Abstract
We made a field survey immediately after the disastrous flood in the Kano river basin on Sep. 26, 1958 and investigated the actual conditions of the flood flows and the flood deposits brought by this flood water. The results are epitomized as follows:
1) The Kano river basin may be divided into two main areas; that is, the area upstream of Shuzenji bridge where scour exceeded deposition and the area below this bridge where deposition predominated scour. (Fig. 1).
2) The condition of inundation by the flood water is shown in Fig. VI-2 which gives the direction and the force of the flood flows at various spots. For the most part, the flood flows ran down straight regardless of river channel. Such a tendency is remarkably recognized in the upper part of the river basin. Also there is an indication that some 5 miles downstream of Shuzenji bridge, one of the flood flows ran along the older channel.
3) Bank collapses mostly happened in the river impinging undercut slope of meander. Especially, rips on the right bank are pronouncing and as damage by inundation was serious in the eastern half of the river basin, it seems that at the time of flood the river Kano had a tendency to shift its channel eastwards.
4) Fig. 3 represents the depth of the flood deposits in the protected lowland measured immediately after the flood. As shown in the figure, a large quantity of sediments deposited in the area downstream of Shuzenji bridge attaining at some places a depth of some 2 meters. Fig. 4 is the distribution map of the largest gravels that were found both in the river bed and in flooded arable land in the foreland. As can be seen from the figure, grain size of the gravels become abruptly smaller below Shuzenji bridge or thereabouts. Accordingly, we can make distinction of river regimen by setting up this point as the boundary. It may be said in this connection that the amount of sediment deposited by this flood reached nearly 4, 000, 000m3 in the area downstream of Shuzenji bridge.
5) The Kano river flood can be broadly divided into 4 types (3 types of bank-collapse and 1 type of overflowing) from the condition of inundation and deposition. Details are given below.
Type (A); Flood flows of this type ran violently down spreading lingulately below the rupture. Also, voluminous sediments were transported to a great distance. This type was the most extensive among the bank-collapse types and the depth of the flood deposits attained its maximum. Type (B); In the narrow valley area where the force of the flood flows was violent, the flood flows completely destroyed embankment along the channel and became a main factor of the disaster. Type (C); Like former two types, the cause of flood of this type was bank-collapse, only the water invaded the upstream lowland in conrtaverse direction to the flow of the river by the influence of geomorphological settings. The flood flows fanned out and the force of the flows was weaker than the former two types. Sediments were transported only in a short distance and deposited not too deeply. Type (D); This type was caused by overflowing of flood water. The direction of the flows was not so clearly indicated. This type was marked two variations; namely, in one type the flood flows invaded low ground below the level of the banks. The water of low ground remained long after the river level returned to its normalcy. Sediments were deep for its scale of flooding. In the other type the flood flows retarded rapidly with the recession of water as the altitude of flooded area was equal to the height of banks. The flood deposits were comparatively coarse and their depth was not so deep.
6) Seeing from the physical character, flood deposits were generally well sorted, although in the upper part of the basin soil class of the flood deposits was coarse sand and their external area per unit weight was extremely small.
