A Vigorous Debris-Flow Incident at Al-Hada Descent and Remedial Measures

Abstract

Al-Hada descent lies in the western region of Saudi Arabia; it begins at an elevation of about 2,000 m, and is characterized by sharp cliffs. The Al-Hada descent road was constructed with an elevation difference of 1,500 m between the highest and lowest heights along the road. The road alignment is intersected by 14 very steep gullies of almost 60–80°. The gullies contain large quantities of consolidated and unconsolidated mud, old levees and large rock blocks. The Al-Hada descent road was hit in April 2012 by heavy rainfall lasting almost 2 h. The rainstorm initiated 11 debris flows in steep gullies, which travelled rapidly down and hit the road with speeds of 25 m/s along the gully channel. Once the debris flow dropped from a height of about 150 m elevation and reached a less confined area at the retaining wall, it partially destroyed the gabions layer, right side gabions, and edges of the retaining walls across the gullies and overflowed them, as they received more rolling, sliding and bouncing rocks from higher steep elevations along the gully. The moving debris flows spread out, lost speed and was deposited beyond the highway descent route, and dropped to the lower side of the gully towards the valley.