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Seepage Flow Rate from a Flow Net

Hardseepageflow net
Seepage beneath a sheet pile is analysed with a flow net having 4 flow channels and 12 equipotential drops. The hydraulic conductivity is 2 × 10⁻⁵ m/s and the total head loss is 6 m. The seepage per metre of wall is most nearly:
  1. A1.3 × 10⁻⁵ m³/s
  2. B4.0 × 10⁻⁵ m³/s
  3. C1.2 × 10⁻⁴ m³/s
  4. D3.6 × 10⁻⁴ m³/s
Show solution

Answer: B. 4.0 × 10⁻⁵ m³/s

Flow-net seepage is q = kH(Nf/Nd) = (2 × 10⁻⁵)(6)(4/12) = 4.0 × 10⁻⁵ m³/s per metre of wall. Inverting the ratio to Nd/Nf overstates the flow by nine times; the number of drops always exceeds the number of channels in a correctly drawn net.
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