The Sea Isn't Level
In all discussions of sea level it is absolutely vital to keep in mind at all times that the sea is a very dynamic ravenous beast that wants to devour your coastal lands.
In passing, if you know that your land is likely to be gone in a few years, find yourself a mug punter and sell quickly. Despite all the evidence - the rapidly growing evidence - that the average global sea level is rising at an accelerating rate, find yourself someone who is so convinced that it ain't happening that he thinks beach front property is a sound financial investment.
Where land at the sea's edge isn't level it is not a sound idea to build level sea defenses. If the land is flat all along the coast then by all means build a level sea wall. But if the land undulates gently along the coast you need a sea defence that undulates in like manner. Otherwise a storm surge at high tide will just roll up those slopes and overtop that pretty little wall and then roll downhill on the other side.
The sea is never level. Due to tidal effects the sea surface slopes from high tide at some point to low tide at another point. The Victorian engineers who built the original sea defenses on the Isle of Sheppey knew this. They built a drainage ditch parallel to and behind the sea wall designed to capture water which might overtop the sea wall and allow it to flow to the other side of the island. The flow of water from a high level to a lower level in a level ditch is faster than a tidal flow over the same distance. In 1953, that drainage system couldn't cope with all the water, but it did reduce the impact of the flooding somewhat.
Another flood defense for Sheppey was the tidal mud flats. The cliffs at Minster would erode and the resulting clay sediments would be deposited along the shores in the Swale and Medway. Those mudflats tend to reduce the energy in flows of water and can have a significant mitigating effect in times of extreme tides or weather.
Modern engineers know so much more than those crazy Victorians. The cliffs at Minster have been protected against erosion, the mudflats adjacent to the former Sheerness dockyard have been converted into an enormous car park and many parts of the drainage ditch have been filled in. That should do the trick nicely. The next storm surge of 1953 proportions will overtop the sea defences and will send a few hundred brand new cars floating up the Medway.
In Medway, many of the old sea defences which have been in place since the Napoleonic wars are well past their expiry date. Just adding more blocks and bricks and sticks and slick tricks doesn't do any good if the foundations are rotten. Raising the sea wall to protect a large swathe of currently empty land to make it look like a good investment is a very bad idea. That water has to go somewhere. Keeping the water away from land means it has nowhere to go except to the lowest, weakest and oldest parts of the flood defences.
That's right, folks: that bit of empty land isn't likely to flood any time soon, but a mere 1 meter storm surge is going to make the streets of Chatham a bit damp, as sure as apples is apples.
