Hyporheic Exchange With Complex Channel Morphologies

While currently available hyporheic exchange models generally consider one- or two-dimensional transport, rivers with complex channel morphologies are expected to produce three-dimensional advective hyporheic flow patterns. Experiments were conducted in a large recirculating flume to observe exchange with naturally-formed alternate bar and meandering channel morphologies. The structure of the stream flow and channel topography were measured in detail, and the sediment properties were very well characterized. Because it is so difficult to make these types of measurements in conjunction with field tracer injection experiments, this represents a unique data set for assessment of hyporheic exchange with complex channel morphologies. A standard two-dimensional advective exchange model was initially used to analyze the experimental results. The two-dimensional model significantly under-predicted the observed exchange, clearly indicating the need for consideration of three-dimensional flows. Introduction of additional length scales to better characterize the channel geometry indicated that exchange occurred by means of three-dimensional pore water flows induced by head variations over the complex channel boundaries. The three-dimensional flow coupling and resulting hyporheic exchange can be analyzed by the application of a computational fluid dynamics code for the stream flow and subsequent application of a finite element model for hyporheic transport. Both models must represent the complex three-dimensional channel morphology.