes, gravitydriven flows, and resuspension. Analyses of Waiapu River shelf seabed data showed that multiple transport mechanisms influence strata formation. Fluvial sediments are initially deposited at water depths shallower than 80 m before being remobilized and deposited at greater water depths. Over the last 100 years fine sediments were retained mainly at water depths between 60 and 190 m, and accounted for 24% of the fluvial load. High shelf accumulation rates (0.2--3.3 cm/yr) were sufficient to preserve pulsed event layers, which were identified by low excess 210Pb and terrestrial delta 13C. Additionally, high subsidence rates on the tectonically active shelf likely influences modern depositional patterns. A three-dimensional numerical model was used to address the mechanisms by which sediment escaped the shelf and to assess the relative importance of the various transport mechanisms. The simulation was able to reproduce time-averaged currents, near-bed sediment concentrations, and bed shear stresses at a tripod deployed off the river mouth at 60 m water depth. Gravity-driven transport was most important on the inner and mid-shelf, whereas dilute transport became more important beyond 65 m depth. Sediments escaped the shelf via dilute suspension to the north of the Waiapu River mouth. Sensitivity experiments showed that transport pathways and depositional patterns were sensitive to floc fraction, waves and currents, and sediment load. Increasing the floc fraction resulted in increased wave-supported gravity-driven transport relative to dilute transport and increased shelf deposition. Coherence between energetic waves and floods increased the importance of wave-supported gravity-flows and shifted initial deposition offshore. Wave-induced bed shear stress increased gravity-driven transport, whereas current-induced bed shear stress increased dilute transport. Deforestation over the last 150 years, which has resulted in an increase annual suspended load, may have resulted in increased shelf sediment retention.

DOI

https://dx.doi.org/doi:10.25773/v5-05pd-dq04

Rights

© The Author

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