The severity of climatic natural disasters has been increasing over recent years, and repetitive flooding incidents have led to losses of numerous lives. In Japan, rapid flooding of small and steep rivers (flash floods) is an especially serious problem, and there are many rivers that may flood within several hours after heavy rainfall. For example, in the year 2016, Omoto River flooded in a few hours, leading to the loss of many lives (Nursing home operator failed to act on typhoon advisory before fatal flooding, 2016), and heavy rainfall in northern Kyushu in 2017 killed many people because of the resulting rapid floods in associated small rivers (Hazarika et al., 2020). Short-term rainfall intensity directly affects the river flow in such rivers, and the frequency of the rainfall exceeding 50 mm/h has increased by 1.4 times compared to 30 years ago in Japan (Japan Meteorological Agency, n.d.). Spurred by the flooding damage, the Japan administrative agencies launched a policy to increase inexpensive and maintenance-free water gauges for various rivers (Sato et al., 2019). Simultaneously, owing to the recent progress in "the Internet of Things," the water gauges are increasingly deployed by thousands every year, and we can monitor water levels via website (Ministry of Land, Infrastructure & Tourism, n.d.-a) in real time. Although these inexpensive water gauges have been widely used to monitor water levels in recent years, it is difficult to execute timely and properly evacuation actions using only information on the water level at a specific point in time; this is because water level increases tend to be rapid during flooding events.