Practical Modeling of Flow Connectivity in Intermittently Flowing Wetlands and Streams

Abstract:

Water flow above and below the ground surface are both important contributors to connectivity between intermittently flowing streams, wetlands and downstream waters. The physical controls on discharge and water depth are well represented by rate laws governing flow above and below the sediment interface; however, in the shallow flows of wetlands and streams, there is not necessarily a sharp threshold that determines when surface flow ceases and subsurface flow dominates. Instead there is a gradual transition between flows governed by surface and subsurface flow controls. We examined the effect of microtopography on flow connectivity by measuring and modeling the effect that a decreasing water level has on cross sectional area and tortuosity of surface water flow paths as microtopographic high points emerge above the flow. Several thresholds were determined as functions of the microtopographic distribution, including a major transition where surface flow ceases altogether and the surface water becomes connected by only subsurface flow between pools. We developed a coupled surface and subsurface model of flow connectivity for application in shallow, intermittently flowing wetlands and streams. This one-dimensional model uses a statistical distribution approach to characterize complex microtopography based on limited topographic surveying at appropriate scales. A test application of the model in the Everglades revealed a broad range of transitional conditions associated with relatively easily measured physical features of streams and wetlands.