What This Solves
Designs bioretention facilities with underdrains for sites where native soil infiltration is insufficient, calculating required area, media depth, storage volumes, and underdrain pipe sizing.
Best Used When
- You are designing a bioretention cell on a site with poorly draining soils (HSG C or D)
- Your local regulations require an underdrain beneath the bioretention media
- You need to calculate both infiltration volume credit and underdrain discharge for a bioretention facility
Do NOT Use When
- The native soil has adequate infiltration and no underdrain is needed — Use Rain Garden Calculator
- You need a subsurface infiltration trench without surface ponding or vegetation — Use Infiltration Trench Calculator
Key Assumptions
- Engineered media infiltration rate is uniform and constant (typically 1-2 in/hr)
- The underdrain pipe has adequate capacity to convey filtered water
- Storage volume is the sum of surface ponding and media void space
- No significant clogging of the media or underdrain over the design life
- Contributing drainage area is correctly delineated
Input Quality Notes
Engineered media specifications (mix ratio, infiltration rate) vary by jurisdiction. Confirm media spec with local stormwater manual. Field-verify native soil infiltration to determine whether underdrain is required.
Design Bioretention with Underdrain
For educational purposes only. Not a substitute for professional engineering judgment.
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)
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Last verified: February 2026