Urbanization causes negative impact to hydrological process, such as declining normal surface flow, increasing storm flow and degradation of habitat. In japan, flood disaster occur frequently in urban area because the noninfiltrating area is increasing. To mitigate these negative impacts of urbanization, it is necessary for urban basin to improve ability of infiltration capacity and water retentive in the whole basin. We focus on remaining infiltration area in the urban area and develop the soil amendment technique using bamboo charcoal and humus for improving the infiltration capacity and water retention potential.

   The constant head infiltration test and the rainfall simulator were conducted to evaluate properties of soil amendments. Combination ratio of improved soils used for experiments were ①only decomposed granite, ②mixed 10% bamboo charcoal, ②mixed 20% bamboo charcoal, ③mixed 30% bamboo charcoal, ④mixed 10% humus, ⑤mixed 20% humus, ⑥mixed 30% humus.

   The results of the constant head infiltration test and the rainfall simulator showed that infiltration capacity is improved by mixing bamboo charcoal or humus, and infiltration capacity increases with increasing the mixing ratio. As an example of the constant head infiltration results, final infiltration capacity is 2.3 times when the combination ratio of bamboo charcoal is 30%. Result of the experiments suggest that these soil amendment techniques could contribute to restore hydrological process in urban area. To practical use, we will accrue knowledge of amendment of soil surface layer by further experiments and evaluate the effect of application to actual watersheds.