5th June 2026
Main conclusion
Mott MacDonald found significant shortcomings in the A555 drainage design, construction and operation, but concluded that these shortcomings were unlikely to have been a major cause of the downstream property flooding in places such as Sandringham Road, Bramhall Green and The Demmings during the July 2019 event.
What went wrong on the A555 itself?
The report identifies a number of problems:
1. The road flooded repeatedly shortly after opening
The A555 opened in October 2018 but experienced several flooding incidents within months, including major closures in March and July 2019.
In July 2019:
- The road flooded at Woodford Road.
- The road flooded at Hall Moss Lane.
- Pump stations failed or tripped.
- Attenuation ponds overflowed.
- Sections of the road had to be closed for days.
2. Pumping systems were not sufficiently resilient
The report found:
- Pump overload trips occurred.
- Floodwater accumulated around pumping stations.
- Temporary pumps were required.
- Existing pumping arrangements became overwhelmed during the rainfall event.
3. Construction defects were identified
Investigators found:
- Blocked grilles.
- Debris in drainage systems.
- Potentially malfunctioning attenuation ponds.
- Bank slippage near drainage structures.
- Incorrectly installed drainage components.
One attenuation pond outlet was found to have been installed 680 mm higher than designed, reducing available flood storage.
4. Questions over whether the drainage design achieved its stated objectives
The report is unusually critical in places.
It notes that some “green” areas were drained directly to local watercourses without attenuation and states:
“It is difficult to justify this approach as meeting the intent of the FRA.”
The report also says:
“the shortcomings of the A555 drainage could have been avoided”
and criticises a planning process that focused on obtaining consent rather than delivering a “holistic and robust drainage design”.
5. Parts of the system may have been designed to lower standards
The report states that:
- Drainage Networks A, B and C were designed up to approximately the 1-in-100-year standard.
- Network E appears to have been designed only to around the 1-in-30-year standard.
- The combined performance of old and new drainage infrastructure was not fully checked.
Did the A555 cause flooding at Sandringham Road?
This is where the report reaches a different conclusion from what many residents suspected.
The investigators note that:
- Only a small proportion of the Lady Brook/Micker Brook catchment drains from the A555.
- The drained area associated with the A555 is roughly 4–5% of the wider catchment.
Their conclusion is:
“this investigation does not indicate A555 drainage deficiencies were a significant contribution to the downstream flooding in the borough”
and
“would have had little effect on the downstream flooding.”
What the report recommends
The report nevertheless recommends substantial work on the A555:
- Correct construction defects.
- Clear and maintain drainage systems.
- Ensure attenuation pond B functions properly.
- Review enlargement of attenuation storage.
- Build drainage models for the whole system.
- Improve resilience to flood exceedance events.
- Review pumping capacity and controls.
In one sentence
The report effectively says:
The A555 drainage system had significant design, construction and operational flaws and was not as robust as a newly built road should have been, but the evidence did not show that those flaws were the primary cause of the downstream flooding that affected Sandringham Road and other properties in July 2019.
What is notable, however, is that the same report also concludes that the flooding experienced at Sandringham Road occurred outside the Environment Agency’s mapped flood extent, indicating that the flood risk to those properties had been materially underestimated.
