EVALUATING POST-RESTORATION EROSION AND STREAMBANK STABILITY UNDER VARIOUS VEGETATION AND FLOW CONDITIONS

Stream restoration is a common and costly practice amongst natural resource managers. While restoration objectives are diverse, erosion mitigation is typically one of several motivations underpinning such projects, as excessive erosion jeopardizes the new channel and has detrimental impacts for water quality. Despite this, the impacts of restoration on streambank stability, particularly the removal of vegetation during full-channel realignment projects, are poorly understood. This study seeks to model streambank stability as vegetation reestablishes at three fully realigned reaches in and near Duluth, MN. The Bank Stability and Toe Erosion Model (BSTEM) is being utilized to model several selected reaches under different flood magnitudes and varying root reinforcement conditions. Each realigned reach in this study has a corresponding control reach. Root counts were performed at the control reaches, and these data are being used as a reference for modelling root reinforcement under a mature riparian community. Factor of safety results will first be compared across the different model runs to determine whether changing root reinforcement meaningfully impacts streambank stability. The factors of safety will then be analyzed as flood frequencies increase to determine if there is a threshold flood magnitude at which the streambanks are at risk of excessive erosion.

To compare model results with erosion, unmanned aerial vehicle and total station surveys were performed at each reach. These will be compared to as-built surveys conducted immediately after the realignment occurred to quantify erosion and deposition at each reach, with results compared to the model outputs. Reaches with very low factors of safety should show the greatest erosion risk, while those with higher factors of safety should have experienced lower erosion since the installation of the project. These results will act as a case study and will seek to answer whether the act of restoration at these specific locations is exposing the channels to excessive instability during high flow events. Because many projections in the Upper Midwest suggest increasing flood magnitudes due to climate change, these case studies can help to highlight the need for restoration practitioners to consider stream resilience in their design.