While it's not strictly about a flash flood at summer camp, it does feature a significant and dramatic flash flood that causes a lot of trouble for the main characters. The movie is based on the novel of the same name by Bette Greene, and it tells the story of a young girl who befriends a German prisoner of war during World War II. The flash flood occurs while they're trying to keep their secret friendship a secret.
It's a powerful story about friendship, prejudice, and the horrors of war, and the flash flood scene is a memorable part of the film.
While the movie isn't based on a true story, it's inspired by real events. The author Bette Greene drew from her own experiences growing up in Arkansas during World War II.