A dam has broader walls at the bottom than at the top as the pressure exerted by the liquid increases with it’s depth. Thus, as depth increases, more and more pressure is exerted by water on the walls of the dam.
A thicker wall is required to withstand a greater pressure, therefore, the wall of a dam is made with thickness increasing towards the base.
In the figure given below, the increasing length of arrows in water represents the increasing pressure on the wall of the dam towards the bottom.
