Abstract
Understanding and modeling the stochastic feature of sediment influx becomes important for longterm reservoir management and restoration program of old reservoirs with the increase of intense rainfall frequency due to recent climate change.In this study, we obtained the variation of landslide area in a sediment yielding valley upstream from the Midorikawa Reservoir for about 40 years using nine sets of aerial photographs, and discussed its correlation with short-duration intense rainfalls as well as the sediment deposition in the reservoir:After distinguishing landslide sites from manmade bright colored parts such as roads, buildings and etc. by eyes, the landslide area was estimated for each set of photographs by assuming relative brightness threshold determined from comparison of actual and estimated values obtained from the photograph in 2016.A numerical model was built by assuming the equation Zi=Zi-1·exp(-αΔt)+βRi, where Δt is time increment of calculation, i is calculation step, Z is landslide area, R is rainfall amount above the threshold, and α and β are some constants.The comparison of calculation and observed Z(t) suggested that the result for the threshold of 80～90mm of rainfall during three hours explained the variation of landslide area very well. In addition, the calculated Z(t) had a good correlation with the sediment deposition amount measured in the reservoir for 40 years.
