Dunes

sand reliefs formed by the wind.

Usually the term “dunes” is applied to all sand build-ups regardless of zonal and climatic conditions; in the USSR the term refers only to forms of nondesert sand relief: on the shores of seas and lakes, on river terraces, in morainic aprons, and so forth. In contrast to the barchans developed in deserts, dunes (in this sense) have a convex form on the steep side, not on the sloping side, and the “tails” are located behind the dunes, on the windward side. The sloping side faces upwind and has an angle of inclination of 8°-20°; the downwind side approaches the angle of repose of dry (32°-33°) or moist sand (up to 40°). Dunes range from 5 to 30 m high and more [on the Courland Spit in the Baltic Sea (in Lithuania) a dune 58 m high is known; in Les Landes, on the coast of the Bay of Biscay (in France) there is one 97 m high]. Dunes may shift in the direction of the prevailing wind at a speed of 10 m a year, depending on the weight of the sand and the wind velocity.

The evolution of nondesert dunes, with a prevailing wind direction, is expressed in the gradual transition from coastal or riverside dune banks, which as a rule are transverse to the wind, into staple-shaped, parabolic, and hairpin-shaped forms. Under seasonal winds directed at angles of less than 90°, spear-shaped dunes form. In areas with convective or interfering wind patterns, rounded, ridge-like dunes develop, with a scattering from the center toward the periphery. Moreover, with time simple shapes turn into complex ones. The movement of denuded dunes, which cause great harm to the economy, is being combatted by the planting of wooded vegetation (mostly pines).