Estimación de la profundidad de socavación local bajo condiciones hidraúlicas impermanentes Effective flow work for estimation of pier scour under flood waves

Abstract

Bridges are important for society, especially for the mobility, economy and development of humanity. The main objectives for the design of bridges are linked to life expectancy, stability, and safety during natural extreme events such as earthquakes and floods. The global climate change increases the magnitude and frequency of extreme precipitations and floods events confirming the necessity for a better bridge-pier design ad-hoc to unsteady hydraulic conditions. Generally during a flood event, the maximum scour depth at peak-flow may be smaller than the equilibrium scour depth, which is commonly estimated using peak-flow conditions for design of bridge piers. If the local scour depth is sub-estimated the hydraulic structure can fail and on the other hand, if this scour depth is overestimated, that construction has no economic justification. The objective of this work was to study the impact of the shape, duration and peak discharge of flood hydrograph on the maximum scour depth under clear-water scour conditions and over natural uniform sediment. Scour experiments were conducted at the Laboratory of Hydraulic Engineering, University of Concepción, with constant discharges until equilibrium and with hydrographs having different duration, shapes and peak discharge. Finally, the pier scour caused by flood waves is analyzed introducing the idea of an effective work by the flow on the sediment bed around the pier. Following dimensional considerations, the author shows different possible formulations for the dimensionless flow work W* and the corresponding dimensionless parameters that govern scouring. In each case, the dimensionless flow work is shown to be a generalization of the flow intensity concept c u u , commonly used in existent scour formulas. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is described and four series of experiments are presented. The first series consists of experiments with constant discharge until advanced stages of scour. The second and third series, of scour experiments with flood waves of different shapes and durations, respectively. The fourth series consists of scour experiments caused by hydrographs with multiple peaks. Results confirm that the dimensionless flow work W* represent a reliable concept for the prediction of scour caused by any hydrograph.