Strange Ferrybank features?

[Wexflyer]

When I was growing up, I would often walk with my grandfather over to Ferrybank, across the then new, new bridge (which replaced what had been the previous "new bridge"). Immediately on crossing the bridge and reaching Ferrybank, just to the right, on the curve, was an area with several curious features, which I have never understood:
- Several circular, brick lined pits? These were sand filled in my day.
- Several odd rectangular walls/banks - vertical on one side, but with angular inclines on other side?
- These were all outside of the ruins of what had been some sort of building right on the sea.
Would love to know what these were! I believe these items are now all long gone.

Just further on was a feature I did understand/know of - the launch slipway into the river from the Great War US Navy seaplane base. Still there, I believe.