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The Effect of Climate Change on Mobility and Stability of Sand Dunes

Wednesday, June 4, 2008
Haim
Tsoar (Ben-Gurion University of the Negev, Israel)

Sand dunes are known to be: (i) free of vegetation and active (ii) partly vegetated and active (iii) fully vegetated and fixed. It is common to believe that low rainfall and high potential evaporation result in sparse or non-existence vegetation cover and hence active sand dunes. The amount of vegetation on sand dunes is also checked by strong wind erosion. Considering the above three factors, equations were developed by several researchers, which calculate the mobility of sand dunes by referring directly to the percent of days during the year with sand moving winds and indirectly to the ratio of mean annual precipitation to mean annual potential evapotranspiration. This equation is widely used to determine whether sand dunes would be active or fixed and the expected effect of climate change on sand dunes. According to the mobility index equation, sand dunes are fixed in humid areas and mobile in arid areas. There are many examples of unvegetated active sand dunes in humid areas and of vegetated fixed dunes in semi-arid and arid regions. Moreover, there are many examples that show the coexistence of mobile and stabilized dunes under the same climate conditions. This incompatibility is the result of the singular characteristics of dune sand where precipitation and evaporation are not so effective on vegetation.

Wind power is the most important factor in sand dune mobility because of the non-cohesiveness of the sand. Wind above a certain wind velocity can erode sand to such an extent that it prevents seeds from germinating in the sand. A much better index for the wind magnitude is the drift potential (DP) of the wind, which refers to the sand transport and wind power equations. Results of field data support the premise that the predominant factor affecting sand dune mobilization is wind erosion.

The relationship between wind power and vegetation cover can be recapitulated by a hysteresis curve. When climate changes, in the form of a decrease in wind power, vegetation starts covering the sand dunes in increasing numbers. However, when this process is reversed, increase of wind power over vegetated dunes will not cause the extinction of vegetation under higher DP.

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