Experimental analysis of flash flood-induced scour on semi-arid hillslopes

Flash floods play a key role in soil erosion and land degradation on hillslopes and lands with low or no vegetation cover, particularly in arid and semi-arid climates. These conditions prevail in most regions of Kerman province, southeastern Iran, where intense, short-duration rainfall generates rapid runoff and substantial sediment transport. Hence, to understand the process of erosion induced by flash floods, this study explored the combined influence of slope gradient (0-0.175) and flood discharge (3, 4.5, and 6 L/s) on scour and sediment yield using controlled flume experiments. A 300 cm glass flume with non-uniform sand (1-4 mm particles) was used to test 18 configurations under three repetition test schemes. Using dimensional analysis, scour depths were measured at 50 mm intervals. The results showed that both scour volume and sediment yield increased significantly with increasing slope and discharge. For example, at a discharge of 6 L/s, the maximum scour volume was observed at a slope of 0.175. When the slope was reduced to 0.15, the scour volume decreased by approximately 30-50%. At the same discharge, for slopes below 0.1, the reduction exceeded 93%. Increasing discharge from 3 to 6 L/s at a 15% slope increased scoured sediment by 36.3%. Moreover, with a 12.5% increase in bed slope, the amount of eroded sediment increased by a factor of 18.5. The results showed that mild slopes (0-0.05) with moderate flows improved bed stability, whereas steeper slopes under high discharges intensified upstream erosion and shifted deposition downstream.