WIND VELOCITY AND ALGAL CRUSTS IN DUNE BLOWOUTS

In two saucer-shaped blowouts wind velocity was measured at 0.18 m hei ght and compared to the wind velocity data from the nearest standard m eteorological station at Valkenburg about 3.5 km to the northeast. In blowout 1 relative velocities are highest when the wind blows from dir ections between E and SSE. In blowout 2 winds blowing from directions between S and WSW are relatively stronger. In general the wind speed i ncreases as its moves along the surface inside the blowout, because th e aerodynamic roughness of the blowout surface is less than that of th e surrounding area. Less important are the influence of the configurat ion of the terrain surrounding the blowout and the influence of the sl opes within the blowout. The frequency distribution of the general win d at Valkenburg is analysed and combined with the horizontal wind velo city pattern within blowouts. A simulation model, producing the spatia l distribution of the wind power shows that the prevalence of algal cr usts at the SW side of blowouts agrees well with the location which re aches the lowest wind power values. In other parts of the blowouts alg al crusts are damaged by moving sand at times of high wind velocity.