Effect of grade-control structures at various stages of their destruction on bed sediments and local channel parameters

Abstract

Grade-control structures (GCSs) represent the typical management of torrential streams, preventing massive bed erosion and bedload transport. The original and present geometric and sedimentary parameters of 18 GCSs at various stages of their destruction since the 1970s were evaluated to determine the relationship between the former and present-day components of the managed Mohelnice River (the western Carpathians, Czech Republic). The latest changes in the GCS geometry, related scour holes, and bed surface grain size of sedimentary wedges were caused by the 2010 flood event of 20–50 R.I. discharge. No relationship exists between the bed surface grain sizes and the present water drop or the present equilibrium channel slope of the sedimentary wedge. A significant downstream coarsening of the largest grain size percentile represented by D95 is detected through the sequence of GCSs. Also, statistically insignificant trends in downstream coarsening were observed for D16, D50, and D84 grain sizes. However, the investigated sequence is still passable for grain diameters up to 200mm during high-magnitude floods similar to the 2010 event, as documented by the development of a confluent gravel bar downstream of the sequence. Bedload transport simulations provide the highest bedload transport rates for the initial stage of the uppermost studied channel reach without the presence of GCSs (30,000kgmin−1 for Q50). Grade-control structures reconstruction in the 1970s significantly decreased transport rates (>2000kgmin−1 for Q50). Owing to the erosion of GCS crests and an increase in related equilibrium channel slope, damage on GCSs can lead to an increase in bedload transport intensity (13,000kgmin−1 for Q50). Significant linear relationships exist among the present parameters of the scour holes (length of scour hole, maximum scour depth, and horizontal distance between the point of maximum depth and the GCS crest). A statistical significant power relationship exists between the parameters of maximum scour hole depth and the present drop height, showing adjustments of maximum scours to the present stage of GCSs and the last flood event.