Three computer models contribute to the forecast procedure for tidal Thames.
- North Sea Model
- Continental Shelf Model
- River Thames Model, known as the ISIS model
The information provided by the models is supplemented by information supplied by the Meteorological Office and real time information provided by the National Tidegauge Network around the east and south coast and tide gauges located on the tidal Thames.
Tides are tracked as they travel down the East Coast (about 36 hours before reaching the Thames Estuary).
Decision to close
The decision to close or not, is based on a combination of three major factors:
- The height of the tide (usually a spring tide) measured at the Thames Estuary.
- The height of the tidal surge, which naturally accompanies each tide.
- The fluvial river flow entering the tidal Thames, measured as it passes over Teddington Weir.
The barrier has no individual trigger level for closure. Hydrological and meteorological data is fed into the control room every 20 minutes by telemetry. The closing process is guided by a mathematical matrix considering fluvial flow, tide and surge from this data. The end decision for closure lies with the Thames Barrier duty controller at the time.
In general terms the Thames Barrier would start to close about 1.5 hours after low water at North Woolwich. Closure of all 10 gates takes about 1.5 hours and creates an ‘empty reservoir’ for fluvial river flow entering the tidal Thames at Teddington.
The Thames Barrier will then remain closed over high water until the water level down stream of the Thames Barrier has reduced to the same level as upstream. This is a managed process to provide for different circumstances and takes about five hours to achieve.
The Thames Barrier is then opened, allowing the water upstream to flow out to sea with the outward-bound tide.