Exploring Large-scale Delta Morphodynamics with a Coupled Fluvial-Coastal Model

Humans have become increasingly reliant on deltas for agriculture, resources, transportation and trade, yet these often densely populated landforms and their inhabitants have become increasingly vulnerable to submergence and natural disasters. Although many of the 'natural' processes that influence large-scale delta morphology have been previously recognized and studied, we do not fully understand the relative importance of anthropogenic influences (e.g., climate and land-use change) in shaping modern deltas. In particular, the processes and feedbacks that shape deltas over large space and time scales (i.e., multiple river avulsions) are not well understood. To investigate the combined effects of sea-level rise (SLR), waves, climate change, subsidence and anthropogenic manipulations, we have developed a new morphodynamic delta model that links fluvial, floodplain and coastal processes over large scales. Using the Community Surface Dynamics Modeling System framework and tools, we couple the River Avulsion and Floodplain Evolution Model (RAFEM) to the Coastline Evolution Model (CEM). The newly developed RAFEM models fluvial processes, including avulsions, river profile evolution, and floodplain deposition. CEM distributes the sediment flux from the river mouth and shapes the coastline using alongshore sediment transport gradients. Preliminary results show that greater wave influence increases the river avulsion timescale, via inhibited channel progradation. SLR nonlinearly affects avulsion timescales: a small SLR rate (relative to a characteristic river aggradation rate) inhibits progradation and decreases avulsions; however, with a relatively large SLR rate, base-level rise causes aggradation rates to dominate over the reduced-progradation effect, resulting in more frequent avulsions. Varying wave energy affects avulsion characteristics and autogenic river flux variations. If fluvial sediment delivery is greater than alongshore sediment transport capacity (i.e., river dominance), channel progradation occurs relatively rapidly, and avulsions cause large changes in river length (and slope) and associated large variations in river flux. We will expand this new model through additional couplings, including sedimentary basin, coastal marsh, and human dynamics modules.