The mountains of the upper drainage area of the Watarase have been wasted remarkably as a result of the destruction of the forest by smelter fumes of the Ashio Copper Mine, so that the bare land has become widely distributed there. This wasted land has greatly influeced flooding in the Watarase drainage area.
Making clear the present conditions of the wasted land and the relationship between the supply of debris front bare land slopes and the sediments on the river bed from. the view point of the Watarase flood control is most urgent problem.
There is considerable literature on the historical studies and on the. reclamation of the vegetal cover of, the wasted land but little work has been done in analysis of the present conditions of the, debris size distribution on the bare land slopes or the influences on the river bed deposits.
The purpose of the present study is to determine how the debris size, distribution on the “tall” is controlled by th rock properties and what-relationship exists between the supply of debris from. the bare land slopes and the sediments in the river bed. Also this paper reports the results of measurement of the transported materials, and their solution, suspension, and saltation in the river.
The general conclusion of this study are as follows:
1) The debris size distribution on the bare land slopes has been controlled. by the rock properties. For example, the debris of qurtzporphry has many boulders and gravels of 250 -64 mrn in diameter with few fine material (Fig. 5-1). The debris of liparite has few boulder and comparatively much gravel of 128mm diameter, size and, much fine material (Fig. 5, -2). The. frequency distribution of granitic debris shows many gravel particles of 32-8mm diameter, size and shows many gravel particles of 150mm in diameter with much fine mateial. That is. to say, the distribution has two peaks (Fig. 5-3). Finally, the debris, of Palaeozoic sandstone has many boulders and. that. of Palaeozoic shale has much fine material (Fig. 5-4).
The facts above mentioned indicate the unit of debris size derived from the-different rock beds and determine the debris size distributions before the river-bed deposition. These determinations are important in the investigations of river bed sediments.
2) There are many “tali” below the bare land: slopes. Flood waters tra-nsport this talus rock debris to downstream locations where the consequent deposition increases lower course flooding.
3) Where the supply of debris. from the mountain side to the river bed is considerable, the mountain river bed shows sediments like mud flow deposits.
4) The particle sizes in the wasted land river bed portion are generally, smaller than those in river bed portion where the drainage area is vegetati-on-covered.
5) The amount of solution on the rivers of the wasted land is extremely small but the amount in suspension is very large.
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