COLUMBIA, S.C. — Days of torrential rains kept much of South Carolina and its capital gripped by floodwaters early Monday as emergency responders promised renewed door-to-door searches for anyone still trapped after a weekend deluge and hundreds of rescues. At least seven weather-related deaths have been blamed on the vast rainstorm. Heavy rain kept falling into the early hours Monday around the Carolinas from the storm that began swamping the Southeast late last week, part of an unprecedented low pressure system that dumped more than 18 inches on one spot alone in Columbia, the South Carolina capital. The rainstorm dumped so much water on South Carolina and parts of several surrounding states that even the weather experts said they were astonished. “The flooding is unprecedented and historical,” said Dr.