Directly after the independence of Bangladesh, the government of Japan was requested to research the possibility of constructing a bridge across the Brahmaputra-Jamuna River.
Constructing a bridge across the river, the most important thing to consider is the shifting of the river course and change of the banklines of the river. The shift from the Old Brahmaputra River to the present Brahmaputra-Jamuna River occurred 180 years ago. It is the fear of technical officials who govern the river in Bangladesh that the river may shift after construction of the bridge.
The government of Japan was requested to fix the whole river course of the BrahmaputraJamuna River. This was, however, impossible for financial reasons because the river is both too long, and too wide. Bahadrabad, Gabargaon, Sirajganj and Nagarbari (Aricha) were selected as proposed bridge sites. The author was requested to select the most stable and suitable site among the four for the bridge based on applied geomorphological research.
The main causes of the shifting of the river are remarkable crustal movement, deposition of sand, variation of discharge, and a bank completely consisting of sand. To make sure of the stability of the river at the four proposed bridge sites, the author investigated (1) the change of banklines and the location of thalwegs and prepared (2) a geomorphological map of the R. Brahmaputra-Jamuna Basin (1: 50, 000) and (3) a geomorphological map of the R. Brahmaputra-Jamuna and R. Ganges Plain (1: 1, 000, 000).
(1) The author researched the change of banklines and the location of the thalweg from 1830 to the present. At the Bahadrabad site, due to the rising of the river bed, the expanding of river width and braiding of river channels are continuing.
Nagarbari (Aricha) is located at the confluence between the Ganges and the Brahmaputra-Jamuna. The peak of the flood discharge of the Ganges generally occurs about one month later than that of the Brahmaputra-Jamuna. Due to the back-water, the bank erosion, change of river, and location of the thalweg are the largest among the the four proposed bridge sites. The change is smaller at the Gabargaon and Sirajganj sites.
(2) The geomorphological map of the Brahmaputra-Jamuna River has been prepared by utilizing the mozaic of the aerial photographs which were taken in 1974 by the Japanese Reconnaissance Team as a base map. The author has prepared the map by utilizing the photographs and conducting field observation by boat and jeep (Fig. 5).
The rivers from the border with India to Sirajganj, with a straight or meandering flow, are similar to rivers in natural levee regions. There are natural levees, back-swamp, and abandoned river courses in the alluvial fan around the Bahadrabad and Gabargaon. The author estimated that the natural levees and back-swamps were originally formed by the R. Tista, Jamuna, and Jhinai and that the alluvial fan by the Brahmaputra-Jamuna River had coverd partly the natural levees, back-swamps, and former river courses after the shifting of the Brahmaputra-Jamuna River. In the future, the area of the natural levee, back-swamp, and abandoned river courses in the fan will be buried by the future shifting of the river course.
Sirajganj and Balktia (which is located across the river from Sirajganj) are situated on the old alluvial plain which was formed by the Tista, Jamuna, Jhinai, etc. more than 180 years ago. The old alluvial plain is separated from the present alluvial plain by the cliff, whose relative height is about 3 m. The narrowest point of the new plain between the old plains is 7 km in width.

