That’s from Woolie towards the beginning of our Secret Society adventures.
One would imagine that there were protections for structures on the mainland from irregularly high tides/tsunamis of natural origin, so those would likely have fared better than the defensive structures further out.
Depending on where exactly the protective line was, it’s possible that the dune lake is leftover from the Silver’s wave changing the shore’s layout. Maybe there’s a few things laying at the bottom of the dune lake from the shore-facing side?