SPECIFICATION AND CALIBRATION OF BAGNOLD'S MODEL FOR SAND TRANSPORT: URAYQ AL BULDAN DUNE FIELD, CENTRAL SAUDI ARABIA
Sand invasion is, and has been, a critical problem for many small, scattered villages in the Northern, Central and Eastern Provinces of Saudi Arabia. Year after year more of the usable arid land of Saudi Arabia is converted to wasteland due to the encroachment of desert sand. More importantly, the productive capacity, of vast dry regions of Saudi Arabia is declining.
The research used wind profile observations over Urayq Al Buldan sand dunes of central Saudi Arabia as a basis for calculating the shear stress and wind shear velocity which are the most important variables influencing sand movement by wind. The measured critical wind velocity at the beginning of sand movement on Urayq Al Buldan sand dunes was found to be about 470 cm sec('-1) and the corresponding critical shear velocity calculated directly from the wind profiles is 24.89 cm sec('-1).
The rate of sand movement was determined with two different types of sand traps modeled after the designs of Horikawa and Shen (1960). Calculation of creep to total sand captured by these traps agree very well with that calculated by Chepil for similar grain size.
Plots of the measured values of shear stress versus total sand transport revealed a strong statistical association between shear stress and sand transport but the proportional relationship between shear stress and sand transport does not appear to be a simple one.
Bagnold's equation for sand transport which was based solely on the threshold velocity and wind speed at a fixed measurement height appears to be very useful in applied studies and capable of very precise predictions of rates of sand transport as long as the model has been properly calibrated.
The analysis of grain size of sand caught by the different traps revealed variations in the symmetrical sediment distributions according to the different traps used.