› Bioretention Cell Stormwater Design Simulator Back
Green Infrastructure

Bioretention Cell Stormwater Design Simulator

Size a bioretention cell — also known as a rain garden — in real time. Adjust the catchment area, imperviousness and design rainfall together with cell area, ponding depth and media specs, and see the runoff capture ratio, drawdown time and pollutant-removal efficiency move with you to support Low Impact Development (LID) decisions.

Parameters
Catchment area
m²
Total drainage area contributing runoff to the cell
Impervious fraction
%
Share of pavement, roads and roofs in the catchment
24-hour design rainfall
mm
Total depth of the design storm event
Cell area
m²
Plan area of the bioretention cell
Ponding depth
cm
Maximum water depth held above the media
Media infiltration rate
mm/h
Steady-state percolation rate of the soil media
Media depth
cm
Thickness of the engineered planting-soil layer
Bottom pipe routed to the stormwater network
Results
—
Design runoff (m³)
—
Total storage (m³)
—
Capture ratio (%)
—
Drawdown time (h)
—
Sizing ratio (%)
—
TSS removal (%)
—
Bioretention cross-section — stormwater flow

Runoff (blue) gathered from the impervious catchment ponds at the surface, percolates through the planting media and discharges as treated water (green) through the underdrain. Ponded depth tracks the parameters.

Water balance — runoff vs storage vs bypass
Drawdown time vs media infiltration rate
Theory & Key Formulas

$$V_{storage} = V_{ponding} + V_{media}\,\phi,\qquad t_{drawdown} = \frac{V_{storage}}{i_{media}\cdot A}$$

φ ≈ 0.30 is the effective porosity of the planting media, i_media is the media infiltration rate (mm/h) and A is the cell area. The design guideline is to drain within 24-48 hours.

$$V_{runoff} = A_{catch}\cdot P\cdot C,\qquad C = f_{imp}\cdot 0.95 + (1-f_{imp})\cdot 0.15$$

The design runoff V_runoff (m³) is the product of catchment area A_catch, rainfall depth P (m) and the weighted runoff coefficient C, which mixes 0.95 for paving with 0.15 for pervious area.

$$\eta_{TSS} = \min(95,\ 60 + 30\cdot r_{capture}),\qquad r_{capture} = \min\!\left(1,\ \frac{V_{storage}}{V_{runoff}}\right)$$

TSS removal efficiency η_TSS scales with the capture ratio r_capture. Bypassed runoff is discharged untreated, so raising r_capture is the most effective way to raise pollutant removal.

