It's a boat or ship with a flat bottom, but 'flat bottom damage' is at best ambiguous English. Maybe the Dutch makes a point (?) of mentioning the flat bottom only because this localises the part of the hull that is damaged. Also, the 'bijkomend' suggest that there was also damage to somre other part(s) of the ship.

Not too mayny hits on Google for 'valkbodem', but it's a bit better with the German equivalent 'Flachboden + Schiff' (see ref).