I am writing a story about London flooding for an English project.
What would the most likely cause be for this huge city to be completely flooded about 10 meters above street level?
London covers ~1000 square miles (according to my old A-Z). Standard definition of Greater London: Everything inside the M25 London Orbital Motorway. Shown a a yellow ring on the map below.
Elevation ranges from sea level by the river to 245m (804ft), but that's Bromley which isn't really London anyway. Take it to 150m to cover almost everything of interest with a couple of little islands. You're still going to have a lot of the taller towers peaking out of the top of this as they range up to 300m.
This sea level rise tool only goes up to 60m, but that shows that when around half of London is flooded you've already completely lost Denmark, the Netherlands, half of Belgium including Brussels, and places like Berlin, Dublin and half of Paris.
I'd suggest playing with a tool like this one to work out how much of London you want to flood and tone down your requirements and remember that no matter what you choose, there are going to be entire cities wiped out before London gets hit.
Remember that London is defended, the Thames Barrier will prevent any "normal" storm surge reaching the city. It doesn't look like much in most photos, but when you kayak through it you realise quite how big it is. There will be major flooding in other places before London gets hit at all.
An effect coming off the North Sea will knock out Amsterdam and The Hague first. Something coming off the Atlantic will take out Southampton and Brighton on the way through.
The most likely scenario would be a Storm Surge, however boring it might be as Pete suggest in his answer meaning no critism. The North Sea has regularly suffered such surges and a recent (in historical terms) one in 1953 was the worse peacetime disaster in recorded British history.
On the 31 st of January 1953 a storm surge hit the Eastern Coast of Britain and the Western one of the Netherlands, Belgium etc. In Britain 307 people were killed in major flooding and large amounts of infrastructure were destroyed. It also lead to major loss of life in the other countries affected by the flooding. Parts of London were flooded by this event which lead to the planning and construction of the Thames Barrier.
This is the British Metrological Offices page on the disaster.East Coast Disaster.
Edit I have just though of another possible cause of a flood that could affect London, the wreck of the S.S. Richard Montgomery, a US Liberty ship that sank in the Thames Estuary in WW2. On board the vessel are still around 1400 short tons of explosives that are unstable and could explode under the correct conditions. This could generate a tsunami which estimates vary in height of around 1-5 meters which would funnel up the Thames towards London.Although this flooding would not be a deep as Ben Poulter wishes it could be a source of disaster if a lower flood was desired, and have the advantage of being highly localised to the Thames River and its surrounds rather than a global event and as a sudden event the Thames Barrier might not be raised quickly enough to protect London particularly if it was under going maintenance at the time.SSRichard Montgomery
A pretty severe mid-atlantic sea-quake would do the job.
The English Channel would form a nice funnel, concentrating the Tsunami, which would flood the relatively shallow North Sea and flood the Thames Estuary.
A slightly more boring alternative would be a storm surge in the North Sea.
A storm surge is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, and the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges.
As others have suggested the best plausible solution is a storm surge up the Thames estuary.
Thames barrier storm surge according to Wikipedia:
London is vulnerable to flooding and from heavy tides closing in. A storm surge generated by low pressure in the Atlantic Ocean sometimes tracks eastwards past the north of Scotland and may then be driven into the shallow waters of the North Sea. The surge tide is funnelled down the North Sea which narrows towards the English Channel and the Thames Estuary. If the storm surge coincides with a spring tide, dangerously high water levels can occur in the Thames Estuary.
This sort of disaster would only create flooding of 1 or 2 metres above street level (and a lot of London is below street level) BUT the initial surge could probably be several times larger than that.
Several contributing factors to make it full blown disaster:
Just a few affects on buildings/infrastructure:
Pretty much, think of all the things that can go wrong and put together the 1 in a 100 year perfect storm and you have a natural disaster like nothing you have seen before... oh wait, that's already happened with Hurricane Katrina in New Orleans, USA.
Your meteor strikes the Southern Bight portion of the North Sea and sends a devastating wave of water and sediment in all directions. Keep in mind that the sediment part;
The narrowing of shape of the outflow region of the Thames could suggest that the positioning of the meteor would cause more damage to London while a tiny bit less damage to everyone in countries like Norway (I cannot save the Netherlands for you, they're effed no matter which plausible answer you choose);
As suggested in the comments, I've moved your meteor to deeper water. This should also reduce the amount of sediment you get in general.