Abstract
The purpose of this study is to investigate the characteristics of sand bars formed in a model flume in connection with the flow condition. As the results of the experiments, the bed configurations generated in experimental flume are classified into the following four types by their shapes and pattern of flow (Table 4).
(1) Type 1: Bed configuration of this type consists of a set of linguoid bars, and is regarded as a combination of alternating bars in raws. (2) Type 2 (Alternating bars with sharp crest lines): Bed configuration of this type shows sand bars with sharp crest lines which are developed alternately, and the axis of main flow meanders from one side to the other in the flume, This type is characterized by abrupt lateral shift of the talweg, that is the longitudinal line connecting the deepest point in the cross section. (3) Type 3 (Alternating bars with obscure crest lines): This type consists of alternate rises which are generated adjacent to the wall of the flume. The talweg meanders continuously from one side to the other, and the meandering tendency of flow is weak. (4) Type 4: Sand bars are not created in this type and the bed consists of ripples and dunes which stand in line parallel to the flow, and meandering of flow is not observed in this type.
Each of the above classification on the bed configuration corresponds to the magnitude of three-dimensional hydraulic characteristics.
Then, hydraulic characteristics concerning the formation of each type were examined. It was found that the bars are formed under the condition of steep slope and shallow water depth. Under the condition of constant slope, three-dimensional characteristics of the bed configuration becomes pronounced as the ratio of width to depth (W/D) of the flow increases.
Satisfactory method for classifying the above types has not been developed. Therefore, dimensional analysis basing on the similarity-law of distorted models was applied, and two parameters, U*/U*c and S•W/D, were obtained. Here U* is shearing velocity at bed, U*c is critical shearing velocity for bed material, and S, W and D denote slope, width and depth of flow respectively.
The relation between. U*/U*c and S•W/D are plotted in Fig. 15. As seen from the figure, it was found that the following relations were applicable to each type of bed configuration.
Type 1. U*/U*c _??_ 1.4 (S•W/D) 1/3 (_??_ in Fig. 15)
Type 2. 3.5(S•W/D)1/3 _??_ U*/U*c>1.4(S•W/D)1/3 (_??_in Fig. 15)
Type 3. 10 (S•W/D)1/3_??_U*/U*c>3.5(S•W/D)1/3 (_??_in Fig. 15)
Type 4. U*/U*c>10 (S•W/D)1/3 (_??_in Fig. 15)
These parameters will be useful for the classification of bed configuration not only in the model flume but in actual river channel.
