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TOOL-017 / STORMWATERReleased calculator

Catchment Area Breakdown & Runoff Summary

Add Rational Method peaks for the catchments you supply, and report the area-weighted composite coefficient.

Method & assumptionsWorked exampleFAQ
Unrouted sum. Peaks are added directly. Time of concentration, routing, and pipe sizing stay outside this summary.
01

Catchment inputs

Supply an ID, name, area, and coefficient for each catchment, plus one intensity.

Input units
Catchment 1
Catchment 2

Switching units converts the stored area and intensity and rounds only the text in the field. A blank field stays blank. The previous result is cleared when an input changes or an entry is rejected.

02

Runoff summary

Enter catchments and calculate.

Ready for calculation

Review the catchments and the shared intensity, then calculate the unrouted sum.

Qtotal = Σ(Ci × i × Ai)
03 / METHOD & LIMITS

Catchment breakdown and unrouted runoff sum

Each catchment uses the Rational Method. The total adds those peaks without routing.

Qi = Ci × i × Ai4th edition Equation 4.1 · Total peak = ΣQi · Composite C = Σ(Ci × Ai) / ΣAi is Equation 4.2

Peak discharge for a coefficient above 0 is calculated by the released Rational Method engine. The composite coefficient and area fractions are calculated by the released composite-coefficient engine. US area uses 1 ft² = 0.09290304 m². US intensity uses that engine's inch, 0.0254 m, which is 25.4 mm/h per in/h.

Input guidance

Catchment area · A
Use a finite area greater than zero. Square metres in SI, square feet in US.
Runoff coefficient · C
Supply a dimensionless coefficient from 0 to 1, including both bounds. A blank or whitespace-only coefficient is rejected and is not treated as zero. The calculator does not select it.
Rainfall intensity · i
Supply one design intensity for every catchment. Millimetres per hour in SI, inches per hour in US.

Applicability and limitations

Q = C × i × A is FHWA HEC-22 Equation 4.1 in the 4th edition (FHWA-HIF-24-006, February 2024, Section 4.2.2). Equation 4.2 in Section 4.2.2.1 is the weighted runoff coefficient, weighted C = Σ(Cx × Ax) / Atotal. The secondary reference is the 3rd edition (FHWA-NHI-10-009, September 2009, revised August 2013), Chapter 3. Equation 3-1 is Q = (CIA)/Ku, and Equation 3-2 is the same weighted coefficient. Ku is 360 when area is in hectares and intensity is in millimetres per hour, and Ku is 1.0 when area is in acres and intensity is in inches per hour. This summary evaluates Equation 4.1 in canonical SI, with i in metres per second and A in square metres. Rainfall may be treated as uniform when the intensity duration is at least the time of concentration. This summary uses one intensity for every catchment and does not calculate time of concentration. Adding the peaks assumes they occur together. It can overstate the combined peak when catchments respond at different times.

Model bounds: area must be finite and greater than zero, intensity must be finite and greater than zero, and each coefficient must be inside [0, 1]. A coefficient of 0 is zero discharge because the Rational Method engine accepts only (0, 1]. The engine does not enforce a watershed-size limit. Many agencies limit the Rational Method to small urban areas, often about 200 acres (80 ha). Pipe sizing, inlet capture, detention routing, and Hazen-Williams are outside this summary. The result is not a code or permit determination.

Method reference

Formula FORMULA-TOOL017-CATCHMENT-RUNOFF-SUMMARY-001, using FORMULA-TOOL003-RATIONAL-001 and FORMULA-TOOL016-AREA-WEIGHTED-COMPOSITE-C-001. Sources SRC-RATIONAL-METHOD-TOOL003-001, SRC-AREA-WEIGHTED-COMPOSITE-RUNOFF-TOOL016-001, and SRC-PROJECT-METHOD-TOOL017-001, each version 1.0.0.

FAQ

Frequently asked questions

What formulas does this summary use?

Each catchment peak is Q = C × i × A, FHWA HEC-22 Equation 4.1 in the 4th edition (FHWA-HIF-24-006, February 2024, Section 4.2.2), https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif24006.pdf. The reported total is the sum of those peaks. The weighted coefficient is Equation 4.2 in Section 4.2.2.1, Ccomp = Σ(Ci × Ai) / ΣAi. The secondary reference is the 3rd edition (FHWA-NHI-10-009, September 2009, revised August 2013): Equation 3-1 is Q = (CIA)/Ku and Equation 3-2 is the same weighted coefficient, https://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf.

Why are the catchment peaks added together?

The total is an unrouted sum. It assumes the peaks occur at the same time. It is not a hydrograph and it does not apply a timing offset. Catchments that respond at different times can make this total larger than the combined peak.

Which units can I enter?

SI areas are square metres and intensity is millimetres per hour. US areas are square feet, converted with 1 ft² = 0.09290304 m², and intensity is inches per hour, converted with the Rational Method inch of 0.0254 m. Results are canonical square metres, a dimensionless coefficient, and cubic metres per second.

Are runoff coefficients of 0 and 1 accepted?

Yes, when you enter the number. A coefficient of 1 is sent to the Rational Method engine. A coefficient of 0 is recorded as zero discharge because that engine accepts only coefficients above 0 and up to 1. A blank or whitespace-only coefficient is required and is not treated as zero. A coefficient below 0 or above 1 is rejected before a result is shown, and any previous result is cleared.

What happens if an area or the intensity is zero, or the catchment list is empty?

Each area must be a finite number greater than zero, intensity must be a finite number greater than zero, and at least one catchment is required. An empty list, a blank scenario ID, a duplicate ID, or a non-positive area or intensity is a validation error, and any previous result is cleared.

Does one intensity apply to every catchment?

Yes. You supply one rainfall intensity and it is used for every catchment. The summary does not calculate time of concentration and does not look up an IDF curve. The intensity duration still needs to cover each catchment's response time.

Is there a catchment-size limit?

The engine accepts any finite area greater than zero and does not enforce a watershed-size cap. Q = C × i × A is FHWA HEC-22, 4th edition, Equation 4.1 (FHWA-HIF-24-006, February 2024, Section 4.2.2), and the weighted coefficient is Equation 4.2, https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif24006.pdf. The 3rd edition (FHWA-NHI-10-009, September 2009, revised August 2013, Equations 3-1 and 3-2) is the secondary reference, https://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf. Many agencies limit that method to small urban areas, often about 200 acres (80 hectares). A larger total is still only a preliminary sum.

Does this summary prove code compliance or size a pipe?

No. Coefficients and intensity are inputs you supply. The result does not select a coefficient, size a storm drain, check an inlet, or establish a permit or code determination.

CONTINUE YOUR WORKFLOW

Check the pieces separately

One catchmentRational Method peak runoff

The same Q = C × i × A relationship for a single area.

Coefficient onlyComposite runoff coefficient

Area-weighted C without a rainfall intensity.

Read the methodRational Method guide

Units, assumptions, and a worked peak-flow example.

Related

Continue from this result

ReleasedRational Method peak runoff

Single-catchment Q = C i A

ReleasedComposite runoff coefficient

Area-weighted C without intensity

ReleasedRational Method guide

Units and a worked peak-flow example

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