As becomes clear from the figures, the general principle of sand storage dams and subsurface dams is similar. In general, sand dams are used when the topographical gradient is high and subsurface dams are used when the topographical gradient is low. If the impervious layer lies near the surface, the storage capacity of the basin will be low. A sand storage dam will enlarge the storage, while a subsurface dam only uses the storage capacity under the surface. When the river is narrow and has high embankments, the storage capacity of a sand storage dam can easily be enlarged. The embankments provide a good anchoring possibility