Exploring Wave and Sea‐Level Rise Effects on Delta Morphodynamics With a Coupled River‐Ocean Model

Deltas, which are often densely populated, have become more vulnerable to flooding due to increasing rates of sea‐level rise and decreasing sediment supply. These landscapes grow through the successive stacking of delta lobes, as abrupt changes in the river's course episodically alter the river mouth's location. The dynamics of these channel avulsions determine the location and size of delta lobes. We use a newly developed coupled model of river and coastal processes to explore how changing wave climates (varying wave heights and offshore approach angles) and a range of sea‐level rise rates affect avulsion processes and large‐scale delta morphology. Although the relative impacts of the fluvial sediment flux and wave influence have long been studied and more recently quantified, we find that the sign and magnitude of wave climate diffusivity both play important roles in determining not only delta morphology but also avulsion dynamics. We also find, surprisingly, that increasing sea‐level rise rates do not always accelerate avulsions. If the river channel is prograding rapidly enough, relative to the rate of sea‐level rise, then avulsion frequency will not be significantly affected by the rate of sea‐level rise. This finding highlights important differences between wave and river‐dominated deltas, as well as between prototypical deltas and experimental deltas created in the lab. The wave climate, sea‐level rise rate, and critical superelevation required to trigger an avulsion all play important roles in determining the degree of autogenic variability operating in delta systems, which has implications for both avulsion dynamics and interpretation of delta stratigraphy. Key Points When waves are sufficiently large to rework fluvial sand, wave height and offshore approach angles affect delta shape and avulsion behavior Time scales for progradation and base‐level rise determine how avulsions are affected by higher sea‐level rise rate Wave climate (height and offshore approach angles), sea‐level rise rate, and critical superelevation affect autogenic variability