Coupled fully three-dimensional hydro-morphodynamic modelling of bridge pier scour in an alluvial bed

Vonkeman, Jeanine Karen (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Local scour at piers has been cited as the main mechanism responsible for the collapse of bridges founded in alluvial beds and yet there is no universally agreed upon design procedure to accurately predict the equilibrium scour depth. The scour process was investigated by a 1:15 scale physical model for a combination of different flows, pier shapes and sediment beds, from which the scour patterns and flow velocities were measured. The experimental data was used to evaluate thirty empirical equations for bridge pier scour, which were found to produce a wide range of unreliable results. No single equation is conclusively superior but the HEC-18 equation is proposed, as well as equations that rely on the pier Reynolds number, a parameter which has been shown to be significant in the horseshoe vortex formation. Subsequently, an improved dimensionless shape factor and armouring factor based on the particle Reynolds number were developed for the HEC-18 equation from field data measurements. Although extensive research has been published on bridge pier scour for more than six decades, comparatively few studies have been presented on the detailed 3D numerical modelling of such processes. The key aim of this study was to develop an improved coupled fully three-dimensional hydro-morphodynamic model with the Immersed Boundary method and Reynolds Stress Model to simulate pier scour. The proposed numerical model computes bed shear stresses from implicit wall functions and adopts an Eulerian multi-fluid model to account for rolling and saltating particles. Numerical instabilities were addressed in the sediment transport submodels which were ascribed to the fine mesh resolution required to resolve the crucial horseshoe vortex and the diffusion resulting from the discretization of the Immersed Boundary method. The Reynolds Stress Model was compared with the 𝑘𝑘-ε turbulence model but it was found that the results from the numerical model are more sensitive to the computational grid than to the choice of turbulence model to resolve the horseshoe vortex and to obtain stability. Despite the perceived limitations of the proposed hydro-morphodynamic model, the model demonstrated that the velocity flow field, the horseshoe vortex and the subsequent maximum bridge pier scour upstream of the pier nose can be modelled successfully to simulate the results from the experimental work. The simplicity of conservative empirical equations may be feasible for the conceptual design of bridges. However, advanced numerical models have the ability to better account for the interaction of several interrelated parameters and the intricate vortex systems responsible for the scour process at bridge piers. It is proposed that the primary subject of future studies for bridge pier scour should be on the comparison of numerical models with one another.

AFRIKAANSE OPSOMMING: Lokale uitskuring by brugpylers is die belangrikste meganisme wat verantwoordelik is vir die faling van brûe wat in alluviale riviere gebou is, maar daar is nog geen universele metode om die ewewiguitskuurdiepte te voorspel nie. Die uitskuurproses is ondersoek deur laboratoriumtoetse met ‘n 1:15 skaal vir verskillende deurstromings, pylervorms en sedimentbeddings, waarvoor die uitskuurpatrone en die snelheidsvloeiveld gemeet is. Die eksperimentele data is gebruik om dertig empiriese vergelykings vir brugpyleruitskuring te toets. Soos in al die voorafgaande studies, het die vergelykings se voorspellings 'n reeks van onbetroubare resultate gelewer. Geen enkele vergelyking is by uitstek die beste nie, maar die HEC-18 vergelyking word voorgestel, asook vergelykings wat op die pyler Reynolds-getal staatmaak, 'n parameter wat beduidend is in die hoefyster werwel. 'n Verbeterde pylervormfaktor en sedimentfaktor gebaseer op die deeltjie Reynoldsgetal is daarom vir die HEC-18-vergelyking met velddata ontwikkel. Alhoewel daar meer as ses dekades lank uitgebreide navorsing oor brugpyleruitskuring gedoen is, is daar relatief min studies gepubliseer oor die 3D numeriese modellering van sulke prosesse. Die hoofdoel van hierdie studie was om ‘n verbeterde gekoppelde en volledig drie-dimensionele hidro-morfodinamiese model met die Immersed Boundary metode en die Reynolds Stress Model te ontwikkel om pyleruitskuring te simuleer. Die voorgestelde numeriese model bereken die skuifspanning deur implisiete muurfunksies en neem ‘n Euleriese meervoudige model aan vir deeltjies wat rol en spring. Numeriese onstabiliteite is in die submodelle van die sedimentvervoer aangespreek, wat toegeskryf is aan die fyn maas wat nodig is om die kritieke hoefyster werwel op te los en aan die diffusie wat voortspruit uit die diskretisasie van die Immersed Boundary metode. Die Reynolds Stress Model is met die 𝑘𝑘-ε turbulensiemodel vergelyk, maar die resultate van die numeriese modellering is meer sensitief vir die maas as vir die keuse van ‘n turbulensiemodel om die hoefyster werwel op telos en om stabiliteit te verkry. Ten spyte van die waargenome beperkings van die voorgestelde hidro-morfodinamiese model, het die model gedemonstreer dat die snelheidsvloeiveld, die hoefyster draaikolk en die maksimum brugpyleruitskuring voor die pylerneus suksesvol gemodelleer kan word vergeleke met die resultate van die eksperimentele werk. Die eenvoud van konserwatiewe empiriese vergelykings kan haalbaar wees vir die konseptuele ontwerp van brûe. Gevorderde numeriese modelle het egter die vermoë om die interaksie van verskillende interwante parameters en ingewikkelde draaikolkstelsels beter te verantwoord. Dit word voorgestel dat die primêre onderwerp van toekomstige studies vir brugpyleruitskuring op die vergelyking van numeriese modelle met mekaar moet wees.

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