The Sedimentology of Tidally-Influenced Fluvial Bars in High Energy River Systems

  • Ashworth, Phil (PI)
  • Best, Jim (CoPI)

Project Details

Description

This research was funded by Exxon Mobil and came on the back of Phil Ashworth's three-year project on Tidally-influenced fluvial zones, also feeding the volume edited by Ashworth, Best and Parsons, Fluvial-Tidal Sedimentology (2015).

Bedforms are ubiquitous within subaqueous environments and are generated by unidirectional, short to long period oscillatory, and combined-flows (currents with unidirectional and oscillatory components), which deform a mobile bed through erosion and deposition of sediment. For centuries, laboratory, field and theoretical investigations have focused on bedform genesis, morphological equilibrium, and
their depositional structures.

One of the most common subaqueous bedforms are dunes whose strata represent a fundamental building block of the rock record. Subaqueous dunes typically possess heights of 0.075 to > 5m, wavelengths from 0.6 to > 100m, and can be compound (possessing crests, stoss-sides, or leesides, populated with smaller-scale superimposed bedforms) or simple (lacking superimposed bedforms) in form. Primary (largest) and
secondary (superimposed) dune morphology is a function of the interplay between growth, migration, and decay, as controlled by varying current magnitudes and orientations Dunes are therefore spatially and
temporally dynamic and follow coupled flow and sediment transport hysteresis loops which result in transient morphologic properties.

Examining the Lower Columbia River, USA, three questions were addressed:
1) How does primary dune morphology vary through the LCR fluvial-tidal transition?
2) Do variations in dune morphology relate to transitions in hydraulic and sediment transport processes, and/or bed grain size?, and
3) How does dune morphology differ from tidally-dominated estuaries and unidirectional current-dominated rivers and flume experiments?

Key findings

The key paper published from this research showed changes in primary dune morphology of the mesotidal Lower Columbia River (LCR), USA, through ~ 90 river kilometres of its fluvial-tidal transition at low-river stage. Measurements were derived from a Multibeam Echo Sounder dataset that captured bedform dimensions within the thalweg (≥ 9m depth; 𝐻 𝐻𝑚𝑎𝑥 ⁄ ≥ 0.7) of the LCR main channel. Measurements revealed two categories of dunes:
i) fine to medium sand ‘fluvial-tidal to tidal’ (upstream-oriented, simple, and 2D) low-angle dunes (heights ≈ 0.3-0.8m; wavelengths ≈ 10-25m; mean lee-angles ≈ 7-11°), and
ii) medium to coarse sand ‘fluvial’ (downstream-oriented, compound, and 2.5- 3D) low-angle dunes (heights ≈ 1.5-3m; wavelengths ≈ 60-110m; mean lee-angles ≈ 11-18°).

At low-river stage, where 𝐻 𝐻𝑚𝑎𝑥 ⁄ ≥ 0.7, approximately 86% of the fluvial-tidal transition is populated by ‘fluvial’ dunes, whilst ~ 14% possesses ‘fluvial-tidal to tidal’ dunes that form in the downstream-most reaches. Thus, throughout the majority of the deepest channel segments of the fluvial-tidal transition, seaward-oriented river and ebb-tidal currents govern dune morphology, whilst strong bidirectional tidal-current influence is restricted to the downstream most reaches of the transition zone.

Two mechanisms are reasoned to explain dune low-angle character: (1) high-suspended sediment transport near peak tidal-currents that lowers the leeside-angles of ‘fluvial-tidal to tidal’ dunes, and (2) superimposed bedforms that erode the crests, leesides, and stoss-sides, of ‘fluvial’ dunes, which results in the reduction of leeside-angles.

Fluctuations in river discharge create a ‘dynamic morphology reach’ at depths where 𝐻 𝐻𝑚𝑎𝑥 ⁄ ≥ 0.7, which spans river kilometres 12-40 and displays the greatest variation in dune morphology. Similar channel reaches likely exist in fluvial-tidal transitions with analogous physical characteristics as the Lower Columbia River and may provide a distinct signature for the fluvial-tidal transition zone.
StatusFinished
Effective start/end date26/06/1031/01/17

Funding

  • Exxon Mobil

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