Hyporheic exchange in a gravel bed flume with and without traveling surface waves

Solute exchange between the pore water of a streambed and overlying flowing surface water can have important effects on the chemical mass balance and biological productivity in the aquatic environment. The base mechanism for driving this hyporheic exchange in a moving current is turbulence at the water- sediment interface and the water surface slope. Many previous studies have shown that the nature of the exchange can be significantly modified when spatial pressure variations, induced by standing surface waves or bed forms, are present in the system. In contrast to these studies, we consider how transient pressure variations induced by moving waves modify hypothetic exchange. This is achieved experimentally by using a vertical array of electrical conductivity probes to track the movement of a conservative solute tracer in the gravel bed of a recirculating laboratory flume under a variety of flow conditions. Our experimental observations indicate that the addition of traveling waves greatly increases the initial rate and depth of solute penetration into the bed. When the analytical solution of a one-dimensional (vertical) advection–dispersion model is fit to the experimental measurements it appears that the presence of traveling surface waves results in a more dispersive hyporheic transport. Through experimental observation, model fitting, and direct simulation we demonstrate that this apparent dispersion dominated transport is due to the vertical “pumping” of interstitial fluid in the presence of traveling surface waves. From these simulations we develop a relation that for a given set of flow and wave characteristics allows us to estimate the apparent dispersion due to traveling waves.