Size a linear trench (channel) drain for surface runoff. Enter the design flow and channel geometry to get the full-flow and grate-adjusted effective capacity from Manning's equation, plus normal depth, velocity and Froude number — so you can confirm the channel and grate convey the flow without surcharging.
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
How the trench drain calculation works
A trench drain is a prismatic open channel, so its capacity is governed by Manning's equation for uniform flow:
Q = (k / n) · A · R2/3 · S1/2
where:
- Q = flow capacity (cfs, or m³/s in metric)
- k = unit conversion factor = 1.486 for US customary units, 1.0 for SI/metric
- n = Manning's roughness coefficient (depends on channel material — see table below)
- A = cross-sectional flow area (ft² or m²)
- R = hydraulic radius = A / P, where P is the wetted perimeter (ft or m)
- S = longitudinal (channel) slope (ft/ft or m/m)
The geometry terms A and R depend on the channel shape. For a rectangular channel, A = b·y and P = b + 2y; for a trapezoidal channel, P = b + 2y√(1 + z²) with side slope z (H:V). The tool computes A, P and R at full depth, evaluates the full-flow capacity, then solves Manning's equation iteratively for the normal depth at your design flow.
Because a grate cannot intercept all of the approaching water, the usable effective capacity is the full-flow capacity reduced by the grate's interception efficiency:
Qeff = Qfull · Egrate, where Egrate = Aopen · fgrate
Aopen is the grate open-area ratio and fgrate is the interception efficiency factor for the grate type. Finally the Froude number, Fr = V / √(g · Dh) where Dh = A / T (hydraulic depth), classifies the flow as subcritical (Fr < 1), critical (Fr = 1) or supercritical (Fr > 1). The design is adequate when the effective capacity meets or exceeds the design flow and the normal depth stays below the channel depth.
Method: FHWA HEC-22 (2009), Eq. 7-1 & Chapter 4; Chow, Open-Channel Hydraulics (1959); ASCE MOP 77 (2006).
Grate interception efficiency by type
The effective capacity of a channel drain depends heavily on the grate. The table below gives typical open-area ratios and interception efficiency factors used by this calculator. Effective grate efficiency is their product (open area × efficiency factor), which is the fraction of full-flow capacity the grate can actually capture.
| Grate type | Typical open area | Efficiency factor | Effective efficiency |
|---|---|---|---|
| Parallel bar | 70% | 0.90 | 63% |
| Reticuline | 65% | 0.85 | 55% |
| Curved vane | 60% | 0.80 | 48% |
| Tilt bar | 55% | 0.75 | 41% |
| Slot drain | 15% | 0.70 | 11% |
Open-area ratios from manufacturer specifications; efficiency factors per FHWA HEC-22 (2009), Chapter 4. Bar grates intercept the most flow; slotted and narrow-opening grates the least. Values shown are typical defaults — enter a custom open area in the calculator to override.
Frequently asked questions
How is trench drain capacity calculated?
This calculator uses Manning's equation for open-channel flow, Q = (k/n) A R^(2/3) S^(1/2), to find the full-flow capacity of the channel, then multiplies that by a grate interception efficiency to get the effective capacity. It also solves iteratively for the normal (uniform-flow) depth at your design flow and reports the Froude number so you can see whether flow is subcritical or supercritical. The method follows FHWA HEC-22 (2009) and Chow's Open-Channel Hydraulics (1959).
Why is the effective capacity lower than the full-flow capacity?
A grate does not capture 100% of the water flowing toward it. The calculator reduces the channel's full-flow capacity by a grate efficiency equal to the grate's open-area ratio multiplied by an interception efficiency factor (E_grate = A_open x f_grate). For example, a parallel-bar grate with a 0.70 open area and a 0.90 efficiency factor gives an effective efficiency of about 0.63, so only ~63% of the theoretical channel capacity is usable. Bar-type grates intercept more than slotted or curved-vane grates.
What flow velocity should a trench drain have?
A common target is roughly 2 to 10 ft/s (about 0.6 to 3 m/s). Below about 2 ft/s, sediment and debris can settle out and clog the channel; above about 10 ft/s, the higher velocity can cause abrasion and erosion at the outlet and may overshoot the grate openings. The calculator flags the full-flow velocity when it falls outside this range.
What is the difference between a trench drain and a French drain?
A trench (channel) drain is a surface drain: a hard-walled linear channel with a grate, set flush with paving, that intercepts sheet flow and carries it away by gravity in open-channel flow. A French drain is a subsurface, perforated pipe in a gravel-filled trench that collects groundwater and infiltration. This calculator is for surface channel/trench drains; use it for paved areas, loading docks, pool decks, and plazas.
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.
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Last verified: February 2026