Coastal squeeze can be seen at Blackwater Farm where the seawall presses the marshes in, so that the marshes cannot be built up further. This can hinder the development upwards of the marshes too, and make them become the long thin strips seen in the photo above.
A loss of marshland can increase the amplitude of flood events. Marshes absorb salt water as it flows into them, and can thus slow down flooding and reduce the damage it causes.
In turn, more severe flood events can cause greater erosion of the salt marsh, which in turn makes floods more dramatic.
Large quantities of land had to be trapped during the World Wars to allow safe and secure food supplies, especially in WWII, when the German forces started bringing down civilian supply vessels. The walling needed to reclaim that land was what lead to the declining quality of the salt marsh and its declining protections.
In response, holes have been broken through the salt marsh. The remains of the walls should not matter, as water in salt marshes travels in channels like in normal rivers; as long as a route is clear, it should grow fine. The soil was not ideal at best because of the high salt content, so poor quality land is being lost. The land that might develop from the marshes eventually, if they can grow and build up normally will likely be of far higher quality.
(Image Source: http://www.essexbiodiversity.org.uk/coordinators-blog; I don’t know if that’s Blackwater Farm, but it’s definitely a similar area)