A Hydraulic Modelling Approach to Study Flood Sediment Depositions in Floodplain Lakes

Abstract

Abandoned river channels on alluvial floodplains represent dynamic systems where sediments, organic matter, and pollutants preferentially accumulate during extreme discharge events. Descriptive models that explain the infilling of these floodplain lakes due to sedimentation processes recognize different stages in their evolution. For example, the threshold for hydrologic connectivity and the transfer of material increases in older lakes as a plug-bar develops. Sedimentary archives collected from floodplain lakes are widely used to reconstruct ecological and hydrologic dynamics in riverine settings, but how floodplain lake evolution influences flow velocities and sedimentation patterns on an event scale remains poorly understood. In this study, we examine a floodplain lake along the Trinity River at Liberty during the extreme flood event associated with the landfall of Hurricane Harvey in August 2017. We combine sediment samples collected in and around a floodplain lake with hydraulic modeling simulations in the HEC-RAS modeling platform from the US Army Corps of Engineers to examine inundation, flow velocity, and sedimentation patterns. Additionally, we develop a series of alternative lake bathymetries to study the influence of floodplain lake evolution on flow velocity patterns during the flood. The hydraulic model reproduces the sediment patterns that we observe around the lake resulting from Hurricane Harvey and matches descriptive models on the behavior of a young floodplain lake. We find that sediments deposited in the lake following the Hurricane Harvey flood become thinner and finer with distance from the lake entrance in accordance with simulated flow velocities that are lower further from the lake entrance. Flow velocity simulations from model runs with alternative plug-bar geometries and lake depths imply that sedimentation patterns will shift as the lake evolves and infills. As the floodplain lake becomes shallower and narrower, flow velocities extend further into the lake resulting in more extensive transport of coarse-grained material into the lake. These simulations are coherent with observed sediment records from lakes in different stages of floodplain lake evolution. The integration of sediment sampling and hydraulic model simulations provides a method to understand the processes that govern sedimentation in floodplain lakes during flood events that will improve interpretations of individual events in sedimentary archives from these contexts.