Riparian Freshwater Lens Response to Flooding

Abstract

AbstractThe freshwater lenses (“lenses” hereafter) within saline floodplain aquifers are sensitive to river flooding events. However, the effects of extensive floodplain inundation on saline aquifers are rarely considered and have not been examined previously under controlled laboratory conditions. We conducted laboratory experiments within a two‐dimensional (cross‐section) sand tank (i.e., representing a saline floodplain aquifer) and built both laboratory‐ and field‐scale numerical models to examine lens responses to flood events. Three sets of experiments were performed to explore different lateral extents of floodplain inundation. The temporal behavior of experimental lenses was quantified and compared to variable‐density numerical models that adopted calibrated laboratory parameters, showing good agreement. Results show that more extensive floodplain inundation leads to larger lenses (as expected). The sensitivity analysis was performed based on field‐scale numerical models, demonstrating that the floodplain inundation extent, hydraulic conductivity, and dispersivity are key factors controlling the post‐flood recession in lens extent and volume. In field‐scale simulations of floodplain inundation, the entire lens was significantly salinized during flood recession due to enhanced dispersion accompanying higher groundwater velocities, which may further split into several isolated freshwater bodies before eventually returning to steady‐state conditions. Importantly, field‐scale numerical results indicated that the salt load to the adjacent river increased immediately following the flood event, consistent with reporting of the River Murray (South Australia). These results provide critical new insights into relationships between flood events and the behavior of lenses, highlighting the significance of flooding events on both intermediate and long‐term conditions of saline floodplains.