Flooding, levels and surface water

What is tide-locked drainage?

• The outfall closes, the rain does not stop. Inflow continues while discharge is zero.
• Neither event alone floods. Ordinary rain plus an ordinary high tide is the dangerous combination.
• Duration is set by the tide, not the storm. The system stays locked until the tide drops.
• It is predictable. Tides are published years ahead, which makes forecasting genuinely possible.

What is physically happening

Most coastal and estuarine drainage discharges by gravity, through an outfall fitted with a flap valve or tidal flap. The valve is a simple one way device: internal pressure pushes it open to discharge, external pressure pushes it closed to stop seawater entering the system.

That works exactly as intended. When the tide rises above the water level inside the drainage system, the flap closes and keeps the sea out. The unavoidable consequence is that it also keeps the drainage in.

For the duration of the high tide, the drainage network has no outlet. Whatever arrives, from rainfall, from pumped flows, from groundwater, accumulates inside the system. Levels rise from the outfall backwards, and the network fills up like a bath with the plug in.

Why the combination is the risk, not either part

This is what makes tide locking easy to underestimate, because assessed separately neither component looks threatening.

A moderate rainfall event that the drainage system handles comfortably on an average day is genuinely unremarkable. A high spring tide that the sea defences contain without difficulty is equally unremarkable. Each has been survived hundreds of times.

It is the coincidence that floods. The system has to store several hours of runoff with no discharge, and its storage was never sized for that, because the design assumed an outlet was available. The flooding that results is inland surface water flooding, appearing some distance from the coast, which is often attributed to a drainage failure rather than to the tide.

The duration is also set by the wrong variable. A conventional storm ends when the rain stops. A tide-locked event ends when the tide falls far enough for the flap to reopen, which may be well after the rain has finished.

The one thing that makes it tractable

Tide locking has a redeeming property that almost no other flood mechanism offers: half of it is known in advance with high confidence.

Tidal predictions are published years ahead and are accurate. That means the window in which the system will be unable to discharge is known before it happens, and the only genuinely uncertain input is the rainfall.

Combining a published tide curve with a rainfall forecast and a live level reading inside the network turns this from a surprise into a scheduled risk. Storage can be drawn down before the gate closes, temporary pumping can be positioned in advance, and staff can be on site for the window rather than called out during it.

The monitoring that supports it is level inside the network near the outfall, so the actual rate of filling is known during the locked period, alongside a local rain gauge so the inflow is measured rather than assumed.