Hydraulic architecture of alluvial floodplains

Abstract

This extended abstract presents our research on the distribution and implications of hydraulic infiltration into an inundated alluvial floodplain. Previous research highlights the heterogeneous distribution of various alluvial sediments (clay, silt, sand, gravels, etc.) in an alluvial floodplain; periodic avulsing of the floodplain's river reworks these sediments, erasing lateral continuity and compartmentalizing the alluvial aquifer. While most compartments are clay-dominated, relic courses of the avulsing river can be sand-dominated, leaving interlaced sand-dominated channel-belts that form networks of lateral preferential flow paths for groundwater throughout the floodplain. To examine how surface water infiltrates into those channel-belts, we surveyed a 46 acre (0.19 km2) site above a suspected channel-belt using electrical resistivity tomography and time-domain electromagnetics. We then used the respective 2-D tomograms and 1-D soundings to confirm the existence of the channel-belt, construct a 2-D model thereof, and simulate half a year of inundation. Unexpectedly, infiltration entered the channel-belt primarily through its side walls (ie. laterally) and only somewhat through its roof (ie. vertically). These results provide a basis for making informed decisions about the placement of production or monitoring wells and about focus areas for contaminated groundwater control. By accounting for the heterogeneous distribution of geomaterials in space as well as the heterogeneous distribution of groundwater in time, geologic realism can be added to the decision-making process for both water management and environmental remediation strategies.