SAND ENCROACHMENT BY WIND IN AL-HASA OF SAUDI ARABIA
For a long period of time al-Hasa oasis of Saudi Arabia has been known by its fertile soil, huge underground water reservoir, numerous springs and for being one of the most productive agricultural areas in Saudi Arabia for dates and rice. However, drifting sand and moving dunes have been devastating the oasis. Archeological investigations and field measurements carried out in this region reveal that sand has greatly lessened the cultivable area of al-Hasa littoral during the past two thousand years. Nearly half of the oasis may have been lost during the past ten centuries. Communities and villages such as Wasit, Nathra, old Kilabiyah, Juwatha, old al-Umran and al-Mohtariqa were known to exist in the past. It is reported that Juwatha once was in the middle of the oasis. Today the only remnant of this capital is a mosque which was found in the 1970's covered by sand in a dune field approximately 5 km northwest of al-Umran. Ancient springs and palm trees in several locations of the region are now covered by moving sand. In some instances only the very top of the well shafts and palm trees appear above the dunes. Mobile sand dunes 4 to 20 meters high with annual average migration rates of 10 to 15 meters are presently moving steadily southward endangering the northern and the eastern parts of al-Hasa oasis. As a result of such sand encroachment the inhabitants of several communities were forced to move southward in front of the advancing sand to new areas which in many cases were less productive than the areas being abandoned. The archeological findings, economic decline in agricultural production and field data collected in this study provide solid evidence of the rapid rate of sand encroachment into the oasis.
Sand drift and sand dune movement rates in the study area are controlled by several major factors. These factors in descending order of importance are: wind characteristics (velocity, duration, and direction), mean sand grain size, dune size, sand volume available for transport, moisture content of the sand and density of vegetal cover. The objective of this study was to determine the rate of sand drift and sand dune movement as it is related to wind characteristics and mean sand grain size. Equipment to record wind characteristics was installed in the al-Hasa dune field during the period between February and June of 1980. Sand traps to measure the volume of sand moved and the direction from which it moved during this period were also installed at the project site. Sand caught in the various traps was analyzed and its statistical parameters such as the mean, sorting, skewness and kurtosis were determined. Rates of sand dune movement were determined. Several equations were finally derived to determine and forecast the hourly volume of sand moved under different wind conditions. The analysis revealed that over 96% of the variation in sand movement rates is explained by the changes in the wind speed. The data indicate that nearly 75% of the total sand drift and sand dune movement which took place during the period of the study was caused by the most frequently occurring wind ranging between 5.6 and 12.3 m/sec. The highest quantity of sand drift was transported from the northern directions. The data show that the northerly sand flow was more pronounced during the period from April to June.