Sediment discharge observation and its analysis by means of a hydrophone
in the upper Tedorigawa river basin
Hiroaki NAKAYA, Kenji TSURUTA and Nobuya YOSHIMURA
Abstract
Operational observation of sediment discharge in steep ravines is pivotal to make
timely flash flood warnings as well as to build robust erosion control plans for
effective disaster mitigation. A hydrophone sediment discharge measuring system
(hereafter ghydrophone system") has been installed in a 200kmQ]scale river
basin in order to develop a practical method for bed load observation. The hydrophone
system counts the times that bed load sediments strike the acoustic sensor of
the system. Five amplification levels (hereafter gchannel") can be tuned
simultaneously to the same sound signal from the acoustic sensor to examine an
appropriate set of the amplifications. A direct sediment discharge sampling apparatus
was installed at the right upstream of the hydrophone acoustic sensor to calibrate
its measuring. The observation of flood]induced sediment discharge shows that
sediment discharge behaves in variance with water discharge as floods come about
each season. In one example, sediment discharge fell in spite of steady rising
water level, while it spiked sharply with water level plummeting at the latter
part of the flood. Sediment supply in the river channel seems to be used up during
the snow melt season, which reduced the sediment discharge in response to the
rising water level. The relationship between sediment discharge and water level/discharge
changes significantly as sediment supply is recharged at the outset of the flood
season. A way of quantitative estimation of sediment discharge from the hydrophone
system was suggested and compared with that of theoretical estimation based on
hydraulic variables. Our result indicates that amplification level 16 for sediment
discharge and amplification level 1024 for suspended load are proper for the upper
Tedorigawa river basin.