[thedoc (OP)]

Kaitlyn Robinson asked the Naked Scientists:
   Could we make the seas deeper to prevent rising sea levels e.g. expanding existing trenches so they could hold more volume?
What do you think?

[Atomic-S]

Well, of course, in theory. The soil would have to be relocated onto dry land. Could be a bit of a challenge, though.

[evan_au]

Sea level has been rising at almost 3mm per year=0.003m per year.
https://en.wikipedia.org/wiki/Sea_level_rise

The area of the Earth is 500x10¹² m², of which about 70% is water.
So the volume of the Earth's oceans is increasing by 500x10¹² x 70% x 0.003 = 10¹² m3 per year.

This is a lot of dirt to dredge, transport and dump onto land, every year - around 4 trillion tons. To give some concept of how big this is, it would fill in the USA's Grand Canyon in 3-4 years.

Bear in mind that this marine sediment will destroy any land it is dumped on; it is loaded with salt, which will destroy the agricultural potential of land surrounding the dumping site.

I suggest it is best not to buy waterfront real estate.

[Tim the Plumber]

Short answer; No.

But as a thinking point, the idea that measuring the sea level is an exact reflection of the volume of water in the oceans is likely to be very wrong.

If the ocean floor moves up, or down, this will cause sea level changes. This could be happening now. We are overdue an earthquake storm. The ocean floor could be buckling up a bit before the release of this tension.