Seawall Flanking and End-Around Erosion on Lake Michigan: Why Walls Fail at the Property Line
Most seawall failures we get called out to look at are not really failures of the wall. The sheet piling is straight, the cap is intact, the tiebacks are holding. What has gone is the ground at one end, usually the downdrift end, where the armored shoreline stops and the natural bank begins. Water found the edge and went around it.
Lakefront owners tend to notice this late, because the damage starts on the side of the property rather than the front. By the time there is a visible notch cut in behind the last panel, the wall has often lost several feet of the backfill it depends on. This is what flanking looks like on Lake Michigan, why it happens, and what can actually be done about it.
What Flanking Actually Is
A vertical seawall does not absorb wave energy. It reflects it. That is the entire trade-off of hard armoring: you get a definite line that holds against a definite force, and in exchange the energy that would have dissipated across a sloping beach bounces back into the water column and travels somewhere else.
At the ends of the structure, that reflected energy meets an abrupt change in shoreline hardness. Wisconsin DNR's design guidance for Great Lakes erosion control structures notes that flat surfaces reflect wave energy, which can cause scouring at the base of a structure or increase wave run-up, and that straight, shore-perpendicular structure ends produce chaotic wave conditions leading to increased wave-based erosion at those corners. The corner becomes the weak point by geometry, not by workmanship.
Once the bank next to the wall starts to cut back, the process accelerates. The eroding notch gives waves a pocket to enter. Each event reaches slightly farther landward. Eventually the erosion line passes behind the plane of the wall, and water begins removing backfill from behind the last few panels. That is the point where a structural problem finally appears, because a seawall with no soil behind it has nothing to retain.
How to Recognize It Early
Flanking has a distinct signature, and it is different from the failure patterns we cover in repair versus replacement.
| What you see | What it usually means |
|---|---|
| A notch or embayment cut into the bank right at the end of the wall | Classic end-around erosion, already underway |
| Turf or soil slumping toward the lake within a few feet of the terminus panel | Backfill is being drawn out laterally |
| The last one or two panels leaning or rotating while the rest of the run stands plumb | Lost lateral support at the end, not a global wall failure |
| A widening gap between the wall cap and the adjacent grade | Soil loss behind the terminus |
| Your neighbor's unarmored bank retreating faster than the general shoreline trend | Reflected energy from your structure concentrating downdrift |
| Fresh sand or fill fanning out lakeward of the terminus after storms | Material is actively being removed from the corner |
The two corners rarely behave the same way. On most Lake Michigan reaches, littoral drift has a dominant direction, and the downdrift terminus takes the worse beating because it is starved of the sediment the wall interrupted. Photograph both ends every year from the same two spots. That habit belongs in the same annual pass as our seawall inspection checklist.
Why It Gets Worse in High Water Years
Flanking is a function of where the water surface sits relative to the top of the bank. In low water years, waves break well lakeward of the terminus and the corner sees relatively little. In high water years, the same waves arrive with more depth beneath them, break closer in, and put their energy directly against the transition point.
That is why owners often describe flanking as sudden. It is not. It is a slow geometry problem that becomes visible when lake levels rise into the range where the terminus is exposed. Anyone who watched the 2019 and 2020 highs on this lake saw the pattern at scale: walls that had sat quietly for a decade suddenly had holes cut around their ends in a single season. Our discussion of Lake Michigan water levels and erosion risk covers how those cycles work.
Ice adds a second mechanism. Shore ice piles preferentially against irregular edges, and a structure terminus is exactly that kind of edge. Ice shove that rides up a bank at the corner can strip vegetation and open the soil to the next open-water season. We covered that failure mode in ice shove and winter damage.
The Fix: Returns, Transitions, and Continuity
There are three real answers, and they scale with how far the erosion has already gone.
A return. The end of the wall turns landward, ninety degrees or close to it, and ties into the upland bank far enough back that wave action cannot get behind it. This is the standard detail and the one most existing walls are missing, usually because the original run stopped at a property line rather than at a defensible point in the bank.
A graded stone transition. Instead of an abrupt vertical end, the structure steps down into armor stone that dissipates energy rather than reflecting it, blending the hard line back into the natural profile. Wisconsin DNR's guidance describes rounding a structure and merging it into the upland as it approaches the littoral property boundary, both to avoid impacts to adjacent property and to minimize flanking around the ends. Sizing and layering matter here for the same reasons they matter in a full riprap revetment, and undersized stone at a terminus simply becomes the next thing that moves.
