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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, F03016, doi:10.1029/2006JF000666, 2007

Sorted bed forms as self-organized patterns: 2. Complex forcing scenarios

Giovanni Coco

National Institute of Water and Atmospheric Research, Hamilton, New Zealand


A. Brad Murray

Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina, USA


Malcolm O. Green

National Institute of Water and Atmospheric Research, Hamilton, New Zealand


E. Robert Thieler

U.S. Geological Survey, Woods Hole, Massachusetts, USA


T. M. Hume

National Institute of Water and Atmospheric Research, Hamilton, New Zealand


Abstract

We employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions. Results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated, a priori unpredictable, behavior. This happens because the system responds to a change in wave characteristics by attempting to self-organize into a patterned seabed of different geometry and spacing. The new wavelength might not have enough time to emerge before a new change in wave characteristics occurs, leading to less regular seabed configurations. The new seabed configuration is also highly dependent on the preexisting morphology, which further limits the possibility of predicting future behavior. For the same reasons, variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms. Spatial variability in grain size distribution and different types of net sediment aggradation/degradation can also result in the development of sorted bed forms characterized by a less regular shape. Numerical simulations qualitatively agree with observed geometry (spacing and height) of sorted bed forms. Also in agreement with observations is that at shallower depths, sorted bed forms are more likely to be affected by changes in the forcing conditions, which might also explain why, in shallow waters, sorted bed forms are described as ephemeral features. Finally, simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms. Instead, variations in sorted bed form characteristics may result from variations in local hydrodynamic and/or sedimentary conditions.

Received 23 August 2006; accepted 12 April 2007; published 14 August 2007.

Keywords: self-organization; sorted bed forms; numerical modeling.

Index Terms: 3022 Marine Geology and Geophysics: Marine sediments: processes and transport; 4460 Nonlinear Geophysics: Pattern formation; 4435 Nonlinear Geophysics: Emergent phenomena; 4485 Nonlinear Geophysics: Self-organization.

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Citation: Coco, G., A. B. Murray, M. O. Green, E. R. Thieler, and T. M. Hume (2007), Sorted bed forms as self-organized patterns: 2. Complex forcing scenarios, J. Geophys. Res., 112, F03016, doi:10.1029/2006JF000666.