Collaborative Research: Biophysical alteration of wetland geomorphology in response to rising sea level
Our objective is to examine the evolution of networks of tidal creeks in salt marshes as they respond to rising sea level. We have documented rapidly growing creeks, incising into the marsh at 1-2 m per year, in coastal marshes throughout the South Atlantic Bight and in similar environments worldwide. Understanding the mechanisms driving the evolution of these drainage networks is crucial for predicting marsh responses to sea level rise under different conditions. The study will examine the physics of tidal flows and sediment erosion, while also considering the significant impact of marsh plants and animals (e.g. crabs) on these processes. Using a combination of field measurements, mesocosm experiments, and numerical modeling, we will 1) evaluate the physical mechanisms underlying sediment erosion in creeks, including variation in flow speed and processes changing the strength of the marsh soils, 2) test the hypotheses that crabs facilitate growth of creeks by loosening sediment through burrowing and removing stabilizing vegetation; and that hydrological conditions at creek heads are preferred by crabs, and 3) develop a numerical model of creek evolution to test the importance of different parameters such as vegetation type, faunal density, tidal range and sediment supply on the morphological evolution of creeks. The model will help us to integrate our observations and generalize our findings over a range of conditions. Salt marshes provide valuable ecosystem services to humanity, but these productive habitats are threatened globally by accelerating sea-level rise. Understanding the impact of sea-level rise on the physical structure of a marsh is crucial to predicting both the response of marsh ecosystems and the larger-scale morphological response of the coastline. The impact of increasing water levels on marshes depends on complex feedbacks between physical and biological processes. Initial studies of this issue focused on interactions between water depth and marsh elevation, which affect sediment deposition and plant growth. Recently, increasing attention is being directed towards the manner in which creek networks evolve as they respond to the larger volume of water they must convey during each tidal cycle at higher sea levels. The evolution of creek networks is a critical component of marsh response to sea-level rise, because creek morphology affects the amount of water and sediment delivered to the marsh surface, and the drainage of the marsh, and thus, the length of time the marsh is waterlogged. In turn, the flooding and drainage of the marsh influences the health of plants and animals in this ecologically vital environment. The ability of creek networks to respond to increasing sea levels depends, in part, on how easy it is to erode the marsh surface, which depends on both sediment type, and on stabilization or loosening associated with flora and fauna. Uncovering the linkages between geology and biology will have bearing on the fields of geology, ecology and climatology, as well as those interested in coastal protection and salt marsh evolution. The project will develop a new modeling tool to examine response of marshes to sea-level rise, which will be broadly applicable to many salt marshes in the U.S. and globally.