Can you operate machinery in the water prior to dewatering?? If so will underwater surface support an excavator?? Gravel or silt??
Tidal water exerts a lot of pressure even at only 4' deep.
I can think of and have seen and done similar projects a number of different ways.
In the Pacific Northwest where I am located a boat ramp would most likely be state or federal funded which double my labor costs as the contractor is required to pay prevailing wage. Because of this we have found that using machinery and innovation is more cost effective in the long run.
Sandbags and plastic is cheap but 85 lineal feet of sandbags 2 to 5 feet high would require roughly 1.5 to 1 slope for support, thats a whole lot of sandbags to be placed and later removed. To me this option seems rather labor intensive especially considering that removing deepest sandbags would require laborers to squat into the water. If conditions allow and machinery can be used prior to dewatering, placement could be easily accomplished using an excavator with a skilled operator but removal would have to be done by hand, unless local authorities will allow the sandbags to be individually cut and emptied leaving only the empty sandbag to be removed??
What about a combination of sandbags and sheet pile? Or sandbags 25' on each side, on the end you could fabricate a U shaped structure out of plate steel, cut the leading edges at an angle that allows the end sheet to be placed so that the top is slanted slightly in towards your work area. This would provide additional support and cause the pressure on the end plate to force the whole structure down. Could be placed with an excavator or crane, even floated to the end and anchored using weights prior to sandbag placement and dewatering. Once your structure is in place pump out the water and additional sandbags could be placed behind the steel structure for additional support if neccesary.
Have seen large precast pads placed directly into water on short driven piles, small casons only used to cut excess piling and fasten piles to pads. [not sure how fastening was accomplished]
Friend of mine rented crabpots from local fisherman, filled the crabpots with sandbags, plastic on the outside and placed them all with an excavator. When he was done just dumped out the sandbags into his dumptruck, took em home and biult a giant sanbox for his kids. I didn't think crabpots could handle the weight but they worked like a charm and he did the whole project entirely by himself.
I myself have used large 3'x4' concrete blocks [available for rent around here] for sides with plastic pond liner on outside and sheet pile on end, all placed with a crane. Total crane rental was 6hrs, portal to portal, actual placement took roughly 2 hrs. This job was simaler to yours, looking at the sandbag method I figure I saved $960, would have been around $1500 if sheet piling that was originally promised would have been available. Savings do not include cost of getting the sandbags to the jobsite.
Have built dozens of sandbag coffer dams in live streams, most built by hand and the manhours they require for placement and removal just kill me. Whenever I can reach them with an excavator I use a combination of concrete blocks or steel sheets and sandbags, saves money and time.
Understandibly your job specifications, labor and equipment costs could be very different from mine. Like you, I get lots of ideas from asking around, hope this helps.
