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

P.E. Ashmore, R.I. Ferguson, K.L. Prestegaard, Philip Ashworth, C. Paola

Research output: Contribution to journalArticlepeer-review


Measurements of the primary and secondary velocity components were made in two, active, braided river anabranchconfluences with a simple Y-shaped plan form, in the gravelly Sunwapta River (D50 of approximately 30 mm). Flowvelocity was measured at regularly-spaced intervals using a bidirectional electromagnetic current meter and themeasured downstream and cross-stream velocities were converted to primary and secondary velocities to yield thesecondary circulation. The primary (downstream) velocity field shows two high velocity streams from the two tributarieswhich merge (and, in some cases, accelerate) into a single high velocity core over the thalweg. Primary flow velocitydeclines 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 atthe bed. This is the first confirmation of this flow structure in confluences, based on field measurements. The strength ofthe secondary cells declines downstream through each confluence, and laterally away from the thalweg area incross-section. There is also a tendency for one cell, from the larger of the tributaries, to override the other. The secondaryand primary flow structure and strength differs slightly between the two confluences and this is reflected in differences in scour hole form.
Original languageEnglish
Pages (from-to)299-311
Number of pages13
JournalEarth Surface Processes and Landforms
Issue number3
Publication statusPublished - 1 Jan 1992


  • Braided streams
  • Confluence
  • Secondary flow


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