Design of guide bond and length , width ,thickness of launching apron ,river training works cpt-7

Guide Bank are earthen embankments with stone pitching in the slopes facing water, to guide the river through the barrage, These river training works are provided for rivers flowing in planes, upstream and downstream of the hydraulic structures or bridges built on the river.To protect the face of the guide bank at the river bed level a thick stone cover is laid on the bed. It is called an apron. When the scour undermines the river bed the apron comes down or launches to cover the face of the scour. The site for guide banks should be selected in such a way that there is no side channel flowing parallel to the guide banks. The side channel if present may breach the approach embankment. The guide banks should be so designed that no swirls are produced.

The top width of bank should not be less than 3 m. Side slopes should be 2:1 and free board 1.25 to 1.50 meters. While providing the free board due weightage should be given for heading up of the water and also for settlement of banks (generally 10 per cent of height). The inside slope should be protected with stone pitching and outside slope with good earth.

The waterway is given by Lacey’s regime perimeter formula:

Pw = 4.825 Q1/2

where Pw is waterway in metres and

Q is discharge in cubic metres/sec. The length of upstream part of the guide bank should be 10 per cent more than the length of a bridge or any other structure between the abutments. The length of downstream part of the guide bank should be 1/5 of the structure. (Fig. 14.6).

Dimensions of Guide Banks

The radius of curvature of the upstream curved head should be such as not to cause intense eddies. The radius of downstream curved head may be kept half that of upstream curved head. The heads should be curved well round to the back of the guide bank. Upstream curved head generally subtends an angle from 120° to 145° to the Centre and downstream head from 45° to 90°. The upstream curved head is also called “impregnable head”.

To protect the face of the guide bank at the river bed level a thick stone cover is laid on the bed. It is called an apron. When the scour undermines the river bed the apron comes down or launches to cover the face of the scour. Hence it is called Launching apron also. The quantity of stone in the apron should be adequate to insure complete protection of the scoured face. Figure 14.7 shows the details of a guide bank. After launching, the apron does not remain uniform in thickness.

Details of Guide Bank

Generally apron thickness is kept 1.25 times thickness of the pitching. For rivers in which deep scour is likely to take place thickness of the apron may be increased to 1.5 times.

3. Spurs or Groynes:
They are the structures constructed transverse to the river flow. They extend from the bank into the river.

Groynes serve following purposes:

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a. They protect the