Plates 12 DAM TROUBLES AND STUDIES      Wave erosion affects part of the wall of a large farm irrigation dam, forming cliffs of earth with a flat terrace below (upper left). Note the terrace Just below water level (upper right).    Middle left (as described in Chapter XX) : Inside slumping of a dam wall may start with little cliffs of an inch or so after a full dam has become empty in irrigation. In a fortnight cliffs three feet high develop.    Lower right: The "wet" dam at "Yobarnie" which caused all the trouble as described in Chapter XX.