River-Ocean Interactions: A Coupled Morphodynamic Delta Model

Abstract:

Society has become increasingly reliant on deltas for agriculture, resource extraction, transportation and trade, yet these landforms and their inhabitants have become increasingly vulnerable to submergence and natural disasters (e.g., flooding, storm surges). Although we know that many ‘natural’ processes influence large-scale delta morphology, we do not yet know 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 delta morphology over large space and timescales (i.e. timescales of multiple river avulsions and the evolution of multiple delta lobes) are not well understood. To explore the long-term combined effects of sea-level rise, subsidence and anthropogenic manipulations, we have developed a new morphodynamic delta model that links fluvial, floodplain, and deltaic dynamics over large space and timescales. Using the framework and tools of the Community Surface Dynamics Modeling System, we couple a new river and floodplain module to the Coastline Evolution Model (CEM, Ashton and Murray, 2001). In the fluvial module, cell width is assumed to be larger than the channel belt width (including natural levees that are maintained at a bankfull channel-depth above the riverbed elevation). The river course is determined using a steepest-descent methodology, and erosion and deposition along the course is modeled as a linear diffusive process. An avulsion occurs when the riverbed becomes super-elevated above the surrounding floodplain, and the new steepest-descent path to sea level is shorter than the previous course. Floodplain deposition is modeled by blanket (uniform) deposition and crevasse splay deposition (after a ‘failed’ avulsion; if the riverbed is super-elevated, but the new steepest path to sea level is longer than the prior path). Preliminary results indicate that anthropogenic manipulations of the river (e.g., levees) can propagate hundreds of kilometers upstream. The CEM distributes the sediment flux from the river mouth along the coastline using alongshore sediment transport gradients. The new medium-detail model will be expanded by further model couplings, including Sedflux 3D (Hutton and Syvitski, 2008), as well as vegetation, marsh, and human dynamics modules.