Complex sentence: The decision to establish a new, big cemetery for Vienna came in 1863 when it became clear that--due to industrialisation--the city's population would eventually increase to such an extent that the existing communal cemeteries would prove insufficient.
Split into simpler sentences: The decision to establish a new, big cemetery for Vienna came in 1863. Around that time, it became clear that--due to industrialisation--the city's population would eventually increase to such an extent that the existing communal cemeteries would prove insufficient.