A field tech walks the cap after a storm and finds a long depression cutting across the side slope. Is that settlement working its way to the surface, or did last week's rain just carve a channel through the cover soil? The two get confused more often than you'd think, and the wrong call means the wrong fix gets scheduled.
What settlement looks like on a cap
Settlement comes from the waste mass compacting and decomposing underneath the cover. It shows up as a depression or a dip that doesn't track with slope or drainage patterns. Differential settlement, where one part of the cell sinks faster than the area next to it, tends to produce a more irregular shape: a sunken bowl, a tension crack along the edge of a dip, or a low spot that holds water even on a slope where water should run off.
A few tells that point to settlement rather than erosion:
- The depression doesn't line up with a drainage path or swale.
- Cracks radiate outward from the low point instead of running downhill in a straight line.
- Ponding shows up in a spot that wasn't designed to hold water, especially after the cell has been closed a few years and waste decomposition is still active.
- Vegetation over the depression often looks stressed or stunted well before the ground shape changes enough to spot by eye, because root zones get disrupted and gas can concentrate in the low spot.
Settlement is a geotechnical issue. It usually calls for a survey comparison against baseline grades, and depending on severity, cover soil placement to restore the design grade and keep positive drainage.
What erosion looks like on a cap
Erosion comes from surface water: runoff that the cover soil can't hold against once it starts to concentrate. Rills and gullies form downhill of something, a swale that's undersized, a bench drain that's clogged, a spot where vegetation thinned out and bare soil got exposed to sheet flow.
Erosion rills have a signature settlement doesn't:
- They run downhill, following the slope, often in a branching pattern like a tiny drainage network.
- They get deeper and wider toward the bottom of the slope, where more water has collected.
- They show up fastest after heavy rain events, not gradually over months.
- The soil loss is visible. You can usually see exposed subgrade, armor, or even geomembrane at the head of a bad rill.
Erosion is fixed with regrading, added riprap or erosion control blanket, and revegetation, sometimes paired with a look at whether the stormwater channel sizing upslope needs adjusting.
Why the difference matters for your corrective action plan
Treating settlement like erosion means you regrade and reseed a spot that's going to sink again in six months, because the compaction underneath never got addressed. Treating erosion like settlement means you order a geotechnical investigation and borrow material for a problem that a bigger swale and some erosion blanket would have solved for a fraction of the cost. Most post-closure permits require documenting cause before corrective action, so getting this right matters for what goes in the annual report, not just for saving a return trip.
Telling them apart in the field
The fastest field check: does the low spot follow drainage, or does it cut across it? Erosion channels nest into the drainage design. Settlement depressions ignore it. Second check: pull soil at the edge of the feature. A clean cut with exposed roots and no accumulated sediment usually means settlement. A buildup of transported sediment at the downhill end points to erosion.
The tricky cases are the ones where both are happening at once, a settled area that then starts eroding because it's collecting water it shouldn't be. That's usually where vegetation stress shows up earliest, often before the ground movement is obvious on a walkover. A quarterly read on vegetation stress across the cap, flagged from the kind of multispectral imagery used on Landfill Cover Monitor, can catch that overlap before it turns into a bigger repair.
If your next quarterly walkover has you guessing at which repair crew to schedule, it might be worth getting a read on the cap before you call one in.