Measurement of hyporheic flow within the granular bed using the MIR flow and PIV methods

The hyporheic zone of the gravel bed is an important habitat for spawning salmonid and aquatic invertebrate, and the significance of hyporheic exchange has been recognized over recent years. Despite some analytical and numerical techniques existed, experimental observations are, however, very limited mainly due to the difficulties for acquiring more detailed information regarding the flow field within the gravel bed and depth of hyporheic exchange. In this study we provide a new method to visualize and measure the hyporheic flow within the granular bed. We set up a 2D granular bed using transparent particles whose index of refraction is identical to that of the fluid such that the matched index of refraction (MIR) interstitial flow can be visualized. Via the Particle Image Velocimetry (PIV) method, we can obtain the flow field within the granular bed and estimate the depth that hyporheic flow could affect. The ultimate goal of this study is to quantify the interactions between hyporheic flow and surface flow type.