Checking What is Inside a Stopbank, Without Digging It Up
- Jul 18, 2024
- 4 min read
A stopbank is only as reliable as what is inside it, and that is the problem: you cannot see it.
Many of New Zealand’s flood-protection embankments were built decades ago, often in different stages and from variable materials. In many cases, the record of how they were constructed is limited. Yet the councils and authorities responsible for these assets need to understand how they are likely to perform when flood levels rise.
The risks that can lead to stopbank failure are usually hidden: soft or poorly compacted zones, seepage pathways, old culverts, buried services and changes in material through the embankment. Digging into the bank to check can be slow, expensive and may disturb the very structure you are trying to protect.
Geophysics offers a way to build a picture of what is happening inside a stopbank without cutting into it. This article explains how Cook Costello used several geophysical methods across a 10 km network of flood-protection embankments to identify internal variability, potential seepage zones and areas needing closer attention.
Why stopbank integrity is hard to assess
Most stopbank failures begin out of sight. Water can find a seepage path through a more permeable layer and gradually pipe material from within the embankment. Soft or poorly compacted zones can settle or slump, and old culverts or buried pipes can create preferential flow paths straight through the bank.
Traditional investigation relies on boreholes or test pits. On a long stopbank, that approach is slow, expensive, and only tells you about the handful of locations you investigate. Every hole also has the potential to create a new weakness in a structure whose job is to remain continuous and watertight.
What flood-protection managers need is a picture of the whole asset, not just a series of isolated spot checks, and ideally without disturbing the embankment itself.
The approach: several geophysical methods, combined
Geophysics measures physical properties of the ground from the surface and uses those measurements to interpret what is happening below. The various methods respond to different ground conditions, so combining methods gives a more useful picture than relying on one technique alone.
Across a 10 km section of stopbanks, we used four complementary techniques:
Multichannel Analysis of Surface Waves, or MASW: measured shear-wave velocity to identify softer, lower-stiffness zones that may be relevant to stability.
Ground Penetrating Radar, or GPR: used a 250 MHz antenna to image shallow layering and identify buried culverts and other man-made features.
Vs30 profiling: extended the shear-wave velocity interpretation to 30 m depth, providing broader geotechnical characterisation of the ground beneath the stopbanks.
Electrical Resistivity Tomography, or ERT: mapped variations in electrical resistivity, helping identify changes in moisture, material type and possible seepage-prone zones.
By integrating these methods, we developed interpreted 2D cross-sections along the stopbanks. These helped engineers understand how the embankments varied internally and identify areas needing closer attention.
What the survey covered, and found
Across the network, the survey collected a substantial dataset:
33 MASW lines
96 Vs30 profiles
106 GPR lines
19 ERT profiles
The results showed significant variability both along and through the embankments, with shear-wave velocities ranging from about 100 to 250 m/s. Shear-wave velocity is a measure of stiffness: the higher the number, the stiffer the material is likely to be. The lower end of that range pointed to softer, lower-stiffness zones within the banks, while the higher end indicated firmer, more competent material.
A spread that wide across a single network is important. It is consistent with embankments built in stages from variable material, with some sections likely to perform differently from others under flood loading.
The water table was interpretable in several locations, and buried culverts and other man-made features were resolved in the data. Being able to detect and match known culverts gave confidence in the geophysical interpretation and helped support its use for identifying other areas needing closer attention.
The survey gave the asset owner a much broader picture of the stopbank network than isolated test points could provide, while also helping target any further intrusive investigation to the areas where it would add the most value.
What it delivered, and why it matters
For the council and engineers responsible for these stopbanks, the survey turned a largely hidden asset into something that could be understood, visualised and managed.
The geophysical data showed how the banks varied internally, identified softer zones, detected buried utilities and highlighted areas where seepage risk may be higher. That gave asset managers evidence to plan maintenance, prioritise further investigation and focus attention where the risk appeared greatest.
The value was not that geophysics replaced all intrusive investigation. It was that geophysics showed where intrusive investigation would be most useful. Instead of relying on isolated boreholes or test pits, the survey provided much broader coverage along the stopbank network. Where the geophysics identified areas of concern, targeted drilling or testing could then be focused in the right places.
For flood-protection authorities, that combination is powerful: broad coverage, non-invasive investigation and a clear steer on where the risks may be. It makes geophysics a practical tool for managing ageing flood-defence assets, especially as flood risk and pressure on infrastructure continue to grow.
Where we come in
Cook Costello carried out this work using our in-house geophysics capability. Because our geophysicists work alongside our geotechnical engineers, the survey was not treated as standalone data collection. The results were interpreted in the context of what they meant for the ground, the embankment and the next stage of assessment.
That is where this approach is most useful: large-scale assets, sensitive structures and situations where understanding the wider pattern matters more than isolated test points.
If you are responsible for stopbanks or other flood-protection assets and need to understand their condition, tell us about the network. We can advise how a geophysical survey could help identify risk areas, target further investigation and support long-term asset management.
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