Understanding Sediment Flux Through Delta-top Channels Using Repetitive Multibeam and Hydrodynamic Modeling

English Abstract:

The growth of any delta is controlled by the available sediment flux from the feeder fluvial system. The manner in which that sediment moves through the tidally- influenced estuarine channels on the delta top will influence the mechanism of growth and collapse of the delta lip. This study addresses channelized delta top sediment transport on the Squamish Delta in Howe Sound, BC.

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. There is a 3-5m tidal range that strongly modulates the flow in the channel and over the adjacent intertidal sand banks. The channel depth at low water ranges from 0.5 to-2m and thus multibeam surveys can only be carried out at high water. The delta front is growing rapidly with about 1 million m³ of sediment being input from the river system annually.

In 2011, the delta top channel was surveyed every 3-4 days 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 neap to springs. The multibeam data capture the instantaneous expression of both the long wavelength channel shape as well as the superimposed bedforms distribution (as preserved at high water). While the long wavelength shape changes over a time scale of about a week, it was clear that the individual bedforms could not be correlated from one tide to the next.

Preliminary observations off the delta lip clearly demonstrate that the current speeds on the delta top were strongly modulated by the tide, ranging from almost stationary at high water to in excess of 5 knots at low water. To quantify the bed shear stress associated with this modulation, a 3D hydrodynamic model was built to predict the flow within the river, the delta top, and adjacent fjord over the complete tidal cycle. This clearly shows that the sediment flux is dominated by the low water period when the off-delta flows are strongest. The peak intensity of those flows is in turn modulated by the spring-neap cycle. This matches complementary work using repetitive multibeam off the delta lip that shows that mass wasting on the delta front is highly correlated with the spring tide and low tide periods.