WEST COLUMBIA, S.C. (Reuters) — Floodwaters from unprecedented rainfall in South Carolina have killed 11 people, closed some 550 roads and bridges and prompted hundreds of rescues of people trapped in homes and cars, officials said on Monday.Governor Nikki Haley warned residents to remain on alert as rain continued to fall in some of the hardest-hit areas, including the state capital, Columbia, which saw its wettest days on record over the weekend.“This is not over,” Haley said at news conference.