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Launching apron, 8000 m3/s, HFL 128.5 m

Short answer

f=1.76, t=0.06×8000^(1/3)=1.20 m, R=7.79 m, D'=1.5R−3.5=8.18 m. Straight-apron length=12.27 m, launched thickness=1.80 m (laid √5 t=2.68 m).

Determine the length and thickness of launching apron for the straight portion of a guide bund in a river. Design flood = 8,000 m³/s; average diameter of river bed material = 1 mm; river bed level = 125.00; highest flood level = 128.50.

Data and design basis

  • Silt factor (Lacey): f=1.76dmm=1.761.0=1.76f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{1.0} = 1.76
  • Flood depth above bed: h=128.5−125.0=3.50h = 128.5 - 125.0 = 3.50 m
  • Design discharge for scour/apron: Q=8000Q = 8000 m³/s

1. Pitching thickness

t=0.06 Q1/3=0.06×80001/3=0.06×20.00=1.20 mt = 0.06\,Q^{1/3} = 0.06\times 8000^{1/3} = 0.06\times 20.00 = 1.20\ \text{m}

2. Scour depth and launching apron

Lacey's normal scour depth:

R=0.47(Qf)1/3=0.47(80001.76)1/3=7.79 mR = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{8000}{1.76}\right)^{1/3} = 7.79\ \text{m}

Maximum scour below HFL: 2R2R at the upstream nose, 1.5R1.5R at the shank (straight portion). The apron must launch to the slope 2H:1V2H:1V, so its horizontal width is 1.5D′1.5D' and, when launched, it covers a slope length 5 D′\sqrt5\,D' with thickness 1.5t1.5t.

PortionScour factorScour depth below HFLDepth below bed D′D'Apron length 1.5D′1.5D'
Shank (straight)1.5R11.68 m8.18 m12.27 m

Thickness: pitching t=1.20t = 1.20 m, so the launched apron thickness is 1.5t=1.5×1.20=1.801.5t = 1.5\times1.20 = 1.80 m. Equating volumes per metre, the thickness as laid over 1.5D′1.5D' width is T=1.5t×5D′1.5D′=5 t=2.68T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.68 m (about 2.25t2.25t).

Answer (straight portion): length of launching apron =1.5D′=12.27=1.5D' = 12.27 m, thickness =1.5t=1.80=1.5t = 1.80 m launched (about 2.68 m as laid).

✓ Answer checked by Claude Opus 5.5 (high effort)

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