滋賀県犬上川河口部の掘削凹地における地形の形成過程

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More than 100,000m^3 of sandy sediment was excavated up until 1993 at the river mouth of the Inukami River, Central Japan. A huge depression was formed at the river mouth, and the sediment transport into the depression induced drastic topographical change around the river mouth as well as marked downward erosion of the riverbed at the lower reach. This study examines sediment transport processes and shoreline changes at the mouth of the Inukami River, and attempts to predict the future change. Analyses of old topographic maps and meteorological data showed that a cuspate delta, formed in 1920, was destroyed by a 50-year flood in 1959. A new delta was again formed by 1968, and the topography of the delta scarcely changed from 1980 to 1992 in spite of a 20-year flood that occurred in 1990. This little topographical change suggests that the delta had reached a dynamic equilibrium condition in 1980, and only a flood larger than the 50-year flood could change the rivermouth topography. A topographical survey in October 2003 showed that, after the excavation, the shoreline at the right bank of the river mouth has retreated drastically, whereas a mound and several spits were formed in the river mouth. Comparison between simulation results on wave incident and wave refraction in and around the river mouth and observation results of wave behavior obtained by a kite-photo technique showed that the simulation can reproduce general patterns of wave incidence and refraction. Results of wave simulation studies under the pre- and post-excavation topographical conditions indicate that, when driven by northwest wind (i.e. prevailing wind around the river mouth), high waves (more than 60cm wave height) occurred both around the right and left bank after the excavation, whereas such high waves occurred only in a limited area before the excavation. In particular, the estimated wave height ranged between 45 to 85% of the offshore wave height around the shoreline at the right bank after the excavation, while it is less than 55% of the offshore wave height around the shoreline before the excavation. Calculation results on littoral drift in onshore-offshore direction under northwest wind conditions showed that sandy sediment is transported offshore both before and after the excavation. The amount of offshore sand-drift in the post-excavation state was not so much different compared with the pre-excavation. In contrast, gravel was found to be transported offshore and onshore before and after the excavation depending on water depth. The amount of offshore drift of sand was one to two orders of magnitude larger than that of gravel before and after the excavation. Calculations on littoral drift along shoreline showed that, since the angle of wave incidence to the shoreline after the excavation was much larger than that before the excavation, the amount of littoral drift after the excavation was almost double. The calculation results also indicate that, after the excavation, littoral drift at the right bank eroded the shore, the sediment was transported in the upstream direction, and then it was further transported into the depression of the right bank by offshore drift. In contrast, littoral drift hardly occurred at the left bank, suggesting that the depression at the left bank cannot be filled within several decades. This limited amount of littoral drift indicates that the delta similar to that before the excavation can be formed only when more than 100,000m^3 of sediment is supplied to the river mouth by a flood larger than the 50-year flood.
日本地形学連合の論文
2007-04-25

滋賀県犬上川河口部の掘削凹地における地形の形成過程

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