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

Abstract

[1] The morphodynamics of large sand-bedded rivers and estuaries are ultimately controlled by the way bed material is moved and the development of large, subaqueous sand dunes that control hydraulic flow resistance. It is widely thought that the primary mechanism for moving sandy bed material in these channels is large-scale coherent flow structures that cause suspension events whose properties vary with flow, especially in tidally influenced environments. Here, we examine mean flow and sediment suspension events over low-angle dunes (lee face angle <30°) in the unsteady flow of the Fraser Estuary, Canada. At high tide, flow nearly ceased and a salt wedge entered the channel, forcing salt water under the downstream-moving fresh water. The salt wedge persisted in the channel until late in the falling tide, causing stratification in the water column and instabilities along the saline-fresh water interface. At low tide, mean velocities peaked and forced the saline water out of the channel. Flow over the low-angle dunes displayed topographically induced patterns previously observed over high-angle dunes, but permanent flow separation was not observed. Large-scale sediment suspension events dominated sediment flux during low tide and became larger scale, yet less frequent, as the tide began to rise. The suspension events appeared to form over the lower stoss of the dunes and grew up over the bed forms and, less commonly, emerged downstream of the crest. Suspension events move~69% of the total sediment in the flow above low-angle dunes when they are present.

Tidal controls on river delta morphology

“…In estuarine environments, the slope of the lee side of dunes is generally low-angled, causing dunes to be more symmetrical 40,41 , which may be attributed to the presence of clay rather than to the tidal motion 42 . Recent laboratory experiments show that bedform dimensions are directly related to clay content: increasing levels of clay content result in progressively smaller bedforms, with reduced height, length, and steepness 43 .…”

Flow structure and resistance over subaquaeous high‐ and low‐angle dunes

“…A morphological difference has been observed between small‐scale dunes formed in shallow and deep flows [ Kostaschuk , ]. Shallow flows produce dunes with an asymmetric shape and a steep angle‐of‐repose leeside of ~30° (“high‐angle dunes”) [e.g., Schindler and Robert , ; Venditti et al , ; Wren et al , ; Naqshband et al , ], while in deep rivers and estuaries, dunes typically have much lower angle lee slopes (average lee slopes between 5° and 12°, “low‐angle dunes”) and more symmetric shapes [ Smith and McLean , ; Kostaschuk and Villard , , ; Roden , ; Carling et al , ; Bradley et al , ; Hendershot et al , ]. Following Kostaschuk [], we refer to all dunes with a lee slope ≥ 30° as high‐angle and dunes with lee slopes <30° as low‐angle.…”

Morphodynamics of river dunes : suspended sediment transport along mobile dunes and dune development towards upper stage plane bed

“…Several time-averaged flow features over dunes are observed in Figure 3.7 that have been described in other studies [e.g., Raudkivi, 1966;Parsons et al, 2005;Venditti, 2007;Coleman et al, 2008;Grigoriadis et al, 2009;Shugar et al, 2010;Bradley et al, 2013]. This includes (1) a zone of lee side flow reversal or deceleration; (2) flow acceleration due to convergence on the stoss side of the dune towards the dune crest with largest mean streamwise velocities at the crest area; (3) flow deceleration due to expansion in the wake region with negative velocities in the flow separation zone; (4) an outer, near-surface region with higher velocities overlying the flow separation zone; and (5) development of an internal boundary layer starting on the stoss side of the dune towards the dune crest.…”

“…Therefore, under same flow conditions, the bypass fraction calculated using the model of Mohrig and Smith [1996] will result in an overestimation of the actual sediment fraction that is not contributing to dune migration, compared to our results. However, it should be noticed that the dunes observed in the field have much smaller lee side slopes (low-angle dunes) that results in a zone of highly intermittent or non-existent flow reversal [e.g., Best, 2005a;Shugar et al, 2010;Bradley et al, 2013]. Therefore, sediment particles with greater excursion lengths than the slip face distance might not be transported back towards the dune lee side and thus not contribute to dune migration.…”

“…In such rivers dunes with much smaller lee side angles (usually < 10°) are repeatedly observed [Roden, 1998;Kostaschuk, 2000;Best and Kostaschuk, 2002;Bradley et al, 2013 and references therein]. Although the geometries of dunes in flumes (high-angle) and in the field (low-angle) may be quite different, several studies have revealed similar mean flow patterns over both types of dune geometries [e.g., Nelson et al, 1993;McLean et al, 1994;Bennett and Best, 1995;Venditti and Bennett, 2000;Best, 2005a;Venditti and Bauer, 2005], except that the well-known separation zone and counter-rotating eddy observed in the lee trough of high-angle dunes are notably weak (intermittent) or absent in the lee trough of lowangle dunes [Best and Kostaschuk, 2002;Bradley et al, 2013]. The presence of a welldeveloped flow separation zone in the lee trough of high-angle dunes in the flume may result in a higher contribution of turbulent sediment fluxes to the total sediment fluxes along high-angle dunes compared to low-angle dunes (see chapter 4).…”