Fluvial Transport of Large Boulders

Fast water can cause extraordinary bedload transport. Of the record floods considered here, the Jul. 26, 2022 urban flash flood on the upper River des Peres, St. Louis, Missouri provides particularly well-constrained data on flow conditions associated with large block movement. Field measurements and a detailed lidar survey show that concrete slabs as large as 3.0 × 2.5 × 0.33 m³ were moved from the open channel to reside 215 to 450 m inside a 6.5 m-diameter drainage tunnel, some to become part of a 10-m long imbricated pile. Peak flows of 160 m³/s were recorded at a gauging station located only 1.6 km upstream of the tunnel entrance, which provides a good estimate of 4.2 m/s for the peak flow velocity in the tunnel. Available observational data and a new theoretical analysis show that the radius of large boulders that can be moved by flowing water is directly proportional to the velocity head.