Why a Harbour Floods on a Calm and Sunny Day

Treat the sections below as a sequence. With tidal flooding in harbour towns, getting the early decisions right makes the later ones much easier.

The tide table is a prediction, not a measurement

Published tide tables are astronomical predictions based on the positions of the moon and sun over time. The actual water level is that prediction plus a residual produced by weather, and the residual can be substantial.

On a day when the residual is positive and the predicted tide is already high, the two add together. That is how a town floods on an afternoon with blue sky and almost no wind at the harbour itself. National tide gauge networks publish the observed level alongside the prediction, which makes the residual visible.

Air pressure pushes the sea down

Atmospheric pressure acts on the sea surface, so a deep low allows the water beneath it to rise. The relationship is close to a fixed amount of sea level change for a given change in pressure.

Along that stretch of coast, a large high-pressure system does the reverse, holding the level down and producing lower tides than predicted. Because pressure systems are large and slow, this effect covers a wide area rather than one harbour. It is the reason a settled anticyclone can make a spring tide look unimpressive and a passing low can do the opposite.

Wind and swell arrive late

Strong winds pile water against a coast, and that piled-up water takes time to relax after the wind eases. Surge generated over a distant sea area travels as a long wave and can arrive well after the storm itself has gone. Long-period swell from the same storm arrives on a similar delay and adds run-up on top of the raised level.

Before the first ferry, this is why the day after a storm is often the day the quay actually floods. Local flood warning services model these effects specifically, which no general tide table attempts to do.

The water comes up through the ground

Harbour town drainage discharges into the harbour, usually through outfalls below the top of the tide. When the sea rises above the outfall, the drains cannot discharge and the sea can travel back up them.

With the tide out, the result is water appearing in the middle of a road behind an apparently intact harbour wall. Non-return valves and pumped systems exist to prevent this, and they are a maintenance cost rather than a permanent fix.

It also means rainfall arriving at high tide has nowhere to go, which is when the worst combinations occur.

What towns do about it

Demountable barriers, flood gates and raised thresholds are common in ports with a long history of nuisance flooding. Some businesses on the quay simply operate with the expectation of water and design their ground floors for it. Flood warning services issue alerts for specific coastal locations, usually with a lead time of hours.

Those services are national or regional, and their thresholds are set for that stretch of coast rather than generally. Sea level rise makes the same physical combination produce flooding more often, which is changing what counts as a rare event.

The visitor's version of the problem

The quay is the lowest ground in most harbour towns and the most obvious place to leave a vehicle. Vehicles on a quay during a high spring tide with a positive surge are the standard local photograph. Warning signs on many quays state the risk plainly, and they exist because the situation recurs.

Checking the local flood warning service and the day's tide before parking takes very little time. It also tells you when to be standing on the quay with a camera rather than parked on it.

The takeaway

Sea level is the tide plus the weather, and the day after a storm is when that sum is largest.

Weather is the itinerary. Everything else is a preference.