Several active, defunct or dead distributary channel systems occur on the delta plain of the Mahanadi delta. They are arranged into four successive growth-stages of the delta. The new development of a distributary channel system has tended to occur on the south of the pre-existing channel system. In addition, within a distributary channel system, it is generally observed that the branch to the south becomes more active than the branch to the north. Such tendencies are interpreted as the result of the control of the prevalent wind. The southerly prevalent wind blows river-bed sand and piles it up on the northern bank of the channel. Consequently the northern bank of the channel becomes higher than the southern bank. At the branching point of a channel, the bed of the northern branch is raised up by the southerly wind action. Hence, the water has a tendency to prefer the southern branch. The southern branch thus becomes an active branch, whereas the northern branch gets choked at the bifurcation and becomes defunct with time. As a result of such effects of the wind action, the southern bank of the southernmost branch of the distributary channel system tends to be breached more easily and the new channel system preferably develops on the south of the pre-existing channel system.

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The particle-water fluidization is simulated numerically and a mechanism of formation of water-escape-structure is investigated. The numerical method employed in this study consists of computation of the two phase flow and that of movements of particles. The latter is based on the discrete element method (DEM) where effects of virtual mass and lubrication due to the movement of particles in water are taken into account. It is found that the water-escape structure is not formed in the case of fixed sedimentary layer, whereas it is formed when there is a water layer under the sedimentary layer. These results agree with previous experimental observations.

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The Late Pleistocene “Miwa Formation” of the Shimosa Group and Hinuma terrace gravel distributed in the western Higashi-Ibaraki Terrace, Ibaraki Prefecture, are examined by an integrated analysis of depositional facies, tephra key beds and geomorphology. The lower cycle, cycle 1 of the “Miwa Formation” consists of muddy estuarine deposits filling an incised-valley of the Paleo-Hinumagawa River. The upper cycle, cycle 2 of the “Miwa Formation” forming the upper terrace surface, is characterized by thick and muddy lagoonal deposits in the western part (Oogoyama), and by wave-dominated shoreface-beach sandy deposits in the eastern part (Asahigaoka-danchi). The following Hinuma terrace gravel is mainly composed of fluvial conglomerate, forming the middle terrace surface. These distribution of depositional fades have controlled geomorphic evolution after postdepositional emergence. Shallow depressions developed in the western part have resulted from differential erosion reflecting the muddy estuarine facies deposited in the back-barrier side during the cycle 2. The NE-SW trending shallow valleys and low ridges distributed in the eastern part, seem to reflect the original surface geomorphology of shore-pararell ridges on an eastward-prograded strand plain contemporanously developed outside of barrier system.

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