Stormwater calculations begin with a fundamental question: how much runoff will this site generate during a storm? The answer depends on which method you use, and the two most common methods — the Rational Method and TR-55 — give different answers because they make different assumptions about how rainfall becomes runoff.

Understanding when to use each method, and when neither is adequate, is essential for stormwater system design.

The Rational Method

The Rational Method is the simplest and most widely used method for estimating peak stormwater discharge from small drainage areas. The formula:

The method assumes that the peak discharge occurs when the entire drainage area is contributing runoff simultaneously, which happens when the storm duration equals the time of concentration (Tc) — the time it takes for water to travel from the most hydraulically remote point in the watershed to the outlet.

When to Use It

• Drainage areas less than 200 acres (some references say 100 acres; local standards vary)
• Storm drain pipe sizing
• Inlet sizing
• Small channel design
• Preliminary estimates for any site

Runoff Coefficient (C)

The C value represents the fraction of rainfall that becomes runoff. Typical values:

For a mixed-use site, the composite C is the area-weighted average of the C values for each surface type. A commercial site with 60 percent impervious (C=0.90) and 40 percent landscaped (C=0.20) has a composite C of 0.62.

Limitations

• The Rational Method gives you the peak flow rate only — not the runoff volume or hydrograph shape. You cannot use it to size detention basins (which require volume calculations).
• It assumes uniform rainfall intensity over the entire drainage area for the entire storm duration. This becomes less valid as the drainage area increases.
• It does not account for storage effects (ponding in parking lots, overbank flooding) that reduce and delay peak flows.
• The time of concentration calculation requires judgment, and small changes in Tc significantly affect the result because rainfall intensity is a steep function of duration.

TR-55 (NRCS Method)

TR-55 (Technical Release 55) is a method developed by the USDA Natural Resources Conservation Service (NRCS, formerly SCS) for computing peak discharge and runoff volume from small watersheds. It uses the Curve Number method to determine runoff volume and then routes that volume through a unit hydrograph to determine peak discharge and the full hydrograph shape.

The Curve Number (CN)

The Curve Number is an empirical parameter that represents the runoff potential of a surface based on its soil type (Hydrologic Soil Group A, B, C, or D) and land cover. CN values range from 30 (thick forest on well-drained sand) to 98 (impervious surface). Higher CN means more runoff.

When to Use It

• Detention basin sizing (because it gives you the full hydrograph, not just the peak)
• Drainage areas from 1 acre to 2,000 acres
• Projects where runoff volume is important (retention/infiltration sizing, hydromodification management)
• FEMA floodplain studies and flood control district submittals
• Projects where the reviewing agency requires NRCS methodology

Limitations

• The Curve Number method is based on 24-hour rainfall totals and does not directly account for storm intensity within that 24 hours. The SCS Type I, IA, II, and III rainfall distributions assign an intensity pattern to the 24-hour storm, but these are statistical distributions, not representations of actual storm patterns.
• CN values were developed empirically from agricultural watersheds in the eastern United States. Their accuracy for urban land covers, arid climates, and steep terrain is debated.
• For small, highly impervious sites, the CN method can underestimate peak flows compared to the Rational Method because the initial abstraction assumption (the first 0.2S inches of rainfall produces no runoff) may not reflect the rapid response of impervious surfaces.

Which Method for Your Project?

In practice, the choice is often dictated by the reviewing agency:

• Storm drain pipe sizing: Rational Method. Almost every public works department accepts the Rational Method for pipe sizing on site development projects.
• Detention basin design: TR-55 or equivalent hydrograph method. You need the volume under the hydrograph curve, which the Rational Method does not provide.
• FEMA studies: HEC-HMS or TR-55 at minimum. The Rational Method is not accepted for floodplain delineation.
• C.3 stormwater control plans: The BASMAA calculator or IMP worksheets, which are based on continuous simulation — a more sophisticated approach than either the Rational Method or TR-55.

Software

Hand calculations using the Rational Method are quick for simple sites. TR-55 calculations are tedious by hand but straightforward in software. Common tools:

• HydroCAD — the industry standard for detention/retention design. Uses TR-55/TR-20 methodology with a graphical interface.
• Autodesk Storm and Sanitary Analysis — integrated with Civil 3D for pipe network design using the Rational Method.
• HEC-HMS — the Army Corps of Engineers' free hydrologic modeling software. More powerful than HydroCAD but steeper learning curve.
• EPA SWMM — free software for combined hydrologic and hydraulic modeling. Used for complex urban drainage systems.