堤防

Tunnels of the moles, Talpa micrura k o beae (Thomas), are said to have done a great role in the breaking down of the banks of the rivers, in the case　of the recent flood which attacked Northern Kyushu in the end of June, 1953. This investigation was done to see how and in what extent they had damaged the banks. For that purpose several places of banks of Onga, Sue, and Chikugo rivers in Fukuoka Prefecture were examined. What was found in this investiga t ion is the fact that unexpectedly great number of moles dwell in the slope of each bank and their tunnels are distributed abundantly. At the place of most compact density of population of the moles, their tunnels are placed like networks under the surface of the slope, so the whole vegetations seem to give the impression that they relief above the ground together with their roots. In addition the tunnels penetrate deep into the earth. The sites of the distributions of the tunnels are usually above the ordinary water level of the rivers but when the water level arises higher by heavy rainfall and the slope of the banks is washed by the water current, the surface vegetal layer of the banks is easily stripped off. It is imagined that, if the ground earthy layer is exposed, the earth is gradually carried away and the destruction of the bank will be brought in. Beside, the tunnels are placed on the outer slopes of the banks and many of them also penetrate deeply into the earth, so when the water come up to high level, it communicates throughout the breadth of the banks passing through the tunnels of the inner and outer slopes. It is easily understood that this passage of water, though small at the beginning, become gradually larger and bring about the destruction of the banks later.