8th April: (b) A Record Tide Height

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Fig.1 (a) NTSLF Storm Surge forecast with tidal height over 5.5m; (b) showing the 0.25m bias in previous day’s forecasts.

Anticipating the Event

I normally wait for an Environment Agency Flood Alert or Warning before publishing any flood advisory on the Itchentides website, however by the weekend of 6/7 April it was looking certain that we were going to have a flood event.  

On the evening of 7th April I posted an “Advanced Warning” of a “5.75mCD” tide on Itchentides which was also emailed to subscribers. This was because the NTSLF storm surge forecast was predicting a total tide (at Portsmouth) of over 5.5mCD (Figure 1a) and, as noted by local resident Nigel Cook, there had been a low bias in the NTSLF forecast in the previous days of about 0.25m.  

This persistent bias suggests that there is an elevated mean sea level along the south coast of the UK, persisting over an extended period.  Sea level researchers at the National Oceanographic Centre (Liverpool) are aware of this, the cause is not known. If it continues it makes flooding more likely.  

I did realise that 5.75mCD was probably an over-estimate for Southampton, but did not correct for the smaller Southampton tides compared to Portsmouth given the amount of variation in the magnitude of the NTSLF surge forecast.

Flood Alerts and Warnings

The sequence of EA Flood Alerts and Warnings is summarised in the Table.

Day and Date (April)/Time(BST) Type Message
Sat 6th/13:44 Alert 5.18m tide that night and warned that Sunday night’s tide and both tides on Monday would be higher but “Currently it is unlikely that it will cause flooding to property
Sun 7th/19:30 Advanced Warning posted on Itchentides website:  5.75m total tide over-night on 8th/9th
Mon 8th/08:09 Alert 5.14m morning tide; the following night time tide “is forecast to be higher
Mon 8th/12:57 Alert an overnight 5.64m total tide “similar to the historic maximum level
Mon 8th/13:23 Warn the “early morning tide at 00:30 on 09/04/2024 and 02:00 on 09/04/2024 ” will be increased by 0.82m and that the resulting 5.64m tide would be “tide will be similar to 14 February 2014 “. This implies the EA were using a 4.82m astronomical tide value which is similar to that listed in the Channel Almanac but with later timing.
Mon 8th pm A note bringing attention to the flood warning was posted through the doors of houses at risk by flood group members.
Tue 9th/08:19 Alert Following the flood, a forecast 5.22m total day time tide height on the 9th.
Tue 9th/09:01 Warn Warning Cancelled: “Storm Pierrick, which brought gale force winds and elevated tide levels along the south coast has now cleared the area. Last night’s tide came in 6 cm over the forecast.”
Tue 9th/15:10 Alert  a 5.17m total tide in the early hours of the 10th.
Observed Time and Height of the Tide

Fig. 2: Recorded tide heights (mCD) from Woolston and from Sotonmet. (a) the tidal cycle. (b) the period of maximum height.

The heights mentioned in the alerts and warnings for the overnight tide on 8th/9th April imply that EA were using a 4.82m astronomical tide value (which is similar to that listed in the Channel Almanac but with later timing) and that the highest level reached was a 5.7mCD total tide.

The data from two local tide gauges are shown in Figure 2.  Woolston Tide Gauge data is available at 15 minute intervals with 1 cm resolution, and Sotonmet (Dock Head) data is available at 5 minute intervals but with only 10 cm resolution.  Both tide gauges show the tidal height peaking at about 23:45BST at about 5.7 mCD in agreement with the EA height value but some 45 minutes earlier than the EA time for astronomical high water.  The advanced tidal peak is caused by the greater water depths during a storm surge.

Accuracy of the Storm Surge Forecast

Fig. 3 (a) Surge Residuals: (red) calculated for Woolston; (green) observed at Portsmouth; (blue) NTSLF forecast for Portsmouth. (b) as (a) but  corrected for the mean 0.25m sea level bias.

Figure 3(a) shows that the observed storm surge at Southampton was similar to the observed surge at Portsmouth (from the NTSLF website). To calculate the Southampton surge values I have used the astronomical tide forecast from “Mr Tides”, an application which seems to perform well when compared to NTSLF tidal curves.  However the maximum astronomical tide from “Mr Tides” was 4.77mCD at 23:50/8th while the EA have a maximum of 4.82mCD at 00:30/9th.  If “Mr Tides” is underestimating the astronomical tide by 5cm, then the storm surge calculated for Woolston (red line) would be lowered by 1 graph division and be even more similar to the surge observed at Portsmouth.

Correcting the NTSLF surge forecast for the 0.25m mean sea level bias brings the forecast surge in good agreement with what was observed at Portsmouth: Figure 3(b).

Extent of Road Flooding

Fig. 4: Approximate extent of the road flooding during the first high water derived from the photos.

Figure 4 shows the extent of flooding observed during the first high water peak superimposed on the “Belsize Project” area land height map.  It suggests that the flood water in the road only reached about 5.5mCD, less than the 5.7mCD recorded at Dock Head and Woolston.  At around midnight water levels in the gardens was observed to be  lowering despite water still flowing into the road.  This is in agreement with the tide gauge peak at 23:45BST.  It suggests that there was not time for the water level in the road to reach the peak level which occurred in the river.  Observations of the 10th March 2008 flood event also suggested that the road flooding did not reach the full potential.

Second High Water

Fig. 5: Observed water levels at Woolston, the astronomical tide (from MrTides), and the implied storm surge.

After midnight, and with the river levels decreasing I,  and I think most others, went to bed.  However the observed heights at Woolston (Figure 5) show that the second high water was about 5.60mCD and maintained that level for longer than the first high water.  The implied storm surge (also shown in Fig.5) peaked at 1.03m at 01:15/9th which is in agreement with the forecast for Portsmouth (Fig.3).  However the debris marking tide lines on the following morning did not indicate that the flooding had been significantly more than on the first high water.

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