A standing wave in an enclosed or partly enclosed basin, set off by wind, pressure jumps, storms or long ocean waves, that keeps sloshing at the basin's natural period after the trigger has passed. Periods range from minutes in small harbours to hours in large lakes. Seiches show as regular wiggles on gauge records, can swing moored ships into damaging motion, and in large lakes can shift water levels by metres from end to end.
There's a 22 minute seiche on the gauge since the front went through, the ships at berth 3 are surging on their lines.
Harbour seiches with periods of a few minutes do most of their damage by surging moored ships on their lines, long before any obvious change in level.
Also heard as
- harbour oscillation
- range action
- basin oscillation
Related terms
Storm surge
A rise of water above the predicted tide caused by a storm's winds and low pressure.
Residual
The observed water level minus the predicted tide, showing the effect of weather, rivers and other non-tidal causes.
Tide gauge
An instrument that records water level continuously at a fixed point, the basic tool of tidal measurement.
Six-minute data
Water level values recorded every six minutes, the standard sampling interval at many tide stations.