Hydrological risk and associated scour risk: case study sensitivity analysis

Abstract

Bridge collapses have been a severe problem worldwide, leading to socio-economic disruption and human casualties. The knowledge of the causes and consequent collapse modes have drawn much attention from researchers, which have also contributed to the gain of experience from real bridge failures in the design of new bridges. Bridges, like other hydraulic infrastructures, are designed to withstand a certain design flood event, ascertained by the establishment of acceptable risk and its design life. Different levels of risk can be accepted depending on the typology, importance and dimensions of a certain bridge, as well as on the consequences of its possible collapse, including possible material damage and loss of human life. Thus, the assessment of the associated hydrological risk plays a vital role in the design process, as well as during their operation, namely when bridges are founded on alluvial riverbeds. For instance, minor bridges are commonly designed with higher risk. In Portugal, the persistence of floods exceeding 8,000 m3/s in three months has contributed to the partial collapse of the 19th century Hintze Ribeiro bridge on the night of 4 March 2001. In structural design, the characterization of the design life of a bridge is preceded by the definition of its functional life, which depends on the maximum traffic capacity desired for that infrastructure. The functional life is often shorter than the expected design life. In that situation, the bridge should be functionally rehabilitated, for example, by increasing its width or building a new one nearby, in order to ensure that the functional quality of the crossing is maintained. In May 2002, the 19th century Hintze Ribeiro bridge was, thus, replaced by the new Hintze Ribeiro bridge, 7.5 m upstream of the old bridge’s position. Although the structural design particularities of the new Hintze Ribeiro bridge were beyond the reach of the authors, a sensitivity analysis may be performed. For instance, assuming the typical 50 years for the expected design life of such a bridge, different return periods could be plausibly used in its design process, depending on the acceptance risks. Hence, a 100-year return period of the design flood event corresponds to a hydrological risk of 40%. For a hydrological risk of 10%, the bridge would be designed to withstand the 500-year flood (Q500), whereas, for a longer design life of 100 years, a hydrological risk between 15% and 20% would be attained. The adopted return period as a design criterion, and associated design flood, also plays a significant role in the assessment of admissible scour risk (i.e., maximum scour depths at the bridge foundations level) for the bridge structural stability, which is also dependent on the priority factor attributed to that bridge. While Q500 (in 50 years) is associated with a lower hydrological risk, the water level regarding that design flood reaches the bridge deck, which adds other sources of instability in addition to the inherent scour risk.