バングラディシュ政府は,日本政府に対しブラマプトラ-ジャでの架橋援助を要請してきた.しかし,この川は河道変遷・河岸変遷が著しく,巨大であるため,河道固定は不可能である.そこで,筆者はあらかじめ選ばれたバハドラバッド,ガバルガオン,シラジガンジおよびナガルバリ(アリチャ)の4地点のうち,いずれの地点が最も安定していて架橋に適するかを応用地形学的立場から調査することを要請された.
筆者は, (1) 河岸線および流心の変化を調査するとともに, (2) ブラマプトラ-ジャ沿岸地形分類図(1/5万), (3) ブラマプトラ-ジャ,ガンジス平野地形分類図(1/100万)を作成した. (1) の結果,ナガルバリはガンジス川の背水の影響を受けて河岸,州および流心の位置の変化が最大であること,また,バハドラバッドも変化が大きいこと,これに比べてガバルガオン,シラジガンジの変化が少ないことがわかった. (2) の結果,バハドラバッド,ガバルガオンは形成開始以来180年しか経過していない扇状地上に位置し,将来河道変遷がおこりうること,シラジガンジ以南は自然堤防地帯河川の特色をもつこと,シラジガンジには地盤の隆起運動によって形成されたと見られる180年以上経過した古い沖積平野による狭さく部があって,河道が一定しており,この狭さく部は今後も存続しうると考えられることなどがわかった. (3) の結果,バハドラバッド,ガバルガオン付近には断層線,線状構造があるが,シラジガンジにはないことがわかった。上記の諸点にもとづいて,筆者はシラジガンジを4架橋候補地点中では最適であると判断した.

抄録全体を表示

The World Bank has made “Bangladesh Action Plan for Flood Control” which consists of 26 components after severe floods of 1987 and 1988. Japanese and UK governments donate the Component No.2, for Northwest Region. The author attended the preliminary study in 1990 as a geomorphologist and found some problems on flood mitigation projects related with characteristics of landform and flood feature.
The northwest region is divided into the following four geomorphological units: 1) Barind Tract, 2) the Himalayan piedmont plains, 3) the Alluvial lowland along the Brahmaputra-Jamuna and 4) the Alluvial lowland along the Ganges. The embankments are frequently broken by floods because river beds are unstable in the Himalayan piedmont plains. In the alluvial lowland along the B.-Jamuna, the river channels are unstable and change their courses frequently during annual floods with breaching embankments, since the B.-Jamuna changed its course in the 18th century. In the alluvial lowland along the Ganges, distinct large natural levees have developed along the Ganges and a broad back-marsh area lies along the lower Atrai Basin. The “Polder Project” for flood protection is progressing in the lower Atrai Basin. However, each polder is too broad to protect the whole area. In order to protect their houses, people often cut embankments for draining inundation water not only from inside of the polder to river, but also from outside of the polder into the protected area.
In the Flood Action Plan, the World Bank proposed two major projects in this area, namely, an interceptor drain from the Upper Atrai into the B.-Jamuna, and a diversion channel from the Lower Atrai into the Ganges. The main technical problem on both the Interceptor and the Diversion channel is the water level at the outfalls. The Interceptor is proposed along the Karatoya-Atrai, a heavy free-meandered river. The Interceptor needs high embankments whole from the Atrai to the B.-Jamuna. The Diversion channel is less feasible ; since the river bed of the Atrai is lower than that of the Ganges.
Apart from these major projects, the author suggests smaller but reasonable projects for this area. In the southern part, the Polder Project is to be improved: smaller units of polders with consideration of geomorphological setting and local land-use conditions. A comprehensive project which contains both structural and non-structural methods for flood control and rural development programs as waterside district will support the development of this area.

抄録全体を表示

バングラデシュではジャに架かる多目的橋建設のための護岸工事の際に斜面崩壊が頻発した。川砂には雲母片が含まれており、これが変形・強度特性に影響を与えることが判明した。そこでこの川砂を用いて、平面ひずみ圧縮試験を行った。モールド開口面を変え、空中落下法で試料を詰めることで現場での圧縮、伸張状態を再現し、拘束圧400kPaで異方圧密後、非排水せん断を行った。緩詰試料では圧縮・伸張ともに非排水せん断直後にピーク強度に達した。ピーク後の延性・脆性特性は再現試験で統一性は見られなかった。緩詰試料圧密時の体積ひずみは圧縮試料で約5%、伸張方向で8%と違いが見られた。伸張方向載荷では体積収縮性が特に高いことがわかった。

抄録全体を表示

　本研究は国内外の河川において，aDcpを用いて測定されたデータ及び岸・黒木が提案したτ*-τ*’関係に着目して流水抵抗の分析を行う．初めに，姫川/山本地点及び千代田実験水路における観測データは既存のτ*-τ*’関係と良く整合した．また，信濃川/臼井橋地点では概ね整合するものの，τ*-τ*’関係の傾きが若干異なり，これは当該地点の相対水深が既存の水路実験のものより大きいことに起因すると推察された．一方，ジャ及びメコン川下流域では既存のτ*-τ*’関係とは整合せず，これは当該地点の相対水深及び浮遊砂濃度が非常に高いためと考えられる．そこで，ジャ及びメコン川下流域における抵抗予測を実施するために，観測データを用いて新たにτ*-τ*’関係を導出した結果，相対水深をτ*-τ*’関係の説明変数として導入することで粗度を良好に予測できた．

