Bioretention Underdrain Calculator

Design bioretention facilities with underdrain systems for sites with poor native soil infiltration. Calculate required area, storage volumes, underdrain capacity, and infiltration split using ASCE and Virginia DEQ methodology.

Design Bioretention with Underdrain

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

Input Parameters

Contributing Drainage Area

Drainage Area

Total area draining to the bioretention facility

Impervious Percentage

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

Design Storm

Design Rainfall Depth

Total rainfall depth for design storm

Bioretention Dimensions

Maximum Ponding Depth

Maximum water depth allowed on surface (typically 6-12 in)

Media Depth

Depth of bioretention media (typically 18-48 in)

Gravel Layer Depth

Depth of gravel storage layer (typically 8-12 in)

Gravel Porosity

Void ratio of gravel layer (typically 0.35-0.40)

Bioretention Media

Media Type

Type of bioretention media mix

Native Soil Infiltration Rate

in/hr

Measured infiltration rate of underlying native soil

Underdrain System

Underdrain Diameter

Diameter of perforated underdrain pipe

Underdrain Material

Type of underdrain pipe material

Underdrain Slope

ft/ft

Slope of underdrain pipe (minimum 0.5%)

Number of Pipes

Number of parallel underdrain pipes

Outlet Control

Outlet Control Type

Type of outlet flow control

Target Drawdown Time

hours

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

IWS Zone Depth

Internal water storage zone for denitrification (optional)

Bioretention with Underdrain Overview

Bioretention with underdrain systems provides stormwater treatment and storage for sites with poor native soil infiltration. Water filters through engineered media for pollutant removal, then exits via both native soil infiltration and underdrain pipes.

Ponding Storage - Surface depression holding runoff
Media Filtration - Engineered soil for water quality treatment
Gravel Storage - Stone layer for storage and underdrain bedding
Underdrain - Perforated pipe to remove excess water
IWS Zone - Optional saturated zone for denitrification

Bioretention Media Properties

Media Type	Min (in/hr)	Typical (in/hr)	Max (in/hr)	Porosity
Standard bioretention mix (60% sand, 20% compost, 20% topsoil)	1	2	4	0.25
High-rate mix (80% sand, 15% compost, 5% fines)	4	8	12	0.3
P-reducing mix with iron/aluminum amendments	1	2	4	0.25

Source: Virginia DEQ Stormwater Design Specification No. 9 (2011)

Underdrain Pipe Roughness

Material	Manning's n
Pvc Perforated	0.01
Hdpe Perforated	0.012
Corrugated Metal	0.024

Source: FHWA HEC-22 (2009), ASCE MOP 77 (2006)