Trench Drain Calculator

Calculate trench drain capacity using Manning's equation. Determines flow capacity, velocity, and grate interception efficiency for linear drainage systems.

Calculate Trench Drain Capacity

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Design Requirements

Design Flow Rate

cfs

Peak flow rate the trench drain must convey

Channel Length

Total length of the trench drain

Channel Geometry

Channel Shape

Cross-sectional shape of the trench

Bottom Width

Width at the bottom of the channel

Channel Depth

Total depth of the channel

Channel Slope

Longitudinal slope (ft/ft or m/m)

Material & Grate

Channel Material

Material affects Manning's roughness coefficient

Grate Type

Grate type affects flow interception efficiency

Custom Grate Open Area

Override default open area ratio (0-1, optional)

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