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

姬川流域位于日本中部的北方，是日本侵蚀潜力和产沙量都极高的地区之一。实际上，该流域以暴雨为主的滑坡、泥石流灾害频繁发生。我们估计了位于盆地西部的白间大池火山的19个流域的化学风化率。这些速率已简单地估计了从每个流域和溶质损失比较新鲜和风化火山岩的流沃茨的溶质通量之间的质量平衡方程，并计算为0.15至3.24毫米/年。化学风化和溶质通量速率最高的流域对应于1911年发生大规模滑坡的地区，泥石流和滑坡至今仍在不断发生。化学风化形成的不稳定沉积物是研究区沉积物灾害发生的重要因素。各溪流的溶质通量可用于各流域滑坡和泥石流的易感性制图。高化学风化速率的原因之一是新鲜岩石中的可溶性元素被先前热液活动氧化蚀变岩石中的黄铁矿产生的硫酸浸出。来自蚀变带的溪流沃茨具有高SO_4/Cl比值的特征。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.