Flood Hydrology

Catchment area (A, km²)

Runoff coefficient (C, –)

0–1

Rainfall intensity (i, mm/h)

from IDF

Formula (SI): Q = 0.278·C·i·A → Q in m³/s, i in mm/h, A in km².

Catchment area (A, km²)

Curve Number (CN, –)

30–100

Storm depth (P, mm)

Time of concentration (T_c, h)

user estimate / Kirpich etc.

Storm duration (D, h)

hyetograph block

\(S = 25400/CN - 254\) (mm), \(I_a = 0.2S\). If \(P>I_a\), \(P_e = (P-I_a)^2/(P+0.8S)\). Lag time \(t_{lag} = 0.6T_c\), \(T_p = D/2 + t_{lag}\). Peak \(Q_p = 0.208 A P_e / T_p\).

Annual peak flows (m³/s, comma/space/line separated)

Return period (T, years)

e.g. 2, 10, 50, 100

Gumbel: \(y_T=-\ln[-\ln(1-1/T)]\), \(K\approx (y_T-0.5772)/1.2825\), \(Q_T=\bar{Q}+K s\).

Calculator

Output

Formulas & derivation

Formulas (step-by-step)