However, depending on the context, you could consider using these options:
General:
* A series of calamities
* A wave of disasters
* A string of misfortunes
* A cascade of upheavals
* A deluge of misfortunes
More specific:
* A tempest of disasters (for natural disasters)
* An epidemic of failures (for man-made disasters)
* A plague of tragedies (for personal tragedies)
Ultimately, the best collective noun will depend on the specific context and the desired effect.