How a properly built wall is supposed to work
A seawall has to do two contradictory things: keep soil in, and let water out. If it only did the first, water trapped behind it when the tide dropped would exert enormous pressure on the panels.
The solution is filtration. Drainage stone and geotextile fabric behind the wall let water pass toward weep holes while holding the soil particles back. Done properly, water leaves and soil stays.
Every washout problem is a failure of that separation. Once water can take soil with it, the wall is no longer filtering — it is just draining, and it is draining your yard into the river.
- 1Backfill lost from the yard — the depression is the last stage, not the first
- 2Open joint — water takes the easiest route out, and it widens each cycle
- 3Sediment carried into the waterway — your soil, in transit
The five pathways
Soil can only leave in a limited number of ways, and each leaves different evidence.
1. Open panel joints
The most common route. Panels interlock, but differential movement, impact and settlement work the joints apart. An opening the width of a pencil moves a surprising volume over a season.
2. Failed filter fabric
Geotextile behind the wall degrades, tears, or was never lapped correctly at installation. Once it fails, the drainage path becomes a soil path.
3. Cracks through the wall
A crack that extends through a panel becomes a conduit. These usually show first as damp patches at low tide, then as visible flow.
4. Scour beneath the toe
Where current, wake or storm flow has excavated sediment at the base, soil migrates under the wall instead of through it. These voids sit deeper and further back.
5. Stormwater from above
Downspouts, deck drains and regraded yards discharging at the top of the wall erode backfill from the land side, independent of anything at the waterline.
Why it accelerates
Soil loss is not linear. Water moving through a small opening carries the finest particles first. Removing those raises the permeability of the soil around the opening, so more water flows through, so more material moves, so the opening enlarges.
On a tidal waterway this cycle runs roughly 700 times a year. A defect that seems trivial in its first season can produce a substantial cavity by its third.
It also explains why the problem seems to appear suddenly. The soil has been leaving gradually; the surface collapse that finally makes it visible is just the moment the remaining crust can no longer bridge the gap.
Blocked weep holes as a hidden contributor
Weep holes relieve the pressure that builds behind a wall as the tide falls. When they silt up or get blocked by vegetation and debris, that pressure does not disappear — it finds another route out.
The other route is almost always an unfiltered one: a joint, a crack, or under the toe. So blocked drainage does not just raise loading on the wall; it actively drives soil through whatever opening is available.
This is why clearing weep holes is a genuinely useful maintenance item rather than a cosmetic one.
What to look for yourself
A few observations from safe ground will tell you a lot.
- Watch the wall on a falling tide and look at what is coming out, and from where
- Clear water from weep holes is good; cloudy water from joints and cracks is not
- Look for sediment fans building up on the bottom below discharge points
- Check whether depressions track the wall line or are scattered across the yard
- Note where your downspouts and yard drains actually discharge
Frequently asked questions
Why does the soil keep disappearing after I fill it?
Because the pathway is still open. Filling restores the surface but adds material to the route that removed the original soil. Until the water is blocked or properly filtered, fill behaves as a consumable.
Could the problem be a broken pipe rather than the seawall?
It genuinely could, and it is worth ruling out. A failed storm drain or irrigation line can produce depressions that look identical. What points at the wall is soil loss that tracks the wall line, concentrates near joints, or worsens with tides rather than with irrigation.
How much soil is usually missing?
Almost always more than the surface suggests. Compacted, root-bound soil bridges cavities well, so the visible depression is typically a fraction of the actual void. Probing establishes the real extent.