The Influence of Antecedent Rainfall to Flashed Flood in a Mountainous River

At a mountainous river called Omoto River located the northern Japan’s main island, many houses have been damaged and more than 20 people have been killed, by a flood caused by the 10th Typhoon, 2016. Our previous report has suggested that, there were rainfalls by another typhoon before the 10th typhoon that caused flood disaster. The river basin has been the wet condition, which increases the flood disasters. Therefore, this paper, to contribute to disaster prevention and reduction, describes the influence of antecedent rainfall in the mountainous river basin.

Firstly, in order to evaluate a rainfall return period of the 10th typhoon, historical rainfall data were investigated. According to these data, there were 2 heavy rainfalls almost the same as the 10th typhoon that caused flood disaster. Comparing the return period of these 3 heavy rainfalls including the 10th typhoon indicated that, the return period of annual daily maximum was almost the same, but the 10th typhoon had the large return period with not only annual hourly maximum but also annual month maximum. According to historical disaster statistics, no flood damage was expected by 2 heavy rainfall excepting the 10th typhoon. Therefore, we should discuss the flood disaster occurred related to not only a short-time rainfall but also an antecedent rainfall.

A physical-based rainfall-runoff simulation model is employed to physically evaluate the influence of antecedent rainfall to river flow rate during a flood. The model consists of a two-dimensional surface flow model by diffusion wave approximation and a three-dimensional water-air two-phase groundwater model capable of tracking fluctuations of groundwater free water level. Those governing equations are fully coupled in one equation system to solve it unified. Thus, this method can estimate the interaction of surface flow and groundwater without using boundary conditions.

A result of simulation indicated that due to the infiltration of antecedent rainfall, a pressure rise has occurred across the basin, except the area directly under the river channel where the pressure has decreased. The groundwater flows towards the area of lower pressure head. the water is then pushed out into the channel on the surface. For this reason, we can say that in the flood disaster caused by Typhoon No. 10 in 2016, flood-discharge was increased due to the effects of antecedent rainfall. In addition, it is possible that the change in pressure head distribution is related to the rapid rise of water level in mountain rivers.