Why a Postcode Can Show Different Flood Risk Levels for Different Sources

A single postcode can show a different flood risk band for rivers, for coastal or sea flooding, and for surface water, all at the same time. This is not an inconsistency in the data. Each flood source is caused by a different physical mechanism and is modelled independently by the relevant agency.

Why sources are assessed separately

River flooding occurs when a watercourse overflows its banks, typically after heavy or sustained rainfall in a wider catchment area. Coastal or sea flooding is driven by tides, storm surges, and sea level, affecting a different, generally narrower set of locations near a coastline. Surface water flooding happens when rainfall cannot drain away quickly enough locally, regardless of nearby rivers or coastline, and can occur almost anywhere, including areas well away from any watercourse.

What this means for a single property

A property can be at low risk of river flooding because it sits well above a nearby river, at low risk of coastal flooding because it is far from the coast, and still be at high risk of surface water flooding due to local drainage conditions, ground slope, or impermeable surfaces nearby. None of these ratings override or inform the others, since they measure genuinely different things.

Why this is shown as separate figures rather than one combined score

Combining these into a single figure would obscure genuinely different types of risk that call for different responses (property-level flood defences for surface water are different from the kind of consideration relevant to river or coastal flood risk). Environment agencies across the UK, including the Environment Agency, SEPA, and Natural Resources Wales, all publish these as separate figures for this reason, and reports reflect that same separation rather than combining them into one number.

What this means for the reader

• Check all applicable flood sources for a location, not just one, since a low rating in one source does not indicate anything about the others
• The relevant agency's own flood risk checker (linked from the report) allows a source-by-source look-up for any specific address