Beaches display water quality classifications that look like a statement about today's sea. They are calculated from samples taken across several previous seasons.
The tests look for indicator bacteria
Sampling programmes measure particular intestinal bacteria that indicate contamination from sewage or agricultural runoff.
Those organisms are not usually the hazard themselves; they are measured because they are abundant, easy to detect and correlate with the presence of harmful material.
Laboratory culture takes time, which is the fundamental reason a sample cannot tell a bather about the water they are currently standing in.
Classification is calculated over multiple seasons
Standard schemes combine several years of samples into a statistical assessment, so a single bad result does not by itself change the classification.
This makes ratings stable and comparable between sites, which is what allows them to be used as a public standard.
It also makes them slow to respond, so a beach whose conditions have recently changed may carry a classification that no longer reflects it.
Short-term pollution is handled separately
Heavy rainfall can overwhelm drainage and cause brief contamination, and this is predictable enough to be managed by forecasting rather than sampling.
Many authorities issue advisory notices after significant rain, and some beaches display them automatically without waiting for a laboratory result.
These short events are usually excluded from the classification calculation where they are correctly identified and reported, since they are not representative of normal conditions.
Award schemes measure more than water
Voluntary beach awards require good water quality as a precondition but also assess lifeguard provision, signage, facilities and environmental management.
A beach can therefore have excellent water and no award, simply because it lacks the facilities the scheme requires.
Criteria differ between schemes and countries and are revised over time, so the meaning of a given flag depends on which scheme issued it.
How to read the information at the beach
The posted classification indicates the general reliability of the site, and the most recent sample results are usually published alongside it.
Advisory notices after rainfall carry more current information than the classification does, and they are the item worth checking on the day.
River mouths and storm outfalls are the usual local hot spots, which is why guidance often suggests bathing away from them regardless of the overall rating.