MODELING THE EFFECTS OF IN-CHANNEL GEOMORPHIC FORMS ON HYPORHEIC INTERACTION

Previous studies have shown the impact of various stream channel features on hyporheic flow patterns through field studies and modeling exercises using real stream reaches. These studies have been limited by the presence or absence of features in the particular stream reaches employed, and thus do not represent the complete range of possible geomorphic conditions. Their results have also been confounded by the simultaneous variation of multiple factors in the field setting. We used simulation models to explore the effects of a wide range of geomorphic structures on the magnitude of surface-hyporheic coupling in a simplified hypothetical gaining stream reach under baseflow conditions. We combined a one-dimensional channel hydraulics model, a three-dimensional groundwater flow model, and a three-dimensional groundwater particle tracking algorithm to simulate the effect of variations in surficial channel features including steps, pools, and several types of in-flow obstructions on the exchange rate, residence time, and depth of surface flow penetration into the hyporheic zone. Results indicate that while a wide variety of in-channel features drive some surface-subsurface interaction, in-channel flow obstructions (e.g., debris dams) appear to be more effective than certain bedform features (e.g., steps). For example, in our hypothetical stream reach, a channel spanning flow obstruction 50% as high as the downstream flow depth increased subsurface exchange nearly 10 times as much as did a vertical step of the same height. In addition, the effect of flow obstructions was enhanced in model runs with increased stream gradient but damped in runs with increased baseflow. Our results suggest that certain commonly used techniques for restoring in-stream habitat may have the additional benefit of being highly effective at enhancing interactions with the hyporheic zone. In addition, restoration techniques designed to increase hyporheic exchange may be more successful in certain geomorphic, hydrologic, or climatic settings.