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The Question
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A levee was losing integrity and nobody knew why. Regular visual inspections revealed nothing unusual. The embankment looked stable, the grass cover intact, the crest elevation unchanged. Yet monitoring data suggested something was happening beneath the surface.
The question was deceptively simple: what is causing the slow but measurable deformation of this levee — and can we find it before it becomes a safety issue?
The Context
The levee in question is part of a flood defence system along [river name], protecting [area] from [risk]. Built in [year], it had performed well for decades — until routine monitoring picked up anomalies.
Understanding why required looking beneath the surface. And that's where geophysics comes in.
The levee section under investigation.
The Approach
We deployed a combination of geophysical methods to image the internal structure of the levee. The primary tool was electrical resistivity tomography (ERT), supplemented by ground-penetrating radar (GPR) along selected profiles.
ERT works by injecting current into the ground and measuring the resulting voltage distribution. Variations in resistivity reveal differences in soil composition, moisture content, and — critically — voids or disturbed zones.
Reference Smith, J. & Van den Berg, K. (2019). Animal burrowing and erosion risk in flood defences. Journal of Hydraulic Engineering. Read paper →
The survey covered a 200-meter section of the levee with electrode spacing of 0.5 meters, providing a resolution sufficient to detect features as small as 30 centimeters in diameter.
Data processing followed a standard inversion workflow, but we added a time-lapse component by comparing measurements taken in dry and wet conditions.
Reference De Vries, M., et al. (2021). Geophysical methods for levee assessment. Engineering Geology. Read paper →
The Insight
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The resistivity images revealed a network of low-resistivity anomalies running through the levee body. These weren't random — they followed distinct patterns, branching from the waterside slope inward toward the crest.
Resistivity profile showing the network of anomalies (dark blue) interpreted as animal burrows. The branching pattern is clearly visible from the waterside slope toward the crest.
Cross-referencing with the GPR data confirmed the interpretation: these were animal burrows, most likely created by muskrats or rabbits. The burrows had created preferential flow paths through the levee, allowing water to penetrate deep into the structure during high water events.
What made this particularly dangerous was the invisibility of the problem. The entrance holes on the surface were small and often obscured by vegetation. But inside the levee, some tunnels extended over two meters in length.
A barely visible burrow entrance on the waterside slope — easy to miss during visual inspection.
Why It Matters
Animal burrowing is one of the most underestimated threats to flood defences. Visual inspection alone cannot detect the extent of internal damage. This case demonstrated that geophysical methods can reveal what lies beneath — literally — and provide the quantitative evidence needed to prioritise maintenance and repair.
The levee section was subsequently repaired, and the geophysical survey approach has since been adopted as part of the regular monitoring programme.
Looking Ahead
This project raised a broader question: how many levees have similar hidden vulnerabilities? With thousands of kilometers of flood defences in the Low Countries alone, systematic screening is impractical with current methods.
The future likely lies in combining remote sensing, drone-based surveys, and targeted geophysical investigations — using data science to identify where to look, and geophysics to look deeper.
Could automated anomaly detection in ERT data reduce the need for expert interpretation? Could machine learning identify burrow signatures in large-scale datasets? These are questions worth exploring — and exactly the kind of challenge where geo, hydro, and data science intersect.
Further reading This story is based on published research. For the full technical details, see the references in this article. Interested in how we approach similar challenges? Browse our other stories →
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GeoHydroData delivers research and consulting at the intersection of Geo, Hydro, and Data Science. We help organisations look beyond the surface — turning complex measurements into the insights that drive better decisions.
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