兵庫県における被災ため池の特色—地形・地質,池の構造および老朽度・改修歴との関連を中心にして—

Abstract

There are many farm ponds in the western part of Japan and they play important and essential roles in rice field agriculture. Hyogo Prefecture has 51, 679 farm ponds, the most of any Japanese prefecture. Of the farm ponds in Hyogo Prefecture, 1, 372 were damaged by the Southern Hyogo Prefectural Earthquake on January 17, 1995. The author attempted to clarify the geographical features of those damaged farm ponds. First research was conducted on relationships between damaged farm ponds in Hyogo Prefecture, excluding Awaji Island, and their topography, geology and structure. Damaged farm ponds are located on valley plains, terraces, small fans and boundaries between two topographies, for example, terrace and valley plain, terrace and back marsh, etc. The subsurface geology of damaged farm ponds are Holocene deposits, terrace deposits (Pleistocene deposits), Osaka Group (Pleistocene deposits), Kobe Group (Pleistocene deposits) and Arima Group (Miocene deposits). Furthermore, they are classified by structure as valley closed ponds (tani-ike in Japanese) that enclose valley plains with embankments and flat ponds (sara-ike) that are surrounded by large embankments on an almost fiat surface. Farm ponds located on valley plains of which the subsurface geology is Holocene deposits are very easily damaged during an earthquake, because their bottom gradients are steep and their grounds are weak or unstable. Next, the author conducted research for owners of damaged farm ponds to clarify their superannuation and history of repair. It was found that most severely damaged farm ponds were old and had not been repaired for many years. Few of the old ponds were only slightly damaged or undamaged. Farm ponds that were built more than 200 years ago and have not been repaired in the past 50 years are very susceptible to earthquake damage.