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.