Hydrologic and Eco-geomorphic Response of Tidal Salt-marsh Platforms to Sea-level Rise and Protective Barrier Design
Howell Taylor, Susan Meredith
The long term stability of tidal marshes is related to interactions among the surface slope of the marsh platform, sedimentation, primary productivity, and the relative rate of sea-level rise. Here I simulate the physical and ecological response of marsh platforms to changes in flood and ebb flows and shoreline barrier design. A hydrodynamic model is developed based on principles of conservation of mass and momentum, and demonstrates that the total drag due to macrophyte stems can be approximated as the sum of the drag on individual stems. Short platform length scales are shown to experience bath tub like flooding, and the flood behavior is controlled primarily by the spatial distribution of biomass over longer platform length scales. This study also examines if the design of shoreline barriers facilitates macrophyte degradation, platform erosion, or platform stability, and concludes that ambient conditions on the marsh platform can be maintained using shorter barriers. The influence of flood frequency and macrophyte productivity is further analyzed through a description of the astronomical and wind driven signals. Simulations show that wind derived low frequency variations in the water level signals constitute a significant part of the observed record, the effect of which is to set the average water level about which the astronomical signal fluctuates. This work suggests that marcrophyte primary productivity is a function of the frequency and timing of flooding rather than the mean water level for a given elevation. Longer time scales of platform evolution are simulated with two marsh sedimentation models that include the inorganic and aboveground and belowground organic processes. Organic deposition influences the interpretation of dating methods (210Pb, 137Cs) due to carbon decay. Marsh stratigraphy response to sediment supply and the rate of sea-level rise, including carbon accumulation, is sensitive to the background sediment supply; if inorganic sediment supply is reduced in a sediment poor marsh, then the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh. Carbon accumulation in marshes is also shown to be nonlinearly related to the supply of inorganic sediment and the rate of sea-level rise.