Upstream, downstream, centerline: reading raise type from satellite imagery

A regulator or an insurer asking "what raise method is this facility" isn't making small talk anymore. Since Fundão and Brumadinho, raise type has become one of the first questions on any tailings file, because it changes where the structure is weakest and how fast a problem can move. The construction records usually answer it. But if you're staring at a stack of weekly or monthly imagery and trying to confirm what you're looking at, or tracking whether a facility is behaving the way its stated method predicts, you're reading geometry, not paperwork.

Here's what each method actually looks like from above, and what to track once you know which one you've got.

Upstream raises: the crest walks toward the pond

In an upstream raise, each new lift sits partly on the beach of settled tailings from the lift below it. The practical result, visible over a sequence of images spanning several raises, is that the crest alignment migrates inward, toward the impoundment, while the outer (downstream) toe barely moves at all. Beach width between the crest and the pond edge shrinks a little with each raise cycle, because the crest is advancing onto ground the pond used to occupy.

Compare imagery across years and the signature is unmistakable: a downstream slope face that holds roughly the same position pass after pass, paired with a crest line that creeps toward the water on every cycle. That's also why upstream facilities draw the most scrutiny on beach width and freeboard specifically. The foundation under the new fill is tailings, not native ground, so a shrinking beach there means the loading path is changing, not just settling into a familiar pattern.

Downstream raises: the footprint grows outward

A downstream raise does the opposite. Each new lift is built on compacted fill or native ground beyond the toe of the previous one, so the whole structure widens away from the pond over time. In a time series, the outer toe advances into the valley raise after raise, and the embankment gets visibly wider and flatter at its base with each cycle. The crest itself may rise in elevation but it doesn't migrate inward the way an upstream crest does.

This is the easiest method to confirm from imagery, because the expansion is outward and cumulative. If the downstream toe in this month's pass sits noticeably past where it sat two or three raises ago, that's consistent with downstream construction, not a structural surprise.

Centerline raises: a little of both, modestly

Centerline sits between the two. Each raise adds fill both upstream and downstream of the prior crest's centerline, so the structure gets taller roughly along the same vertical axis rather than marching in one direction. From a sequence of images you'll see small, roughly symmetric advances on both the upstream and downstream toes, neither one dominant. It's a subtler pattern than upstream's inward crawl or downstream's outward sprawl, and it's easy to misread as "no change" if you're only glancing at a single pass instead of comparing against the baseline footprint.

Why the method changes what you watch

None of this is academic once you've confirmed the raise type. An upstream facility built on tailings makes beach width and seepage staining near the toe the indicators that matter most week to week, because the loading path runs through material that hasn't fully consolidated. A downstream facility, built on firmer ground, generally tolerates more crest movement before it means anything, so freeboard trend and crest survey points carry more weight there. Centerline sits in between, and the modest symmetric growth pattern is itself worth tracking, since a sudden asymmetric shift (one toe advancing while the other doesn't) is the kind of departure from the expected pattern that's easy to miss without a running comparison.

That comparison is the part a single walkdown or a one-off image can't give you: whether this week's beach width, crest position, and staining look like the last twenty weeks, or like something new. Tailings Dam Monitor builds that running log against your own facility's baseline, so a crest that's drifted an inch toward the pond shows up next to the ten passes before it, not in isolation.

If you'd like to see what that weekly read looks like for your own facility, get in touch about a weekly log for your site.