What This Solves
Calculates the flow capacity and sizing for a surface trench drain (channel drain with grate) using open channel flow principles.
Best Used When
- You need to size a surface-level linear drain for a parking lot, driveway, or loading dock
- You are designing a channel drain with a grate to intercept sheet flow before it reaches a building or low area
- You need to determine whether an existing trench drain has adequate capacity for the design flow
Do NOT Use When
- You need a buried subsurface drain rather than a surface channel drain — Use French Drain Calculator
- You are sizing a catch basin or inlet structure at a point location rather than a linear drain — Use Catch Basin Calculator
- You are designing a vegetated open channel or swale — Use Swale Calculator
Key Assumptions
- Flow follows open channel hydraulics using Manning's equation
- The channel has a uniform cross-section and slope along its length
- Grate interception efficiency can be estimated from the open area ratio
- Flow is steady and uniform (not rapidly changing)
- The grate is clean and not clogged with debris
Input Quality Notes
Grate interception efficiency depends heavily on grate type and maintenance. Consider using a safety factor for clogging, especially in areas with leaves or other debris.
Calculate Trench Drain Capacity
For educational purposes only. Not a substitute for professional engineering judgment.
Trench Drain Design Overview
Trench drains (channel drains) are linear drainage systems used to collect and convey surface runoff from paved areas. They consist of a channel with a grate that intercepts sheet flow.
- Channel Capacity - Calculated using Manning's equation for open channel flow
- Grate Efficiency - Fraction of approaching flow intercepted by the grate
- Normal Depth - Uniform flow depth that will develop for the design flow
- Froude Number - Determines if flow is subcritical, critical, or supercritical
Manning's Roughness Coefficients
| Material | Min n | Typical n | Max n |
|---|---|---|---|
| Concrete | 0.011 | 0.013 | 0.015 |
| Polymer Concrete | 0.010 | 0.012 | 0.014 |
| Fiberglass | 0.009 | 0.011 | 0.013 |
| HDPE | 0.009 | 0.011 | 0.012 |
| Galvanized Steel | 0.012 | 0.014 | 0.016 |
| Stainless Steel | 0.011 | 0.013 | 0.015 |
| Cast Iron | 0.012 | 0.014 | 0.017 |
Source: FHWA HEC-22 (2009), Table 7-1
Grate Interception Efficiency
| Grate Type | Open Area | Efficiency Factor |
|---|---|---|
| Parallel Bar Grate | 70% | 90% |
| Reticuline Grate | 65% | 85% |
| Curved Vane Grate | 60% | 80% |
| Tilt Bar Grate | 55% | 75% |
| Slot Drain | 15% | 70% |
Source: FHWA HEC-22 (2009), Chapter 4
Was this calculator helpful?
Last verified: February 2026