The hydrological observations were carried out during a year at two points in a drainage basin implied a golf cource downwards, in the south of Shiga Prefecture, the central Japan, in order to compare with the hydorological characteristics between lawn and forest. The catchment areas covered by forest and covered by lawn are 23.7ha and 53.8ha, respectively. The former is granite in geology and 21.7° in mean slope angle, and the latter is tertiary and 4.12°. The yearly evapotranspiration was 836.2mm in the former, and 536.7mm in the latter. The catchment areas covered by lawn increased the yearly discharge of about 300mm. The HYCYMODEL (Hydrological cycle model) was adopted for the daily, the hourly and the representative 20 minutes records, in order to analize runoff characteristics of both coverages. Consequently, the runoff characteristics of the catchment area covered by lawn are that the most part of rainfall flows quickly down as a component of direct runoff and that the hydolograph widely fluctuates in comparison with forested area.

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In Japan with its many mountains, debris flow disasters on alluvial fans, as exemplified by the 1982 Nagasaki Disaster, are major problems. To prevent such disasters, it is necessary to predict areas likely to be flooded by debris flows. It is also necessary to ascertain the effects produced by countermeasure works for debris flows when arranging them so as to protect lives and properties. Following matters were studied by a debris flow simulation model. (1) Confirmation of the reproducibility of the simulation model with regard to an area flooded by a debris flow through comparison with hydraulic model tests of a debris flow on an alluvial fan. (2) Evaluation of the effects of the countermeasure works for debris flows. (3) Influence of step time of the calculation to results. (4) Comparison with other simulation models. As a result, it is confirmed that the simulation model is adequate for practical use in predicting the areas flooded by debris flows and evaluating the effects of the countermeasure works for debris flows. It also can be used in making future erosion control plans.

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