permanent pool dock is at 30 inches out off water. bank is steep and just black dirt no rock. Dock I have in will stay in the water and will be submerged for the 7 to 10 days it is under water. Not much of a current but 6 to 10 inch wave action. Would like to add to the current dock but there is plenty of room to make a new dock. When water is up there would be no boats on the water, to hard and muddy to get close enough to unload a boat. I would only want a hand rail on the walkway to the floating dock. Rolling dock would not work for me. I would have to pull it out every time they call for a heavy rain. In the spring that's every 5 to 7 days and we only get a 1/3 of them. Most of the time the lake rises 5 to 7 feet only, but this last spring we had some large rains where it was 15 to 20 feet over main pool. So what ever I do it would have to be able to take that movement.
The fact that your existing dock/pier is submerged at high water could present some design challenges/difficulties. From what I gather from your verbal description I'm guessing it might be easier to build something from scratch to handle such a large water level range.
My first thought would be to examine some docks that are already in use in areas where there is a comparable water level fluctuation. As mentioned, the Eastport or Lubec areas of Maine have a tidal range that can be up around 25 feet or so. I think you'll find that a variety of approaches have been used successfully there. A bit of searching on the Internet should yield some results.
I'd also recommend contacting an experienced dock builder in an area where they have to deal on a regular basis with large water level changes. They should know from experience what issues need to be addressed and what the pitfalls might be. I would think they could suggest a few different options. It's also possible that they could provide the appropriate hardware for your dock project if you plan to build it yourself.
There are a lot of different dock anchoring systems to consider...shore-to-dock cable anchoring systems, stiff-arm systems (which I like), connected floating dock sections, pilings, etc. There is no one answer as to what could work. Other possible factors in the equation are things like how you want it to look, what your budget is, how much of the work you want to do yourself, whether it will be left in year around or removed seasonally, type of usage, boat size/type, other activities on the dock, site topography, bank composition and slope, etc.
I've built a couple of docks in tidal areas, including one permanent wharf and one seasonal floating dock, but not being familiar with all of your requirements and not having seen your site, I couldn't say for sure if they would work well for you or not. Do you have any photos of the site and the existing dock/pier?
In the meantime, I'll see if I can reactivate a few of my old gray cells and come up with an idea or two.
