Inputs
weighted
ha
m³/s (pre-development peak)
i = a / (t + b)^n; mm/h, t min
min
min
Modified Rational Method
- For each storm duration td, compute i(td) from IDF: i = a/(td+b)n (mm/h).
- Inflow peak Qi = (1/360)·C·i·A (Q in m³/s, i in mm/h, A in ha).
- Hydrograph: trapezoidal — rises in tc, plateau td − tc, falls in tc. Modified Rational uses simplified triangular: rise td, fall td.
- Required storage S(td) = (Qi − Qa)·td · 60 (m³/s · min · 60 → m³).
- Worst-case td giving maximum S is the controlling design duration.
- This is a screening tool — use proper hydrograph routing for final design.
Results
Controlling Duration td*—
Inflow Peak at td*—
Required Storage V*—
Equivalent depth—
Storage in m³ / acre-ft—