BuildMeter guide · Excavation & Drainage
Stormwater Infiltration Planning Guide
Plan runoff capture, soil infiltration, storage volume, overflow and maintenance for onsite stormwater control.
Direct answer
What to know first
A reliable plan separates measured quantity from design decisions, allowances, package rounding, accessories, labor and field verification. Record each assumption so estimates and supplier quotes can be compared consistently.
Open Dry Well Size CalculatorStart with runoff volume, then test whether the site can infiltrate it
Stormwater infiltration planning combines hydrology and soil/site constraints. A rain garden or dry well must receive a defined runoff volume, temporarily store that volume and release it by infiltration or controlled overflow. The calculation is not complete until setbacks, seasonal groundwater, soil infiltration and an emergency overflow path are considered.
| Input | Purpose | Typical source |
|---|---|---|
| Drainage area | Area contributing runoff | Roof/impervious plan |
| Design rainfall depth | Event volume | Local stormwater criteria |
| Runoff coefficient | Loss adjustment | Surface type/design method |
| Infiltration rate | How fast stored water can enter soil | Field test/design data |
| Storage void ratio | Usable volume in stone/media | Material/design data |
Worked runoff example
A 1,200 ft² roof receiving 1 inch of rain sees 100 ft³ of rainfall volume because 1,200 × 1/12 = 100. Using a 0.95 runoff coefficient gives about 95 ft³, or roughly 710 gallons, directed toward the infiltration system. A dry well filled with stone cannot use its entire excavation volume as water storage because the stone occupies most of that space; the Dry Well Size Calculator therefore needs a realistic void ratio.
Recommended workflow
- Map the actual contributing roof or paved area.
- Select the design rainfall depth required for the project objective.
- Calculate runoff volume with the Rainwater Runoff Calculator.
- Compare storage volume with the Dry Well Size Calculator or Rain Garden Size Calculator.
- Verify infiltration rate, drawdown time, groundwater separation, setbacks and overflow path before construction.
Common mistakes
- Using excavation volume as though it were fully available water storage.
- Ignoring roof areas that drain to the same downspout.
- Assuming sandy-looking surface soil represents the infiltration rate at the bottom of the facility.
- Providing no safe overflow route for storms larger than the design event.
Professional limits
Local stormwater rules, soils, groundwater, slopes and building setbacks can control whether infiltration is appropriate. Do not infiltrate runoff where it can threaten foundations, slopes, wells or contaminated soil.