为防止水涝，我们必须使河水改道。

To prevent flooding, we shall have to divert the river from its course.

修筑了土墙和排水沟来防止风沙和水涝。

Walls and drains have been built to keep out sand and water.

沙子当然是松散的泥土，但如果没有经受水涝的话，它们还不会变成流沙。

Sand is of course loose soil but unless water logged will not act like quicksand in this regard.