Coupling of repetitive multibeam surveys and hydrodynamic modelling to understand bedform migration and delta evolution
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This study addresses channelized delta top sediment transport on the Squamish estuary in Howe Sound, British Columbia. The mechanism of bedform migration and delta evolution is affected by the manner in which the available sediment flux from the feeder fluvial system is distributed. The present study is complementary to a parallel project looking at the sediment migrating on the delta slope as landslides or turbidity currents.
The termination of the Squamish River consists of a single channel that flows between flanking intertidal sand bars and over a mouth bar at the lip of the delta. The delta front is growing rapidly with about 1 million m[superscript 3] of sediment being input from the river system annually. There is a 3 to 5 m tidal range that strongly modulates the flow in the channel and over the adjacent intertidal sand banks.
In 2011, the delta top channel was surveyed every 3 to 4 day at high water, over a period of 4 months during which the river discharge waxed and waned and the tides ranged from springs to neaps. In 2012 and again in 2013, the channel was surveyed daily over a week while the tides increased from neaps to springs.
In order to understand the sediment transport mechanism in this estuary, this research parameterized the short wavelength bedform morphology and the long wavelength channel shape on the delta top, extracted the shape of the delta lip, and used volumetric characterization of the sediment on the delta top and the delta lip vicinity. A three dimensional hydrodynamic model was also built to predict the flow within the river, the delta top, and adjacent fjord over the complete tidal cycle so that the bed shear stress associated with tide modulation and river discharge could be quantified.
This research shows that the short wavelength bedform characteristics and long wavelength channel shape are primarily a result of the low water period when the offdelta flows are strongest. The flow fields of the research area are dominated by the tidal modulation. However the river surge also plays a role during the high flow regime.
Good correlation was demonstrated between flow conditions (parameterized by the Froude number) and all of: the bedform roughness, the bedform mobility, the 1D bedform roughness spectra and the off-delta sediment flux. These relationships indicate that a single snapshot of the riverbed morphology could potentially be used to estimate sediment transport conditions.