Survey of Gravel Movement during Floods Using Low-Frequency Communicator in the Tama River in Japan

A gravel-bar restoration project was started in 2001 in the Nagata District of the Tama River. As a part of the project, gravel augmentation was conducted as a trial in 2001 and the work is continuing. The objective was to increase the riverbed height and to maintain gravel-bar area, but there were some concerns about the downstream region, such as local deposition of sediment which would reduce the discharge capacity. This paper reports on the following field study. First, a cross-sectional survey was carried out to confirm whether gravel augmentation was effective for gravel-bar restoration. Secondly, gravel movement was measured with a low-frequency communicator to identify where imported gravel accumulates after a flood. From these observations, it was found that most of the imported gravel flowed downstream during a flood and tended to stop around the lower area of the Nagata District. However, local deposition of sediment was not observed downstream of the Nagata District.