Example Data Table
| Scenario | Area | Rain | C | Safety | Void | Required storage | Notes |
|---|
Formula Used
- Runoff Volume = Area × Rainfall Depth × C × Safety Factor.
- Required Storage (void) = Runoff Volume (no infiltration credit).
- Excavation Volume = Required Storage ÷ Void Ratio.
- Plan Area = Excavation Volume ÷ Depth.
- Circular Diameter = √(4 × Plan Area ÷ π).
- Rectangular Length = Plan Area ÷ Width.
- Drawdown Time ≈ Storage ÷ (Infiltration Rate × Infiltration Surface Area).
How to Use This Calculator
- Select your unit system and choose Size or Check.
- Enter drainage area, design rainfall depth, and an appropriate runoff coefficient.
- Set a safety factor and void ratio that match your stone or chamber system.
- For sizing: enter a practical depth (and width for rectangular) to get suggested dimensions.
- For checking: enter existing dimensions to see if storage is adequate.
- Optionally add infiltration rate to estimate drawdown time and compare to your limit.
- Use the CSV/PDF buttons in the results to export a clean report.
Professional Notes
Runoff volume drivers
Runoff volume depends on drainage area, design rainfall depth, and the runoff coefficient. Roofs typically use C near 0.90, while turf areas may use 0.20 to 0.35. This calculator multiplies area × rainfall × C, then applies a safety factor to cover clogged stone, uneven grading, and rainfall variability. If downspouts connect directly, treat the contributing roof footprint as fully connected area.
Storage vs excavation
Dry wells store water in voids, not in solid stone. Typical washed aggregate void ratio ranges from 0.35 to 0.45, meaning only 35–45% of the excavated volume is usable storage. Required excavation volume equals required storage divided by the selected void ratio, producing realistic dig sizes for garden installations. In practice, add space for bedding, wrap, and any chamber ribs that reduce net volume.
Geometry and sizing outputs
For circular pits, the plan area is excavation volume divided by depth, then diameter is computed from √(4A/π). For rectangular trenches, the calculator keeps your chosen width and solves length as plan area ÷ width. Suggested dimensions are excavation dimensions; cover soil thickness is separate. Keep access in mind for maintenance and verify that slopes do not collapse during wet seasons.
Infiltration and drawdown checks
Drawdown time is estimated using infiltration rate and total infiltration surface area (sidewalls plus bottom). The model is intentionally simple: it assumes a steady intake rate and uniform wetting. Use measured field rates where possible, and select conservative values for clayey soils or compacted backfill zones. When results are marginal, increase surface area, reduce loading, or provide overflow to a rain garden.
Planning, placement, and maintenance
Place dry wells downslope of buildings, protect them from sediment, and include an overflow route to a safe discharge point. Add geotextile to slow fines migration and consider a cleanout or pretreatment sump for roof leaders. Re-check performance seasonally; reduced drawdown often signals surface clogging. Simple pretreatment, like a leaf screen and a small settling basin, can extend service life significantly.
FAQs
1) What rainfall depth should I design for?
Use a local design storm or a practical event such as 25–50 mm (1–2 in). If you expect intense downpours, increase rainfall depth or safety factor to reduce overflow risk.
2) How do I choose a runoff coefficient?
Select C based on surface type: roofs 0.85–0.95, pavers 0.60–0.85, compacted soil 0.40–0.60, and lawns 0.15–0.35. Mixed areas can use an area-weighted average.
3) What void ratio should I use for stone?
Washed angular stone often ranges from 0.35 to 0.45. Rounded gravel may be lower. If using chambers or crates, use the manufacturer’s storage fraction for the chosen system.
4) Why does the calculator ignore infiltration for storage sizing?
Storage sizing without infiltration credit is conservative and easier to verify. Soil intake can drop over time due to fines, roots, or biofilm. Drawdown is still estimated separately to help you judge performance.
5) What drawdown time is considered acceptable?
Many guidelines target 24–72 hours after a storm, depending on soil, climate, and mosquito concerns. Use the allowed drawdown field to match your local requirements or your project’s risk tolerance.
6) Can I split one large dry well into several smaller ones?
Yes. Increase the “number of wells” to divide runoff and required storage per well. Splitting can improve distribution and reduce excavation depth, but ensure each well has stable soil support and safe overflow routing.