Bell's guide bund plan, sections, design steps

A Bell's bund is a guide bund (guide bank) on the pattern recommended by Bell, in which a pair of embankments, with curved heads upstream and short tails downstream, is laid symmetrically about the axis of a bridge or barrage so that the river approaches the waterway axially. The waterway is based on Lacey's regime width.

Plan and sections (3 marks)

 U/S                                       D/S
   ______________________________   |   ____
  /   upstream shank 1.0-1.5 W   \  |  /    \ tail
 (  curved head, R = 0.5W,        )=|=(      ) 0.25 W
  \ sweep 120 deg                 /  |  \____/
   \_____________________________/   |
         waterway W (Lacey)        axis of bridge

 Section at shank              Section at nose
     __6 m__                      __6 m__
    /       \  HFL+1.5           /        \
 HFL/_________\ 2:1          HFL/__________\ 3:1
  / pitching t \                / pitching t \
 /___ apron ____\ (1.5R)    /____ apron (2R) ___\

Design steps (5 marks)

1. Design discharge $Q$, HFL, bed level and median grain size $d$.
2. Silt factor $f = 1.76\sqrt{d_{mm}}$, and waterway $W = 4.75\sqrt{Q}$.
3. Length of upstream shank $1.0$ to $1.5W$, downstream tail $0.2$ to $0.5W$.
4. Radius of upstream curved head about $0.5W$ with sweep $120$ to $145^\circ$; tail with a smaller radius, sweep about $45^\circ$.
5. Top level $=$ HFL $+ 1.5$ m, top width 6 m, side slopes 2:1.
6. Pitching thickness $t = 0.06Q^{1/3}$.
7. Scour depth $R = 0.47(Q/f)^{1/3}$; max scour below HFL: $2R$ at nose, $1.5R$ at shank.
8. Launching apron: depth below bed $D' =$ scour depth $-$ flood depth, width $1.5D'$, launched thickness $1.5t$.
9. Draw plan and sections.