Complex sentence: However, the new design was not intended to cope with the subsequent increases in London population over 150 years, and the concreting of huge amounts of London green spaces causing substantial rainwater run-off into the sewerage system which was expected to soak into the ground.
Split into simpler sentences: However, the new design was not intended to cope with the subsequent increases in London population over 150 years. The concreting of huge amounts of London's green spaces causes substantial rainwater run-off into the sewerage system which had been expected to soak into the ground.