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Elements of Tidal Marsh Circulation


Flows in tidal marshes arc not described well by the classical equations of open channel flow. The geometry of the marsh complicates analysis of the system. Water depths in tidal marshes are usually small, varying from a few centimeters or even dry at low tide to a meter or more at high tide. At low tides, the flow can be channelized and is often essentially one-dimensional. At high tides the system frequently must be described using a more complete two-dimensional approximation. A model is thus required that permits transitions between differing areas of the system. It must accurately represent the changes of the areas inundated during the tidal cycle. It must also be capable of operating to the resolution necessary to characterize the flow regime in the marsh.

Vegetation within the marsh presents a variable resistance to flow particularly in this range of depths. A parameter like Manning's n is insufficient to characterize the variable friction resistance to flow. A new formulation is required which accounts for head losses within the vegetation as well as flow transitions between the channels and vegetated marsh surface. This formulation will be independent of the model used and is relevant whether the model is one-dimensional or two-dimensional. Factors that contribute to head losses in marshes are: turbulent energy losses, bottom friction, wind stress, drag from the erect plants that obstruct the flow, and secondary currents within the vegetation. The relative magnitudes of these effects have not been quantified impervious research. Direct observation of small scale hydrodynamic phenomena is difficult because of the shallow depths, frequently inaccessible field conditions during high tides and the spatial heterogeneity over the marsh surface. Most computation of flow in tidal marshes is carried out using the lumped friction parameter of classical open channel flow. This parameter is a calibration coefficient and has only a limited physical basis.

An additional problem that must be addressed is the influence of tidal marsh inundation on the circulation within the estuary proper. Typical estuary models build the equivalent of sea wall on the estuary perimeter. Depths at these locations typically do not go to zero and the marsh behind these arbitrary boundaries is excluded from the analysis. A model is thus required that can cost effectively and accurately incorporate these areas into the estuarial analysis.

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