Localized Scour Around Structures Under Transient Flow Conditions

Abstract

In recent years, extreme natural phenomena such as tsunamis or storm surges have affected populated coastal regions around many coastal regions around the world and with particular impact for countries bordering the Indian, Pacific and Atlantic Oceans. Such extreme events caused significant loss of life and extensive damage to coastal communities. Field surveys conducted from the 1992 Nicaragua Tsunami to the relatively recent 2011 Tohoku Japan Tsunami have recorded lots of evidence of scour around damaged buildings and bridge foundations. It is documented that scouring is one of the significant causes of coastal structural damages. In post-tsunami forensic engineering surveys of India and Thailand following the 2004 Great Sumatra Andaman tsunami, researchers were able to document local scour occurrences not only at buildings located close to shore, but also hundreds of meters inland. (Yeh and Li 2008, Nistor et al. 2012 and Li et al. 2012). The most damaging 2011 Tohoku Tsunami provided an outstanding opportunity to document (Chock et al., 2013) a variety of failure modes of infrastructure including coastal and onshore structures. This research program encompassed a comprehensive experimental and numerical investigation and analysis for the scour mechanism and vortex structures due to inland-propagating tsunami-like bores on a dry or flooded horizontal mobile bed, representing comprehensive tsunami-induced scour study. The primary objective of this thesis was to investigate the influence of different parameters on the bore propagation characteristics and how they affect the scouring process. To achieve this objective, a series of hydraulic bores consistent with the dam-break wave theory were generated. Extensive experiments were conducted to perform parametric analysis on both bore propagation and local scour (e.g., influence of structure size, bore height, bed initial condition, bed material size and second tsunami wave). The secondary objective involved development of a numerical model to simulate local scour and predict the threedimensional (3D) flow pattern around a structure induced by tsunami–like bore. The open source TelemacMascaret numerical model was used to determine flow characteristics adjacent to the structure and the associated bed evolution and resulting local scour. The 3D hydrodynamic Telemac model couples internally with the sediment transport module SISYPHE to solve bed evolution equations. The efficiency of the numerical model was assessed and compared quantitatively and qualitatively with the results of the physical experiments. The studies show a longer duration of a turbulent bore induced more scour depth compared to that generated by solitary or long waves. The short duration and very turbulent nature of the bores induced rapid scour. It was shown that the location of maximum scour depth and sediment deposition pattern downstream of the structure are well captured by the numerical model, however; magnitude of scour depth is underpredicted compared to that of the experimental work. It was found that the ASCE (2016) prescriptions on estimating scour underpredicts the maximum scour depth and thus, the use of the ASCE (2016) provisions, may lead to undesirable underestimations of maximum scour depth. The results of this study have contributed to the American Society of Civil Engineers (ASCE) 7 Chapter 6, which is the first standard written in mandatory language in the world, which specifically prescribes guidance for the design of tsunami resilient critical infrastructure using also a probabilistic framework for the determination of the extent of coastal inundation.