A Japanese pipe hydrophone is an acoustic sensor that is widely used in Japan to measure bedload transport. Here we report experimental and theoretical tools for calculating bedload transport using a Japanese pipe hydrophone, and discuss the limitations based on laboratory tests. We found a linear relationship between the momentum of colliding particles and the maximum amplitude of the acoustic wave, and found that the grain size can be calculated using this linear relationship. We determined the range over which this linear relationship holds, and found that it depended on the amplifier gain; we then used it to calculate the maximum flow depth. The median frequency and the form of the frequency distribution of the acoustic wave depended on the grain size, slightly on the flow discharge. The bedload transport rate can be calaculated using the velocity of the bedload and the sediment volume in the bedload layer using the linear relationship between the momentum of the colliding particles and the maximum amplitude of the acoustic wave.
On May 5, 2012, a rockslide occurred on the western cliff of Annapurna IV and triggered floods, which
ran down the Seti River causing serious damage to villages and tourist spots along the river. Considering
the seriousness of the disaster and the necessity of investigating the phenomenon for disaster mitigation in
Nepal in the future, the NPO Nepal-Japan Friendship Association for Water-induced Disaster Prevention
(NFAD) dispatched a survey team 1 month after the disaster, which remained on site from June 2 to 10,
2012. The survey was carried out smoothly and satisfactorily owing to the kind cooperation of the
Department of Water-Induced Disaster Prevention (DWIDP), The International Centre for Integrated
Mountain Development (ICIMOD) and others. Good weather, despite the rainy season, and the carefully
arranged logistical support were also key factors in the success. A helicopter survey focused on the
western flank of Annapurna IV where the rock slides and rock avalanches that triggered the disaster
occurred. Soil materials were collected and compared with the deposits in the downstream areas.
Reconnaissance along the Seti River confirmed a “dam up” at Kapuche and severe bank erosions at
various places along the river. The deposits along the river were confirmed to be identical to the materials
collected during the helicopter survey. Interviews at affected villages, district offices, schools etc., were
useful for understanding the actual situation at the time of the disaster. All interviewees responded to our
abrupt questions without prior notice with various pieces of information. We are certain that progress has
been made in research on the mechanism of rock slides/rock avalanches/mudflows, using practical
approaches such as helicopter surveys, reconnaissance surveys, and interviews. We hope that this report
will further advance research to mitigate the effects of this type of disaster in Nepal in the future.