1953年南山城水害によって現れた京都府南部不動川本谷石積み堰堤の堆積物

Abstract

1874年デ・レーケにより南山城の不動川上流に作られた石積み堰堤は1953年の南山城水害の際決壊し, 下流に大災害をもたらした.その時池を埋積した78年間の地層の露頭が出現し, 翌年京都大学学生実習として実測地形図と柱状図が作成された.その資料を検討し, 埋積過程を考察した.堆積層は下位よりユニットI〜Vに区分される.ユニットIは堰堤建設以前に堆積した土石流堆積物である.ユニットIIは堰堤建設直後, 池上流の浅い水域を急速に埋め立てた含礫粗粒の花崗岩質砂質堆積物が主体である.前端が急斜し段丘状地形を作るが, ユニットIIIはその下流側の深い水域を埋め立てたシルト・細粒砂などの細粒堆積物が主体である.顕著な2枚の礫質粗粒砂層が含まれるが, これらはユニットIIの砂層と同様洪水に伴う水中重力流堆積物と考えられる.ユニットII, IIIの堆積により池の奥行きは70mに減少し, 当初200mあった池の6割が10数年で埋め立てられた.これは1887年頃の古い見取り図とほぼ一致する.この上を覆う2回の洪水堆積層がユニットIVである.この堆積層は縮小した水域ではデルタの前置層, 底置層的な堆積に移化する.その後堤体は破損し, 木製用水樋の腐朽と相まって漏水により水位は1m以上低下した.段丘化した埋め立て地は侵食の場となった.この時期の顕著な洪水堆積層は連続性の悪いユニットVの細〜中粒砂層のみである.池はシルト・極細粒砂でゆっくり埋め立てられ, 堰堤決壊時には奥行きは2-30mになっていたが, 集中豪雨により右岸の用水樋付近が決壊した.また粗粒砂層と過去の洪水との比定も試みた.ダム湖の6割近くが埋積された1887年ごろまでに発生した大きな洪水は4〜5回であり, 粗粒堆積層とおおむね対応する.

The stone-dam, 55m long and 6m high, was constructed in 1874 at the upper stream of River Fudogawa, Kyoto Prefecture, western Japan. The dam-pond was at about 200m long. The dam was destructed by heavy rain in 1953, and the sediments accumulated in the pond during 78 years were eroded and exposed by violent stream. Examining the measured topographic map and columnar sections made immediately after the destruction, the sedimentary process of the deposits was clarified. The sedimentary body is divisible into five units, I to V. Unit I is debris flow deposits on the former river-floor, consisting mostly of subangular cobbles of hornfels of the Ryoke metamorphic rocks and granite. Unit II is represented by coarse-grained, gravelly granitic sands which buried rapidly the shallow upper reaches of the pond. The unit shows a terrace-like sedimentary form ended with steep frontal margin. On the contrary Unit III is made mostly of fine-grained materials, such as, silt and fine sand deposited on deep bottom in front of Unit II. Two coarse-grained, gravelly sand layers are contained in Unit III, which, together with coarse sand beds of Unit II, are considered to be subaqueous high density gravity flow deposits. The basal part of Unit III is possibly a continuation of Unit II. Two-thirds of pond area was buried with Units II and III. Based on the old map it was around 1887, only ten and several years later than the construction of the dam. Unit IV is represented by two sets of flood sediments on the buried pond surface. It changes into deltaic foreset and bottomset facies in the remained pond area. The stone dam was damaged by repeated floods and the pond surface was lowered by more than 1m. Most of pond area became to river terrace suffered from erosion. The pond was gradually buried with fine clastic materials. Fine to medium grained sands (Unit V) distributed limitedly in the upper reach of the pond are only spill-over flood deposits from the river before the destruction of the dam.