FAQs from Fair in the Square

Responses to some commonly raised questions at Saturdays Science Fair in Saxon Square provided by Professor Vincent May

When the tide rises, does it affect the saltiness of the water in the harbour?

This depends, first of all, on how far into the harbour the flood tide is able to progress.  During periods when the river flow is very high, typically during winter floods, very little salt water will enter the harbour because the river flow is too great.  During these flood events, it is possible to see a plume of fresh muddy water out as far as a line from Beerpan Rocks to Highcliffe.  When river flows are low, salt water does flow into the harbour and because it is denser than fresh water flows in the lower parts of the water column.  It can form a wedge beneath the freshwater which becomes gradually thinner as it moves upstream.  It does not usually get much further upstream than alongside Stanpit Marsh.

Does much mixing take place between the fresh and salt water?

In the low water flows very little mixing occurs as the water moves upstream, but as its velocity lessens, and the tide turns there is more mixing.

Are there differences in the water temperature between the salt and freshwater, and does this affect the location of fish in the harbour? 

Yes, usually simply because the temperature of the water flowing downstream  depends upon the temperature of the groundwater and the surface runoff which feed it.  The sea temperature is usually different because it depends largely upon the seasonal heating by the sun. In addition, if the seawater flows over the mudflats which have been exposed to sunshine it can absorb heat from their higher temperatures and this can affect the incoming water temperatures over the saltmarshes.

(I don’t know enough about the fish to answer that part of the question, but the local fishermen might have a view.)

Why isn’t the actual water level always the same as the forecast tide level?

The tide forecasts are based upon the planetary motions which can be forecast well into the future. However, water levels are also affected by atmospheric pressure, so during low pressure water levels will cause the water levels to be higher than the forecast level. In addition if there are onshore winds this can also raise water levels close to shore. When tide, pressure and wind combine we have storm surges when water levels can cause significant flooding. In addition, the onshore winds can force waves further up the shore, especially where we have built slipways and so water levels can easily exceed the forecast tide.

Does the Isle of Wight cause the double tides?

No, although it may affect the detailed flows. This can be added to if needed.