Gabion guide bund and apron, 3000 m3/s

Data and design basis

• Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.1} = 0.557$
• Flood depth above bed: $h = 245 - 240 = 5.00$ m
• Design discharge for scour/apron: $Q = 3000$ m³/s

1. Waterway and length of guide bund

Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{3000} = 260.17$ m. Adopt $W \approx 260$ m. Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt

$$\begin{aligned} L_{us} &= 1.25W = 1.25\times 260 = 325\ \text{m} \\ L_{ds} &= 0.25W = 65\ \text{m} \end{aligned}$$

Curved heads: upstream radius $R_u \approx 0.5W = 130$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 65$ m with sweep about $45^\circ$. Arc length of upstream head $= R_u\theta = 130\times 2.094 = 272$ m, so total length on the upstream side $\approx 597$ m.

2. Thickness of stone pitching

$$t = 0.06\,Q^{1/3} = 0.06\times 3000^{1/3} = 0.87\ \text{m}$$

Gabion slope protection: use gabion boxes (1.0 m thick) in one layer, so adopt $t_g = 1.0$ m (next standard size above 0.87 m) over a 0.15 m gravel filter.

3. Scour depth and launching apron

Lacey's normal scour depth:

$$R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{3000}{0.557}\right)^{1/3} = 8.24\ \text{m}$$

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 $t = 0.87$ m, so the launched apron thickness is $1.5t = 1.5\times0.87 = 1.30$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 1.93$ m (about $2.25t$).

Summary

Gabion details

• Slope pitching: gabion boxes $2\times1\times1$ m laid in one layer (1.0 m thick) on a 0.15 m geotextile or gravel filter, wire mesh 8x10 cm, galvanised and PVC coated.
• Launching apron: launched thickness $1.5t = 1.30$ m; laid thickness $\sqrt5\,t = 1.93$ m, so adopt 2.0 m (two layers of 1 m gabion boxes or four layers of 0.5 m mattress) over a width $1.5D'$ = 11.04 m (shank) and 17.22 m (nose).
• Gabion boxes are flexible, so they follow the scour hole and launch without breaking.