Flow Structure and Channel Stability at the Site of a Deep Scour Hole, Mackenzie Delta, Canada

Abstract

Unusually deep scour holes in distributary channels of the Mackenzie Delta are of concern for oil and gas resource development, particularly with respect to buried pipeline crossings. Surveys of one such hole, carried out in 1985 and 1992, indicated vertical stability and slight lateral movement. The present study examines how the hole may have changed by the mid-2000s and documents the complex local velocity field and related bed material properties. Small discrepancies between isobaths of different years suggest a dynamic stability that involves short-term fluctuations in erosion and deposition. This suggestion was corroborated by detailed measurements of the highly three-dimensional velocity field, which revealed major eddy structures and flow reversals that help maintain sizeable velocity magnitudes despite low mean velocities. The composition of the bed material suggests cohesive behaviour, but the literature indicates a range of critical shear stresses that spans two orders of magnitude. The more probable lower end of this range is consistent with the observed dynamic stability of the scour hole.

Flow structure caused by a local cross‐sectional area increase and curvature in a sharp river bend

“…These recirculations can be interpreted as a reversed Venturi 10.1002/2014JF003334 effect essentially resulting from the local increase in cross-sectional area. Similar flow patterns were previously observed, mainly in small streams [Andrle, 1994;Nanson, 2010;Schnauder and Sukhodolov, 2012] and in a larger channel in the Mackenzie Delta [Gharabaghi et al, 2007;Beltaos et al, 2012]. Often these flow patterns are associated with slow lateral erosion and deposition [Beltaos et al, 2012;Vermeulen et al, 2014a].…”

“…Flow in river bends often separates laterally causing horizontal flow recirculation [Carey, 1969;Woodyer, 1975;Page and Nanson, 1982;Hickin, 1986;Andrle, 1994;Ferguson et al, 2003;Blanckaert et al, 2013;Beltaos et al, 2012]. In general, flow separation is the result of two opposing effects in the boundary layer.…”

“…These results were confirmed in laboratory experiments [Blanckaert et al, 2013] where a wide outer bank promotes separation. Several field studies, however, have challenged this understanding of flow separation in bends [Andrle, 1994;Beltaos et al, 2012].…”

“…Horizontal separation can sometimes occur at the sides of deep scours at the entrance of the bend [ Andrle , ; Beltaos et al , ]. The deep scours create a large increase in cross‐sectional area decreasing the mean flow velocity.…”

Sharp bends associated with deep scours in a tropical river: The river Mahakam (East Kalimantan, Indonesia)

“…In large rivers sharp bends have only been observed occasionally [Gharabaghi et al, 2007;Beltaos et al, 2012]. Flow in sharp bends of the Mackenzie delta also features two horizontal recirculation zones in a cross section which is remarkably large [Beltaos et al, 2012]. The scarcity of field-based evidence of deeply scoured sharp bends in large rivers suggests that these features are rare and only occur in an Arctic region.…”