SOME METHODOLOGICAL DEVELOPMENTS IN THE ANALYSIS OF SEDIMENT TRANSPORT PROCESSES USING AGE DISTRIBUTION OF FLOODPLAIN DEPOSITS

Floodplain areas are primary storage sites for river sediment. In the Saru River, ages of floodplain surfaces were examined by tree ring ana lysis, and vertical and horizontal configurations of floodplain deposi ts were measured by field and aerial-photo surveying. A flood in 1992 provided a good opportunity to examine depositional and erosional proc esses of floodplain sediment before and after the flood. Predominant d isturbances were observed in the unconstrained, wide reaches where flo odplains develop. This event indicated that eroded areas of floodplain deposits in each age class linearly increased with sediment volume an d that the proportion of the total area eroded decreased exponentially with increase in the age of sediment. We constructed basic equations expressing continuity of age distribution in order to analyze river se dimentation in a time series according to the results of the 1992 floo d. The floodplain disturbance rates determined by this analysis showed similar temporal changes with sediment transport rates monitored at t he Iwachishi Reservoir. A sharp increase in sediment discharge was see n after 1962 associated with the historical maximum rainfall in the te mporal analysis of floodplain sediment. The time series approach prese nted here is useful for evaluating the speed of sediment waves and the cumulative impact of sedimentation in a river basin.