Thread by @henryrobertshaw

A thread on what's happened at @afcwimbledon, drawing on a few years experience working on flooding.

#AFCW

Despite reports, I don't think the flooding was from the River Wandle....

🧵

River levels on Sunday night were the highest on the record for the gauge (3.23m vs 3.11m in Jul 2007 - it wasn't built in 1968), but below the level where the water would come over the banks of the river and cause flooding (3.48m - the grey line)

This is also the risk of river flooding in the area (darker blue = higher risk), which shows that there are a few places nearby which are at higher risk of flooding from the river than the stadium, but which evidently didn’t flood earlier in the week.

Instead, the flooding was most likely what is called surface water flooding. This is where rainwater does not drain away or soak into the ground as it should do, and instead just sits there. We mainly see this in urban areas where...

...extreme sudden downpours fall on tarmac or concrete, the water flows very quickly into the drainage system which is overwhelmed, causing the water to back up. It’s normally highly localised, difficult/impossible to predict, and getting worse with climate change.

(It can also be exacerbated by poorly maintained/blocked drainage - although impossible to say if that is the case here)

Unfortunately the closest available rain gauge is in Mitcham (1.5 miles away), but that shows ~36mm (about three weeks worth) of rain in two hours.

From the impacts at Plough Lane I’d be surprised if it didn’t receive more, or the rain was concentrated over a v short period.

It therefore seems that the flooding was caused by the local drainage system being overwhelmed by a short period of extremely heavy rainfall, causing the water to pool on the surface.

Responsibility for this sort of flooding lies with the local council, who - in time - should also have to produce what’s called a section 19 report. This will looks in detail at what caused the flooding and - importantly - what could be done to prevent it happening again.

At this point, it’s also important to note that the Environment Agency, Thames Water and the council all signed off the surface water drainage plans for the stadium and surrounding development as part of the planning process.

As for the damage to the pitch, an alternative (this time completely non-expert!) theory as to how the damage has been caused…

My understanding is that the pitch consists of layers of grass, soil, and gravel. Below the drainage system is a series of pipes running underneath the pitch, connected to large tanks which help collect the water, before releasing it into the public drainage system. Example below

The problem here being… because of the surface water flooding issues described above, the public drainage system is already at capacity. This presumably means that water backs up within the tanks and the rest of the system.

Whether this caused a pipe or tank to burst, collapse or whatever, I don’t know, but that might explain why it looks like the pitch has collapsed in a couple of straight lines in the photos above. Hopefully the club are getting to the bottom of it now.

But whatever happened, it is hopefully something that can be fixed with a few spare parts rather than something that’s caused wider problems for the whole system.

Anyway, all we can do for now is donate a few quid here (well done @MisterGLS):

£95,000 🤯

@WomblesDream @9yrspodcastjustgiving.com/crowdfunding/g…