Guide bund purposes and design components

A guide bund is a pair of embankments, built along both banks upstream (and a short length downstream) of a bridge or barrage, which guide the river through the opening without allowing it to outflank the structure.

Purposes

1. Confine and guide the river axially through the waterway.
2. Prevent the river from attacking the approach embankments and abutments (flanking).
3. Prevent the formation of eddies and cross-currents near the structure.
4. Keep the scour holes and deposits away from the structure.
5. Maintain a smooth, symmetrical entry of flow, so the afflux is minimum.

Design criteria (Lacey and Garg)

Plan

• Waterway $W = 4.75\sqrt{Q}$.
• Length of upstream shank: $1.0$ to $1.5\,W$; downstream tail: $0.2$ to $0.5\,W$.
• Upstream curved head: radius about $0.5W$ and sweep $120^\circ$ to $145^\circ$; downstream curved head: smaller radius, sweep about $45^\circ$.

 U/S                                 D/S
   ___________________________  |  ____
  /  shank 1.0-1.5 W           \ | /    \ tail
  \____________________________/=|=\____/ 0.2-0.5 W
  curved head                     axis
        <------ waterway W ------>

Cross-section at the shank

• Top width 6 m (3 to 6 m) and top level $=$ HFL $+$ 1.5 m free-board.
• Side slopes 2:1 on the river side (pitched) and 2:1 on the country side.
• Pitching thickness $t = 0.06\,Q^{1/3}$ on a filter 0.15 to 0.30 m thick.

Launching apron

• Scour depth $R = 0.47(Q/f)^{1/3}$; maximum scour below HFL is $2R$ at the nose and $1.5R$ at the shank.
• Depth of apron below bed $D' =$ (scour below HFL) $-$ (flood depth).
• Width $= 1.5D'$ and thickness $= 1.5t$ when launched on a 2:1 slope (laid thickness about $2.25t$).

 Section at shank                 Section at nose
      ____6 m___                      ___6 m___
     /          \ HFL+1.5            /         \
 HFL/____________\                 HFL/___________\
   / pitching t   \                  /             \
 _/_______apron____\_             _/____apron 2R_____\_

At the nose the scour is deeper (2R), so the pitching continues around the head and the apron is wider and thicker; sections are flatter (3:1) at the head.