Complexity in Overmarsh Circulation

Overmarsh circulation is highly complex with variations in current direction over very short timescales and flowpaths that differ substantially between flood and ebb cycles. Understanding such flow complexities in salt marshes is important because they inherently impact the distribution of sediment, larvae and nutrients throughout the marsh, and hence, the ecogeomorphic feedbacks that control landscape evolution. The purpose this Ph.D. research is to quantitatively evaluate the effects of salt marsh topography on overmarsh circulation in a Georgia salt marsh. This dissertation is comprised of three parts. First, digital elevation models (DEMs) were compared to characterize how well airborne lidar (light detection and ranging) data depict the microtopography of a salt marsh. 72,000 GPS points and 700,000 lidar points from a 1km2 salt marsh island were linearly interpolated using identical DEM configurations. 1D comparison between the two surfaces indicates that lidar is a robust tool for mapping intertidal landscapes. However, in a 2D or 3D sense, lidar DEMs do not adequately resolve the microtopographic variations of a salt marsh, and for research questions that require accurate depiction of small scale tidal creek networks and subtle terrain features lidar data should be augmented with other information. Therefore, in the second part of this work we apply a high resolution, high precision GPS DEM to numerical simulations in Delft3D to characterize complexity in overmarsh circulation over a 1.4km2 salt marsh basin. The model results provide a robust representation of field conditions and reveal that at times not necessarily linked to the transition between flood and ebb, the current magnitude and direction were highly dynamic, with variations occurring on the order of tens of minutes. Model results show simultaneous flow divergence and convergence, concentrated flow, as well as large-scale rotational flow. Also, all spatial scales show strong differences between flood and ebb pathways, which gives rise to 53% transport of the dye export via overmarsh and intertidal creek flows, and 32% exiting through the central subtidal channel system; 15% remained in the system after five days. The complexity in overmarsh circulation arises from the submergence and emergence of salt marsh topography. In the final part of this work I systematically smooth the high resolution GPS DEM to answer the question: How much topographic detail is needed to accurately simulate overmarsh circulation? Topographic smoothing progressively reduced the basin drainage density by 97%, and in the course of averaging removed small scale intertidal creeks and creek networks. Nevertheless, simulation results show nominal effects up to a 50m x 50m grid smoothing whereupon a system-wide change from ebb to flood dominance occurs. Moreover, assessments of dye circulation reveal that tracer residence time appears to be set by the geomorphic structure of the basin. Overall, smoothing out small scale intertidal creeks and creek networks reveals their minor role in overmarsh circulation, while excessive smoothing caused the system to change from ebb to flood dominance