Bioretention Cells — Stormwater Management & Green Infrastructure Design

🙋
A "bioretention cell" — that's the sunken little planted bed you see at the edge of parking lots, right? What is it actually doing when it rains?
🎓
Exactly — the planted strips in the corner of a parking lot or around a shopping center. It looks like a regular planter, but inside it is really an engineered natural filter and a short-term buffer tank. When it rains, dirty runoff from pavement and roofs first ponds on the surface, then slowly percolates through the planting media, gets filtered by roots and soil, and either recharges groundwater or leaves through an underdrain pipe at the bottom. The US EPA and Portland, OR have promoted them since the 1990s, and they are now the poster child of Low Impact Development (LID).
🙋
How is it different from just a tank? Wouldn't a buried concrete cistern do the same job?
🎓
Good question. A concrete tank only stores volume — it does nothing for water quality. Bioretention is special because it tackles peak-flow control and water-quality treatment together. Parking-lot runoff carries tire rubber, heavy metals (zinc, copper), oils and nutrients, but a well-designed cell removes 80-95% of TSS, 70-90% of zinc and 30-50% of nitrogen. On top of that, plants transpire and the cell cools the surroundings. That is why cities are shifting from grey infrastructure (concrete pipes) to green infrastructure worldwide.
🙋
So I should just build a big one, right? Bumping the "sizing ratio" up clearly raises the capture rate. But in practice land is tight…
🎓
That is the design tension. The rule of thumb is 5-10% of the catchment area. Above 10% the land use becomes inefficient; below 5% a 75 mm event overwhelms it. There is also the drawdown trap: if you pick low-permeability media, the storage volume cannot drain in 24-48 hours, plants drown and mosquitoes breed. That is why this tool flags drawdown over 72 hours as not acceptable. A typical recipe is 50-70% sand, 20-30% compost, 10-20% topsoil, targeting 50-100 mm/h.
🙋
The underdrain is optional in the tool. How do I decide whether to install one?
🎓
It depends on the underlying soil. If the in-situ subsoil is sandy with infiltration above ~13 mm/h, a fully infiltrating cell with no underdrain can recharge groundwater. On clay (under ~5 mm/h) you must use a filter-type cell with an underdrain that pipes the treated water to the storm network. Get it wrong and the cell turns into a multi-day puddle and residents complain. Many Japanese urban areas sit on silty-clay soils so underdrains are standard, while sandy soils in Florida often run infiltration-only — it is decided by the geotechnical report.
🙋
Can I plant whatever I like, like a regular flower bed?
🎓
That is a major misconception — you need a dedicated planting plan. The plants must tolerate cycles of inundation and drought, and ideally take up nitrogen and phosphorus. In the US, switchgrass, blue flag iris and certain sedges are staples. Lawn grass alone is too shallow-rooted to remove much nitrogen, and putting a tree in the center will eventually destroy the underdrain. The standard recipe is deep-rooted herbaceous plants and small shrubs in the basin, with any trees placed around the perimeter.

Frequently Asked Questions

A bioretention cell (also called a rain garden) is a shallow, vegetated, engineered depression that temporarily holds, infiltrates and filters stormwater runoff from impervious surfaces such as parking lots, roads and roofs. It is built up from a surface ponding zone (5-30 cm), an engineered planting-soil media layer (40-90 cm of a sand-compost-topsoil mix), a gravel choker layer and a perforated underdrain. It is a core technology of Low Impact Development (LID) and is widely implemented by the US EPA, Portland (OR), Seattle (WA) and many other cities.
A common sizing rule is to allocate 5-10% of the contributing catchment area to the bioretention cell. For a 1,000 m² parking lot that means 50-100 m² of cell. This tool calls that the "sizing ratio". Too small and the runoff capture ratio drops; too large and you waste land. For a design event such as a 24-hour 75 mm storm, aiming for 80% capture typically keeps the sizing ratio between 5 and 15% depending on imperviousness and rainfall depth.
Drawdown time is how long it takes a full cell to empty through the media. EPA and ASCE design guidance recommends 24-48 hours. Longer than that and the plants drown, mosquito breeding becomes a problem and there is no free volume for the next storm. This tool computes drawdown as totalStorage / (mediaInfiltrationRate × cellArea); above 72 hours the design is flagged as not acceptable. If the rate is too low, increase the sand content of the media or add an underdrain.
A properly designed bioretention cell typically removes 80-95% of TSS, 40-60% of total phosphorus, 30-50% of total nitrogen, 70-90% of heavy metals (zinc, copper) and 80-95% of petroleum hydrocarbons. The mechanisms combine physical filtration (TSS), plant uptake (N, P), media adsorption (metals) and microbial degradation (hydrocarbons). Bypass volumes that exceed storage are not treated, so raising the capture ratio is the single most important lever. This tool estimates TSS and TN removal from the capture ratio.

Real-World Applications

Urban parking lots and commercial sites: In Portland and Seattle, new parking facilities can only be permitted if they include bioretention cells. The planting islands are intentionally graded into sunken basins so that runoff from the surrounding pavement is captured and filtered. Large chains such as Walmart and Starbucks use bioretention to score LEED credits, and similar approaches are increasingly required under stormwater retention regulations in Tokyo and other Asian cities.

Streetside planters and "green streets": Long, narrow bioretention strips installed between the sidewalk and the road — popularized by New York City's Green Streets programme — capture small catchments but, replicated hundreds of times across a city, sharply reduce combined-sewer overflows (CSOs). Tree pits enlarged into "tree trenches" with stormwater functionality are a closely related concept.

Residential rain gardens: Roof downspouts are diverted into a planted depression in the garden so the runoff infiltrates on the lot. Prince George's County, Maryland pioneered the residential rain-garden movement in the 1990s, and DIY guidance is now widely available. Smaller-scale designs (catchments of 100-300 m²) are also being installed at temples, schools and parks in Kyoto and Kanazawa under the local label of "rain gardens".

Campus and park sustainable landscapes: Universities and city parks increasingly weave bioretention into their grounds for both performance and education. The University of Pennsylvania and UC Berkeley involve students directly in stormwater-landscape design projects, and similar initiatives are visible at the University of Tokyo Kashiwa Campus and at several Waseda campuses.

Common Misconceptions & Pitfalls

The biggest misconception is that "any flower bed receiving runoff becomes a rain garden". The heart of a bioretention cell is its engineered media, not ordinary garden soil. A typical Bioretention Soil Media (BSM) is roughly 50-70% sand, 20-30% compost and 10-20% topsoil by weight, balancing 50-100 mm/h infiltration against TP, TN and metal adsorption. Ordinary garden soil drops to under 10 mm/h and fails drawdown rules, while excessive compost can actually leach phosphorus and worsen receiving-water quality. In the US, BSM is procured and quality-controlled to recognized specifications.

A second pitfall is assuming that "with an underdrain, the subsoil does not matter". Even with an underdrain, if the subsoil percolates at only a few mm/h the media-subsoil interface stays saturated, goes anaerobic and the plants suffer root rot. Always run a percolation test (double-ring infiltrometer or equivalent) before design; aim for at least about 13 mm/h native infiltration. Below that, either thicken the gravel storage layer or wrap the cell in an impermeable liner and rely fully on the underdrain as a "filter only" cell.

Finally, "bioretention is maintenance-free" is a costly myth. The natural look hides the fact that surface media accumulates litter, fines and oils, and infiltration can drop to half of the as-built rate within 3-5 years. Inspect at least once or twice per year, remove surface scum and weeds, replace dead plants, and rejuvenate the top 5-10 cm of media every 3-5 years. US case studies show repeated functional failure after roughly five years where maintenance budgets were not planned alongside the capital cost.

How to Use

  1. Enter catchment area in hectares using catchNum (typical range 0.5–5 ha for residential/commercial sites)
  2. Set imperviousness percentage (impNum: 40–100% for parking lots, roofs, compacted soil)
  3. Input design storm rainfall in mm (rainNum: 25–100 mm depending on local IDF curve and return period)
  4. Specify bioretention cell surface area in m² (cellNum: typically 5–15% of catchment area)
  5. Simulator calculates design runoff volume, storage requirement, and TSS removal efficiency in real time

Worked Example

Suburban commercial site: 2.5 ha catchment, 65% imperviousness, 50 mm design storm (10-year ARI), 250 m² bioretention cell. Design runoff = 81.25 m³. With 0.6 m media depth and 150 mm/h infiltration rate, total storage = 150 m³. Capture ratio = 54%. Drawdown time over 24 h = 16 hours (meets most municipal standards). TSS removal = 87% (typical for engineered sand-compost media with 1.2 m³/m² pore space). Sizing ratio = 10%, acceptable for water quality targeting.

Practical Notes

  1. Increase catchment imperviousness incrementally to model retrofit scenarios; observe nonlinear effect on runoff volume and drawdown time
  2. For clay soils (infiltration <50 mm/h), reduce cellNum or add underdrain system; drawdown time extends beyond 48 h without it
  3. TSS removal improves with media depth >0.75 m and vegetation establishment; sandy clay loam media outperforms coarse sand
  4. Size ratio of 12–15% ensures capture of 90th percentile storm in most Australian/US climates; check local stormwater guidelines