Everyone does know the dams are there. But in an otherwise flat city they are just big mounds of dirt running for miles. They don’t hold water most of the time. Even with heavy rain any minor flooding goes away in a couple days. I never really understood why a 20-50 foot tall dam was needed. I remember once joking with a friend that if the water ever filled up over the dam that means the entire state of Texas behind the dams is under water. It actually did not make sense to me why they built the dams so big until Harvey.
In any case, someone knew and should have been a required disclosure when buying a home. However, I don’t think it would have helped. People would have bought there anyway because this kind of flooding was unthinkable. Although they would have known they needed flood insurance. But then, insurance companies probably wouldn’t offer it.