Every case can be slightly different, but my guess would be that it's mostly in terms of the structural strength of the wharf. For the deteriorated infrastructure you'd have old timber and deteriorated concrete and the like, which would be a lot more prone to wave action, whereas for new construction you might have steel members or brand new concrete that resists waves and flooding a lot better.
It's not necessarily the height, but the structural strength. Where the height comes into play is with respect to damage on infrastructure that's on top of the wharf, like your bait sheds and your traps, and the electrical systems and the like.