Flooding: Do we need to change our approach to Basement Design?

Reports of London underwater have been a regular occurrence over the past few months. Pictures of a flooded Sloane Square underground station and the devastation caused to Brian May’s West End townhouse basement, and the irreplaceable mementos it contained, were particularly poignant images.

Whilst many of the media outlets have put their own spin on who is to blame for the problem, what cannot be denied is that a month’s worth of rain fell within a few hours. But was this just a freak one-off isolated event or is heavy rainfall and subsequent flooding becoming a more regular event? Well the science seems to suggest there is plenty more of it to come…

Climate Change 2021 Report

The much publicised UN report, ‘Climate Change 2021: The Physical Science Basis’ released this week states:

“It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred…. Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened.”

What does this mean? Well the very same report tells us that:

“Globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s (medium confidence). It is likely that human influence contributed to the pattern of observed precipitation changes since the mid-20th century, and extremely likely that human influence contributed to the pattern of observed changes in near-surface ocean salinity. Mid-latitude storm tracks have likely shifted poleward in both hemispheres since the 1980s, with marked seasonality in trends (medium confidence).”

Why is this significant to us?

Both examples given at the beginning of this article have certainly grabbed the most headlines, where both underground structures and many members of the Association specialise in keeping water out of below ground structures. In fact a number of PCA members have informed us that they were called out to flooded basements as a result of these recent heavy rainfall events.

These are not isolated events and it has clearly affected water ingress into basements. Should we be shrugging our shoulders, declaring it somebody else’s problem and, arguably, is it this attitude that got us here with climate crisis in the first place?

Alternatively is this an issue that we should be considering as becoming more commonplace, as the science suggests, and start to give some consideration when we are designing our systems to keep water out?

What additional considerations would we need to make?

Whilst most systems are designed very well and are perfectly adequate at dealing with water from the ground, do they need to consider the potential for water coming over the top? One anecdotal report claimed flooding in the basement had only been prevented due to a raised concrete step at the threshold of a property – which flies in the face of modern building practice where level access is desirable.

Clearly a solution such as this is not going to be suitable in all instances, but is it simply just an indication of how a little thought may have prevented many of these flooded basements?

Type C Systems are desirable for many installers as they are more tolerant of workmanship errors, but what capacity do these systems have for all this extra rainwater we are seeing and undoubtedly will be seeing more of? Are they being designed to ensure they have sufficient capacity, not just for today, but with the possibility of much greater levels of water ingress that can be anticipated with our changing climate?

Realistic approaches to basement design

Should we perhaps accept that basements will flood – we can’t be expected to predict an unprecedented local storm. A more realistic approach might be to design basements that are made habitable with as little inconvenience to the occupants as possible?!?

Climate change is not a problem that will be fixed overnight. Tackling the issue requires a global effort and can only be achieved by altering a mindset that has been engrained into us for decades. Whilst perceptions seem to be changing, we must all be awake to the fact that at least for the foreseeable future, increased levels of flooding are inevitable and the question is should we be taking this into consideration when designing below ground waterproofing systems…?

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