The role of grain size, water depth and flow velocity as scaling factors controlling the size of subaqueous dunes
Flemming, B.W. (2000). The role of grain size, water depth and flow velocity as scaling factors controlling the size of subaqueous dunes, in: Trentesaux, A. et al. Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings.
In: Trentesaux, A.; Garlan, T. (Ed.) (2000). Marine Sandwave Dynamics, International Workshop, March 23-24 2000, University of Lille 1, France. Proceedings. Université de Lille 1: Lille. ISBN 2-11-088263-8. 240 pp., more
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Document type: Conference paper
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| Abstract |
The dimensional parameters (height, spacing) of subaqueous flow-transverse bedforms (ripples and dunes) define a highly correlated exponential relationship which has universal character. However, site-specific data sets rarely conform to this global trend. While such disagreements do not mitigate against the global trend, they do require explanations on the basis of the locally prevailing conditions. Amongst such local factors are changing flow depths, rapidly changing flow velocities, inadequate sediment budgets, and storm wave action. In addition, measuring errors can distort the scatter plots. Evidence is provided which precludes water depth as a primary control factor. It will simply terminate further dune growth once flow acceleration above the dune crest reaches a grain-size dependent critical suspension velocity. In depth-limiting flows dune height (or spacing) and water depth are therefore inherently correlated. In deep water, by contrast, dune growth is not limited by water depth. Dunes will continue to grow in response to increases in mean flow velocity until the critical suspension threshold for a given grain size is reached. Since critical suspension thresholds increase with increasing grain size, maximum dune size must increase with increasing grain size. For example, for a mean grain size of D = 0.063 mm Hmax ≈ 0.03 m and L ≈ 0.14 m, for D = 0.125 mm Hmax ≈ 0.8 m and L ≈ 7 m, for D = 0.25 mm Hmax ≈ 9 m and L ≈ 125 m, or D = 1 mm, Hmax ≈ 35 m and L ≈ 600 m. From this follows that, in order to achieve maximum size for a given grain size, the water depth has to be correspondingly deep and the flow velocity correspondingly high, bedform growth proceeding in steps by which smaller dunes amalgamate to form larger dunes. Taken together, the critical factors involved in the development and growth of flow-transverse bedforms would appear to be best accommodated by a kinematic wave theory. |
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