Simulating Galloway's famous triangle: Testing the hypothesis that rivers, waves, and tides control delta morphology

Abstract:

River deltas are among the most dynamic and important depositional environments on Earth, hosting large populations and valuable natural resources. Despite their economic and ecological significance, the relationship between process and form in river deltas is still poorly understood. In his famous ternary diagram, Galloway (1975) proposed that delta morphology is primarily controlled by the relative strengths of three environmental forcings: fluvial input, wave energy flux and tidal energy flux. This conceptual model lacks a quantitative framework yet remains the standard for delta classification. This model was recently quantified using sediment flux ratios (Nienhuis et al., 2020) such that a delta can now be quantitatively placed on the Galloway diagram. Here we use this framework in conjunction with a variety of morphometrics to provide a quantitative test of Galloway’s hypothesis and investigate the relationships between process and form in simulated and real-world river deltas. Numerical deltas are generated in Delft3D using an array of boundary conditions that cover the spectrum of relative forcings within ternary space. Morphometrics are calculated for each simulated delta and for a number of real-world deltas using remote sensing data. Preliminary results suggest that shoreline morphology (as measured by delta shape, shoreline length and rugosity, and shoreline angle) is controlled primarily by the relative strengths of the river, waves, and tides. However, metrics describing channel network morphology (number of distributary channels, nearest edge distance, island aspect ratio) can vary significantly between two deltas with nearly identical forcings. This is particularly true in certain regions of the ternary space, suggesting that other factors (such as sediment properties) may be more important than differences in forcing conditions for predicting channel network morphology in those regions.

Galloway, W., (1975). Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional system. SEPM, Special Publication No. 31. 127-156.

Nienhuis, J.H., Ashton, A.D., Edmonds, D.A., Hoitink, A.J.F., Kettner, A.J., Rowland, J.C., Tornqvist, T.E., (2020). Global-scale human impact on delta morphology has led to net land area gain. Nature, v. 577, 514-518.