Giuseppe Oliveto

Local Scour Downstream of Positive-Step Stilling Basins

This note focuses on the temporal and spatial evolution of local scour below low-head spillways. Steady-flow experiments were carried out in a 1-m wide and 20-m long rectangular straight channel. The jet was generated by an ogee-crest spillway followed by a positive-step stilling basin. Nearly uniform sandy beds were generally tested, but additional tests were also performed with a special bed of lead spheres. To circumvent the combination of local and general scour phenomena, tailwater depths were set such that tailwater flow intensities were below the threshold of sediment motion. As a consequence, for each run a submerged hydraulic jump formed. Tests were of long durations (of order of days) mainly to achieve conditions of quasi-equilibrium. Based on the data collected, literature approaches are discussed. Then, empirical models are proposed to estimate: (1) the maximum scour depth at the quasi-equilibrium stage and its horizontal distance from edge of stilling basin; (2) the time variation of scour depth; and (3) the axial scour profiles. The proposed equations agree well with experimental data. Findings also highlight that affinity rather than similarity may be the typical property of low-angle eroding jets.

Morphological Evolution of Dune-Like Bed Forms Generated by Bridge Scour

This research intends to provide new insights on sediment transport at bridges, exploring the features of migrating dune-like bed forms generated immediately behind piers or abutments by local scour processes. An extended VAW (Laboratory of Hydraulics, Hydrology, and Glaciology, Swiss Federal Institute of Technology Zurich) data set collected from 1999 to 2002 was analyzed. In total, 63 temporal runs involving 480 observations for cylindrical piers, and 54 runs with 384 observations for rectangular-shaped abutments were considered. It was found that the main dimensionless parameters governing the dune processes are the relative pier diameter (or abutment width), densimetric Froude number, wash-out parameter, and relative time. The dune height is mainly controlled by the relative width of the scour element, and its relative length depends on the wash-out parameter. The morphologic dune evolution and its downstream propagation are described using straightforward equations as a function of the key parameters. The results apply to practice and are suitable to assess the performance of computational modeling.

Time-dependent local scour downstream of positive-step stilling basins

Local scour downstream of spillways is an important problem in hydraulic engineering. The objective of this experimental study was to investigate the temporal evolution of local scour downstream of a spillway followed by a positive-step stilling basin. Experiments were conducted in a rectangular channel using four different sediments for the mobile bed. Additional tests concerned the incipient motion of sediment particles. The main parameters governing the scour process were found to be the tailwater densimetric Froude number and the relative tailwater depth. No experimental evidence was found on similar scour profiles. Scour starts at a much reduced tailwater densimetric Froude number when compared with Shields. The temporal variation of the scour hole geometry was estimated using simple power-law equations relating the scour geometry to the controlling scour parameters.