Critical wind speed for loose laid aggregate scour on flat roofs with parapets

A series of large-scale wind tunnel tests were conducted to measure the critical velocities at which gravel removal occurs from flat rooftops. Results are presented for a range of parapet heights and wind angles with a focus on cornering winds (wind angles of 35° to 45°). Critical velocities are reported for sporadic and continuous motion initiation, upwind and downwind removal, and scour hole formation. A deep learning-based image analysis tool is utilized for measuring the size, shape, and growth rate of the resulting scour holes. Results are qualitatively and quantitatively consistent with prior studies. The scour hole formation and shape are then compared to contours of pressure and shear stress coefficient data collected using the same building and wind field as the removal tests. This analysis clearly shows that the process of roof gravel removal is driven by surface shear stress. Furthermore, it is shown that the application of Shields’ formulation demonstrates a consistent non-dimensional critical shear stress across all wind angles and parapet heights, within the bounds of measurement uncertainty. This result means that the critical wind speed for rooftop scour formation can be calculated based on surface shear stress measurements made at lower wind speeds.