Thames flood control barriers.

TIL: The Thames flood control barrier, across the mouth of the Thames, is used more often to protect against flooding from high river flows than from seawater storm surges.

How can blocking the outlet of a river PREVENT floods from high rainfall?

By blocking the outlet after low tide, it preserves a basin for the river to keep flowing into, which would normally be stopped by the high tide. Once the high tide has retreated to river level, it's opened again and the draining continues.

It seemed very counter-intuitive to me, but obviously it works.

Typical British process. 1953 disaster, 20 years of discussion and planning, then just 8 to build and deploy.