Investigating sediment transport on the Waipaoa margin: linking in situ observations with preserved deposits
Hale, Richard Prescott
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Connecting the processes that control sediment transport to the deposits they leave behind is critical for understanding sedimentary system dynamics on modern timescales. Small mountainous river systems (SMRs) have been the focus of intense investigation for several decades, as they provide an ideal setting in which to study the conditions responsible for generating and preserving individual event deposits. In this study, bottom-boundary-layer tripods were deployed in three locations on the continental shelf seaward of the Waipaoa River (New Zealand), to monitor sediment and water movement from January 2010 to February 2011. Four research cruises were conducted at approximately four-month intervals during this time period to service and repair instrumentation, as well as to collect short (<50 cm) sediment cores at repeat locations across the shelf. Cores were examined for their textural and radiochemical properties as evidence of recent sediment deposition. Typical of SMRs, water and sediment discharge from the Waipaoa River is episodic, with the potential for relatively major (>2000 m<super>3</super> s<super>-1</super>) events to occur in any season. Sediment transport on the Waipaoa margin is similarly event driven, with the majority of near-bed sediment transport occurring in 36 discreet events (defined in this study by U<sub>*wc</sub> >0.03 m s<super>-1</super>) of <60 h duration. Events which result in a net aggradation of the seabed tend to have longer duration than erosive events (~100 h), though on an annual timescale this is still rapid. In general, short-duration events with relatively low-height, long-period waves result in seabed erosion on the mid-shelf. Longer-duration events with tall-height, short-period (i.e., steep) waves typically result in sediment deposition. Event duration is particularly important in the case of large floods of the river, as the associated sediment requires a minimum of ~120 hours of event-level shear velocity before being deposited on the mid-shelf. When events (both discharge and U<sub>*wc</sub>) occur coincidentally, and are of sufficiently long duration, wave-supported fluid mud (WSFM) can form at the mid-shelf. Near-bed suspended-sediment concentrations (SSC) >50 g l<super>-1</super> are observed in the near-bed layers (<26 cm above bed), which is well above the theoretical threshold for downslope travel under the influence of gravity. Gravity-driven velocity is slow at the mid-shelf due to the shallow slope, however material travelling from the mid-shelf under the influence of gravity alone can reach the shelf break in ~50 h during periods of highest SSC. The deposit from a WSFM that occurred in early July, 2010, was plainly visible in x-radiographs of sediment cores collected at 17 locations in September 2010, and again at the same locations in February 2011. The deposit is identified as relatively low-bulk-density material, 1-12 cm thick, which tends to be physically stratified, especially at greater thicknesses. Using a newly developed method for estimating the degree of physical lamination, 10 locations show more surface variability in September than in February, while the opposite is true in 5 locations. Visual inspection of the cores agrees with these results, which demonstrate how biological mixing of event layers destroys strata, and prevents long-term event-bed preservation. This new method for using down-core bulk-density to analyze quantitatively sediment x-radiographs over a decadal-to-century scale faithfully reproduces the results of qualitative analyses performed in this location by previous studies.
- Oceanography