The study of the development and movement of sand ripples and dunes has received considerable attention in recent years, largely due to its relevance to several meteorological, environmental and engineering problems. Several authors have developed various mathematical and physical models to study the deformation of sand beds consisting of non-cohesive particles under the influence of sheer flows. For a comprehensive review the reader is referred to the papers by Kennedy (1969) and Allen (1974). In general, two approaches have been followed in the theoretical treatment of the movement of sand. The first and older approach is to start directly from the equations of hydrodynamics, and adopts one of the numerous empirical laws for the transport of sediment. This approach was first started by Exner in 1925 using a hydraulic model and has subsequently been developed by several authors (Kennedy 1963, Reynolds 1965, Engelund 1970, Engelund and Fredsoe 1971, Reynolds 1976, Richards 1979). Although this approach has been followed for more than half a century, it is still not fully developed, due to the lack of sound knowledge of the mechanism of the transport of sediment. The second and more recent approach adopts statistical methods to describe the formation and particle distribution of ripples and dunes.