火山活動終了後の雲仙普賢岳・水無川流域におけるガリの一次元河床高変動計算

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The 1990-1995 eruption of the Unzen volcano in southwestern Japan produced great quantities of pyroclastic flow deposits. Many gullies formed on these pyroclastic deposits. Elevation variation between 1998 and 1999 were measured for an 800-m-long gully that formed in the Mizunashi River watershed using a digital elevation model (DEM) with a horizontal grid spacing of 2m. The DEM was obtained by time-series measurements of the bed height of the gully by photogrammetry. Between 1998 and 1999 there was no bed variation. The bed variations obtained by DEM agreed closely with bed variations interpreted from aerial photographs. Previous studies attributed the absence of change to removal of pyroclastic fine deposits by surface erosion and rapid growth of vegetation. To confirm these explanations, one-dimensional simulation was performed for the bed variation. First, the rainfall runoff process was simulated by introducing a tank model with two parameters : final infiltration capacity (I_f) and precipitation storage by vegetation (H). The greater the infiltration effect by removal of pyroclastic fine deposits, the greater the I_f. The greater the precipitation storage by vegetation, the greater the value of H. Second, by the rainfall runoff analysis, hydrograph simulation data were obtained at the upstream end of the gully. Finally, the riverbed variation was simulated using the obtained hydrograph. As a result, assuming that I_f has been smaller than 3mm/hr (i.e., pyroclastic fine deposits have not been removed), the simulation resulted in no bed variation except when H≧22mm was given. H≧22mm means that H is larger than the normal storage capacity of vegetation. Assuming H=0mm, or no vegetation storage effect (i.e., no vegetation), the simulated result was no bed variation for 12mm/hr≧I_f. The I_f=12mm/hr means that it is much smaller than the field observation value in the study area, according to the previous study. Consequently, the absence of bed variation could not be explained exclusively by the vegetation growth effect, but could be explained by the infiltration effect by removal of pyroclastic fine deposit.
2005-07-25

火山活動終了後の雲仙普賢岳・水無川流域におけるガリの一次元河床高変動計算

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