Analysis: the 10th March 2008 Event ( part 1)

march10thmapOne of the highest water levels yet recorded for Southampton occurred on the 10th March 2008. During the period of high tide there was partial flooding of roads near Priory Road Hard and at Woodmill. A number of gardens were flooded and the river encroached on Riverside Park. Examining what caused the high water on that day will help us anticipate potential flood events in future.

Part 1 of this analysis will consider the effect of local atmospheric pressure and any larger scale storm surge. The effect of changes in the arrival time of the tide and duration of the surge are considered in Part 2.

The effect of low air pressure

The flooding occurred at about mid day when low air pressure (965mb) caused by a deep depression centred over the Irish Sea coincided with a 4.8m tide. The graph (below left) shows in light blue the normal height of the tide during the day as given by the tide tables (the “astronomical tide”). The actual water level recorded at Dock Head is shown by the dark blue area. In other words the dark blue represents the extra water height that gave rise to the flooding. Throughout the day the actual water level was higher than forecast.

Much of the increase in water level was due to the air pressure being low. Because the air was not pressing down on the sea so much, the water was able to rise higher. A surface air pressure of 965mb will cause the water level to be higher by about 0.48m. We can allow for this effect and our predicted water level is now shown by the light blue area (below right). It is much closer to the actual levels shown in dark blue; for two short periods it is actually higher (yellow shading).

Larger Scale Storm Surge

surgetimeseriesThe top graph (a) shows a time series of water levels from mid-day on the 8th March to mid-day on the 11th March. The red (dashed) line shows the water level predicted by the tide tables (the “astronomical tide”).  The black line shows the effect of correcting for air pressure alone, and the blue line shows the observed water level.

Up until the first high water at around midnight on the 9th March, correcting for pressure gave a good estimate of the observed water level. However for much of the 10th March the water level was even higher than correcting for air pressure would predict. This “residual”value is shown by the purple line in plot (b). The exact value of this residual appears to vary rapidly with time. However if we draw a smoothed line (shown in black) we see that the peak excess residual is about 0.4m at mid-morning on the 10th with a decrease of about 0.3m early on the 11th.  The most likely explanation of this residual is that the storm created a storm surge which travelled along the English Channel as a wave with the peak arriving at Southampton around low water on the 10th with the corresponding trough arriving around midnight that day.

Summary (part 1)

The high water levels on 10th March 2008 can be ascribed to two factors, a large scale storm surge and locally low air pressure, both adding to a large, 4.8m astronomical tide.

There would appear to have been a large scale storm surge in the English Channel which increased the water level by around 0.4m. The maximum of this surge coincided with the morning low tide, however it probably contributed about 0.3m to the raised water level at the time of high tide.

 Although the centre of the depression passed north of Southampton, local air pressure fell to 965m causing an increase in water level of about 0.5m.  Combined with the large scale surge the high tide level was raised by about 0.8m resulting in a 5.6m tidal height.