Rainfall Trigger Checker
Checks measured antecedent or event rainfall against a site trigger threshold and reports whether the rainfall TARP action is triggered, the margin above or below the threshold, and (optionally) the average rainfall intensity. Used in slope, tailings and open-pit ground-control monitoring.
Enter Values
Minimum 0
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the measured rainfall depth (mm) accumulated over the monitoring period.
- Enter the trigger threshold (mm) from your site's rainfall TARP.
- Optionally enter the duration in hours to also get the average intensity in mm/hour.
How it works
The trigger is met when the measured rainfall depth reaches or exceeds the threshold. The margin (rainfall − threshold) is signed: positive means the threshold has been passed and by how much, negative means how much headroom remains.
If you supply the period duration, the tool also returns the average intensity = rainfall / duration (mm/hour) — a mean over the whole period, coarser than a true peak intensity. Rainfall thresholds are site- and regolith-specific and come from the ground-control plan.
Worked example
80 mm against a 50 mm trigger over 24 hours. Rainfall 80 mm ≥ threshold 50 mm, so the trigger is EXCEEDED with a margin of +30 mm. Average intensity = 80 / 24 = 3.333 mm/hour.
Common mistakes
- Using a generic rainfall threshold instead of the site/regolith-specific value from the ground-control plan.
- Ignoring antecedent moisture — the same event rainfall is far more hazardous on already-wet ground.
- Reading the average intensity as a peak intensity; short bursts within the period can be much higher.
Frequently asked questions
What is a rainfall trigger?
A rainfall depth (over a defined period) above which a specific ground-control action is required, because saturation raises pore pressures and reduces slope stability. It forms part of the site's rainfall TARP.
Why does the duration matter?
The same total depth is more hazardous when it falls quickly. Entering the duration lets the tool report the average intensity (mm/hour) so short intense bursts can be compared against longer soaking events.
What is antecedent rainfall?
The rainfall that fell in the days before the event of interest. It sets how wet the ground already is, strongly influencing how the current rainfall affects stability.
Are rainfall thresholds transferable between sites?
No. They depend on soil and rock type, weathering, slope geometry and failure history, so a threshold is specific to the site and must come from its ground-control management plan.
Related tools
- Prism Movement TARP Checker
- Prism Movement Rate Calculator
- Pore Pressure Ratio Calculator
- Saturated Slope FoS Calculator
- Crack Displacement Calculator
- Piezometer Trend Calculator
Explore more in Geology, Geotechnical & Ground Engineering.
Cite this tool
Zerdly. (2026). Rainfall Trigger Checker. Retrieved from https://www.zerdly.com/tools/rainfall-trigger-checker/
Tip: Enter any known values to calculate the remaining results.
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