Abstract
It has long been well known that types of flood vary greatly according to topographical elements mountains, basins, plains and so forth. Accordingly if we subclassify the topography of those flood-visited areas, we can tell their flood-types. For example, in the case of the plain, most of the plains in Japan are alluvial, formed by sand and gravel transported by the frequent flood-waters. So the micro-relief of the plain and the state of the accumulated sand and gravel show the history of the floods. From this point of view, the writer surveyed the topography of the City of Isahaya and its vicinity by means of aerial photographs and by a field survey. And he has drawn up a topographical survey map of the City of Isahaya and its vicinity showing the classification of flood stricken areas. Furthermore, he surveyed the great flood which visited the City of Isahaya on July 25th, 1957 and has drawn up a map of the City of Isahaya and its vicinity showing the state of flood there on July 25th, 1957. He found that the depth of the stagnate water, its period of stagnation, the direction of the current, the velocity of the current, the erosion and deposit vary remarkably according to its area. In such a case, the following factors—the valley plain, the fan and the delta—have a decisive influence upon deciding the type of flood. As a result of this research, the flood-types in this district were divided into the following six types.
(1) The flood-type seen in the valleys situated radially at the foot of the volcano.
A lot of land-slides occurred in the upper courses of the valleys made up of the following rocks—hypersthene andesite, lava and block-mud, and situated radially around the extinct crator of Taradake Volcano, and those land-slides have furnished debris to the valley-plains. The distance of the movement of the debris is not so long.
(2) The flood-type seen in the valley plains surrounding the foot of the volcano.
Here the Honmyo-gawa-river and Suzuta-gawa-river which run around the foot of the volcano have tributaries coming from the northeastern part but have no tributaries from the western and southern part. The velocity of the transmission of the flood-waves in these valleys was very fast and the water reached its highest level in almost all the places of the valley plains. This phenomenon is caused by the following factors: 1) it rained heavily along the valley, 2) the flood-water from the radiate valley plains flowed into the main stream about the same time, 3) the velocity of the transmission of flood-waves was very fast. No stagnation water was seen because the valley plain has steep slopes. The banks were destroyed in many places by the flood water and here and there the streams took short cuts changing their meander courses. The surface soil drifted away with the flood-water and the layer of sand and gravel was exposed here and there, but the distance of the movement of the gravel was very short.
(3) The flood-type seen in the fans at the lower part of the canyon.
The Honmyo-gawa-river cuts the foot of the volcano in the western part of the City of Isahaya and forms a small canyon there. At the lower part of the canyon where the City of Ishaya lies, the flood was very disastrous and the flood water flowed away radially to the east. The deposited gravel furnished from the canyon, banks and river-bed were seen at the area where banks were destroyed. Next to the deposited gravel was seen accumulated sand, and silt was seen beyond. The velocity of the flood-water was fast and the depth of the flood-water was 3 to 4 meters deep.
(4) The flood-type seen in the fan situated at the foot of the volcano.
There is a fan facing the coast of Omura Bay to the west of the Taradake Volcano. The fan is composed of the volcanic products when the Taradake volcano went into the eruption in ancient times, and is quite different from those fans formed by the frequent flood water.
