Hydraulic Study on Purification Function of Forest-Soil Layr

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Turbid water flowing into forest area is purified by the forest-soil layr. In this study, mathematical models to quantify this purification function of forest-soil layr were proposed. The process of gully formation which governs the erosion in the mountain area was also investigated. The results are summarized as follows :(1) It is experimentally shown that fallen leaves on the soil layr can prevent the soil layr from being plugged by suspended sediment.(2) A model to predict purification of turbidity in forest-soil layr was proposed. This model was combined with a model of two-dimensional unsaturated seepage flow. A numerical simulation for the process of purification was performed. The results of the simulation agree reasonably well with those of the laboratory tests.(3) The relation between forest-nonforest ratio and the concentation of turbidity in rivers was observed in fields.It is found that the concentration of turbidity is 1000-5000ppm for 0% of forest-nonforest ratio while 100 and 10ppm for 20 and 100% of forest-nonforest ratios, respectively.(4) It is found that the concentration of turbidity is correlated to that of phosphorus. This implies that nutritious salts as well as turbidity are trapped in forest-soil layr.(5) A mathematical model for the ability of forest to prevent sand transport due to wind was proposed. The results of the model agree reasonably well with those of the wind tunnel experiments.(6) Field experiments of turbidity flow were performed. It is found that it is important to render the flow as uniform as possible to prevent piping erosion.(7) Erosion due to seepage flow on slopes are studied. An instability analysis was performed to obtain the spacing between gullies formed on slopes. The results of the analysis agree reasonably well with those of experiments.