I have sent you a copy of our email to Stephen Belli about this new flooding hazard for Seaton. I am now sending you the supporting diagrams and explanatory argument. Much of this is discussed on our blog at http://www.tescowatchseaton.blogspot.com/, but this had no pictures and these are essential to understanding our concerns.
Look at T1 above This is taken from the sucessful Tesco planning application and shows the overall layout of the whole regeneration site. Important here is the role of the flood relief channel, which I have marked.
This runs from Harbour Road at the bottom of the picture (south) to the marshes at the top (north). Harbour Road is the lowest part of Seaton and most susceptible to flooding from the sea or river. This danger is greatly increased by raising the regeneration site and so stopping flood water draining north to the marsh. To reduce this danger, the Tesco plan provides a flood relief channel (also called a monsoon drain or berm) running north-south right through the development site at low level, so cutting the site into two halves.
When the site is raised and a flood comes, water can flow from Harbour Road to the marshes along this channel. Without the channel, raising the site means a serious increase in the flood risk for the whole area, which is largely residential. We were not terribly happy about this solution, but it is better than nothing.
Now look at T2. This shows Tesco's proposal to raise the whole site by pumping in infill from the sea. The idea is to build a high wall right round the site, pump in a mix of seawater and aggregate along the black pipeline grid, allow it to settle, collect the seawater in a lagoon (blue arrows) and then pump it back out to sea.
The flood relief channel (FRC) is visible (and dotted by me), along with all the later buildings, so they seem to have just pasted their proposals on top of an earlier version of T1 without paying attention to the details. They clearly have no understanding of the function of the FRC, as both the black pipeline and the blue water arrows flow straight over it, and it will be filled up with the rest of the site. Without the FRC, the only flood protection device is lost and the whole area is at enormously greater risk of flooding.
We complained about this to the Environment Agency. The correspondence is covered in the blog but, basically, they said leave it to the Tesco engineers. But we can see no economic solution, and after 5 weeks, nor (it seems) can they.
Consider the options.
1. Fill the site in two halves, on either side of the FRC. This will require two separate filling operations, with separate piping and drainage lagoons. It might be possible, but it is going to be very slow and very much more expensive.
2. Fill the whole site and then dig out a new FRC. This has two objections.
- While the site is being filled, there is no FRC. If a flood comes along during the filling process, Harbour Road has no protection.
- Digging a channel through slippery sediment deposited from seawater is not going to be easy - and may be actually impossible.
We want EDDC and the EA openly to acknowledge the problem and tell us how they propose to deal with it. Under these circumstances, we can only appeal to the power of the press and public opinion to bring this about.