Tadashi Utami, Masaharu Awae, Manabu Sawada
Faculty of Systems Engineering, Wakayama University
Ryosaku Kinoshita
Jiyuu-Gakuen, formerly
Address for correspondence;
Tadashi UTAMI, Faculty of Systems Engineering, Wakayama University
930 Sakaedani, Wakayama 640-8510, Japan
Phone:
+81-73-457-8370, Fax: +81-73-457-8371
Aerial photographs of 1998 flood flow in the
Tone River was analyzed by the cross- correlation method to obtain velocity
vectors distribution over the whole flow surface. The structure of large-scale
vortices observed in the photographs were made clear by comparing the streamline
patterns, and the map of the vorticity and the divergence calculated from the
obtained data. On the other hand, the topographical map of the river bed shows
characteristic configurations that the contour lines shows wavy patterns. In the
conclusion, it was shown that these river-bed configurations are formed by the
local sediment deposition due to large scale vortices.
Keywords: aerial photograph, flood flow, river-bed topography, large-scale vortex, sediment deposition
Large-scale vortex motions were observed in the aerial photographs of the September 17th flood flow just downstream of the channel bend in the Tone River in 1998. The main channel width is 300-350m, while the diameter of the vortices is about 200-400m. On the other hand, the river-bed topography at that location shows wavy patterns, that seems to be formed by sediment deposition. The turbulence structure of flood flow was analyzed by picture processing of the photographs, and the deposition process was examined.
Aerial photographs were taken of the flood flow surface with the overlap rate of 70%. An example of the photographs is shown in photo.1 in which the analyzed area is shown with rectangle. The cross-correlation method was applied in the overlapped area to obtain velocity vectors over the whole flow surface. Velocity vectors were obtained every 6m in span-wise direction and every 12m in longitudinal direction over the whole flood flow surface. Figure 1 shows the distribution of the obtained velocity. In this figure the main channel is shown by dotted lines. The maximum velocity in the main channel is 2.7m/s, while most of the velocity on the flood plain is below 0.5m/s.
Figure 2 gives streamlines drawn by using the obtained velocity vectors. In this figure, the upper part of Figure 1 is omitted and the location of the main channel is shown by dotted lines. Large-scale vortex motions are observed near the main channel bank. The diameter of these vortices is 200-400m.
The distribution of the vorticity calculated by using the obtained velocity vectors is shown in Figure 3. The high vorticity area is observed near the right-hand bank of the main channel. The locations of the local peak of the vorticity are shown by + mark in figure 1. It is noted that the velocity at the local peak is 0-1.2m/s, and the high-vorticity areas are moving downstream. It is also noted that the location of the local peak of the vorticity do not coincide with the location of the center of large-scale vortices shown in Figure 2.
Figure 4 shows the streamlines that is viewed from the frame moving at the velocity around the local peak of the vorticity in Figure 3. The locations of the vortex motions visualized in this figure coincide with the high-vorticity area in Figure 3, but do not coincide with the locations of the large-scale vortices in Figure 2.
By comparing Figure 4 with Figure 2, it is found that large-scale vortex visualized in Figure 2 is comprised of some middle-scale vortices as visualized in Figure 4. The middle-scale vortices move in the large-scale vortex, while the large-scale vortex is almost stationary.
The distribution of the 2-dimensional divergence du/dx+dv/dy is shown in Figure 5. The high-divergence regions observed around the vortex motions in Figure 5 were plotted on Figure 4, the streamline patterns viewed from the moving frame.
Thus we come to the conceptual map concerning the relationship between divergence and vortex motions as shown in Figure 6; row of vortices appears near the main channel bank. Each vortex has longitudinal rotating structure. A part of the flow in the main channel is taken into these vortices and sink toward the river bed.
Figure 7 shows the map around the analyzed area that is shown by thick rectangle. The main flow direction is from left to right, and the main channel bends to the right about 1km upstream from this map. The high-shear layer generated at the convex bank at the channel bend would flow downstream forming the vortices mentioned above.
The cross-sections of the main channel measured every 500m are shown as shadowed area in this figure. The river-bed configurations interpolated from these cross-sections is shown by contour lines. In this figure, characteristic river-bed configurations are observed; comparatively high river-bed topography appears cyclically near the main channel bank and the contour lines show wavy patterns.
It is also noted that the negative-divergence areas shown in figure 5 appear just upstream side of the convex topography of the main channel bank. As sinking flow occurs in the negative-divergence area, sediment is deposited in this flow area and the convex topography of the main-channel bank is formed.
The high-shear layer generated at the convex bank would flow downstream forming vortex motions. These vortices take into themselves a part of main-channel flow highly concentrated with sediments and the sediments are deposited in the negative divergence area which is attached to the vortex structure. These vorteces move in the large-scale vortices which is almost stationary. Thus local deposition occurs and the wavy river-bed topography is formed near the bank.
Acknowledgement
It is gratefully acknowledged that valuable data and aerial photographs were supplied by Work Office of the Tone River’s Lower Reach, Kanto Regional Construction Bureau, Ministry of Construction, Japan.
Fig. 1 Velocity distributions obtained by picture processing of aerial photographs
Fig. 2 Streamlines
Fig. 3 The vorticity distribution
Fig. 4 Streamlines viewed from the moving frame. Shadowed areas show the high-divergence area.
Fig. 5 The vorticity distribution.
Fig. 6 A conceptual model of a vortex structure.
Fig. 7 Topographical map of the analyzed area.
Photo. 1 An aerial photograph of the september 1998 flood flow in the Tone River.
(Supplied by the Ministry of construction, Japan.)