A dam is thicker at the bottom than at the top because the forces exerted are strongest close to the ground. Water pressure increases proportionally with depth, so the lower portion of the dam has greater water pressure exerted upon it. The increasing thickness of the lower dam also helps it support its own weight.
Dam construction reflects the variety of forces that act upon it. Some, such as water pressure and the weight of the dam itself, become steadily greater toward the bottom of the dam. Other forces, such as those exerted by earthquakes and ice formation, are intermittent and often difficult to predict. Earthquake forces are particularly difficult to plan for because they happen unexpectedly and may travel in any direction.
The ideal size and shape for a dam depends on its size, purpose, location and the available building materials. Gravity dams rely upon their extreme weight to resist water pressure. A similar design, called the embankment dam, consists of soil and other natural materials and also resists water pressure by virtue of its great weight. Large embankment dams include a water-resistant core.
According to the Association of State Dam Safety Officials, the design most effective in narrow, rocky locations is the arch dam. It is also one of the strongest options due to its curved surface. Unlike embankment dams, arch dams are made entirely from concrete, masonry and other man-made materials.