# Integration of ERT and Geotechnical Investigation for River Restoration: A Case Study of Dam Removal Site Characterization

## TL;DR

This paper shows how combining electrical resistivity tomography and geotechnical methods helps map subsurface conditions for river restoration after dam removal.

## Contribution

The novel contribution is an integrated methodology using ERT and geotechnical surveys to characterize subsurface erosion risks in post-dam removal river systems.

## Key findings

- Low-resistivity zones (<100 Ωm) indicate water-saturated fine-grained sediments prone to erosion.
- Moderate-resistivity layers (100–600 Ωm) correspond to stable coarse-grained materials like gravel bars.
- High-resistivity zones (>1000 Ωm) reveal competent bedrock and erosion patterns post-dam failure.

## Abstract

This study presents a comprehensive subsurface characterization methodology integrating electrical resistivity tomography (ERT) geophysics with traditional geotechnical investigations for river restoration planning at a former dam site. The investigation was conducted on Little Sugar Creek (contributing watershed area of 222 km² and mean annual discharge of 7 m³/s) in Bentonville, Arkansas, following the 2021 failure of the 5-m tall Lake Bella Vista dam. Eleven ERT surveys, incorporating both terrestrial and underwater electrodes, were validated with thirteen borehole investigations to characterize subsurface conditions across the study area. Results showed that near-surface low-resistivity zones (<100 Ωm) corresponded to water-saturated fine-grained sediments are prone to erosion, while moderate-resistivity layers (100–600 Ωm) indicated more stable coarse-grained materials. High-resistivity zones (>1000 Ωm) marked competent bedrock. The findings indicate that the creek is currently undergoing distinct geomorphic adjustment phases in different reaches after dam failure, characterized by the accumulation of coarse-grained deposits such as gravel bars and increased susceptibility to erosion in cases of severe flooding. The “event-driven” phase of adjustment is particularly erosive in the upstream area, where thick sediment deposits and shallow shale bedrock provide less resistance to erosion. The integrated approach enabled detailed mapping of bedrock topography, identification of erosion-susceptible areas, and characterization of subsurface material distribution, providing valuable information for restoration design. This methodology demonstrates the value of combining geophysical and geotechnical techniques for comprehensive subsurface characterization in river restoration projects. Additionally, the approach outlined in this study can be adapted to other river systems undergoing geomorphic adjustments, particularly in post-dam removal environments, to better inform restoration and erosion mitigation strategies.