Why did a 72 million litre flood happen in a non-floodplain?

In September 2024, around 72 million litres of water submerged the A421 in Bedfordshire. What makes this event so concerning is that, officially, the area isn't designated as a floodplain. So how did this happen?

When it comes to planning permission, developers and local authorities typically consult the Environment Agency's Flood Zone maps. These maps classify land from Flood Zone 1 (low probability of flooding) to Flood Zone 3b (highest risk). This area of the A421 is classified as Flood Zone 1.

The categorisation is based almost exclusively on proximity to waterways, such as rivers, and the coast, however it doesn't take into account other occurrences such as low lying land or areas susceptible to flash flooding during heavy downpours, snow melt, etc.

The National Planning Policy Framework (NPPF) classifies essential utility infrastructure, including electricity supply systems, as "Essential Infrastructure" that must remain operational during flooding. This classification demands that such infrastructure be designed to function, even in Flood Zone 3 – the highest risk level. To allow for this, a control panel in Flood Zone 3 would normally be put above the 1 in 100 year flood level ensuring it is not exposed to even severe flooding.

More information can be found here: Get flood risk information for planning in England - Flood map for planning - GOV.UK

However as part of the works to make the new road, there is an underpass on the A421 to allow for a adjacent road. This creates a low point acting as a basin that during heavy rainfall, such as we had in September 2024, can lead to large volumes of water very quickly collecting. Normally this wouldn't be a problem, however as the control panel was located at the bottom of the underpass, as the water started to pool it quickly reached the electrical control panel leading to a short.

Once the panel shorted, the pumps stopped and then nothing was removing the water so it turned into a multimillion litre flood in a short space of time causing the road to be shut and hundreds of tanker visits to clear the flood.

So the real lesson from this is: while following the regulations, like it appears all parties did in this case, that may not always be enough. Sometimes, it needs some engineering thinking to think through the different scenarios.

News coverage from National Highways here: Update: A421 in Bedfordshire remains closed in both directions due to significant flooding - National Highways