Contents preface (VII) introduction 1—37

12.5.6. Flush Bunds
Subsidiary channels of a braided river may flow very close to one of the banks of a river and result in bank erosion. Flush bund may be constructed across such channels as close to its offtake as possible. Top of the flush bund should be kept lower than the top levels of the surrounding shoal/islands to prevent outflanking of the bund on both sides when the water level in the main river rises (17).
12.5.7. Secondary Current Generating Structures
In addition to the above-mentioned conventional river training methods, there are other methods which depend on their capability to generate secondary currents to train a river. One such method is bandalling that has been successful in improving navigation conditions of rivers in Assam. Bandals are in the form of vertical mats or screens made of bamboos. These bandals are supported by bamboo poles that are driven into the river bed. The bandals are so erected that their upper edge is above the water surface and the lower edge extends upto about one-third to one-half the flow depth. The bandals are inclined to the stream to divert the surface currents towards the navigation channel. Bandals usually require replacement almost every year.
Surface panels consist of an assembly of deflectors suitably connected to an anchored barge. These panels cause double helical circulation stretching downstream along the river channel to be deepened for navigation purposes. Submerged bottom panels are placed on the river bed near the bank of the channel to be deepened. These panels perform rather poorly and are not as reliable as other panelling methods. Compared to the submerged bottom panels, the submerged vanes seem to be more effective.
The submerged vanes are small river training structures placed on the bed of a curved river channel. In the curved reach of a stream, the streamlines are curved and the centrifugal

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forces (proportional to the square of the local velocity) are exerted on river water. These forces are largest at or near the water surface. Therefore, the water near the water surface is driven towards the outer concave bank of the river and, to satisfy continuity requirements, the water near the river bed moves towards the inner convex bank. Thus, a spiral (or secondary) motion is imparted to the water flowing in a channel band. Since the sediment concentration is the highest near the river bed, the secondary flow near the river bed moves sediment towards the inner bank and deposits some of it near the inner convex bank. The sediment-deficient water coming from upper layers causes erosion of the outer convex bank. Short vertical submerged vanes, placed at suitable intervals in the outer half of the river bend and suitably inclined to the channel axis, are very effective in nullifying the secondary currents responsible for scouring of the outer convex bank of the curved channel (25). Submerged vanes, simple in design as well as construction, do not cause any significant change in either the area of channel cross-sections or the longitudinal slope of the water surface. Therefore, the stream characteristics upstream and downstream of the curved reach remain unaffected due to the submerged vanes.

The vane height above the stream bed is about 0.2 to 0.4 times the bank-full flow depth. The vanes are placed in arrays along the outer bank of the river. Each array may have two or more vanes. The vanes in an array will be spaced laterally at a distance of two to three times the vane height. The stream-wise spacing between the arrays will be about 15 to 30 times the vane height. The vanes should not be farther from the bank by more than four times the vane height.