Continuity across the property line. On a straight reach with cooperative neighbors, extending armoring so the terminus lands somewhere geologically sensible rather than at a legal boundary is the strongest answer. It moves the vulnerable corner away from the middle of an actively eroding bank, and it usually costs each owner less than two separate mobilizations.
Where the bank above the wall is tall or already unstable, the terminus fix has to be planned alongside the slope itself. Cutting a return into a bluff toe without addressing drainage can trade one problem for another, which is the whole subject of bluff stabilization on Lake Michigan.
The Neighbor Problem, Honestly
This is the uncomfortable part of shoreline armoring and it deserves a straight answer. A wall built to the property line protects the ground behind it and hands the reflected energy to whoever is next in the drift. That is not a moral failing on the part of the owner who built first, but it is a physical consequence, and it is one reason regulators look hard at adjacent-property impacts.
Wisconsin DNR specifically lists assessing the impacts of a planned project on adjacent properties as one of the things an experienced coastal engineering professional brings to a shoreline project. Michigan's EGLE water resources program reviews shoreline work under the same general concern. Practically, that means a permit application that ignores what happens at the ends is a weaker application, and a design that addresses them is a stronger one.
It also means the conversation with the neighbor is worth having before the design is final rather than after the notch appears. Shared terminus details, a coordinated return, or a joint project all become much harder once someone has lost ten feet of yard and hired a lawyer. Permitting timelines on the lake are long enough that early coordination costs nothing but a phone call. The state-by-state process is laid out in our Lake Michigan seawall permit guide.
What We Would Look At on Your Lot
- Both terminus corners, at the same water stage. Compare them to each other and to last year. Asymmetry tells you the drift direction.
- Whether a return exists at all. Many walls on this lake simply stop. If there is no landward leg, flanking is a matter of time and lake level.
- The condition of the adjacent bank. Vegetated and stable, or freshly cut and raw? Raw means it is happening now.
- Where the backfill is going. Soil loss behind the cap at the ends is different from soil loss along the run, and it points at flanking rather than at drainage.
- What the neighbor has, or is planning. A new wall next door changes your terminus conditions whether or not you were consulted.
If the notch is small and the wall is otherwise sound, a return or stone transition is a contained project. If the erosion has already reached behind the last panels, the terminus section usually has to come out and go back in with proper embedment, which is a different scope and a different budget.
Free Seawall Consultation
We will look at both ends of your wall, not just the face, and tell you whether what you have is a terminus problem, a structural problem, or both.
Request a ConsultationFrequently Asked Questions
What is seawall flanking?
Flanking is erosion that works around the end of a shore protection structure instead of attacking its face. Waves that the wall deflects carry energy past the terminus, scour the unarmored bank next to it, and then begin cutting behind the wall from the side. The wall itself can be in good condition while the property beside it disappears.
Why does a seawall erode my neighbor's shoreline?
A hard wall reflects wave energy rather than absorbing it. That energy has to go somewhere, and it concentrates at the downdrift end of the structure where armored shoreline meets natural bank. Wisconsin DNR guidance notes that straight, shore-perpendicular ends produce chaotic wave conditions and increased erosion at those corners.
How do I stop end-around erosion at my seawall?
The standard fix is a proper return: the end of the wall turns landward and ties into the upland bank, or transitions into graded stone that dissipates energy instead of reflecting it. Wisconsin DNR describes rounding and merging the structure into the upland as it approaches the property line. Retrofits are possible on most existing walls.
Is flanking damage covered by my seawall warranty?
Usually not, because the wall has not failed. Most contractor warranties cover materials and workmanship on the structure itself, and flanking is erosion of adjacent ground. Read the terms before assuming coverage, and photograph both terminus corners annually so you can document when the loss started.
Do I need a permit to extend or return the end of a seawall?
Almost certainly. Work at or below the ordinary high water mark on Lake Michigan is regulated by the state agency in your state and often by the U.S. Army Corps of Engineers. Extending a structure is a change, not maintenance, so plan on a permit review. Confirm the current process with your agency before designing anything.
Can two neighbors build one continuous seawall?
Yes, and it is usually the better engineering answer on a straight reach of shoreline. A continuous run moves the terminus corners to the ends of a longer armored section instead of putting one in the middle of an eroding bank. It also splits mobilization cost. Each owner still needs their own permit coverage.