EVALUATION OF SEDIMENT TRANSPORT CHARACTERISTICS IN A MOUNTAINOUS TORRENT BASED ON SEDIMENT TRANSPORT MONITORING WITH TURBIDITY SENSORS AND HYDROPHONES

The quality and quantity of sediment transport in watersheds must be evaluated for comprehensive sediment management. However, sediment movement in a mountainous torrent is extremely difficult to predict with sediment transport formula. Field observations are thus important. Direct sampling has accuracy, while it needs lots of cost. Indirect methods, such as the use of pipe hydrophones, to observe bed load and the use of turbidity sensors, to observe suspended and wash load are relatively low cost and continuous observations are available.

In this study, we applied these indirect methods to Ashiarai-dani watershed, located in central Japan where sediment transport is very active. Total catchment area is 7.2km², and we installed these indirect methods to the exit of Ashiarai-dani and three tributaries.

This study reports two main results of observation. First, we effectively estimated sediment transport routes during rainfall events by comparing the turbidity curves and changes in hydrophone pulses at two different observation points. Similar shapes of curves in different observation points demonstrated the same sediment group's movement. Particularly, turbidity curves, expressing suspended sediment, had obvious agreement. Second, the sediment source area of suspended and wash loads was estimated by classifying the relationship between turbidity and water level. We classified rainfall events in following three types. Earlier type; the peak time of turbidity is over 30 minutes earlier than that of water level, simultaneous type; the peak time of turbidity is 30 minutes compared to that of water level, later type; the peak time of turbidity is over 30 minutes later than that of water level. In earlier types, sediment was supplied from torrent bed, and in simultaneous and later types, sediment source was bare slope. In addition, earlier types appeared in thawing season, and simultaneous and later types appeared in the rainy season after spring. At Ashiarai-dani, we had heavy snowfall and the process of freeze and thaw in slopes possibly repeated. We recognized the following change of sediment supply area with the seasonal variations, the eroded sediment with this process deposited torrent bed in winter and discharged in a few spring thaw events, and then sediment supply from bare slopes became dominant.