THE DEGRADATION OF RIVER BED IN THE LOWER STREAM OF THE ABE-KAWA IN SHIZUOKA PREFECTURE

Abstract

As to the lower stream (0-17km) of the Abe-kawa in Shizuoka Prefecture, the author estimated the amount of the degradation of river bed in recent years by considering (a) fluctuations in the mean heights of river bed and (b) quantities of deposition or scouring in the river bed. Regarding the causes of the degradation, he considered (a) the decrease of tractional loads to the construction of sand catch dams and (b) the effect of gravel mining in the river bed. Conclusions are summarized as follows: 1) The degradation of river bed in the lower stream of the Abekawa has been increasingly remarkable since about 1954, and the quantity of degradation during the past 10 years (1954-1964) is larger in the lower reaches below the 9km. point from the river mouth, the maximum being about 2 meters at the 6km. point. 2) The yearly records of total deposition or scouring in the river bed show a huge amount of scouring for every year since 1954 excepting 1957, and that of the last year (March 1963—March 1964) amounted to about 2×106m3 of scouring. 3) In the upper stream (above the 34km. point) of the Abe-kawa, four large sand catch dams were constructed between 1951 and 1958. From the quantity of gravels deposited in those dams, mean annual volume of the tractional loads at the 34km. point (Ôgôchi Dam) is calculated to be 2.5×105m3, corresponding to about 20% of the total sediment transportation. The effect of those dams to the river bed in the lower stream, however, is indirect and has a minor importance as compared with that of the gravel mining. 4) The relation between the degradation of river bed and the gravel mining is direct and decisive. Based on the analysis of data from 1952 to 1959, the author pointed out that there is a close correlation between quantities of scouring in the river bed and quantities of mined gravels, in the reaches of 1-6km. Moreover, there is a considerable correlation too in the reaches of 2-6km. between the lowering of river bed calculated from the quantity of mined gravels and the degradation -based on the actual measurement. Therefore, it is to be said that gravel mining has been definitely too excessively undertaken in recent years and the scale of operation is still expanding. If this trend is not reversed in the immediate future, serious social problems will be caused by the degradation of river bed.