Secondary flow in anabranch confluences of a braided, gravel‐bed stream

Measurements of the primary and secondary velocity components were máde in two, active, braided river anabranch confluences with a simple Y‐shaped plan form, in the gravelly Sunwapta River (D₅₀ of approximately 30 mm). Flow velocity was measured at regularly‐spaced intervals using a bidirectional electromagnetic current meter and the measured downstream and cross‐stream velocities were converted to primary and secondary velocities to yield the secondary circulation. The primary (downstream) velocity field shows two high velocity streams from the two tributaries which merge (and, in some cases, accelerate) into a single high velocity core over the thalweg. Primary flow velocity declines as the flow expands and diverges at the downstream end of the confluence. The secondary circulation is dominated by two helical cells, back‐to‐back, plunging over the thalweg and diverging at the bed. This is the first confirmation of this flow structure in confluences, based on field measurements. The strength of the secondary cells declines downstream through each confluence, and laterally away from the thalweg area in cross‐section. There is also a tendency for one cell, from the larger of the tributaries, to override the other. The secondary and primary flow structure and strength differs slightly between the two confluences and this is reflected in differences in scour hole form.