How to prevent flooding.
As I am writing this article in February 2014 the news is inundated with film of floods throughout the South of England. The picture below shows a typical scene experienced by many home owners recently, the good news is that this water was dispersed in a few hours, leaving the garden totally drained, the even better news in that this occurrence will never happen again, The problem hasn't just been moved to someone else's property, the problem has been solved! Read on to find out how it was done.
Firstly we need to understand why so much water is not draining away. Most of you are fully aware that the ground is totally saturated, and will therefore not accept any more water, therefore it has nowhere to go. We are continuously being told that the water tables are so high there is nothing that can be done. This statement is incorrect! Although the water table has risen to levels that are now above ground level there is a solution.
A water table is created because an impermeable layer of clay or rock prevents the water draining any further.
For the purpose of explaining what is happening in layman's terms, we will imagine that the ground beneath your property is like an enormous bath filled with gravel. The bottom of the bath is the impermeable clay, and the top of the bath is ground level. If you look at the bath in normal circumstances it would seem that it is full of dry gravel, the water would be half way up the bath, but not visible from the surface. But if you keep filling the bath with water it will eventually come to the surface and the water will overflow. That is exactly what is happening at the moment.
So, what is the solution?
The solution is very simple, you simply drill a hole in the bottom of the bath and let the water drain away, this then allows the surface water to re enter the bath and disappear.
So how does this work in real life?
The geology throughout the UK varies greatly so for the purposes of this article I'm going the concentrate on London and the South East, but the principle remains the same.
The areas surrounding the Thames River are generally an upper layer of alluvial silts and clays overlying gravels, the water contained in the gravels is in continuity with the river. So as the Thames rises and falls with the tide, the levels within the gravels will rise and fall. They do not fall as quickly as the river level as it takes time for the water to transmit through the sand and gravel geology. This gravel layer is presently full and so cannot accept any more water, and so it floods. The geology holding the water is known as the London Clay. The layer of clay varies in depth, The thickness of the clay in West London can be up to 100 metres thick, as you move to the East of London the thickness decreases, by the time you get to Dartford in Kent there is no London Clay. Beneath the London Clay there are other layers known as the Woolwich and Reading Beds (or Upnor Formation), then the Thanet Sands and finally the Chalk. The chalk is another aquifer which contains its own water table at a much lower level, and is not in continuity with the water contained in the upper gravels.
So we can now imagine that there is another enormous bath many 10's of metres below ground level. The solution is to drain the top bath into the bottom bath, by means of a borehole.
Other areas such as the Chalk Downs can be covered in a layer of impermeable clay, these areas have no permeability and the rain water simply skims across the surface to find the lowest point and then floods that area. Again, a borehole penetrating through the clay into the chalk will readily accept vast quantities of water.
So now you're asking yourself, why isn't this being done then? The only answer can be that the Environment Agency does not want to potentially allow contaminants from the surface to contaminate the water contained within the chalk aquifer, as this water is used by the water companies to supply your tap water. To be honest, that is a very good reason, and I can totally see why they've been reluctant to pursue such a solution. But with the doom and gloom predicted by global warming scientists, and predictions of more rainfall events such as this, desperate times call for desperate measures.
I believe that the solution is to install filters which will drain the upper gravels lowering the water table, and pump the filtered water into the chalk aquifer, or where feasible back into the Thames when the tide is ebbing. This will then provide massive storage capacity for when it does rain. In many other areas a shallow borehole just penetrating through the clay layer into chalk will get rid of the water and the chalk will naturally filter the water as it makes its way slowly down through many 10's of metres of chalk before finally getting to the aquifer as naturally filtered water.
The obvious advantage of this is that the water levels in the chalk would slowly rise and we wouldn't get the threat of hose pipe bans every time we get a prolonged hot spell in the summer. At present all of this water is simple going out to sea. Just imagine if we now had three months of no rainfall and a hose pipe ban was enforced, we would become the laughing stock of The World. Sounds unlikely doesn't it? But then again, who would have predicted such a large amount of rainfall this year?
No, it's not going to be a cheap solution, but when you consider the disruption and cost of insurance claims for this one event, it will seem a drop in the Ocean,
By the way, its not just a question of whacking a borehole into the ground. Each borehole has to be initially designed using information on the geology specific to that area, it also needs to be tested and installed correctly, using appropriate installation materials specific to that geology.