Basic Sea Protection Info
Miko,
Good answer and pleased to see that you have confidence in your Engineers.
Unfortunately the basics of this type of protection construction can be better simplified by the following:
Essentially, the relevant formulas say in simplified words that the grain size (diameter) in the coarser layers of the protection must not be more than about 4 times that in the finer layer. 12-ton rock on Caribbean sand does not meet this requirement! Landlubbers would think this problem acts only in the direction of gravity, but we old mariners know the frightful forces acting on all things near waves. In fact, even if the big rocks were put in a deeper trench, there would still be sand sucked out and thus the likelihood of settlement! Large stones never remain visible for long on a sandy shore, they sink and disappear below the surface. (Your previous concrete lumps, some perhaps still lost, are a prime example). If there is a substantial surf, which there can be in that area of shore break in your vicinity, it might/will erode the formation in front of the revetment, causing additional subsidence; that is why clever people put a "toe" (scour protection) in front of such things, i.e. a blanket of suitable stones laid on a geotextile layer along the foot of the main rock slope.
In practical terms, you would start the construction of any revetment on a sandy shore such as the one you have, with a geotextile layer, (heavy duty type, with very good overlap joints), then some small stones/rock armour, then some medium stones/rock armour, then the final armour layer, observing the filter size criteria mentioned above.
It is not my intention to be too critical but I would hate to see you lose a lot of money on a wasted endeavour. I am, however, pleased to see that your Engineers will be using geo-textile and if it's placed properly, with the right sized rock armour layers in the right places then at least it will have a chance of outlasting previous attempts at protection. Believe me when I add that I am trying to be a little helpful here but having suffered ill health through the successes, failures and stresses associated with deep water ports & harbours, sea walls, dredging channels, etc. for very demanding clients for many years I finally got to retire and just kick back and I'd prefer to keep it that way.
Wanna meet and chat over a beer then that would be fun. We're in Cabarete. PM us.