The influence of dunes on mixing in a migrating salt‐wedge: Fraser River estuary, Canada

Abstract

Dune bedforms and salt-wedge intrusions are common features in many estuaries with sand beds, and yet little is known about the interactions between the two. Flow visualization with an echosounder and velocity measurements with an acoustic Doppler current profi ler over areas of fl at-bed and sand dunes in the highly-stratifi ed Fraser River estuary, Canada, were used to examine the effect of dunes on interfacial mixing. As the salt-wedge migrates upstream over the fl at-bed, mixing is restricted to the lower portion of the water column. However, as the salt-wedge migrates into the dune fi eld from the fl at bed, there is a dramatic change in the fl ow, and large internal in-phase waves develop over each of the larger dunes, with water from the saltwedge reaching the surface of the estuary. The friction Richardson number shows that bed friction is more important in interfacial mixing over the dunes than over the fl at-bed, and a plot of internal Froude Number versus obstacle (dune) height shows that the salt-wedge fl ow over the dunes is mainly supercritical. Such bedforms can be expected to cause similar effects in interfacial mixing in other estuaries and sediment-laden density currents, and may thus be infl uential in fl uid mixing and sediment transport.

Flow and sediment suspension events over low‐angle dunes: Fraser Estuary, Canada

“…These results illustrate how the mean flow field changes over low‐angle dunes in variable flow associated with tidal cycles, reveal patterns of suspension events over low‐angle dunes, and show that these events are important in sediment transport. These observations substantially extend prior results on flow over the low‐angle dunes in the Fraser Estuary [ Kostaschuk , , ; Kostaschuk et al ., ; Kostaschuk and Church , ; Kostaschuk and Villard , , ; Villard and Church , ; Kostaschuk and Best , ; Kostaschuk et al ., ] in as much as those studies were mainly restricted to the documentation of mean flows and mean sediment concentration in the dune field. The most recent studies have deployed aDcp instruments but have not succeeded in resolving sediment plumes well or to associate them with individual flow structures.…”

“…[29] Early in the tidal fall (Upper Falling Tide, Figure 6a), the salt wedge intrusion was evident in the lower portion of the profile, causing stratification in the flow. A pycnocline, recorded as a peak in backscatter due to the sharp density gradient between the fresh and saline water [Geyer and Farmer, 1989;Tedford et al, 2009;Kostaschuk et al, 2010], is obvious in the vertical profile of <WCB> x from 9 to 11 m above the bed. Downstream velocity <u> x was greatest in the upper fresh water layer but sharply declined below the pycnocline.…”

“…It has been suggested that this occurs because mixing between the wedge and the fresh water is heightened by the presence of dunes along the boundary [cf. Kostaschuk et al, 2010].…”

Velocity Mapping Toolbox (VMT): a processing and visualization suite for moving‐vessel ADCP measurements

“…Gartner, 2004, Kostaschuk et al, 2004, Dinehart and Burau, 2005a, 2005bTopping et al, 2006;Wall et al, 2006;Shugar et al, 2010), spatial variations in shear-stress (Sime et al, 2007), and bedload transport rates (through quantification of a moving bed condition and the differences detectable between the instrument-derived boat velocity from bottom track and GPS-derived boat velocity) (Rennie et al, 2002;Rennie and Millar, 2007;Jamieson et al, 2009). Thus, ADCPs have become multipurpose instruments for examining flow, morphology and sediment flux, thereby demonstrating considerable promise for an improved process-based understanding of geophysical processes in a wide range of aquatic environments Parsons et al, 2005;García et al, 2007;French et al, 2008;Jackson et al, 2008;Lane et al, 2008;Bartholomä et al, 2009;Kostaschuk et al, 2009Kostaschuk et al, , 2010Shugar et al, 2010;Wright and Kaplinski, 2011).…”

Bedforms: views and new perspectives from the third international workshop on Marine and River Dune Dynamics (MARID3)

“…Kostaschuk et al (2010b) address quantification of these processes through a detailed field investigation of sediment transport over a large dune in the Río Paraná, Argentina. Kostaschuk et al (2010b) address quantification of these processes through a detailed field investigation of sediment transport over a large dune in the Río Paraná, Argentina.…”