大規模崩壊地を有する渓流における豪雨時の土砂流出と河床変動の実態: 富士川水系早川の春木川を例に

摘要

To clarify sediment dynamics in mountain rivers during the heaviest rainstorms, we observed river water levels and riverbed level at Harukigawa, Yamanashi prefecture. Due the large-scale landslide occurred in more than a hundred years ago, large steep devastated area is located in the upper end of Harukigawa watershed. Here we successfully described detailed sediment dynamics in three large rainstorms due to typhoons in 2011, and, several results were found. About sediment movement of mountain stream with a landslide, we found the following things. At steep mountain river, whether or not to occur riverbed variations is determined by the water level conditions. In addition, when the continuous rainfall exceeds the number 100 mm, sediment discharge will be frequent. After that, the high concentration sediment discharge with riverbed rise occurs more than once, and the more the water level is high, a large erosion and deposition occurs. During floods, the duration of the high concentration sediment discharge with riverbed rise was about up to about one hour. Therefore, temporal change of sediment discharge and river bed variation has been found that not determined by only the hydraulic conditions. The timing of the riverbed change to 800 m downstream from the confluence of the tributary is affected by the sediment discharge waveform from the tributary. If the scale of the sediment discharge is small, the impact distance will be smaller.%河床変動計算は，将来の災害の予測や過去の災害のメカニズムの検証など様々な用途で実施されてきた(例えば，中川ら，2001)。河床変動計算を実施する対象範囲は，目的に応じて設定される。例えば，山地流域の土石流による災害の再現•予測に関する河床変動計算では，土石流の発生•流下区間を含むように，10°以上の地点に計算区間の上流端が設定されることが多い。(例えば，中川ら，2001；西口ら，2011)。一方，流域面積が数10〜数100km~2の広範囲の流域の土砂動態の再現•予測を対象とした場合は，土石流の発生•流下区間以下の相対的には緩勾配河道の河床変動を対象に，10°ないしはそれ以下の地点に計算区間の上流端が設定されることが一般的である(例えば，井戸ら，1999)。以下では，土石流の発生•流下区間と考えられる概ね勾配10°以上の区間を「急勾配渓流」と呼び，土石流の発生•流下区間以下の河道を「緩勾配山地河道」と呼ぶ。