Geotechnical performance of the Gounokawa River levee during the 2018, 2020, and 2021 flooding events

Significant sand ejecta repeatedly appeared during the 2018, 2020, and 2021 flooding events at the Gounokawa River levee in the Shimonohara district of Shimane Prefecture, Japan. Detailed site investigations were conducted, including the recording of a local resident’s eyewitness account, soil borings, electric resistivity tomography, and trench excavations. These methods clarified the event timelines, foundation soil profiles, and origins of the sand ejecta. Topographic changes, analyzed using digital elevation models derived from unmanned aerial vehicle photogrammetry, were used to determine whether backward erosion piping (BEP) had occurred. It was confirmed that high water pressure had propagated through the gravel layer and had broken through the overlying layers. Sand volcanoes formed in locations where an underlying gravel layer had existed and ground surface elevations had been low. The water head difference across the levee, when the sand boiling began, was estimated fairly accurately. This provided a valuable opportunity to verify the validity of the methods used to predict the initiation of sand boiling and BEP. The head difference was the highest in the 2018 event and decreased in the subsequent events. Despite the large amount of sand ejecta, no ground surface subsidence was observed. A grain-size distribution analysis revealed that the underlying gravel contained an unstable sand fraction, and thus, was suffusive. Most of the large amount of sand ejecta is considered to have originated from the gravel layer. Detailed trench wall observations facilitated the development of a scenario describing the ground deformation and sand-clod formation during the repeated flooding events.