A seawall is protecting the chalk coast from what?

The seawall it protecting it from the erosion of the water with the sand.

A seawall blocks chalk cliffs from erosion.

this is caused by the fact that the sea water is mildly acidic due to the presence of salt.

in England, there are very few seawalls and lose up to a meter a year of the coast.

here are some pics of the erosion in Yorkshire.