The "total destruction" radius is - afaik - about 100m, at 1800m glass windows will still be broken. The rule by thumb is, when the defuse a bomb, 1m for every kg, so they evacuate 1800m for the 1800kg of that bomb.
The special situation in Koblenz is that the bomb is in the river - and the slopes of the river valley are all covered with houses right to the tops. Therefore the bomb is figuratively in the center of a funnel, so no "obstacles like building or terrain [that] will deflect/modify it".
I've seen it last Saturday from a balcony right above it - being there on a visit. The probability is *really* low, but still they are *really* nervous...
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