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細粒土砂による河床の目詰まりが河川間隙水域の機能を低下させていることが近年報告されているが、河川間隙水域における細粒土砂の動態はほとんど理解されていない。そこで本研究は、出水に注目し、河川間隙水域内の細粒土砂量の変動を調査した。河川間隙水域は地下に存在するためにその調査が難しく、流路内環境との関係性を見出すことが有効である。水深や流速、土砂堆積量といった流路内環境と細粒土砂濃度との関係性も調べた。...調査は、福岡演習林を源流にもつ新建川で、2012年10月－11月の平水時に2回、2012年12月の出水時に2回行った。河川間隙水域の細粒土砂濃度は平水時に比べると出水時の方が高かった。平水時に湧水傾向にある調査区間において、出水時には河川水の伏流がみられ、河床へ流入する表流水が河川間隙水域への細粒土砂の流入を促していることが示唆された。細粒土砂濃度は、流速×水深(m^2/s)が大きい出水時ほど高く、平水時においても流速×水深が大きいほど高かった。出水時において河床の土砂堆積量が多い地点ほど高い傾向がみられたが有意ではなかった。ばらつきは大きいものの、流路内環境の違いによって、河床間隙水域の細粒土砂濃度の傾向が異なることが示唆された。これらの結果から、河川間隙水域の細粒土砂量は、出水によって変動し、河川地形によって空間的にばらつくことが考えられる。
It has been reported that clogging of the hyporheic zone is hindering its function in some rivers. In this study, we used the concentrations of sediment in the hyporheic water, taken from piezometers, as an indicator of fine sediment content and studied the dynamics of fine sediment in the hyporheic zone. The concentrations of sediments in the hyporheic water was measured during baseflow and high flow events. The difficulty of the hyporheic study is that it is located belowground, and thus linking the hyporheic condition to the in-channel parameters is very useful. The relationship between sediment concentrations and in-channel parameters, such as water velocity and sediment accumulation, were also examined. The study was carried out in the Shintate River, near the Kasuya Research Forest, and two samplings during baseflow in October-November 2012 and two samplings during rain events were carried out in December 2012. The results showed that the sediment concentrations of hyporheic water were higher during high flow event, comparing to baseflow period. The downwelling trend observed during high flow suggests that downwelling stream water may carry fine sediment into the hyporheic zone. Sediment concentrations of hyporheic water were higher during high flow and in the locations with higher velocity x depth (m^2/s) during baseflow. Streambed sediment accumulation did not show significant relationship with hyporheic sediment concentration, but during high flow, higher sediment accumulation showed higher concentration. These results suggest that the in-channel parameters may influence hyporheic water sediment concentration. This study indicated that high flow event influence the temporal variation of fine sediment concentration, and geomorphic features may influence spatial variation.続きを見る
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