Littoral steering of deltaic channels

doi:10.1016/j.epsl.2016.08.018

Earth and Planetary Science Letters

Volume 453, 1 November 2016, Pages 204-214

https://doi.org/10.1016/j.epsl.2016.08.018 Get rights and content

Highlights

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Feedbacks between waves, fluvial sediment, and bypassing set channel orientation.
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Limiting bypassing of river mouth deflects channels downdrift.
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Fluvial sediment flux increase can switch channel migration from downdrift to updrift.

Abstract

The typically single-threaded channels on wave-influenced deltas show striking differences in their orientations, with some channels oriented into the incoming waves (e.g., Ombrone, Krishna), and others oriented away from the waves (e.g., Godavari, Sao Francisco). Understanding the controls on channel orientation is important as the channel location greatly influences deltaic morphology and sedimentology, both subaerially and subaqueously. Here, we explore channel orientation and consequent feedbacks with local shoreline dynamics using a plan-form numerical model of delta evolution. The model treats fluvial sediment delivery to a wave-dominated coast in two ways: 1) channels are assumed to prograde in a direction perpendicular to the local shoreline orientation and 2) a controlled fraction of littoral sediment transport can bypass the river mouth. Model results suggest that channels migrate downdrift when there is a significant net littoral transport and alongshore transport bypassing of the river mouth is limited. In contrast, river channels tend to orient themselves into the waves when fluvial sediment flux is relatively large, causing the shoreline of the downdrift delta flank to attain the orientation of maximum potential sediment transport for the incoming wave climate. Using model results, we develop a framework to estimate channel orientations for wave-influenced deltas that shows good agreement with natural examples. An increase in fluvial sediment input can cause a channel to reorient itself into incoming waves, behavior observed, for example, in the Ombrone delta in Italy. Our results can inform paleoclimate studies by linking channel orientation to fluvial sediment flux and wave energy. In particular, our approach provides a means to quantify past wave directions, which are notoriously difficult to constrain.

Graphical abstract

Introduction

Major channels of wave-influenced deltas tend to be straight, or gently curving, rather than meandering, with orientations that can diverge from the upland river course. It has been hypothesized that a delta's channel orientation arises from the interaction between fluvial channel-building processes and littoral sediment transport at the shoreline (Bhattacharya and Giosan, 2003, Pranzini, 2001). However, the controls on channel orientation are not straightforward as, on some deltas, channels turn into the waves, whereas, on other deltas, channels migrate away from the waves (Fig. 1). The presence of the channel itself affects coastal processes, as river mouths can limit bypassing of littoral sediment (Nienhuis et al., 2016). As such, a mechanistic understanding of the basic controls on channel orientation has been previously lacking. To investigate the mechanisms and controls that set the channel orientations on wave-influenced deltas, we have conducted experiments using an exploratory model of plan-view delta evolution. In these experiments, we allow local shoreline dynamics to determine the channel orientation, while also controlling the quantity of littoral sediment that can bypass the river channel. We compare these model experiments to natural examples in a mechanistic framework, which not only allows us to predict the channel orientation for modern deltas, but also, as the channel orientation of wave-influenced deltas is preserved in the morphology of deltas and eventually stored in the stratigraphic record, has the potential to inform us about past and present fluvial and alongshore sediment transport fluxes.

Section snippets

Asymmetric wave-influenced deltas

In the absence of waves, river deltas often develop intricate networks of distributary channels resulting from mouth-bar formation and channel avulsions (Geleynse et al., 2011, Wright, 1977). However, waves inhibit mouth bar formation and move sediment alongshore, and as such can suppress the emergence of small-scale distributaries, generally leading to the growth of a single major channel (Wright and Coleman, 1973) and a cuspate delta shape (Grijm, 1960).

Alongshore transport of fluvial

Coastline evolution model

To investigate the controls on channel orientation, we modified the existing plan-view model of shoreline dynamics CEM (see Ashton and Murray, 2006a). CEM assumes a constant shoreface cross-sectional profile such that the divergence of littoral fluxes along the coast corresponds directly to advance or retreat of the shoreline position (Ashton and Murray, 2006a, Ashton et al., 2001). Assuming refraction over shore-parallel shoreface contours, the wave energy and wave direction then drive a

Styles of channel orientation

We have modeled delta formation under different scenarios by varying fluvial sediment supply ( $Q_{r}$ ), wave energy, angular wave distribution, and alongshore sediment bypassing (β) to investigate morphologic control on deltaic channel orientation. After ∼10 model years under constant forcing ( $Q_{r}$ , β, and wave climate), modeled deltas reach a dynamic steady state at the river mouth, with intermittent variability in river channel orientation arising from the stochastic wave angle selection. At this

Implications for delta predictions and paleo-environmental reconstructions

Model explorations performed here show how deltaic channel orientation can respond to long-term environmental conditions via feedbacks with wave-driven alongshore sediment transport (Fig. 5). For known directional wave climate, fluvial sediment supply, and alongshore sediment bypassing, we can calculate U and D (eqs. (3) and (6)), which determine the resulting steady-state channel orientation in accordance with our model simulations and natural examples (Fig. 7B).

Following the same approach,

Conclusion

In this study we have investigated how feedbacks between the directional wave climate, fluvial sediment supply, and alongshore sediment bypassing can determine the channel orientation of wave-influenced deltas. Model results enabled us to formulate key criteria for updrift and downdrift channel migration. In particular, we found that limiting alongshore sediment bypassing of river mouths should tend to drive downdrift channel migration. On the other hand, deltaic channels are expected to

Acknowledgments

This study was supported by NSF Grant EAR-0952146. We acknowledge helpful reviews by an anonymous reviewer and the editor An Yin.

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Littoral steering of deltaic channels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Asymmetric wave-influenced deltas

Coastline evolution model

Styles of channel orientation

Implications for delta predictions and paleo-environmental reconstructions

Conclusion

Acknowledgments

Comput. Geosci.

Ocean Model.

Geomorphology

Mar. Geol.

Earth Planet. Sci. Lett.

Anthropocene

Appl. Geogr.

Geomorphology

Mar. Geol.

Some data on the long-shore drift of sand near natural obstacles

Tr. Inst. Okeanol. Akad. Nauk SSSR

Wave-angle control of delta evolution

Geophys. Res. Lett.

High-angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes

J. Geophys. Res.

High-angle wave instability and emergent shoreline shapes: 2. Wave climate analysis and comparisons to nature

J. Geophys. Res.

Formation of coastline features by large-scale instabilities induced by high-angle waves

Nature

The coastline of river-deltas

Support of subtidal tracer studies to quantify the complex morphodynamics of a river outlet: the Bevano, NE Italy

J. Coast. Res.

Coastal dune fields at the São Francisco River strandplain, northeastern Brazil: morphology and environmental controls

Earth Surf. Process. Landf.

Rational theory of delta formation

Am. Assoc. Pet. Geol. Bull.

Wave-influenced deltas: geomorphological implications for facies reconstruction

Sedimentology

The São Francisco strandplain: a paradigm for wave-dominated deltas?

Geol. Soc. (Lond.) Spec. Publ.

Large-scale dynamics of sandy coastlines: diffusivity and instability

J. Geophys. Res.

Long term sediment dynamics of Danube delta coast