Abstract
Ⅰ Introduction
A heavy rain mainly in Asakura city in Fukuoka prefecture in July 2017 and Hita city in Oita prefecture caused heavy rupture of the mountainous body and flooding and flooding of the river, many damage occurred. In order to grasp the characteristics of this heavy rainfall and its influence, the laboratory reviews the rainfall mechanism while comparing it with the disaster occurred in the area in the same area, visits the afflicted area after the occurrence of the disaster, and inspects the damage situation , And rivers, and attempted to consider changes in the water environment accompanying this disaster.
Ⅱ Research method
It visited the site four times in total in July, August, September and December 2017. The main survey target is a river, and the field survey items are AT, WT, pH, RpH, EC etc. Samples collected at the site were processed in the laboratory and analyzed for TOC and major dissolved components.
Ⅲ Results and Discussion
1. About this heavy rain
The precipitation became intense from around noon on May 5, and there was intermittent precipitation rain until about 22 o'clock. The cause of precipitation was that the atmosphere became extremely unstable near the Tsushima Strait due to the southern Baiu front, so a linear precipitation zone occurred. Even in heavy rain disasters in the area that occurred in 1990, the occurrence of local fronts has been pointed out (Hirano et al., 1991), so it can be said that concentrated areas are prone to torrential rains in this area.
2. Status of afflicted areas
The point where we could go around the afflicted area was that the place where flooding and flooding occurred was not only in the mainstream but also in the middle and upstream part of the tributary and it can be seen that the heavy rain this time was so intense. It is also characterized by the fact that there were a large number of driftwoods in addition to earth and sand at places where rivers flooded and flooded. Looking at the mountains in the upstream part of the river, it can be confirmed that collapse occurred in various places, and it is thought that driftwood was derived from the trees growing on the collapsed ground. As time passed, we were able to confirm the progress of reconstruction, but it is estimated that it will take a considerable amount of time to completely restore, and it seems that the collapse is progressing further in the place where the mountain body collapsed It is an area where disintegration is very likely to occur from the observation, and it is expected that damage will once again occur once there is heavy rain.
3. Water environment of affected area
Looking at the state of the river, there were rivers that were cloudy due to restoration work and other rivers. And water temperature tended to be high in muddy rivers, especially in December, there was a difference in water temperature with or without turbidity. The pH is on average at least 7.0 and is alkaline. Especially the tendency was higher in Hita city. As for EC, on average there are almost 100 μS/cm points, but some points exceed 200 μS/cm. Although there is no regional difference, the EC value tends to be large at 100 μS/cm or more even at the relatively upstream point of the tributaries in Hita city, together with the high pH value, the more groundwater comes from the collapse of the mountain It is thought that the contribution of the groundwater to the water quality of the river becomes large as it starts to be discharged.
Ⅳ Conclusion
I was able to grasp the characteristics of the heavy rain in the northern part of Kyushu and the water environment of the afflicted area to a certain extent. I am planning to continue the survey and to see how the water quality will change.
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