DrainageCalculators

SCS Triangular Hydrograph Calculator

Generate SCS triangular unit hydrographs for simplified runoff analysis. Calculates peak discharge, time to peak, and runoff volume per TR-55 methodology.

What This Solves

Generates a simplified triangular unit hydrograph based on SCS/NRCS methodology, providing peak flow rate, time to peak, and recession time for a watershed.

Best Used When

  • You need a quick estimate of peak flow and hydrograph shape for a small to medium watershed
  • You want a simplified hydrograph for preliminary detention sizing without full curvilinear analysis
  • You are performing hand calculations or need a simple hydrograph shape for routing

Do NOT Use When

Key Assumptions

  • The hydrograph is approximated as a triangle with a rising limb, peak, and falling limb
  • Time to peak equals 0.5 * storm duration + 0.6 * time of concentration
  • The recession limb duration is 1.67 times the time to peak
  • Total volume under the triangle equals the runoff depth times the drainage area
  • The peak rate factor is 484 (standard SCS assumption for average watershed conditions)

Input Quality Notes

Time of concentration has the largest effect on peak flow. Verify Tc using appropriate methods for your watershed size and land use. The peak rate factor of 484 can be adjusted for flat or steep terrain.

Input Parameters

Watershed Parameters

Basic watershed characteristics

mi²

Contributing watershed area

hours

Tc for the watershed

in

Depth of runoff to route

Advanced Parameters

Optional hydrograph parameters

hours

Duration of unit rainfall (default: Tc/5)

484 US typical, 2.08 SI typical

hours

Time step for hydrograph output

Triangular Hydrograph Relationships

  • • Recession time Tr = 1.67 × Tp
  • • Time base Tb = 2.67 × Tp
  • • Volume = 0.5 × qp × Tb

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

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Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

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