We use a similar system on docks on the lakes around here that are highly variable in water level, except our poles usually don't telescope, The dock has a pair of rings in which the pole slides. The poles just rest on the bottom, and are not anchored except by their own weight, and usually have a windlass system to raise the poles when necessary to move the dock out when the water level really drop. Some gangplanks telescope, but not many. The alternate system is a gangplank anchored on shore and two guy wires to shore to triangulate the dock system with the shore.