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

Hime河流域位于日本中部北部，被认为是日本侵蚀潜力和产沙量极高的地区之一。实际上，主要由暴雨引发的滑坡和泥石流在盆地中频繁发生。我们估计了位于盆地西部的Shirouma-Oike火山的19个流域的化学风化速率。根据各流域河流水的溶质通量与风化火山岩的溶质损失之间的质量平衡方程，简单地估算了这些速率，计算范围为0.15~3.24 mm/年。化学风化速率和溶质通量最大的分水岭与1911年大规模滑坡发生的区域相对应，泥石流和滑坡一直持续发生。化学风化作用产生的不稳定沉积物被认为是研究区泥沙灾害发生的重要因素。每条河流的溶质通量有助于绘制每条分水岭内滑坡和泥石流的敏感性图。化学风化速率高的原因之一是新鲜基岩中的可溶性元素被先前的热液活动氧化蚀变岩中的黄铁矿产生的硫酸淋失。蚀变带的溪水具有高SO4/Cl值的特点。这种简单的水化学特征也可用于探测覆盖着植被和厚土层的热液蚀变带。

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.