抄録全体を表示

洪水災害を軽減したり，適切な河道管理を行っていくにあたっては，河川地形やその影響を十分理解しておく必要がある。また，河川管理施設の調査・計画・設計・施工の際にも，河川地形について考慮しなければならない。しかしながら，マニュアルに従って検討するだけで，十分地形のことや現地の状況が配慮されていない事例が見られる。また，最近川そのものについて検討・考察している研究が少ないという残念な状況もある。そこで，河川地形について，マクロの視点，ミクロの視点，土砂特性，河床変動特性から見て，気を付けなければならない点について列挙した。これらの事柄は机上だけではなく，現地へ行って確認して初めて，生かされるものであるので，現場で又は洪水時に川を見る時に，是非注目して頂きたい。なお，河川地形は内容が多岐にわたるため，3回に分けて報告を行う予定である。

抄録全体を表示

バングラデシュのほぼ中央部，ジャの支流であるロハジョン川沿いに位置するタンガイル県，ドッキンチャムリア村（Dakshin Chamuria）における土地開発過程の検討を行った． 　現地点での結論を以下にまとめる．ジャ中流域では，1.約12～11千年前に形成された洪水氾濫堆積物（自然堤防状の微高地）を利用するかたちで，それ以降に，生産域と居住域の開発が行われた．その後も2.幾度かの洪水に見舞われながらも，盛土の主体部の維持管理はマティ・カタによって、恒常的に実施されてきた．さらに，少なくとも3.約1.3千年前までには当該地域において，生産域としての水田開発に連動する形で近隣地域に屋敷地が形成されたものと推定した．

抄録全体を表示

Bangladesh faces the Bay of Bengal and its most territory comprises the Bengal Lowland. The Bengal Lowland is located in the lowest reaches of the Ganges and Brahmaputra-Jamna rivers. The Bengal Lowland is subdivided into Ganges Delta, Brahmaputra-Jamna river flood plain, and Sylhet Basin (Umitsu 1987). The land forms of the lowland consist of Pleistocene uplands and Recent alluvial lowlands. The recent sediments in the lowland are subdivided into five members by Umitsu (1987); namely lowest, lower, middle, upper, and uppermost members. The lowest member in the Bengal Lowland consists of gravel bed with the thickness of about 10 m. The lower member consists of the sandy sediments with scattered gravels. The sediments of the middle member are fine in their lower horizon and become coarser in the middle and upper horizons. There is an unconformity between the lower member and the middle member. The sediments of the upper member are fine in the lower and upper horizons and coarse in the middle horizon. The uppermost member is characterized by the fine sediments of silt or clay with peat. The author estimates that the lowest and lower members of the recent sediments form a main aquifer in the Ganges Delta. In Khluna district, the groundwater in the main aquifer may be under confined conditions. The groundwater monitoring data of the main aquifer at Jessore show that the groundwater levels tend to have been declined since 1990. The changes in groundwater level are characterized by sharp decline in dry season. This may indicate that the ground-water pumpage in dry season has been increased as well as the groundwater recharge from rain and river through the confining layers has been decreased.

抄録全体を表示

The Taj Mahal is surely one of the most beautiful pieces of architecture in the world, but we should not overlook the garden that was once beautifully created by Shah Jahan for the memory of his beloved queen Mumtaz Mahal.
Compared with the original plan, the garden has changed through the ages, but if we observe the garden carefully, we can still recognize the original spacial design of the garden. The marble tank which is placed in the center of the garden especially breaks the simplicity of the large plain garden.

抄録全体を表示

 We analyzed changes in the relationship between geo-environment and agricultural land development around the central Jamuna (Brahmaputra) River, central Bangladesh. Based on sedimentary facies analysis and AMS radiocarbon dating, some conclusions are as follows: (1) Natural levees along the Jamuna River formed until ca. 12 to 11 k cal yrs BP; (2) Homesteads (bari-bhiti) were repaired by public works (mati-kata) following some flood events; (3) Paddy fields and homesteads were created since ca. 1.3 k cal yrs BP around central Bangladesh (central Bengal Delta).

抄録全体を表示

2007年7月にBangladesh, Sirajganj DistrictのKhokshabari付近で発生した大規模な河岸浸食について報告をする。この河岸浸食は、Jamuna川の歴史的な西への遷移の一部をなすものとも考えられる。一方、いくつかの人口建造物の影響も調査する必要がある。2008年以降Jamuna川の流路の一部がIchamoti川と合流することに対する懸念が高まっている。

抄録全体を表示

A fundamental geodetic survey over Bangladesh was carried out by a Japanese Survey team headed by Dr. Tajima with Survey of Bangladesh (SOB) from 1992-1995. Levelling survey and tidal observations were carried out to determine the height of the 461 bench marks along the 2, 386 km leveling route. The mean sea level of the Bay of Bengal was determined through 2 years of tidal observations by a tidal gauge installed at the mouth of the Karunafuri River in Chittagong City supported with observations from an auxiliary pressure type tidal gauge installed offshore of Chittagong. The levelling survey was eecuted with bar-xcode levels on the general surveying route. However, river crossing levelling was carried out by the tilting screw method and reciprocal levelling, depending on the length of sight. Two sites of river crossing levelling done by the tilting screw method were compared with the results by levelling over the bridge constructed afterward and the results coincided very well. Adjustments after the entire all survey revealed that the levelling observation was done very accurately, of which standard deviation for the unit weight (1 km) is as small as 0.9 mm. The height of vertical datum is determined as 6.4292 m from the results of these tidal observations and levelling surveys.

抄録全体を表示