DrainageCalculators

Tailwater Analysis Calculator

Calculate tailwater depth at culvert outlets with support for multiple channel types (rectangular, trapezoidal, triangular, natural). Analyze normal depth, critical depth, Froude number, flow regime, and outlet submergence. Optional rating curve generation.

What This Solves

Calculates the downstream water surface elevation (tailwater depth) at a culvert or pipe outlet based on the receiving channel geometry and flow conditions.

Best Used When

  • You need tailwater depth as input to culvert outlet control or outlet protection calculations
  • You are analyzing the downstream channel to determine whether a culvert outlet is submerged
  • You want to generate a tailwater rating curve for multiple flow rates in the downstream channel

Do NOT Use When

Key Assumptions

  • Normal depth in the downstream channel is used as the tailwater elevation
  • The downstream channel has a uniform slope, roughness, and cross-section near the outlet
  • No backwater effects from downstream structures or confluences affect the tailwater
  • The channel is stable and not actively aggrading or degrading

Input Quality Notes

Field survey of the downstream channel cross-section is strongly recommended. If backwater from a downstream structure or confluence is possible, tailwater should be determined from a water surface profile analysis.

Manning's n Values for Channels

Channel Typen
Concrete lined0.013
Riprap lined0.035
Grass lined0.030
Natural - clean0.030
Natural - weeds0.050
Natural - brush0.070
Floodplain - pasture0.035
Floodplain - brush0.070

Source: Chow (1959), Table 5-6

Tailwater Control Types

Control TypeDescription
Normal DepthUniform flow in downstream channel
Critical DepthControl section at or near outlet
Known WSESurveyed or calculated elevation
Downstream PoolPond, lake, or detention basin
TidalCoastal tidal influence

Source: FHWA HDS-5, Chapter 4

Input Parameters

Flow Parameters

cfs

Flow rate at culvert outlet

ft

Channel bottom at outlet

Channel Geometry

ft

Base width of channel

Horizontal to vertical ratio

ft/ft

Longitudinal bed slope

ft

For submergence analysis

Hydraulic Properties

Tailwater Control

Rating Curve (Optional)

Ready to Calculate

Enter channel parameters and click Calculate to analyze tailwater conditions.

Disclaimer & Terms of Use

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.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the Tailwater Analysis Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

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

Disclaimer & Terms of Use

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.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.