The Fleet connects to the sea via a narrow channel at Ferrybridge into Portland Harbour. Sea water can also enter the Fleet by percolation through Chesil Beach under some tidal conditions. Under storm conditions water from Lyme Bay can come through and over Chesil Beach.
Fresh water can enter the Fleet from direct rainfall and indirectly via a number of small streams and by run-off from the surrounding fields.
At Ferrybridge the water of the Fleet is fully marine with a salinity around 35 parts per thousand (ppt). In the upper Fleet at Abbotsbury the salinity can fall below 16 ppt.
The Fleet is strongly tidal at Ferrybridge with a range of up to 2.4 metres on spring tides but the effect of the tides diminishes above The Narrows. At Abbotsbury the effect of the daily tidal cycle is very small and a much greater effect is that of the monthly cycle of spring and neap tides. The combined effect of several narrow stretches and wide basins delay the arrival of tides at Abbotsbury by up to 6 hours and also reduce the tidal range.
Storms and storm surges in the Channel can increase water levels significantly in the Upper Fleet. This, combined with the complex nature of the Fleet, makes it almost impossible to predict water levels in the upper Fleet.
Tidal currents are strongest through the Narrows and under Ferrybridge where currents exceeding two knots can occur.
The water in the Fleet is generally warmer than the surrounding sea in the summer and cooler in the winter. Summer temperatures can be as high as 26°C while in a severe winter the water will freeze. The water temperature in the lower Fleet can vary by up to 4ºC during a twelve hour tidal cycle due to the interaction of tidal flows with exposed sandbanks.
The plot to the right shows the water temperature in the Lower Fleet through the year over a ten year period. The temperature was measured using a Met Office sea temperature bucket at 2pm on each visit to the site. Each point is the temperature measured on a site visit.
Ebb tide under Ferrybridge
Incoming tide through the Narrows
Plot of water temperature in the Lower Fleet sampled across a ten year period.