In the observation of bed load transport using a hydrophone, a bed load measuring pit is often installed for the calibration of the hydrophone's pulse data. A small pit, however, poses a problem as it tends to fill up before a flood event reaches its peak. Because past observations have revealed a tendency for bed load to exhibit different flow patterns before and after the peak of a flood, we need to establish a relationship between the hydrophone pulses and bed load transport after the peak. For this purpose, a device for measuring bed load transport has been developed with a mechanism that opens the slit of the measuring pit automatically at the peak of a flood. This device uses water|level measurements obtained by an ultrasonic water gauge as a trigger to activate a hydraulic jack which moves the lid installed over the slit so as to allow the pit to start collecting bed load in the middle of a flooding event.

Key wordsFautomatic opening pit bedload sampler, hydrophone, Osawagawa river