Geomorphological Maps of Alluvial Plains and their Utilization for Mitigation of Natural Hazards:

After World War II Japan faced the simultaneous problems of food shortages and flood hazards. To resolve these problems, agricultural and civil engineers were requested to obtain information on alluvial plains which provide land for major food production in the Japanese Islands.<br> Fortunately, Japanese geographers had already begun to study the alluvial plains as depositional geomorphology, greatly aided by the availability of aerial photographs. These circumstances gave birth to the Geomorphological Survey Map Showing Classification of Flood Stricken Areas.<br> The maps enable us to estimate the features of flooding not only of the past but also of the future. The reason why such a survey map serves the purpose of defining the type of flood is that the irregular surface of the plain, however slight, as well as the sandy and gravelly deposits, were formed by repeated floods. Consequently, the micro-topography of the plain, i.e., fans, natural levees, deltas, etc. tell the history of past floods. From this point of view, the author compiled the first “Topographical Survey Map of the Kiso River Basin (Nobi Plain) Showing Classification of Flood Stricken Areas” in 1956.<br> The accuracy of the map was actually confirmed by the high tide caused by the Typhoon Ise-Bay in 1959, i.e., three years after the preparation of the map. The results of the flood were almost the same as those predicted by the map. It is especially noteworthy that the area of invasion of flooding, caused by the high tide, coincides exactly with the delta area. This close relationship between high tide and geomorphology is manifested in many other cases, for example, in the routes of high tides and features of flooding in each geomorphological unit.<br> Utilizing the combination of geomorphological units, the flood type is classified into the following three types: overflow type, concentration type, and a combination type. An example of the overflow type is seen in the lower reaches of the Kiso River and Han River, the concentration type in the Chikugo River, and the combination type in the Vientiane Plain along the Mekong River.<br> The map is useful for estimating not only flooding but also soil liquefaction sites caused by earthquakes, and for the selection of bridge sites.