Hornchurch has a real flood risk, and the rivers are barely part of it. Hardly any postcodes fall in the river flood zones. About one in five still rate high on some flood measure. The water that worries the maps here mostly arrives from above. A clean river map answers a smaller question than the one a buyer is asking.
The Environment Agency's river-and-sea mapping puts about 1% of Hornchurch's postcodes in flood zone 3, the high-probability band, and about another 1% in flood zone 2, the medium-probability fringe beyond it. The old parish was drawn between the Ingrebourne and the Beam, and both rivers have mostly kept to the edges ever since.
Surface water is the other story, and it is the bigger one. When heavy rain lands on a town of roofs, roads and paved driveways faster than the drains can take it, the water collects wherever the ground dips, and that has nothing to do with how far away the nearest river is. The river-zone figures leave it out entirely, which is why a house can be clear of every zone and still sit in a postcode flagged for flooding. The Environment Agency's long-term flood risk service shows surface-water risk for a single address, and for any house in Hornchurch that is the map to look at.
Allowing for climate change, the Environment Agency's climate-adjusted flood zone takes in about 2% of postcodes here. The river picture grows a little on that forward look, and stays small.
Insurance follows the measure the insurer uses, and insurers look at surface water as well as rivers. A house outside every river zone can still draw a higher premium or a larger excess if its postcode is flagged for surface water, so a quote on the specific property tells a buyer more than any general reassurance about the town.
Counting every measure together, rivers, sea and surface water among them, about 21% of Hornchurch's postcodes rate high on at least one, about 24% medium on at least one, and about 43% low on at least one. A postcode can appear in more than one of those groups, since each counts any of several measures. The gap between a near-empty river map and one postcode in five flagged high is the whole of this answer.
Counting homes gives a narrower picture. In the 33 postcodes the Environment Agency lists as carrying some river or sea risk, there are 838 properties, and 57 of them are rated high risk, 24 medium and 94 low. Of the 57 at high risk, 54 are homes. Postcodes with no river or sea risk at all do not appear in that list, so it says nothing about surface water.
On defences, the Environment Agency's modelled high-risk flood extent covers about 1% of postcodes with defences left out and about 1% with defences in place. A defence present in the model is a line on a map, and here it barely changes the outline. "Defended" in a sales conversation about Hornchurch describes the drawing more than the street.
Flood risk is Hornchurch's weak spot. The rivers are well behaved, and the rain that lands on the town is the thing a buyer has to price in.