Discharge Determines Avulsion Regime in Model Experiments With Vegetated and Unvegetated Deltas

Abstract

AbstractThe dynamics and morphological evolution of deltas and their channel networks involve interactions between many factors, including water and sediment discharge and cohesion from fine sediment and vegetation. These interactions are likely to affect how strongly vegetation influences deltas. Altering water or sediment discharge may affect channel mobility, magnitude of deposition and erosion events, and may result in the delta being less suitable for vegetation colonization. Using the numerical model DeltaRCM Vegetation, we explore how water and sediment discharge affects delta evolution and under which conditions vegetation exerts a stabilizing effect on the channel network. We propose new insights into delta evolution under different discharge conditions. First, we observe a regime shift in avulsion dynamics, driven by delta‐scale water surface slopes, with increasing water discharge: from a few active channels undergoing occasional complete, global avulsions (with low discharge) to many active channels experiencing frequent partial, local avulsions (with high discharge). Second, with vegetation, increased sediment discharge results in more frequent switching of the dominant channels but also prevents vegetation from establishing in non‐dominant channels, resulting in more frequent channel reoccupation and therefore greater stability in channel network planform. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate and human modifications of fluxes of sediment and water to the coast, especially for restored or engineered deltas with controlled water or sediment discharges.