Flooding is at the forefront of all of our minds at the moment, and even more so in the St Denys area after the flood on valentine’s day. So how can we tell when the tidal Itchen will flood? Ian Turner explains.
Definitive information is always available from the Environment Agency Floodline pages and there are many other excellent sources of information such as www.itchentides.org.uk but the purpose of this article is to provide a brief rule of thumb guide to when the risk might be greater. Using this method the valentines floods looked likely the evening before the flood however the actual flood warnings were issued at 5.30pm that day, so just 5 hours before the flood occurred. This by no means removes the need to monitor the official pages but it can help as an early warning mechanism.
The tidal River Itchen runs from Woodmill down to Southampton water at Dockhead. Above Woodmill is not tidal and different rules apply.
Within the tidal river the level is influenced by many things but the main two are Tide and Surge. To get an idea of what might happen you need to check both:
Tide level
Tides operate on a lunar monthly cycle. Big tides (known as spring tides) occur a couple of days after a full or new moon. Lower tides occur in between times (known as neap tides). Tide height information from Southampton is available from many sources such as this site where you can view the next week's tide information or buy a pocket book guide for under £2. Keep an eye on this information and watch out for upcoming especially high tides. A Spring tide can be as high as 4.9m, a Neap tide maybe only 3.8m.
Surge
Surge is the big variable factor and is very important. Surge is influenced by several factors of which the most significant are atmospheric pressure and wind. Keep an eye on the weather forecast and look at the pressure charts. For every 1mb the pressure drops below 1013mb the tide will be 1cm higher so if the pressure drops to 960mb this will add 53cm of water to the forecast tide height. Wind is harder to predict but strong south, south west or south easterly winds will add water depth as well as waves which can overtop defences. About 24 hours before a suspect high water it is worth checking the actual surge forecast. There isn’t a forecast for Southampton and Portsmouth is the nearest so keep in mind the level may be slightly higher. This information can be found here.
Total
Add the surge figure for Portsmouth at high tide to the high tide height figure and you get the total prediction. On valentine’s day the tide was only a 4.4m tide but there was 1.2m of surge so that made a 5.6m tide… Any total over 5m is worth being aware of and keeping a close eye on the Environment Agency for Flood Warnings.
I hope this is a useful and easy rule of thumb. Much more information is available online and I would urge everyone to look at and keep an eye on www.itchentides.org.uk which is a local website (originating in the CCATCH coastal flooding program), run from here in our community. It has more technical information and full explanations of what you need to know. Also have a read of the Belsize Flood Resilience Project which again applies directly to our community. In addition Southampton City Council is also engaging in a Flood risk consultation – have a read here.
Finally, all these sites and ideas are great but of most importance is the Environment Agency site for our area.
Stay safe!
Ian Turner
Pictures: flood in Priory Road on February 14, 2014, by L. Weedy.