BED FORMS IN THE YORO RIVER AT AZU, CHIBA PREFECTURE

It is well known that among bed forms on the sandy river-bed dunes play important roles in fluvial processes relating to channel geometry, hydraulic variables, . and sediment trans-port. Though many experimental studies concerning bed forms have been carried on, they are not sufficient enough to make clear interrelations between the shape of dunes, flow phe-nomena, and channel characters in a natural river. The purpose of this study is to present the arrangement of the shapes of duns for various stages and the change of dunes with the stream discharge in a natural river, and to discuss these processes in connection with hy-draulic conditions. Observations of bed forms and flow characteristics were made in the Yoro River at Azu, Chiba . Prefecture (Fig. 1), for the various river stages occurred during September 20 to Octo-ber 18, 1970. Fig. 2 shows a sketch plan and cross-sectional profiles in the study reach having 270m in length. The dimensions and shapes of dunes were investigated by an echo-sounder in the observation reach of a 50-meter length from sections D to I. Measurements of flow char-acteristics were carried out either at the narrowest section G or at the widest one I in the observation reach. These data are summarized in Table 1, and plotted in Figs. 4 and 5 in order to represent the relations of hydraulic variables to discharge. Based on the indices such as the number of dunes per 10-meter length, dune height, and dune steepness, which is a proportion of dune height to dune length, the dimensions and shapes of dunes were examined from all available charts of echo-sounding profiles (Photo. 2). As shown in Fig. 7, the dune characteristics varies both transversely and longitudinally in the observation reach. The longitudinal variations of them may be related to the cross-sec-tional form of the reach. That is, in the narrower and deeper reach between sections F and G, the dune height and steepness are larger. And these graphs are assumed to describe the arrangement of dune characteristics in response to hydraulic condition at a given discharge. The change of dunes according to the stage of flow was also investigated during the runoff event which occurred on 30 September 1970 (Table 2). At the falling stage, the values of dune characteristics as a whole were affected by the generation of new dunes on the resi-dual dunes. That is, the mean height of dunes became smaller, but the mean steepness of dunes became larger. While at the rising stage, considering from the relation shown in Fig. 6, the shape of dunes may become flatter because of disappearance of small dunes. In order to discuss the observed dune characteristics hydraulically, it seems necessary to make clear relationships between the shape of dune and hydraulic variables. Since bed forms are an element of roughness, it may be assumed that the resistance to flow in alluvial chan-nels depends on the shape of dune, especially dune steepness. From Fig. 9 plotted by the data on flow measurements for each dune in Table 3, the following relation is obtained; f=0.136 (h/λ) 0.54 where f is friction coefficient and h/A is dune steepness. The correlation suggests that at a given discharge or flow intensity the difference between the values of friction factor at sections G and I, as illustrated in Fig. 5 D or 8, originates from the variation of . dune steepness occured at these two locations (Fig. 7C), and that with decreasing discharge the increase of friction (Fig. 5D) is also caused by the increase of dune steepness. Fig. 10 plotted by the data on flume experiments and natural rivers shows that the similar rough relation exists independently of grain size.