Design steps of a guide bund

The design of a guide bund (guide bank) follows Lacey's regime theory and Garg's recommendations. The steps are listed first and then worked for an example.

Design steps

1. Find the design discharge $Q$, HFL, bed level, bed material size $d$.
2. Silt factor $f = 1.76\sqrt{d_{mm}}$.
3. Waterway $W = 4.75\sqrt{Q}$.
4. Length: upstream shank $= 1.0$ to $1.5W$, downstream $= 0.2$ to $0.5W$.
5. Curved heads: radius and sweep angle.
6. Top level $=$ HFL $+ 1.5$ m (free-board); top width 6 m; side slopes 2:1.
7. Pitching thickness $t = 0.06\,Q^{1/3}$.
8. Scour depth $R = 0.47(Q/f)^{1/3}$; maximum scour $2R$ (nose), $1.5R$ (shank) below HFL.
9. Apron: $D' =$ scour $-$ flood depth; width $1.5D'$; thickness $1.5t$ (launched).
10. Draw plan and sections.

Example

$Q = 5000$ m³/s, bed material $d = 1$ mm, HFL $= 100.0$ m, bed $= 96.0$ m, so the flood depth is $h = 4.0$ m and the silt factor is $f = 1.76\sqrt{1} = 1.76$.

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{5000} = 335.88$ m. Adopt $W \approx 336$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 336 = 420\ \text{m} \\ L_{ds} &= 0.25W = 84\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 168$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 84$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 168\times 2.094 = 352$ m, so total length on the upstream side $\approx 772$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 5000^{1/3} = 0.06\times 17.10 = 1.03\ \text{m}$$

Adopt $t \approx 1.1$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{1.76}\right)^{1/3} = 6.66\ \text{m}$$

Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion). The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

Thickness: pitching $t = 1.03$ m, so the launched apron thickness is $1.5t = 1.5\times1.03 = 1.54$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.29$ m (about $2.25t$).

Result: $W \approx 336$ m, length of upstream bund $= 420$ m, $t = 1.03$ m, shank apron width $= 8.98$ m and nose apron width $= 13.97$ m, launched thickness $= 1.54$ m.

 Plan:  ________ shank ________   bridge
       (  curved head           |=|
        \____________________   |=|
 Section: top 6 m, HFL+1.5, slope 2:1, pitching t, apron