Guide bank and launching apron, 5000 m3/s

Data and design basis

• The average size of the river sand is taken as 0.25 mm (0.25 m would not be sand).
• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.25} = 0.88$
• Flood depth above bed: $h = 254 - 250 = 4.00$ m
• Design discharge for scour/apron: $Q = 5000$ m³/s

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:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{5000}{0.88}\right)^{1/3} = 8.39\ \text{m}$$

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 $2.25t$).

Summary

Sketches

Plan of guide bunds:

 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/

Section at the shank (straight portion):

           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)