Effects of topographic structure on salt marsh currents

Abstract

The purpose of this study was to assess the spatial and temporal variability in overmarsh and in-channel currents, and to assess that variability in the context of salt marsh topographic structure. We created a high accuracy digital elevation model of a salt marsh island using a real time kinematic Global Positioning System. We measured currents with two velocimeters at the heads of adjacent first order creeks of two separate intertidal creek networks, 30 m apart. We also monitored in-channel currents with one current profiler. All measurements were taken during a 5-day period with spring, mixed tides. Observations reveal that during the high-high tides three current reversals occurred, one during flood, one near peak tide, and another during ebb, as opposed to the usual single reversal near peak tide. These reversals took place in shallow overmarsh flows and in deeper in-channel flows. This current complexity appears to be related to tide elevation and the submergence or emergence of salt marsh topography, and it likely reflects interactions between local topographic forcing and larger scale barotrophic pressure gradients. Moreover, when the marsh platform was inundated but under ebb conditions the major components in overmarsh flow patterns between creek heads indicated a regular exchange of water between tidal creek systems. Hence the characterization of overmarsh flows as having flow divides between intertidal creek networks deserves greater scrutiny. Taken together, these observations show that the subtle topography of a salt marsh landscape exerts important control on the temporal and spatial variability on currents at centimeter- to creek network-scales. More research is needed to characterize marsh topography, overmarsh circulation, and their combined effect on material transport and sediment accretion. Copyright 2007 by the American Geophysical Union.