The wall needs to be 39 feet tall. Jumping from 39 feet will break leg bones. 30 feet is marginal.
Not if there are punji stakes for 10 feet at the base of the wall.
“30 feet is marginal.”
Actually, there is a pretty high risk of death from that height, especially if you are not close to a hospital. True, if you are over 40 feet, mortality starts cracking 50%, but there are other variables, like what kind of surface is the landing. If you are not killed, you are very unlikely to be OK.
It is also a sickeningly scary height for most people to climb. It is a serious obstacle to the great majority of people.
But probably more than that, it is a practical height to build, because they will use precast concrete panels, which must be set into the ground at least six feet (more in places with a slope). If you can’t transport the one million or so panels needed for a sea to sea wall on regular 40 foot tractor trailers, your costs go way up, fewer contractors would be able to work with them, and it will take longer.
30 feet isnt marginal. But 40 feet would be even less marginal.
Of course it you put hard concrete on our side it makes a guarantee somethig is going to be damaged.