Estimation of Sediment Depth for a Dam Removal Project Using Combined Underwater and Land Electrical Resistivity Tomography (ERT)

There are many dams around the world where the cost of maintenance or renewal exceeds the expected future benefits. These dams may be considered for removal due to various reasons such as economics, dam safety, recreation, change in water stream direction, or ecosystem restoration to provide fish passage. Sediment management is one of the main challenges for dam removal projects, which can effectively control the budget and safety of the project for the next use of the river and dam site. The released sediments after dam removal can impact the stability or erosion of the riverbanks and increase water turbidity. Based on these concerns, it is critical to estimate the sediment volume and evaluate any associated issues before starting dam removal activities. In this study, a series of electrical resistivity tomography (ERT) survey lines were conducted perpendicular to the axis of the river to evaluate the depth of the fine grain and gravel soil deposited along with the bedrock depth for the project site to support river restoration planning. Each ERT survey line was composed of 56 electrodes starting from the east bank of the river and ending on the west bank including 14–20 underwater electrodes, depending on the width of the river at each cross-section location. Boring logs and field observations were implemented to validate the results of the ERT survey. Data are presented in 2D cross sections showing the electrical resistance of different soil layers. Based on the interpretation of the results, low resistivity near-surface layers are in close proximity to the riverbanks which mostly contain fine grain soils and could be sensitive to erosion and instability concerns following the removal of the dam. The depth of the soil layers resolved in the ERT cross sections agreed well with the boring logs, confirming the ERT method can be a useful tool for estimating the sediment depth in dam basins. This initial result can be used with supplemental field measurements for better sediment management for the project.