Sorted Bedforms

Unlike most bedform that develop solely as an interaction between bathymetry and flow, recent work implicates a sorting feedback as the mechanism for the development of inner continental shelf ‘sorted bedforms’ Murray and Thieler, 2004). Though the work of Van Oyen et al. (2010; 2011) suggests that a flow-bathymetry feedback may be important for some varieties of sorted bedforms, my work focuses on bedforms where the sorting feedback is the dominant process.  The sorting feedback is initiated by wave generated ripples whose size is a function of seabed composition and hydrodynamic forcing conditions. Regions covered with fine sediment support small wave-generated ripples while areas mantled by coarse sediment support large ripples. Large wave ripples on coarse domains generate strong turbulence, leading to enhanced erosion of fine material. In addition, strong turbulence acts as a barrier to fine sediment deposition. As a result fine sediment is preferentially eroded from coarse domains and preferentially deposited where the seabed is composed of fine sediment. A superimposed bottom current allows for these two processes to interact, and results in spatially extensive sorting process.

SB Schematics

Through self organization this sorting feedback leads to spatially extensive (10 m – km scale) patches of segregated coarse and fine grained sediment  that have only slight bathymetric relief (cm – m scale).

Previously, I used an existing morphodynamic model (Coco et al., 2007a2007b; available in the CSDMS model repository) to investigate the long-term plan-view evolution of sorted bedform patterns under different forcing conditions (Goldstein et al., 2011). Recent work focused on the development of a new ‘hybrid’ model of sorted bedforms that uses parameterizations of wave ripple geometry (Goldstein et al., 2013) and suspended sediment reference concentration that are built directly from field and lab data (Goldstein et al., 2014). The new model is available on GitHub.

SOB model figure

I have also been part of work to integrate our understand of sorted bedform pattern evolution with other natural systems that display pattern formation/evolution (Murray et al., 2014a).

The sorted bedform model has also been used to investigate issues surrounding complexity, emergence, top-down causality and model uncertainty. These themes have appeared in all of the above papers, as well as Murray et al. (2006)Coco and Murray (2007)Huntley et al. (2008)Murray et al. (2014b) and Murray et al. (2016).