The threat has increased over time due to the slow but continuous rise in high water levels over the centuries ( 20 cm ( 8 inches ) \ / 100 years ) and the slow `` tilting '' of Britain ( up in the north and west , and down in the south and east by up to 5 cm ( 2 inches ) \ / 100 years ) caused by post-glacial rebound .
The threat has increased over time due to the slow but continuous rise in high water level over the centuries ( 20 cm ( 8 inches ) \ / 100 years ) and the slow `` tilting '' of Britain ( up in the north and west , and down in the south and east ) caused by post-glacial rebound .