That sits on the bottom and fairly easy to cushion it from dangerous junk already there without disturbing it.
Sinking bridge foundations are another matter. Part of of the reason for dumping it there was the depth, so need to be very tall piers just to reach water line, can have fewer with bigger spans, but that causes different problems. It can get rather windy in that sea.
Non-starter done for political stunt
The water isn't really deep by the standards of bridges under design and engineering consideration today.
The weather isn't particularly bad compared to what large condeep type oil platforms endure every day in the Northern North Sea and arctic oceans.
In Norway they want to put bridge piers in 1500m of water.
245m of water is absolutely nothing.
Building this bridge is also likely to be far cheaper than building, and paying for the operation of, the electrolysis capability to provide NI-GB zero carbon flights in future.
Also the munitions is sort of a red herring, since the size of the bridge foundations will be a tiny portion of the bridge track.
So anything that might be dangerous can be cleared by UUVs or divers before we drop the GBS type structures in.
Or we can use TLP type structures for bridge piers, as proposed in Norway, to further reduce the footprint of the bridge in the area.
EDIT:
Take the Tsing Ma bridge.
~1377m combined road-rail span.
Bridge towers are apparently ~208m above sea level.
Taking a track across the crossing from vicinity Whitehead to vicnity Portslogan, the maximum depth is only 245m apparently.
That gives us a maximum pier height, even if the piers are not optimised to reduce this value, of 453m.
The Troll A condeep platform structure is 472m tall.
So the depths are easily achievable.
