Yes, Harlow has a real flood risk, and the river accounts for very little of it. About half of the town's postcodes carry a high flood risk on at least one measure. Barely any are in the river flood zones. The rest is rain finding the low point of a road. For a Harlow buyer the question is surface water, every time.
The Environment Agency's river-and-sea flood zones put about 2% of Harlow's postcodes in flood zone 2, the medium-probability fringe outside the core, and under 1% in flood zone 3, the high-probability core itself. Those are two separate bands, so the small zone 3 figure is a share of its own and isn't counted inside zone 2. The River Stort runs along the northern edge past Harlow Mill, and the agency's zone maps give it a very small footprint across the town.
Surface water drives the high figure, and it shows in the gap between the measures. The river zones cover a sliver of Harlow, yet once rainfall pooling and running off the ground is counted, about half the postcodes reach a high rating on something. Surface water follows the shape of the land and the drains, so it can affect a street that has no river anywhere near it. The agency has 60 recorded flood events across the area. The latest, in January 2021, was a main river overtopping a channel with no raised defences. The record is history, and it doesn't predict what the next storm will do.
The Environment Agency's climate-adjusted zone, which projects how the river-and-sea zones grow under future conditions, takes in about 2% of postcodes. That barely moves the river picture. Climate projections for the river leave the town's main flood exposure, which is surface water, largely where it is today.
Insurers price surface water at street level, so two houses a few roads apart in Harlow can get quite different quotes. Put the exact address into the Environment Agency's long-term flood risk service and read the surface-water result as well as the river one. That second result is the one Harlow buyers most often miss.
Across the postcode indicators, about 49% of Harlow's postcodes rate high on at least one measure, about 21% rate medium and about 64% at least low. Each postcode is tested against several measures, surface water included, and counted under a level if any single measure reaches it. A postcode can therefore appear in more than one group, so the three shares don't add up to a whole.
The agency's rivers-and-sea mapping, counted as properties, lists 22 Harlow postcodes with some river or sea risk, holding 690 properties. Of those, 35 are at high risk, 87 at medium, 167 at low and 1 at very low. Only 5 of the high-risk properties are homes. Postcodes with no river or sea risk don't appear in that list at all. Five homes at high risk from the river is a small number, and a buyer can check whether the house being viewed is one of them.
The modelled high-risk river extents are tiny in Harlow either way. Under 1% of postcodes are in an undefended high-risk extent, and under 1% in a defended one. "Defended" there means a defence appears in the agency's model. It says nothing about whether that defence keeps water out of a particular kitchen, and the flood that matters most here wouldn't come over a defence anyway.
Flood risk is Harlow's weak point. The river is a minor worry, but surface water reaches half the town's postcodes, and that needs checking for each address.