Calculator Inputs
Example Data Table
| Shape | Length | Key dimensions | Water depth | Fill | Estimated volume |
|---|---|---|---|---|---|
| Rectangular | 30 m | Width 0.8 m | 0.5 m | 100% | 12.0 m³ |
| Trapezoidal | 18 m | Bottom 0.6 m, slope 1H:1V | 0.4 m | 80% | 4.15 m³ |
| Circular | 12 m | Diameter 1.0 m | 0.6 m | 100% | 5.64 m³ |
Examples are illustrative. Site dimensions and water profile can change along the run.
Formula Used
Rectangular
Area: A = b × h
Volume: V = A × L × f
Trapezoidal
Top width: t = b + 2s h
Area: A = (b + t)/2 × h
Circular Segment
Radius: r = D/2
Area uses a standard circular-segment equation based on depth.
Where b is bottom width, h is effective water depth, L is length, s is side slope (H:V), and f is fill fraction.
How to Use This Calculator
- Select the unit system that matches your field measurements.
- Pick the trench cross-section shape that best represents the excavation.
- Enter trench length and water depth, then set freeboard if needed.
- Provide the required shape dimensions, such as width or diameter.
- Use fill percentage to model partial flooding or isolated ponding.
- Press Calculate Water Volume to view results above the form.
- Use the CSV or PDF buttons to save the latest report.
Practical Guidance for Trench Dewatering
1) Why water volume matters
Knowing the standing water volume helps you size pumps, tanks, and discharge controls before excavation work continues. Even small misreads can delay pipe bedding, compaction, and inspections. A few cubic meters may be handled with portable pumps, while larger volumes can require staged pumping, multiple sumps, and continuous monitoring.
2) Typical field dimensions and variability
Trenches vary along their length due to overbreak, soft spots, and bedding changes. A 0.6–1.0 m working width is common for many utility installations, but side slopes can widen the top substantially. If the cut opens from 0.6 m at the base to 1.4 m at the top, water area can nearly double at the same depth. Measuring at several stations improves the estimate and reduces surprises.
3) Using shape options effectively
Rectangular sections match shored or boxed excavations where walls are near vertical. Trapezoidal sections suit open cuts where side slopes follow soil stability requirements, often expressed as H:V. Circular segments are useful for culverts, pipe trenches, and partially filled round structures where water forms a segment rather than a full circle. Selecting the closest section minimizes systematic error.
4) Converting volume into pumping effort
Translate volume into time using your planned flow rate and realistic site losses. For example, 12 m³ equals 12,000 liters. At a steady 200 L/min, that is about 60 minutes of pumping. Add allowance for inflow from groundwater, rainfall, and hose friction that reduces delivered flow. When inflow is persistent, plan for bypass pumping and standby capacity.
5) Quality checks and reporting
Use freeboard to exclude the dry zone above the waterline, and apply fill percentage when flooding is partial. Record input dimensions, method, and date for traceability. Re-check after each rainfall event, major excavation stage, or change in shoring. The CSV and PDF exports support daily logs, subcontractor reporting, and consistent documentation across shifts.
FAQs
What is the most accurate way to measure trench dimensions?
Measure length and water depth, then confirm width or slope at several stations. Use average values for long runs, and document any sections that are significantly wider or deeper than the rest.
When should I use fill percentage instead of changing dimensions?
Use fill percentage when the trench is only partly flooded or has isolated standing pools. Keep geometry based on excavation size, then scale the volume to match the observed water coverage.
How does freeboard affect the result?
Freeboard subtracts a dry zone from the measured depth, reducing effective water depth. It helps when you measure from a reference point above the waterline or want to exclude air space intentionally.
Why does trapezoidal volume increase quickly with depth?
With side slopes, top width expands as depth increases, so area grows faster than a rectangular section. That widening effect can make small depth changes produce large volume changes.
Can I use this for groundwater inflow calculations?
This tool estimates standing volume, not inflow rate. For inflow, track how volume changes over time and divide by the time interval, then adjust for pump flow and rainfall.
Which unit system should I choose?
Choose the unit system matching your field measurements. Metric inputs are treated as meters, and imperial inputs are treated as feet. The calculator reports results in multiple common volume units.
What safety steps should be considered during dewatering?
Maintain stable trench walls, keep pumps and hoses clear of walkways, control discharge to prevent erosion, and follow local requirements for sediment management. Stop work if conditions change or instability is observed.