How is a composite runoff coefficient calculated?

C_composite = Σ(C_i × A_i) / A_total, where C_i is the runoff coefficient for each sub-area and A_i is its area. For example, a watershed with 40% lawn (C=0.30) and 60% pavement (C=0.85) gives C_composite = 0.40×0.30 + 0.60×0.85 = 0.63. Used per ASCE 36 rational method guidelines.

How does antecedent moisture condition affect runoff coefficient?

The rational method C is typically based on average antecedent conditions. For wet antecedent conditions (AMC III in SCS/NRCS terminology), CN and corresponding C values increase by 10–15%. For dry conditions (AMC I), they decrease. The NRCS TR-55 method explicitly handles AMC through CN adjustments.

Runoff Coefficient Calculator

Determine composite runoff coefficients for mixed land-use watersheds for use in rational method stormwater calculations.

2D stress state at a point. All stresses in MPa.

σx — Normal Stress X (MPa)

σy — Normal Stress Y (MPa)

τxy — Shear Stress (MPa)

⚙ Mechanical Note: Results are based on standard textbook formulas and ISO/AASHTO references. Always verify critical designs with applicable codes (ISO 898, AASHTO 1993, AISC, Eurocode 3) and a qualified engineer.

How to Use

Enter the values for the patient or scenario you are assessing. Determine composite runoff coefficients for mixed land-use watersheds for use in rational method stormwater calculations. Use the runoff coefficient result to inform your clinical assessment.

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Frequently Asked Questions

How is a composite runoff coefficient calculated?

How does antecedent moisture condition affect runoff coefficient?