Large-scale coastal and fluvial models constrain the late Holocene evolution of the Ebro delta, Spain

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Nienhuis, Jaap H. ; Ashton, Andrew D. ; Kettner, Albert J. ; Giosan, Liviu (2017)

The distinctive plan-view shape of the Ebro Delta, Spain, reveals a rich morphologic history. The degree to which the form and depositional history of the Ebro and many other deltas represent autogenic (internal) dynamics or allogenic (external) forcing remains a prominent challenge for paleo-environmental reconstructions. Here we use simple coastal and fluvial morphodynamic models to quantify paleo-environmental changes that affected the Ebro delta over the late Holocene. Based on numerical model experiments and the preserved and modern Ebro delta shape, we estimate that a phase of rapid shoreline progradation began approximately 2100 years BP, requiring a large increase (doubling) in coarse-grained fluvial sediment supply to the delta. We do not find evidence that changes in wave climate aided this delta expansion. River profile models suggest that such an instantaneous and sustained increase in coarse-grained, beach-compatible sediment to the delta would require a combination of flood discharge increase and increased sediment input into the river channel from upstream drainage basin erosion. The persistence of rapid delta progradation throughout the last 2100 years suggests an anthropogenic signal of sediment supply and flooding intensity. Our findings highlight how scenario-based investigations of deltaic systems using simple models can assist first-order quantitative paleo-environmental reconstructions, elucidating the effects of past human influence and climate change and allowing a better understanding of the future of deltaic landforms.
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