What We Mean by Forensic Engineering

By Simon Crowther, FPS Environmental  | July 2026

At FPS Environmental, we often describe our approach as “forensic engineering”.

That phrase matters to us because good flood risk, drainage and environmental engineering isn’t just about applying a standard detail, reading a map, or quoting for a product. It’s about understanding what is actually happening on the ground, why it’s happening, and what the evidence is telling us.

The question that interests me most is usually why?

Why did it flood? Why did the ground behave that way? Why was the system designed like that? Why did the existing setup behave that way? Why does the mapping say one thing when the site evidence suggests another? What is the paving sub base?

Most good engineering decisions start there.

Similar to Building Pathology

In building surveying, there’s a well-known term called building pathology.

Building pathology is the holistic, forensic study of building defects, their causes and their remedies. Much like a medical diagnosis, it means looking beyond the visible symptoms to find the underlying root cause. A good surveyor doesn’t see damp staining and immediately recommend repainting. They ask why the damp is there, where the moisture is coming from, how the building is behaving, and what repair will actually address the cause rather than just mask the damage.

Forensic engineering in flood risk and drainage works the same way.

A flooded basement, a surcharging manhole, a failed non-return valve, an unexpectedly wet site: these are all symptoms. The visible problem is rarely the whole story. Our job is to investigate what’s actually happening, test it against the available evidence, and only then review mitigation options tha reflects the real conditions.

Starting With the Site

Our starting point is always the site.

Flood maps, sewer records, hydraulic models, drainage drawings, soil maps and planning documents are all valuable, but they’re not a substitute for understanding the physical place. We look at levels, ground conditions, existing infrastructure, flow routes, outfalls, drainage connections, maintenance history and how the site may have changed over time.

Sometimes the mapping broadly matches what’s happened on the ground. Sometimes it doesn’t.

When it doesn’t, the important question is why.

Has the landscape changed? Has a flood wall, bund or highway scheme altered the flow path? Has a drainage system been modified? Is the model based on assumptions that don’t reflect the site? Is the sewer behaving differently to how it was expected to behave? Are the soil maps giving a general indication while the actual ground conditions tell a more complicated story?

This is where engineering judgement matters.

Evidence Before Solutions

We regularly see situations where a solution has been chosen before the problem has been properly understood.

A management company asks a contractor to fit a non-return valve. A developer asks for a drainage strategy based on a drawing that doesn’t reflect what’s actually on site. Sometimes those solutions are appropriate. Often, the real issue is that nobody has asked the right questions first.

A non-return valve, for example, can be a useful part of a sewer surcharge strategy, but only if it’s in the right location, protecting the right pipework, accessible for maintenance, and suitable for how the system actually works.

Just this week, I reviewed a block of flats where non-return valves had already been fitted, yet the lower ground floor continued to flood from sewer surcharge. The issue wasn’t that a product had failed. The issue was that the investigation hadn’t been done properly in the first place: the valves had been installed in the wrong location for the way the drainage system actually behaved.

The forensic investigation should have happened before the remedial works, not after.

Primary, Secondary and Tertiary Evidence

A forensic engineering approach means reviewing the evidence properly.

That includes primary evidence: site observations, levels, drainage records, photographs, CCTV surveys, flood marks, eyewitness accounts, as-built information. It includes secondary evidence: Environment Agency flood mapping, sewer asset plans, local flood risk records, planning documents, historic mapping, previous reports. And it includes wider context: soil mapping, catchment data, historic land use, nearby development, highway drainage changes, maintenance records and model assumptions.

The skill isn’t simply collecting information. It’s understanding how much weight to give each piece of evidence, where the evidence conflicts, and what that tells us about the likely cause of the problem.

Why This Matters

Flood risk and drainage problems are often expensive, disruptive and stressful.

Jumping straight to a solution can lead to unnecessary cost, over-design, or works that simply don’t solve the issue, and in some cases, make it worse. A forensic engineering approach reduces that risk. It means we understand the mechanism of flooding or drainage failure before recommending a remedy. It means designs are proportionate. And it means our work is better able to stand up to scrutiny, whether that’s from a planning authority, a client, an insurer, a management company, a resident group, or another professional reviewing our conclusions.

Good engineering isn’t just about producing a drawing or a report. It’s about understanding the site, interrogating the evidence, and making decisions that reflect what’s actually happening on the ground.

That’s what we mean by forensic engineering.

Engineering Grounded in Real Conditions

At FPS Environmental, our work covers flood risk assessments, drainage strategies, SuDS, flood investigations, property flood resilience, sewer surcharge reviews and wider environmental consultancy. Across all of those areas, the same principle applies: we don’t believe in applying standard answers to complex sites.

We look at how water moves, how systems perform, where constraints exist, and how the site has changed over time. We then use that evidence to develop practical, proportionate solutions.

Because in flood risk and drainage, the most important question is often not “what product do we need?” or “what drawing do we need?”

It’s: why is this happening?

Once you understand that, the engineering design needed becomes much clearer.