Kunitoshi Watanabe

ANALYTICAL STUDY OF THE TOPORAPHICAL CHANGES OF SAND-BAR AND THE DESTRUCTION OF HERBACEOUS VEGETATION DUE TO FLOODING IN THE KITA RIVER

This paper describes the topographical changes of sand-bar and the destruction of herbaceous vegetation on the sand-bar in the Kita River. The sand-bar in the Kawasaka district was excavated with vegetation in 2001 through the special emergency project after the sever disaster due to flooding by Typhoon No.19 in 1997. A flood on Oct. 2004 destroyed again almost all vegetation in a recovery process on the sand-bar. Nowadays, herbaceous vegetation is restored over all the sand-bar because of no flooding since 2008. In the paper, the destruction of herbaceous vegetation by the topographical changes of sand-bar was simulated through the 2-D analysis of the riverbed evolution. It was considered from the simulation that almost all vegetation will be destroyed by the flood over the maximum discharge of 4,000m/s. Moreover, it was found that vegetation will not be destroyed but grow thick by the flood under the maximum discharge of 2,000m/s.

Effects of Relative Groine Height in Bed Configurations and Flow Structure in an Open Channel Flow

A lot of non-overflow groynes that do not sink under the water at the ordinary water flow are used, and it is known to it does working that keeps away the water current form the river bank..On the other hand, when the water level is surpassing of the height of the groyne, it becomes to sink.. In this study, the height of the groyne was made constant, depth was changed systematically, and it aimed to clarify the characteristic of the bed level variation and the flow velocity profile of the flowfield with a single groyne by the laboratory experiment under the condition to assume the Froude number to be 0.2. As a result, the maximum depth of scour has become small as the relative groyne higher. The groyne height when relative height grows, the ratio of the main flow velocity to the mean flow velocity has grown.

RESEARCH ON THE CHANGE OF TOPOGRAPHICAL FEATURES AND VEGETATED REGION ON FLOODPLAIN IN KITA RIVER

This paper describes the relation among the flood, topographical features and the vegetated region on the floodplain at Honmura in the Kita River. The past changes in topographical features and the vegetated region were measured from the aerial photographs. The grain sizes of the gravels were investigated at the several points on the floodplain. As a result, the followings have been understood. 1) Gravels piled up due to the flood in 1997, fine sand mixing clay piled up during 1983-1997 and gravels piled up due to the flood in 1982 are accumulated in the vicinity of the 11.7km section. 2) Gravels on the bare ground of floodplain moved for the flood of 2,000m3/s in discharge. However, the large scale movement of bed materials did not occur for the flood of 3,500m3/s in discharge if the floodplain was covered by the thick vegetation. 3) The forest zone in the river bend had the effect that the topographical feature of floodplain was stabilized. 4) The state that the herbaceous vegetation covered the whole area of the floodplain, recovered from the state of the bare ground when the condition of 1,600m3/s or less in discharge continued for two and half years. 5) The state with thick vegetation recovered when the condition of 1,800m3/s or less in discharge continued for five years.

INTEGRATED WATER MANAGEMENT SYSTEM IN SAGA LOWLAND

The hydrological problem of Saga area is governed by the fact that almost the area is below the Ariake sea level with the highest tide in Japan. Another problem is difficulty of supplying huge quantity of water for agriculture because of less water resources in Saga area. Therefore, open channel network system, Saga creeks, have been developed in this area. In this paper, an old system of water management on Saga creeks is described. The water management was a kind of integrated system by making use of function of Saga creeks such as drainage, storage. On the other hand, the main objective of modern and general water management is to protect flood damage caused by main river. The paper describes that, according to the modern concept, there has been many problems on drainage system in lowland especially in Saga area. Also, it is described that many factors to be cleared such as the drainage characteristic, time constant of design rainfall and the storage effect of channel network are examined. Finally, it is pointed out that the integrated water management system should be developed especially in lowland such as Saga area.