Signs Your Dam Wall Is About to Fail

I’ve spent years building and repairing earthen dams across Queensland and New South Wales, and the same pattern comes up repeatedly. Landowners see a warning sign, think it might be nothing, and put it on the back burner. Then a solid rain comes through and the wall fails overnight. In most cases, early action would have saved the dam and the paddock below it.

This post runs through the main warning signs that a dam wall is in trouble, what each one means structurally, and what you should do about it.

Why Earthen Dam Walls Fail

Most earthen dam walls fail because of internal erosion (piping), overtopping, or slope instability. Overtopping is the most dramatic: water crests the wall and scours the downstream face. Piping and slope failure are often more insidious because they develop slowly and can show very few external signs until they’re well advanced. Knowing which failure mode you’re dealing with changes what you need to do next. That’s why the warning signs matter.

Seepage and Wet Patches on the Downstream Face

Seepage through or under an earthen dam wall is the number one warning sign I look for on any inspection. A wet patch on the downstream face, or boggy ground at the toe, tells me water is moving through the embankment. That movement carries fine soil particles with it. Over time, those particles form channels. Once a channel forms, flow accelerates and the wall fails fast.

Some seepage through the base of a well-built wall is acceptable if it’s filtered correctly. But seepage that appears on the downstream face of the wall itself, or that discharges muddy water, is never acceptable. Muddy seepage means soil is being carried out of the wall. That is active piping, and it needs immediate attention.

If you find wet patches or seeps, monitor them closely. If they grow or the water runs discoloured, contact a dam consultant that day.

Cracks in the Wall

Longitudinal cracks running along the length of the wall often point to differential settlement or slope movement. Transverse cracks cutting across the wall from crest to toe are more serious because they can create a direct flow path for water through the embankment. Surface drying cracks are common and usually minor, but they allow water to enter the wall during filling and can trigger deeper issues.

Any crack wider than about 20mm, or one you can’t see the bottom of, deserves a proper inspection. Cracks also commonly appear around outlet pipes or spillways where the structure meets the embankment. These interface cracks can funnel water directly along the pipe barrel, which is a piping failure waiting to happen. Our dam repair team regularly deals with exactly this issue on older structures.

Slumping and Deformation

If the crest of the wall has dropped in a section, or the downstream face has bulged outward, the embankment is moving. Slumping usually indicates the soil in the wall has become saturated and the slope has exceeded its stable angle. This happens after extended wet periods, after a sudden rise in water level, or when original compaction was inadequate.

Walk the crest after every significant rain event and look for depressions or low spots. View the downstream face from a distance, because a bulge is often more visible from 50 metres away than from standing on the wall. Any movement that wasn’t there before is a red flag.

Animal Damage and Vegetation

Wombats, rabbits, and other burrowing animals are a genuine threat to dam walls. A burrow that connects through the wall from the wet side to the dry side is a ready-made piping channel. I’ve seen walls fail because of wombat activity the landholder never fully investigated. Tree roots present a similar issue: they create voids as they die and decay, and cause cracking from wind loading. Keep large trees off the wall itself and well clear of the toe.

Grass on the wall is good. Bare patches, erosion rills, or areas where grass has died off are worth investigating. Sometimes bare patches indicate seepage just below the surface that’s too saline or waterlogged for grass to survive.

Spillway and Freeboard

A dam without an adequate spillway is a dam waiting to overtop. Check your spillway after every major rainfall. Is it clear of debris? Is it eroding? Has the channel below cut back towards the wall? If your spillway has ever been overwhelmed and water has gone over the wall, inspect the downstream face for scour damage immediately.

Freeboard is the height between the water level and the crest. Most farm dams should maintain at least 600mm of freeboard during normal operation. If your dam is consistently close to the crest, you’re running without a safety margin. A well-designed dam sealing or remediation project sometimes includes spillway upgrades to address exactly this.

What to Do If You Spot Warning Signs

Note what you’ve seen, where it is, and how large or extensive it looks. A photo with a reference object for scale is worth a lot. Check the current dam level, because a high water level dramatically increases risk if the wall is compromised. Then don’t wait for the next dry spell to act. Earthen dam failures can progress from manageable to catastrophic in hours during a rain event. Early intervention almost always costs far less than repair after failure, and avoids the safety and environmental consequences of an uncontrolled release.

Frequently Asked Questions

How quickly can a dam wall fail once warning signs appear?

It depends on the failure mode. Piping failures can develop over days or weeks then accelerate to total collapse within hours once a channel forms. Slope failures can happen within minutes of triggering. The safest assumption is that any active warning sign can escalate rapidly, especially during or after rainfall.

Can a cracked dam wall be repaired without draining the dam?

Minor surface cracks can sometimes be addressed with targeted grouting or bentonite treatment without fully dewatering the dam. Cracks that run deep or show signs of internal erosion usually require dewatering and proper reconstruction of the affected section. The right approach depends on the crack’s depth, orientation, and cause. A site assessment is needed first.

Is seepage at the base of a dam wall always a problem?

Not always. Some seepage through the foundation of an earthen dam is expected and manageable with a properly designed drainage system. The concern is seepage that emerges on the downstream face of the wall, carries sediment, or increases suddenly after a rise in water level. Muddy or increasing seepage anywhere is a serious warning sign.

For technical guidelines on farm dam design and water management in Australia, the NSW Department of Primary Industries and Agriculture Victoria publish authoritative reference materials.

Spotted any of these warning signs on your dam wall? Book a site inspection with Big Ditch before the next big rain event.