How this calculation works
Peak drainage rate and harvested water volume answer different questions. A short intense burst can create a large peak without producing the largest daily water total. This page estimates a peak rate using the rational-method coefficient, intensity and contributing area.
Peak L/s = rainfall mm/h × Σ(area m² × coefficient)/3600A worked example
The example has 100 m² at coefficient 0.9 and 50 m² at 0.6, giving 120 effective square metres. At 100 mm/h, peak flow is 12000/3600, or 3.3333 L/s. The area-weighted coefficient is 120/150, or 0.8. Comparing with an independently supplied four L/s capacity leaves 0.6667 L/s of modeled headroom, not a pipe-sizing certification.
Good to know
Rational peak-rate scenario only; no local rainfall lookup, catchment timing solve, hydrograph, routing or drainage approval. A drainage designer must establish where the method applies and which duration, frequency, coefficient and blockage assumptions are appropriate.
Reference: TxDOT — rational method procedure (opens in a new tab) ↗
What the model answers
Peak drainage rate and harvested water volume answer different questions. A short intense burst can create a large peak without producing the largest daily water total. This page estimates a peak rate using the rational-method coefficient, intensity and contributing area.
Multiple surfaces are combined by summing area times runoff coefficient. This gives an area-weighted coefficient rather than simply averaging the roof and paving coefficients as if their areas were equal.
The Rainwater daily tank balance calculator tracks stored harvest, overflow and unmet demand over days. That storage ledger is distinct from this short-duration drainage peak.
Measure and enter consistently
Use contributing horizontal drainage areas appropriate to the chosen method. Coefficients are user-supplied assumptions describing runoff response; they are not automatically chosen from a surface name.
The rainfall intensity must come from a duration and frequency relevant to the catchment, commonly linked to its time of concentration. An arbitrary daily rainfall average is not a substitute. The page does not look up local intensity-duration-frequency data.
Interpret the result
The capacity metric subtracts the estimated peak from a capacity established elsewhere. It does not calculate the capacity of a gutter, downpipe, inlet or channel from dimensions. A positive number is therefore conditional on that supplied capacity being valid.
There is no hydrograph or timing interaction between subcatchments. Storage, routing, inlet blockage and local design requirements can change a real drainage assessment beyond this compact peak-rate model.
Work through the numbers
The example has 100 m² at coefficient 0.9 and 50 m² at 0.6, giving 120 effective square metres. At 100 mm/h, peak flow is 12000/3600, or 3.3333 L/s.
The area-weighted coefficient is 120/150, or 0.8. Comparing with an independently supplied four L/s capacity leaves 0.6667 L/s of modeled headroom, not a pipe-sizing certification.
Where this model stops
Rational peak-rate scenario only; no local rainfall lookup, catchment timing solve, hydrograph, routing or drainage approval.
A drainage designer must establish where the method applies and which duration, frequency, coefficient and blockage assumptions are appropriate.
A few questions, answered
Can I enter daily rainfall depth as intensity?
No. Intensity is a rate for a relevant storm duration, not a whole-day depth without a time basis.
Does this size my downpipe?
No. It estimates runoff and compares a separately established capacity.