Collier Row has a real flood risk, and almost none of it comes from rivers. Only a sliver of the area falls inside the river flood zones. About a third of postcodes still show high risk once rainwater is counted in. The one flood on record in recent years started in the drains. That is the risk to understand here, and the river maps barely show it.
On the Environment Agency's river-and-sea mapping, about 1% of Collier Row's postcodes fall in flood zone 3, the highest-probability band, and about 2% more fall in zone 2, the medium-probability fringe outside it. The two bands don't overlap, so the second figure adds to the first. On that measure, rivers and the sea pose very little risk to Collier Row.
Surface water is where the risk lies. The Environment Agency has four recorded flood events in the area. The most recent, in June 2016, was logged against a main river but caused by local drainage and surface water, and about 1% of postcodes fall inside historic flood outlines. A suburb of paved front gardens and roofs that drain straight to the road needs the drains to cope, and in 2016 they couldn't.
The Environment Agency's climate-adjusted mapping, which looks ahead to wetter decades, puts about 3% of postcodes in the river-and-sea flood zones. That's a modest increase on the present picture, and it still covers rivers and the sea only. The rainwater risk is assessed separately.
For insurance, the useful point is that underwriters price flood risk at each address, and surface water is included. A home well clear of any river can still be quoted as a flood risk if its road collects runoff. For any house on a shortlist, enter the exact address into the Environment Agency's long-term flood risk service and read the surface water result, which is the one that matters here.
The fuller picture counts a postcode once it reaches a given level on any of several measures, with rivers, sea and surface water all included. About 35% of Collier Row's postcodes are high on at least one measure, about 29% medium and about 49% low. That is far more than the river zones suggest, and almost all of the difference is rainwater.
Counting homes gives a smaller figure. The Environment Agency's rivers-and-sea register lists 446 properties, and only in the 18 postcodes that carry any risk at all, because the rest of the area isn't listed. Of those, 24 properties are at high risk, 20 of them homes, along with 7 at medium risk and 99 at low. On rivers, the risk comes down to a couple of dozen addresses. On rainwater, it spreads much further.
Flood defences make no difference to the modelled picture. About 1% of postcodes fall in the high-risk extent with defences left out of the model, and the same 1% with defences included. Even where a defence does appear in that model, it shows only that the structure exists, which isn't the same as the house behind it being safe. Here the defences don't shift the line at all. Defence walls do nothing about rain falling on a cul-de-sac.
Flood risk is Collier Row's weak point. For a buyer, the river zones matter much less than finding out where the rainwater goes on their street.