The harbour gives, the harbour takes. About 15% of postcodes carry a high flood-risk flag on at least one measure. Another 14% sit at medium. Environment Agency flood zone 3 covers about 3% of the area. Climate uplift takes that to roughly one in six. The risk is geographic, not generic.
About 3% of the area sits inside Environment Agency flood zone 3, the highest-probability zone for river or coastal flooding, and another 4% sits inside zone 2, the medium-probability ring around it. That is the formal classification of where the harbour can come up over a couple of decades on a bad combination of tide, surge and rainfall. Historic flood records show about 1% of postcodes have already taken water at some point on the public register, which is the small but non-zero share that has already been proved by experience rather than modelled.
Surface water is the bigger headline for the wider postcode set. About 15% of postcodes carry a high surface-water flood-risk indicator, with another 14% sitting at medium, and a further third sitting at low risk. Surface water flooding is the kind that arrives during a heavy summer downpour rather than during a winter tide event, and it is the kind insurance assessors increasingly look at first because the drains are the thing that age between underwriting cycles.
The forward look is harder. With a future-climate allowance applied to Environment Agency flood zone modelling, the share of land at risk rises to about 17% of the area, or roughly one in six. That is the share that planners and insurers will be modelling against by the 2050s. Emsworth sits at the head of Chichester Harbour, and over a third of postcodes are within 500 metres of the coast, with the closest barely six metres from the water. The current Shoreline Management Plan policy for the area is Hold The Line across the twenty-year, fifty-year, and hundred-year horizons, which is the policy committing the public purse to defending the existing coastline rather than retreating from it. The Portsmouth tide gauge, the nearest long record, shows a sea-level trend of about two millimetres a year over the 1962 to 2023 window. That is small per year and meaningful over a mortgage.
On insurance, the practical point is that flood cover for a property in a high-risk band runs higher than a property a few streets inland, and that gap has been widening as the models update to the new climate scenarios. Flood Re, the reinsurance scheme that backstops most affordable domestic flood cover, sits behind policies for properties at material risk. Flood Re does not cover homes built since 2009, so anyone buying a more recent build close to the water should price the cover before they price the property. Older period stock is generally inside the scheme. Anything with a slab and a build date in the last fifteen years is not.
Flood risk here rates as average, which is the honest middle of the spectrum and not the same as low. The risk concentrates where the geography puts it: close to the harbour and on the low-lying ground around it. The further you walk uphill, the smaller the number gets, and the closer you walk to the water, the more the surveyor will want to talk about.