| Metric | Value | Units |
|---|---|---|
| Units mode | Imperial (feet, inches, ft³) | — |
| Inflow volume during storm | 175.000 | ft³ |
| Pipe outflow during storm | 175.000 | ft³ |
| Infiltration during storm | 1.563 | ft³ |
| Net storage needed | 0.000 | ft³ |
| Trench storage capacity | 60.000 | ft³ |
| Storage surplus (capacity - need) | 60.000 | ft³ |
| Estimated time to drain after storm | 0.00 | hr |
Inflow volume: V_in = C × A × h, where runoff coefficient C, contributing area A, and storm depth h (from intensity × duration). Units consistent (ft, m).
Trench storage: V_storage = L × W × D × n_v, where n_v is gravel void ratio.
Pipe capacity (Manning full flow): Q = k/n × A × R^{2/3} × S^{1/2}, with wetted area A = \pi d^2/4, hydraulic radius R = d/4, slope S, roughness n; factor k=1.49 in US customary, k=1.0 in SI.
Infiltration: V_infil = A_\text{{bottom}} × f × t, with bottom area A_\text{{bottom}}, infiltration rate f, and storm duration t.
Net storage needed: V_net = \max(0, V_in - (Q\,t + V_infil)). Design succeeds if V_storage \ge V_net.
| Name | Units | Area | Intensity | Duration | C | Void | Len | Wid | Dep | Pipe d | Slope % | Infil | Action |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Light yard storm | Imperial | 1500 | 0.8 | 0.5 | 0.45 | 0.40 | 40 | 1.2 | 1.8 | 4 | 1.0 | 0.20 | |
| Typical design | Imperial | 2000 | 1.5 | 1.0 | 0.70 | 0.40 | 50 | 1.5 | 2.0 | 4 | 1.0 | 0.25 | |
| Heavy driveway event | Imperial | 3500 | 2.2 | 1.5 | 0.85 | 0.45 | 70 | 2.0 | 2.5 | 6 | 1.5 | 0.30 |
Use the Load button to copy a row into the form, then press Calculate.
- Void ratio typically 0.30–0.45 for washed gravel.
- Use slope between 0.5% and 2% where practical.
- Runoff coefficient varies: turf 0.2–0.4, pavement 0.7–0.95, roofs 0.9–1.0.
- Consider geotextile to limit fines migration.
This tool provides conceptual sizing; verify against local codes and detailed hydrology where required.