Modeling the Role of Compaction in the Three-Dimensional Evolution of Depositional Environments

Natural environments such as coastal wetlands, lowland river floodplains, and deltas are formed by sediment, transported by watercourses and the sea, and deposited over century to millennium timescales. These dynamic environments host vulnerable ecosystems with an essential role for biodiversity conservation, coastal protection and human activities. The body of these landforms consists of unconsolidated sediments with high porosity and compressibility. Consequently, they often experience significant compaction due to their own weight, that is, autocompaction, which creates an important feedback within the geomorphological evolution of the landform. However, this process is generally oversimplified in morphological simulators. We present a novel finite element (FE) simulator that quantifies the impact of natural compaction on landform evolution in a three-dimensional setting. The model couples a groundwater flow and a compaction module that interact in a time-evolving domain following landform aggradation. The model input consists of sedimentation varying in time, space and sediment type. A Lagrangian approach underlies the model by means of an adaptive mesh. The number of FEs gradually increases to accommodate newly deposited sediments and each FE changes its shape, that is, becomes compressed, following sediment compaction. We showcase the model capabilities by simulating three long-term depositional processes at different spatial scales: (a) vertical growth of a tidal marsh, (b) infilling of an oxbow lake, and (c) progradation of a delta lobe. Our simulations show that compaction is the primary process governing the elevation and geomorphological evolution of these landforms. This highlights that autocompaction is an important process that determines the resilience of these low-lying landforms to climate change.