Hornsey's flood risk comes from rain, and almost none of it from rivers. No part of the district is in a river or sea flood zone. Close to three in ten postcodes still rate high on at least one flood measure. The difference is surface water, running downhill into a valley whose river now runs underground. A clean river map and a soggy street can belong to the same address.
The Environment Agency's river-and-sea mapping puts 0% of Hornsey's postcodes in Flood Zone 3, the highest-probability band, and 0% in Flood Zone 2, the medium-probability band that lies beyond it. The district sits in the valley of the River Moselle, which has been culverted, and on the river maps it does not register at all. A sales listing can describe a Hornsey flat as outside the flood zones and be completely accurate, while describing only part of what can happen.
Surface water is the real exposure. That is rain that cannot drain away fast enough, collecting on roads and in low spots before it ever reaches a watercourse. In a valley this built up, with a river that now runs in a pipe, the water follows the old slope of the land whatever the maps say about rivers. The Environment Agency's long-term flood risk service lets you look up surface-water risk for a single address, and that lookup is the one that matters here, because the risk changes from one end of a street to the other.
Looking ahead, the Environment Agency's climate-adjusted zones also put 0% of Hornsey's postcodes at risk from rivers and the sea. Even allowing for a wetter future, the river maps stay empty.
For insurance, the clean river-and-sea position helps, but insurers price surface water too, and an address in one of the high-rated pockets may see that reflected in the quote or in the excess. A ground-floor or basement flat is where any such questions will be asked most closely.
At postcode level, taking several flood measures together and counting a postcode if any one of them applies, about 29% of Hornsey's 680 postcodes rate high on at least one measure, about 21% medium and about 32% low. Surface water accounts for that picture, given the river and sea measures are all at zero. "Low" here still means some chance in any given year.
The question of defences barely comes up. The modelled river-and-sea flood extents put 0% of postcodes in the high-risk area with defences and 0% without, so there is nothing for a defence to hold back on those maps in the first place. A defence present in the model would never have meant a dry house anyway, and here the point is academic.
The nearest tide gauge on record is at Portsmouth, where sea level rose by about 2 millimetres a year over the record from 1962 to 2023. For a district in a north London valley, that number belongs on a different page.
No flooding has been recorded within the district in the historic flood records. That is reassuring as far as it goes, though a surface-water flood that soaks a few basements does not always make it into the official record.
Hornsey has a weak flood position for a place with no river on the map. The risk is surface water, it is common across the postcodes, and it falls house by house.