HR: 0800h
AN: H41C-0310    [Abstracts]
TI: Response of an Ecomorphodynamic Model of Tidal Marsh Development to Changes in Sea Level Rise Rates and Sediment Supply
AU: * Kirwan, M L
EM: mlk13@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Box 90229, Durham, NC 27708 United States
AU: Murray, A
EM: abmurray@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Box 90229, Durham, NC 27708 United States
AU: Morris, J T
EM: morris@biol.sc.edu
AF: University of South Carolina, Dept. of Biological Sciences, Columbia, SC 29208 United States
AU: Pratson, L F
EM: lincoln.pratson@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Box 90229, Durham, NC 27708 United States
AB: We have developed a three dimensional model of tidal marsh accretion and channel network development that couples physical sediment transport processes with biomass productivity. A new, simplified, water routing technique facilitates application of the model over the large spatial and temporal scales necessary to address morphologic and biologic responses to changing environmental conditions. The model algorithm, representing a tidal cycle, begins with a basin flooded to high tide water depth. Water parcels in each model cell are routed out of the basin according to a parametrically represented water surface. Outside the boundaries of the channel network, the surface features a constant curvature and flow directions determined by the surface's gradient (e.g. Rinaldo et al., 1999). Inside the channel network, water is routed to cells with progressively decreasing along-channel distances from the outlet. The cumulative volume of water flowing through each cell is divided by a characteristic time of strong flow during a tidal cycle to determine local discharge (Fagherazzi and Furbish, 2001). A local rate of erosion, related to discharge, and deposition, proportional to high tide water depth, is calculated in each cell after every drainage. Biological productivity is additionally calculated in each cell as a function of bed surface depth below high tide and influences the rate of deposition and slope driven transport processes such as creek bank slumping. Changes in bed elevation after each drainage iteration are extrapolated to one month of tidal cycles. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea level rise, meaning water depths and biological productivity remain temporally constant. Establishment of vegetation plays a critical role in developing a horizontal platform incised by a well defined channel network. In early stages of basin filling, or if vegetation influences are not included, channels are broad and grade gradually into inter-channel surfaces. An increase in the rate of sea level rise, or a reduction in sediment supply, causes equilibrium depths in unvegetated regions to increase and channels to deepen and widen. If the new equilibrium depths in vegetated regions of the marsh exceed the optimal depth of biological productivity, reduced biological productivity will cause lower deposition rates on the platform. In this situation, the marsh system is in an unstable condition where channels expand laterally and platform vegetation is rapidly converted to open water. If the new equilibrium water depths only approach the optimal depth of biological productivity, the marsh platform will respond with increased productivity and greater deposition rates. Vegetation on the marsh platform can promote a meta-stable equilibrium where the platform maintains elevation relative to the rising sea level, although disturbance to vegetation could cause irreversible drowning of the marsh platform. Dynamics between biological productivity and predicted equilibrium depths thus have a strong control on the fate of salt marshes in the context of sea level rise and changes in land use/sediment supply and these dynamics may explain patterns of marsh loss.
DE: 4815 Ecosystems, structure and dynamics
DE: 4556 Sea level variations
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting