What sizing means in practice
Sizing a rain garden means determining how large the basin surface needs to be — in square metres — to fully infiltrate the stormwater volume generated by a chosen design storm before the next event arrives. For Polish residential contexts, the typical target is complete drain-down within 24–48 hours of a 10-year return period event.
The calculation has three parts: estimating the runoff volume from the contributing catchment, determining how fast the underlying soil can accept water, and dividing one by the other to get a basin surface area.
Step 1 — Estimate runoff volume
The runoff volume from a roof surface is estimated using the rational method adapted for small catchments:
Where:
- V — runoff volume (litres or m³)
- A — catchment area in m² (the roof footprint)
- P — design rainfall depth in mm (see table below)
- C — runoff coefficient (0.9–0.95 for tiled or metal roofs)
A 100 m² roof in Warsaw receiving a 25 mm design event produces approximately 100 × 0.025 × 0.95 = 2.375 m³ of runoff — roughly 2,375 litres directed toward a downspout.
Step 2 — Measure or estimate infiltration rate
The infiltration rate of the native soil below the basin governs how quickly the stored water drains away. The most reliable method is a falling-head percolation test conducted at basin depth (typically 300–500 mm below surface). Three to five test holes averaged give a workable field value.
When field testing is not possible, published values for Polish soil texture classes provide a starting point:
| Soil texture (PTG 2009) | Typical infiltration rate (mm/hr) | Design rate (50% safety factor) |
|---|---|---|
| Sand (Ps) | 150–300 | 75–150 |
| Loamy sand (Pgl) | 60–150 | 30–75 |
| Sandy loam (Gl) | 25–80 | 12–40 |
| Loam (G) | 10–30 | 5–15 |
| Clay loam (Gc) | 2–10 | 1–5 |
| Clay (I) | <2 | Not suitable without underdrain |
A 50% safety factor on the measured rate accounts for biological clogging and seasonal variation over the basin's first three years of establishment.
Step 3 — Calculate basin surface area
With volume and infiltration rate known, basin area follows from the target drain-down time:
Where k is the design infiltration rate in m/hr and T is the target drain-down time in hours.
For the Warsaw example: 2.375 m³ ÷ (0.020 m/hr × 24 hr) = 4.95 m². A 5–6 m² basin is a practical result for this catchment in a sandy loam soil. On a clay loam site the same roof might require 20–30 m² — or an underdrain system.
Basin depth and ponding allowance
Standard shallow basins in Poland are designed with 150–250 mm of surface ponding depth. This ponding volume supplements native soil infiltration. For the calculation above, add ponding volume to V before dividing by the infiltration term — this reduces basin footprint modestly and is appropriate when a defined ponding area with level overflow is practical.
Overflow design
Every rain garden should have a defined overflow path for events exceeding the design storm. A level spreader or overflow pipe routed to the street drain or a secondary infiltration area prevents erosion and ensures the basin does not back up toward the building foundation. Overflow elevation is typically set at 100 mm below the lowest adjacent ground surface.
Worked example — 80 m² roof, Wrocław
- Roof area: 80 m²
- Design event (10-yr, 60-min): 26 mm
- Runoff coefficient: 0.92
- V = 80 × 0.026 × 0.92 = 1.91 m³
- Soil: silty clay loam, measured rate 8 mm/hr, design rate 4 mm/hr (0.004 m/hr)
- Drain-down target: 36 hr
- Basin area = 1.91 ÷ (0.004 × 36) = 13.3 m²
A 14 m² basin is the minimum for this site. In practice, 16–18 m² with shallow ponding and a perforated underdrain is a more reliable design for Wrocław clay soils.