Effect of Tides on the River Level

tidechanges-244x300Tides with accompanying storm surges are the most likely source of flooding in St Denys and the West Itchen area.

The River Itchen is tidal between Southampton Water and Woodmill with water levels rising and falling naturally over the course of the day in relation to the tide. Tides are caused by the gravitational attraction of the moon and the sun.  When the sun and moon are in ‘conjunction’ or ‘opposition’ (new moon or full moon), the pull is strongest and the highest and lowest tides occur, called spring tides.   In Southampton the two sets of spring tides during each month occur at, or soon after, midday and midnight, during the few days following the full or new moon.

Due to the irregular depths and width restrictions of the English Channel between the Isle of Wight and the Cherbourg Peninsula, the tidal regime for the coast between Dorset and the eastern Solent is very unusual. The diagram shows the change in water level in Southampton during a typical spring tide. There is a double high water and also a young flood stand tidal-curve-300x257(where the water level hardly increases for up to 2 hours during the mid part of the flood tide phase). The double high water causes water levels to drop after peak high tide and rise slightly again before the tide goes out. The flood tide (when the tide is coming in) lasts approximately 6 hours in contrast to the ebb tide (when the tide goes out) which is much more rapid at around 4 hours. As a result, the tidal currents are stronger on the ebb tide than they are on the flood tide.

The times of tides are shown in Tide Tables. These give the depth of the water above the lowest low water level in the area (known as “Chart Datum”).  At high water on a spring tide the river level will be around 4.5m to 5m above that level. A storm surge occurring at the same time might add a further 0.5m or even more. At high water on a Neap tide the water level is around 3.5m so even if there is a storm surge, the river level will still be less than during a spring tide. Thus it is on the days when tides are close to or at spring tide levels that flooding is most likely to occur.

Sources of Tidal Information

img778-201x300The official tide table for Southampton is issued by the Port Authority (ABP). A guide for using the Tide Table can be found here.  Useful tidal information available on the web includes:

  • Tidal Table predictions available for Southampton (and for other sites including Portsmouth) for the coming 28 days together with a diagram showing the next 7 days tides.
  • Estimate of any Surge – A forecast of how much the actual tide will differ from the value shown in the Tide Table is available for Portsmouth and Bournemouth. Most of this difference is due to the expected weather conditions (mainly the air pressure) so this “surge forecast” only covers the next day or two.
  • Comparison of actual and forecast – the sea level gauges at Bournemouth and Portsmouth show time series of the predicted “Tide Table” value and, for comparison, what the present water level really is. High tide occurs at Bournemouth before Southampton so it can be an indication of what to expect.
  • Local tide gauge measurements are available from the Southampton Dock Head Tide Gauge but be aware that differences from the predicted value may be due to changes in the timing of the tide and don’t predict the difference to be expected at high tide.

Other Sources of Tide Information

For those that want to dig a little deeper and understand all of the different data sources around on tidal gauges and tide times the following may be of interest:

  • The Southampton VTS web site only gives the most recent weather and water level reading from the Dock Head Tide Gauge; for a tide time series view the “Sotonmet” (Dock Head) or “Bramblemet” (Bramble Bank) web sites which give similar format data for the two locations.
  • The Woolston Tide Gauge might be useful if the other gauges aren’t working but be aware that this site may not be displaying the latest data.
  • The TideTimes web site is useful for tidal predictions outside the 28 day period on the NTSLF web site. In contrast the Hydrographic Office “Easy Tide” web site only gives free tidal predictions for the next 7 days; for other times you have to pay.
  • Various applications are available to run on your computer or mobile device. For example Mr Tides for Macintosh devices – however be aware that these tidal prediction applications may use fewer components for the tidal calculations and may be less accurate than the other web sites listed here.