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Overland flow path calculator

Describe the channel or flow path, its grade and surface, and what it carries: by default the major storm, 1% AEP. The calculator solves Manning's equation at both ends of the surface's roughness range, gives the depth, velocity and hazard class, and checks the water stays 300 mm below the floor.

NCC 2025 calls up AS/NZS 3500.3 for stormwater drainage (Volume One F1D3 for Class 2 to 9 buildings, Volume Two H2D2(1)(a) for houses and Class 10 buildings), and this calculator applies its Manning's equation and 300 mm floor check; the hazard class comes from Australian Rainfall and Runoff, for information.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clauses 5.4.2 and 5.4.9, Equation 5.4.9 and Table 5.4.9 (Manning n); Clause 5.2.3 Note (1% AEP overflows)
  • NCC 2025 Volume Two H2P1(2) (1% AEP surface water must not enter a house)
  • Australian Rainfall and Runoff (Version 4.2), Book 6 Chapter 7, Table 6.7.4 (hazard classes)

The design storm

An overland flow path carries the storm the pipes cannot. Overflows at 1% AEP should not endanger people or damage property (Clause 5.2.3 Note), and on a house surface water from a 1% AEP storm must not enter the building (NCC 2025 Volume Two H2P1(2)); so the path is worked for the 1% AEP storm unless you choose a minor one. Add any flow from neighbouring land (Clause 5.4.2).

Manning's equation

Q = (1000/n) A R^(2/3) S^(1/2), in L/s (Equation 5.4.9). The calculator solves it for the depth at which a trapezoidal, triangular or rectangular path carries the flow.

Roughness

Table 5.4.9 gives n from 0.009 for plastics to 0.075 for grassed channels as vegetation grows. Both ends of the range are worked in one pass: the rougher end for the depth, levels and capacity, the smoother for the velocity; the hazard class is the worse of the two.

Keeping water out

The water must be at least 300 mm below the floor or damp course of any building beside it (Clause 5.4.9(e)). Depth, velocity and their product give the Australian Rainfall and Runoff hazard class, for information.

A worked example

The calculator's example inputs, worked through step by step.

Depth of flow

181 mm

68.4 L/s runs 181 mm deep and 1.95 m wide at the rougher end of the range, and up to 0.69 m/s at the smoother, on a 1:100 grade.

  1. Equivalent impervious areaΣCA = 1.0 Ar + 0.9 Ai + Cp Ap = 400.0 + 0.9 × 600.0 + 0.00 × 0.0 = 940.0 m²AS/NZS 3500.3:2025 Clause 5.4.6
  2. Design flow, 1% AEPQ = ΣCA × I / 3600 = 940.0 × 262 / 3600 = 68.4 L/sAS/NZS 3500.3:2025 Eq 5.4.8; Clause 5.2.3 Note
  3. Normal depth, rougher endsolve Q = (1000/n) A R^(2/3) S^(1/2) for depth = n 0.075, S 1/100 = 181 mmAS/NZS 3500.3:2025 Eq 5.4.9; Table 5.4.9
  4. Normal depth, smoother endthe same, at the low end of the range = n 0.025, S 1/100 = 107 mmAS/NZS 3500.3:2025 Eq 5.4.9; Table 5.4.9
  5. Velocity, rougher and smootherv = Q / A = 0.0684 / 0.2218; 0.0684 / 0.0991 = 0.31 m/s; 0.69 m/s
  6. Top width of flow = 1.95 m
  7. Depth × velocityd × v = 0.181 × 0.3084; 0.107 × 0.6906 = 0.074 m²/s

Questions

How do I size an overland flow path?

Work out the 1% AEP flow, the major storm, from the site and any neighbouring land that drains across it (AS/NZS 3500.3:2025 Clauses 5.2.3 and 5.4.2), choose the path's shape and surface, and solve Manning's equation for the depth. Keep the water 300 mm below adjacent floors, and check its depth and velocity for safety.

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