How to set a TARP trigger threshold for tailings dam freeboard

Most TARPs we see have a freeboard line item that reads something like "Amber: freeboard within 1.0 m of design minimum." The number is defensible on paper. The problem is where it came from. Too often it's copied from a neighboring facility's EPRP, or it's the number the engineer of record used on the last project, adjusted slightly because someone felt it should be adjusted. A trigger threshold has to tie back to your own pond behavior, your climate, and how often you look at the structure. Set it from someone else's data and you've written down a placeholder, not a control.

Start from your design freeboard, not an arbitrary number

Your design freeboard already accounts for the probable maximum flood or the operating basis flood, whichever governs for your consequence category. That number came from somebody doing the hydrology properly. Express the trigger threshold as a fraction of that margin, calculated from your own data, rather than borrowing a flat value from somewhere else.

A useful starting question: how much freeboard do you lose in a bad week? Pull your rate-of-rise data from wet season operation, not the annual average. If your worst observed week closes 0.3 m of freeboard, an amber trigger set 0.4 m above the design minimum gives almost no warning. That's a number that looks fine in the TARP document and fails the first time conditions turn.

Red triggers get the attention in these documents, but amber does the heavier lifting: it has to fire early enough that lowering the pond, opening additional decant capacity, or calling the EOR can change the freeboard trajectory before the reading crosses into red.

Build in a buffer for how often you look

This is the part most TARPs skip. A threshold calculated against continuous or weekly surveillance is a different number than one calculated against quarterly walkdowns, and a lot of facilities set the first kind of threshold while running the second kind of inspection program.

If your crest and freeboard get a physical look four times a year, size your amber trigger for the worst case that can develop between those visits, using your fastest observed rate of rise rather than the average. Work backward: multiply that rate by the number of weeks between inspections, and that's the minimum buffer your amber line needs above red. For a lot of facilities, that math lands the amber threshold uncomfortably close to design freeboard itself, and that's usually the honest answer.

Closing that inspection gap, rather than padding the number to compensate for it, is the more defensible fix. A facility checked weekly can run a tighter trigger threshold than one checked quarterly, because the surveillance catches a slow-building trend before it crosses the line instead of discovering it already crossed. That's the gap our weekly imagery read against your own baseline is built to close, so the threshold you write down can reflect how the pond behaves rather than how rarely you're able to look at it.

Tie the number to an action, not just a color

Every freeboard line in the TARP needs a clear, named action behind it: who gets called, what gets inspected within what timeframe, and what operational change happens (pond lowering, decant adjustment, spillway check) before the next reading. An amber trigger without an owner and a clock attached is just a number on a page.

Review the threshold annually, or sooner if pond operation changes, if a new discharge point gets added, or if a surveillance interval changes in either direction. A trigger level set for quarterly walkdowns stops being valid the day you move to a tighter inspection cadence, and it stops being conservative enough the day you move to a looser one.

Set the number from your own data, attach it to an action with an owner, and check it often enough to know if the pond is trending toward it before the next scheduled walkdown. If you want that trend visible every week instead of once a quarter, Tailings Dam Monitor delivers a weekly log against your own baseline so a slow-building freeboard problem shows up long before it reaches the line you drew.