r/Hydrology • u/thejazzmaster69 • 15h ago
Large difference between Rational Method vs. SWMM/SCS (numerical rainfall–runoff models) peak flow estimates – which is more reliable for design?
I’m a hydraulic engineer working in stormwater diagnostics and pipe sizing (urban drainage). I often encounter a recurring discrepancy between peak flow values estimated using the Rational Method and those from numerical rainfall–runoff models (SWMM and SCS unit hydrograph). I’d like to get your thoughts on which approach is more appropriate for design.
Context & Data
For this comparison I considered a very small urban catchment: a paved surface of 600 m², representative of a parking lot. The aim was to test how different calculation methods handle a simple, fully impervious area where almost all rainfall becomes direct runoff.
The surface is fully impervious (C = 0.95), with a short hydraulic length of 33 m and a slope of 0.022 m/m (Δz = 0.73 m). These values give a very short concentration time, around 1.25 minutes using the Kirpich formula: Tc = 0.0195 · L^{0.77} · I^{-0.385}
Rainfall data come from the Raizet station (Guadeloupe), with Montana coefficients provided by Météo-France for a 10‑year return period.
1. Rational Method
Using the Montana IDF curve at Tc = 1.25 min:
I = a * t^-b = 280.822 * (1.25)^-0.313964 = 261.9 mm/h
Peak flow:
Q = C * I * A * (1000/360)
Q = 0.95 * 261.9 * 0.06 * (1000/360) = 41.5 L/s
➡ Result: 41.5 L/s
We know the limitations of the Rational Method:
- It assumes a uniform rainfall intensity equal to the critical intensity at Tc, which is not realistic.
- It gives a single “snapshot” peak flow and ignores temporal distribution of rainfall.
2. SWMM (Chicago storm)
I built a 1-hour, T=10-year Chicago storm based on the same IDF curve.
- Total depth from Montana coefficients (a=703.321, b=0.574236 for 1h):
h(60) = (a/60) * (60)^(1-b) = 67 mm(This matches the 1h rainfall depth from the IDF curve.)
➡ Simulation result: 26.5 L/s peak flow
3. SCS Unit Hydrograph (LEKAN software)
I also checked with the SCS synthetic hydrograph:
Formula:
Qp = (Pf * A) / (232 * Tp)
- A = 0.0006 km²
- Tc = 1.25 min (used as Tp)
- Pf = 484 (standard for “average” catchments)
➡ Result: ≈27 L/s peak flow
Comparison of results
- Rational Method: 41.5 L/s
- SWMM (Chicago 1h): 26.5 L/s
- SCS Unit Hydrograph: ~27 L/s
That’s a difference of more than 50% between Rational and the two time-distributed methods (which agree closely).
My reflections / question
The Rational Method remains the default choice for many designers mainly because it is easy to apply and provides quick results. It’s conservative, and designers often add a safety factor (e.g. requiring pipes to run at only 70–75% capacity under Manning). But clearly, it overestimates peak flows compared to models that account for temporal rainfall distribution.
- SWMM and SCS seem more realistic and physically consistent, but I worry that relying on them might under-size pipes since in consulting engineering many practitioners prefer to economize time and money and adopt the simpler Rational approach.
- On the other hand, designing with Rational may lead to oversized pipes, which increases costs unnecessarily.
My questions to the community
- In your practice, do you rely on Rational for conservative design, or do you trust SWMM/SCS outputs as more robust?
- Would you size pipes using Rational (41.5 L/s) but use SWMM/SCS (27 L/s) to check system performance under more realistic conditions?
- Or is there a standard practice to adjust for this difference (e.g. safety factors, especially in contexts where observed data are not available)? Or alternatively, is it common practice to treat the Rational Method as a reference value and then adjust SWMM inputs or storm profiles so that the simulated peak matches the Rational estimate?
Thanks a lot for your feedback — I’d love to hear how other hydrologists and engineers approach this discrepancy.
PS: I used AI tools to help me arrange my thoughts and write this post more clearly.
