Infiltration Trench Calculator

Design infiltration trenches for stormwater management. Calculate required trench dimensions, storage volume, infiltration capacity, and drawdown time using EPA and ASCE methodology.

Design Infiltration Trench

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

Input Parameters

Contributing Drainage Area

Drainage Area

Total area draining to the infiltration trench

Impervious Percentage

Percent of drainage area that is impervious (0-100)

Design Storm

Design Rainfall Depth

Total rainfall depth for design storm

Trench Dimensions

Trench Width

Width of the infiltration trench (typically 24-48 in)

Trench Depth

Depth of the infiltration trench (typically 36-60 in)

Aggregate Properties

Aggregate Type

Type of stone fill used in trench

Native Soil Properties

Soil Infiltration Class

Native soil type for infiltration rate

Safety Factor

Factor applied to infiltration rate (typically 2-3)

Design Options

Pretreatment Type

Pretreatment for sediment removal

Target Drawdown Time

hours

Maximum time to drain stored volume (24-72 hours)

Include Sidewall Infiltration

Include sidewall infiltration

Include infiltration through trench sidewalls (conservative to exclude)

Infiltration Trench Design Overview

Infiltration trenches are linear rock-filled excavations that capture stormwater runoff and allow it to infiltrate into the surrounding soil. They provide both storage and treatment through infiltration.

Storage Volume - Water stored in aggregate void spaces
Infiltration - Water percolates through bottom and sidewalls
Pretreatment - Sediment removal prevents clogging
Drawdown Time - Typically 24-72 hours maximum

Aggregate Porosity Values

Aggregate Type	Min	Typical	Max
1.5-2.5" Clean Washed Stone	0.35	0.4	0.45
0.75-1.5" Clean Washed Stone	0.32	0.38	0.42
Crushed Angular Stone	0.3	0.35	0.4
Rounded Gravel	0.25	0.32	0.38

Source: EPA Stormwater Manual (2004), ASCE MOP 77 (2006)

Soil Infiltration Rates

Soil Class	Min (in/hr)	Typical (in/hr)	Max (in/hr)
Sand	2	8	12
Loamy Sand	1	4	8
Sandy Loam	0.5	2	4
Loam	0.25	1	2
Silt Loam	0.15	0.5	1
Sandy Clay Loam	0.1	0.3	0.5
Clay Loam	0.05	0.2	0.4

Source: EPA SWMM Manual (2020), NRCS Soil Survey