Area commander, Supt Nathan Clark, said the explosion "was always a possibility". However, he said the mitigation, including the sand barrier, put in place "appears to have worked".
He said: "If you imagine what that explosion would have looked like without that [mitigation] it really would have been a catastrophe, a 250kg bomb.
"We knew if that was detonated it would have caused serious damage and threat to life and that is why the military, in partnership with Norfolk public services, have done what they've done."
The force said agencies had "assessed the damage caused by the explosion with a number of car windows broken, the top of the scaffolding on the bridge affected and some damage to the flood wall".