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Assumptions, units & notation

Units (baseline)

  • Discharge: \(Q\) in \(m^3/s\) (cumecs)
  • Length: \(m\) unless stated
  • Area:
    • user inputs often in hectares (ha)
    • internal conversions may use \(km^2\)
  • Time: minutes for hydrology time bases
  • Slope: dimensionless (\(m/m\))

Common symbols

  • \(A\): catchment area
  • \(I\): rainfall intensity from IDF, matched to duration (typically \(T_c\))
  • \(T_c\): time of concentration (min)
  • \(CN\): SCS curve number
  • \(S\): potential maximum retention (mm)
  • \(P\): rainfall depth (mm)
  • \(P_e\): excess rainfall (mm)
  • \(n\): Manning roughness coefficient
  • \(R\): hydraulic radius
  • \(S_0\): culvert bed slope proxy (dimensionless)

Even if the legacy implementation allowed wide input ranges, engineering-safe validation typically includes:

  • non-negative \(A, L, Q\)
  • \(0 < CN \le 100\)
  • \(0 \le C \le 1\) for Rational method
  • physically plausible slopes and roughness values by material

Reproducibility

For auditability and repeatable results, record:

  • all user inputs
  • selected method branches (Rational vs SCS, Tc method, ARF application)
  • fitted IDF coefficients and fit quality metrics
  • solver step sizes / iteration limits for any iterative depths