Barchan dunes. Man climbing a large barchan dune ('mega barchan') in Morocco. A Barchan dune forms the shape of an arc and possesses two horns on either side which face downwind. The concave windward slope in-between these horns stands at the angle of repose at approximately 32 degrees. Whilst the convex lee slope is packed by the wind and lies at an angle of about 15 degrees. These large barchans move between 3 to 6 metres a year.