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DRAFT Stormwater Pipe Flow Calculator

Not to be used for real-world calculations. This is a learning tool.

Pit 1 Inflow

Pipe 1 (Pit 1 -> Pit 2)



Pit 2 Inflow

Pipe 2 (Pit 2 -> Pit 3)



Pit 3 Inflow

Pipe 3 (Pit 3 -> Outfall)



Pipe Flow in Pipe (L/s) Flow in Pipe (m³/s) Full-Flow Capacity (L/s) Friction Loss (m) HGL at Upstream Pit (m) Pipe Crown Elev @ Upstream Pit (m) Status
Pipe 3 (Pit 3 -> Outfall)
Pipe 2 (Pit 2 -> Pit 3)
Pipe 1 (Pit 1 -> Pit 2)

HGL at Pit 1: - m

HGL at Pit 2: - m

HGL at Pit 3: - m

Calculation Logic Summary

  1. Pipe flows:
    • Pipe 1 carries \(Q_1\)
    • Pipe 2 carries \(Q_1 + Q_2\)
    • Pipe 3 carries \(Q_1 + Q_2 + Q_3\)
  2. Full-flow capacity (Manning’s): \[ Q_{\text{full}} = \frac{1}{n} \cdot A \cdot R^{2/3} \cdot S^{1/2} \] where \(A = \pi (D/2)^2\) and \(R = \frac{D}{4}\) for a full circular pipe, \(S\) is slope in decimal form, and \(n\) is a roughness (assumed 0.013 here).
  3. Friction loss each pipe: \[ h_f = \frac{(Q \,n)^2 \; L}{A^2 \; R^{4/3}} \]
  4. HGL Step (Downstream \u2192 Upstream):
    • Outfall HGL = 0 (reference)
    • Pit 3 HGL = Outfall + pipe 3 friction loss
    • Pit 2 HGL = Pit 3 HGL + pipe 2 friction loss
    • Pit 1 HGL = Pit 2 HGL + pipe 1 friction loss
  5. Invert & Crown Elevations:
    • Invert at Pit 3 = 0 (just an example reference)
    • Invert at Pit 2 = Pit 3 invert + \(L_3 \times S_3\)
    • Invert at Pit 1 = Pit 2 invert + \(L_2 \times S_2\)
    • Crown = invert + pipe diameter (in meters)
  6. Undersized Check:
    • If \(Q\) > \(Q_{\text{full}}\), pipe is surcharged (undersized).
    • If HGL at upstream pit > (invert + diameter), HGL is above the crown (undersized).