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In order to examine the relationship between the landslides and the subsurface structure of the slope, the cone penetration test were carried out on the weathered granite hillslope in Aichi Prefecture, where a heavy rainfall caused many landslides in 1972. The results are shown in Fig. 2, where N10 represents the number of the impacts necessary to drive a cone resister at each incremental depth of 10cm. Broken lines and chained lines in Fig. 2 show the depths where N₁₀ exceeds 10 and 50, respectively.
At the middle and the lower segments of the slopes, where landslides frequently occur, the soft layer (N₁₀<10) is much thicker and the transient zone (10_??_N₁₀<50) is extremely thin. So the subsurface structure can be regarded as the two-layered, which means that the bedrock underlies the soft layer without the transient zone. when the landslide occurs, the soft layer slides down, and the bedrock is exposed. The net rate of wasting _??_ is represented by
_??_
'
where _??__l is the average wasting rate by landslides, and the wasting rate by slow and steady process of erosion (soil creep, rain wash, etc.). Each of the wasting rates is defined in the normal direction of the slope.
It seems that the landslide can not occur until the thickness L of the soft layer reaches the critical thickness L_cr.
In the case of the straight slope with slope angle θ, L_cr is approximated by
_??_ (2)'
where C is the cohesion, φ the angle of internal friction, _γs the saturated unit weight, and _γb the submerged unit weight of the soil.
Because the heavy rainfall which can induce the landslides is frequent in Japan, it can be said that the landslide will occur immediately after L becomes equal to L_cr. Therefore, the return period T of the landslides nearly equals to the time for the soil thickness to grow up from zero to L_cr. Thus, Eq. (1)' becomes
_??_ (3)
The time change in the thickness L of the soft layer is given by
_??_ (4)
where υ_w is the descending velocity of the weathering front.
Rearranging Eq. (4) and integrating it with the conditions L=0 at t=0 and L=L_cr at t=T, we obtain
_??_ (5)
Under the assumptions that υ_w is expressed by
_??_
with A and L₁ being constants and that _??_ is constant, we can calculate _??_ from Eq. (5). Therefore, Eq. (1)' becomes
_??_ (8)
Denoting L_cr/L_i as a and _??_/A as β, Eq. (8) is rewritten as
_??_ (8)'
The relationship between β and V/A is shown in Fig. 7. The line R-S in this figure represents the condition that the landslide can not occur, for L is always less than L_cr. The interval P-R represents the condition that the landslides take place on the slope. It is notable at this range that the increase in β brings about less preferable conditions for the occurrence of the landslide and leads to smaller value of V/A. The decrease of _??_/A is remarkable near the point R.
When _??_ is equal to zero, we can obtain the relative rate of slope retreat _??_ as a function of slope angle from Eqs. (2)' and (8). The relationship between θ and V_r/A is shown in Fig. 8.
If L₁ is large enough, V_r/A decreases with an increase of θ after it reaches a maximum at a certain slope angle.

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The weaving industry in eastern Mikawa began in the latter half of the eighteenth cen tury. It differs from the newly risen weaving regions in size of operation. The size of operation in eastern Mikawa varies, while it is small in the newly risen weaving regions, which depend mainly on family labor. The consideration of the small-scale operation in the former weaving region makes possible to show clear differences from the latter regions. On the other hand, this is useful to analyze the social division of labor in textile manufacturing region.
The greater part of existing small operation of weaving in eastern Mikawa was established in 1950s without governmental assistance, making use of the advantages of the traditional manufacturing region. Generally, weaving operators of small size who established their works after 1950s had previously been employed in weaving works in this region and had acquired the weaving techniques. This experience enabled the small operators of weaving to start with less investment in equipments and to make an increase or exchange of looms without difficulty.
Since the end of 1960s, looms were improved to large scale, and a variety of textiles appeared. In response to such trend, smll-scale production became a dominant form in weaving. During the same period, a shortage of laborer, especially young workers, was becoming serious. The percentages of middleaged and aged workers in the small works are higher than those in the large factories. As a result of such trend, weaving operators were forced to reduce the size of operation, and, therefore, they farmed out such parts of weaving processes as winding, warping, warp-sizing, and drawing-in to specialized operators. This brought the social division of labor to this textile manufacturing region.
In eastern Mikawa, various wholesalers in this weaving region supply yarn to weaving operators and collect textile from them. The trade relation between textile wholesalers and weaving operators is not fixed or exclusive. The trade patterns are spatially complicated.
As mentioned above, the processes of formation and the structure of productive circulation in eastern Mikawa are similar to those in the districts of various small industries in the metropolitan areas, and present a contrast to those in the newly risen rural industrial regions.

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