My groundwater pond fluctuates about 4-5'. I couldn't find the post from my original dock build, but here is the one from "phase 2" when I added a 16' section. You can see how I did my hinge 3 or 4 pics down. I did something similar to Catscratch except I used a section of 1/2" black pipe from HD, only about $10. This hinge allows a great deal of postitive and negative articulation with no stress on the hinge joint. I was going to add criss-crossed cables anchored under the gangway to add lateral stability but I didn't think it was necessary. I has withstood 2 hurricanes (CAT-1's) and some wicked straight line storms and it remains solid. Take a look, it may work for you.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=33275&Number=437779#Post4377792O2S
