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.
Size a bioretention cell with an underdrain for sites with poor native infiltration. Enter the drainage area, design storm and cell layers to get the required surface area, layered storage volume, underdrain pipe capacity and the resulting drawdown time — checked against your target.
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)
How the bioretention underdrain sizing works
The calculator follows the volume-based sizing approach in ASCE MOP 77 and state stormwater specifications (e.g. Virginia DEQ Spec No. 9). It works in four moves: estimate the runoff to capture, convert the cell layers into an effective storage depth, divide to get the footprint, then check that the underdrain and native soil can empty the cell in time.
- Runoff volume — the design storm runoff is
V_runoff = C × P × A, using a composite runoff coefficientC = imp% × 0.95 + (1 − imp%) × 0.20. A separate water quality volume is taken from the first 1 in (25 mm) of rainfall. - Effective storage depth — only voids and open ponding hold water:
d_eff = d_pond + d_media × n_media + d_gravel × n_gravel. - Required area —
A_bio = V_runoff ÷ d_eff. Layered storage volumes (ponding, media voids, gravel voids) are then reported for the sized cell. - Underdrain capacity & drawdown — each pipe is sized full-flowing
with Manning's equation
Q = (k/n)·A·R^(2/3)·S^(1/2)(k = 1.486US,1.0SI;R = D/4), multiplied by the number of pipes. Drawdown time ist = V_total ÷ (Q_out × 3600)in hours, whereQ_outcombines native infiltration and underdrain (or an orifice-controlled outlet,Q = C_d·A·√(2gh)withC_d = 0.62).
Variable key: C = composite runoff coefficient; P = rainfall depth; A = contributing drainage area; n_media / n_gravel = layer porosity (void ratio); n = Manning's roughness; R = hydraulic radius; S = pipe slope; D = pipe diameter; C_d = orifice discharge coefficient; g = gravitational acceleration; h = head on the outlet.
Bioretention media infiltration & porosity
Design infiltration rates and porosities for common bioretention media mixes. The cell media rate must stay high enough to drain the ponded water but is intentionally limited to give the engineered soil time to filter pollutants.
| Media mix | Min (in/hr) | Typical (in/hr) | Max (in/hr) | Porosity |
|---|---|---|---|---|
| Standard (60% sand, 20% compost, 20% topsoil) | 1.0 | 2.0 | 4.0 | 0.25 |
| High-rate (80% sand, 15% compost, 5% fines) | 4.0 | 8.0 | 12.0 | 0.30 |
| Phosphorus-reducing (Fe/Al amendments) | 1.0 | 2.0 | 4.0 | 0.25 |
Clean gravel storage layers are modeled at a porosity of about 0.35–0.40. Source: Virginia DEQ Stormwater Design Specification No. 9 (2011); ASCE MOP 77 (2006), Ch. 12.
Underdrain pipe roughness (Manning's n)
The roughness coefficient used to size the perforated underdrain. Smoother pipe carries more flow at the same diameter and slope.
| Underdrain material | Manning's n |
|---|---|
| PVC, perforated (smooth wall) | 0.010 |
| HDPE, perforated (smooth wall) | 0.012 |
| Corrugated metal, perforated | 0.024 |
Source: FHWA HEC-22 (2009), Table 7-1; ASCE MOP 77 (2006).
Assumptions & when to use it
Key assumptions
- Uniform media infiltration rate across the facility
- Underdrain pipes remain unobstructed (clean stone, geotextile or choker layer)
- Native soil rate measured by infiltration testing, not assumed
- Groundwater table at least 2 ft below the gravel layer
- Steady-state infiltration; no surface clogging over time
Best suited to
- Tight clay soils (under ~0.5 in/hr) or high water table
- Combined water-quality treatment with volume control
- Urban retrofits with limited footprint
Not for unmitigated hotspot runoff, karst without a liner, or contaminated groundwater. Always confirm sizing against your local stormwater manual.
Frequently asked questions
When does a bioretention cell need an underdrain?
An underdrain is added when the native subsoil cannot drain the stored water fast enough on its own — typically when the measured infiltration rate is below about 0.5 in/hr (tight clays, fill, or a high seasonal water table). The perforated pipe in the gravel layer guarantees the cell empties within the target drawdown window (commonly 48 hours, sometimes 24–72) so it is dry and ready before the next storm. Where native soils infiltrate well, an underdrain-free (infiltrating) rain garden may be preferable.
How is the required bioretention area calculated?
The tool first computes the design runoff volume as V = C × P × A, where C is the composite runoff coefficient (0.95 for impervious and 0.20 for pervious cover, area-weighted by your impervious percentage), P is the rainfall depth, and A is the contributing drainage area. It then divides that volume by the effective storage depth — ponding depth plus the media and gravel depths multiplied by their porosities — to get the minimum cell footprint: A_bio = V_runoff ÷ d_eff.
Why do the media and gravel layers count for less than their full depth?
Only the void space in those layers can hold water. Standard bioretention media has a porosity of about 0.25 and clean gravel about 0.35–0.40, so a 24 in media layer contributes roughly 6 in of effective storage and a 12 in gravel layer about 4.5–4.8 in. Surface ponding, by contrast, is open water and counts at its full depth. This is why the effective storage depth is always less than the physical depth of the cell.
What controls the underdrain discharge capacity?
The underdrain is sized as a full-flowing pipe using the Manning equation, Q = (k/n)·A·R^(2/3)·S^(1/2), with k = 1.486 in US units (1.0 in SI). Capacity therefore rises with pipe diameter (area A and hydraulic radius R = D/4) and the square root of slope S, and falls with the roughness n — 0.010 for smooth PVC versus 0.024 for corrugated metal. Adding parallel pipes multiplies the available capacity. An orifice or weir at the outlet can deliberately restrict this flow to extend the drawdown time for water-quality treatment.
What is the IWS (internal water storage) zone for?
An internal water storage zone is created by elevating the underdrain outlet so a saturated, low-oxygen layer is held permanently in the bottom of the cell. That anaerobic zone promotes denitrification, removing nitrate that aerobic filtration alone leaves behind. It is optional and most useful where nitrogen is a target pollutant; it requires adequate residence time and is not credited the same way in every jurisdiction.
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Last verified: February 2026