Estimation of Velocity of Chemical Weathering and Amount of Sediment Production and Evaluation of Danger Degree of Torrents Endangered by Debris Flows.

姬河流域位于日本中部北部，是日本侵蚀潜力和产沙量极高的地区之一。实际上，以强降雨为主引发的山体滑坡和泥石流在该盆地时有发生。我们对位于盆地西部的四马大池火山19个流域的化学风化速率进行了估算。这些速率是通过各流域水流溶质通量与新鲜和风化火山岩溶质损失之间的质量平衡方程简单估算出来的，计算结果为0.15至3.24毫米/年。化学风化速率和溶质通量最高的流域与1911年发生大规模滑坡的区域相对应，泥石流和滑坡持续发生至今。化学风化作用产生的不稳定沉积物被认为是研究区发生泥沙灾害的重要因素。每条河流的溶质通量可用于绘制每一流域的滑坡和泥石流易感性图。化学风化速率高的一个原因是以前热液活动蚀变岩中黄铁矿氧化产生的硫酸浸出了新鲜基岩中的可溶性元素。蚀变带水体具有高SO_4/Cl比的特征。这种简单的水化学特征也可以用于探测覆盖着植被和厚土层的热液改变区。

The Hime River Basin is located in the northern part of central Japan and is known as one of the areas where both erosional potential and sediment yield are extremely high in Japan. Landslides and debris flows triggered by mainly heavy rainfalls have frequently occurred in the basin, actually. We have estimated the chemical weathering rates for nineteen watersheds in the Shirouma-Oike Volcano located in the western part of the basin. These rates have been simply estimated by the mass balance equation between solute fluxes of stream waters from each watershed and solute loss comparing fresh and weathered volcanic rocks and were calculated to be ranging from 0.15 to 3.24 mm/yr. A watershed showing the highest rate of chemical weathering and solute flux corresponded to the area where the large-scale landslide occurred in 1911 and debris flows and landslides have continually occurred until now. Unstable sediments yielded by chemical weathering are thought to be an important factor of sediment disaster occurrences in the research area. Solute fluxes of each stream could be useful for susceptibility mappings of landslides and debris flows in each watershed.A cause of the high chemical weathering rate is the leaching of soluble elements from fresh bedrocks with sulfuric acid produced by the oxidation of pyrite in altered rocks by previous hydrothermal activities. Stream waters from the altered zones are characterized by high SO_4/Cl ratio. Such a simple hydrochemical signature could be also useful for detection of hydrothermally altered zones covered with vegetation and thick soil layers.