Complex sentence: The rally has sometimes being marred by flooding during a thunderstorm period, meaning route changes was necessary even before the start, flooding also made the rally far more difficult with normally treacherous roads becoming booby traps with unseen and terrifying hazards.
Split into simpler sentences: The rally has sometimes been marred by flooding during a thunderstorm period, meaning route changes was necessary even before the start. Flooding also made the rally far more difficult with normally treacherous roads becoming booby traps with unseen and terrifying hazards.