Thesis (M.Eng.)--Memorial University of Newfoundland, 2003. / Bibliography: leaves 204-212.

Coastal areas in many parts of the world are vulnerable to tsunami waves. Large tsunamis are strong enough to bring about a substantial amount of sediment mobilization. Several post-tsunami field investigations performed in recent years have documented destruction induced by scouring process. For example, the 1993 Nicaraguan earthquake centred 100km off the Nicaraguan coast caused devastating tsunami-induced scour around structures and bridges (Satake et al., 1993). Differences in the scour depths were related to soil properties, shapes of structures, and tsunami hydrodynamics (Jayaratne et al., 2016). Furthermore, depending on the soil permeability, the flow and pressure propagate at different speeds within the soil, which affects water table fluctuations and the soil strength (e.g., Tonkin et al., 2003; Yeh and Li 2008). The primary objective of this research was to study the effect of different inland-propagating dam-break bore heights on pore pressure variations and scour evolution in saturated beds with two different bed slopes (i.e., zero and +5% slope) by performing comprehensive laboratory studies at a 1:40 scale. To achieve the objective, tsunami-like dam-break bores generated by rapidly opening a swing gate and propagated towards and over a sediment section and hit a structure centred within a sediment bed. The secondary objective of this experimental investigation was finding a relation between scour depths and pore pressure values as a function of still-to-impoundment water depth ratio. The results of this experimental investigation showed that effective pore pressures were consistently greater in the front face of a model than in the side face. Besides that, the highest effective pore pressures took place near the saturated bed surface. Such that, due to the propagation of supercritical bores the maximum effective pore pressure in the bottom of the front corner was 50% larger than the exact same location in the side face. While, this difference decreased to 10% in the case of subcritical bores. For the same hydrodynamic bore conditions, the maximum difference between effective pore pressure in the two faces of the model reduced by 70% in the inclined bed test than the horizontal bed tests and this difference was only 15%. However, the peak effective pore pressure around the model doubled in the inclined bed tests compared to the horizontal ones. The 5% upsloping decreased the maximum scour depths by two times as a result of the same hydrodynamic loading conditions.

Student Number : 9700136A - PhD thesis - School of Civil and Environmental Engineering - Faculty of Engineering and the Built Environment / A sediment budget model in which each steady discharge scours sediment along a trajectory towards ultimate target storage or deposits sediment towards the same ultimate target storage has been conceptualized and developed. The method is aimed at routing sediment in morphologically diverse bedrock-controlled channels in which sediment transport and storage is not a continuous process in space and time and mostly occurs in response to discrete discharges. The relative value of the ultimate stable scour depth (Huss) for each steady discharge with respect to the current scour depth after adding sediment supply determines the potential to scour or store sediment. Scour depths measured at discrete locations along the longitudinal profile of a laboratory pool at discrete times until changes in scour were not discernible for each steady discharge and sediment size have been integrated to provide the Huss and storage depletion curve. The experimentally established dependence of scour depth on critical flow depth, settling velocity and sediment supply formed the basis of generating dimensionless Huss and storage depletion curve from these parameters using the Buckingham π theorem. The optimization of experimental results to generate the storage depletion curve gave the exponent of time (φ) and the exponential decay factor (k) as 0.5 and 0.0040207 respectively. Regression fit of dimensionless Huss and critical flow intensity gave a linear relationship with a gradient of 0.90214, y-intercept of –1.4766 and R2 of 96%. The suitability of the model for budgeting sediment dynamics in a series of connected storage units, the validity of using the relative values of Huss and the current scour depth after adding sediment supply to determine scour potential and the existence of active storage associated with sediment supply for each steady discharge have been confirmed experimentally. Modelling with equivalent steady discharges computed from unit stream power principles on the rising and the falling limbs of the hydrograph resulted in scour on the rising limb of magnitude dependent on the magnitude and sequence of the flood event, and less or no scour on recession. The modelling concepts and approach have thus been validated and the potential to reasonably simulate sediment storage changes in bedrock-controlled rivers demonstrated.

Sediment material around the base of a bridge pier is moved by the flow velocity and associated turbulence. This phenomenon is generally termed as local scour and can lead to undermining the structure and increase its possibility of failure. Numerous factors can affect bridge pier scour and they have been investigated for decades. Debris jams, one of these factors, could significantly contribute to bridge failure as some field examples and experimental investigations pointed out. Woody debris accumulation on the front of either single or multiple bridge piers can result in deeper pier scour and extra load exerted on the pier. Several studies have already investigated the influence of woody debris on pier scour in terms of static woody debris. In addition, HEC-18 (2012) also proposed a design code to estimate scour depth in the presence of woody debris jam. However, in these studies, the woody debris jam was considered to be static, whereas a woody debris jam accumulates piece by piece, growing to a debris jam with a shape most akin to a half-cone, and then may even eventually break up and be carried in pieces downstream. Therefore, this research investigated the evolution of the loading onto and scouring around a bridge pier in the presence of dynamic debris jams. In this study, the temporal evolution of the bridge pier scours was monitored during the development of dynamic debris jams. Experimental modeling was conducted to explore the influence of dynamic debris jam on bridge pier scour using a scale of 30 by employing both dowels and seedling trees. It was found that the dynamic debris jam of dowels could last 10-20 minutes and reach a critical size, then fail and subsequently reform. In addition, the first debris jam had an obvious influence on scour depth which correlated to the blockage generated by the debris jam; however, the influence of the subsequent debris jam depended on its size compared to the previously formed one. For the dynamic debris jam using seedling trees, the debris jam lasted for a longer time once it formed, and it could lead to twice the maximum scour depth compared to that generated in the absence of the debris jam, which is the same with dowels debris jam. In addition, the hydraulic head induced by the debris jam was correlated to the blockage of the debris jam and the flow Froude number irrespective of whether the dynamic debris jam was made of dowels or seedling trees. Additionally, blank control tests in the absence of a debris jam were used along with previous data gleaned from the literature to develop and test new multigene genetic programming (MGGP) models for the temporal evolution of scour. The MGGP model, using the non-dimensional variables from the empirical equations, can reach a better accuracy than the empirical equations, which indicates the ability of the model to optimize the empirical equations. The temporal evolution of load exerted onto the bridge pier with a dynamic debris jam was also measured. Experimental tests were performed to investigate the additional debris jam drag force exerted onto the bridge pier using both dowels and seedling trees in the presence of a fixed flume bed. Likewise, the dynamic debris jam of dowels lasted for about 10-20 mins, while those formed by the seedling trees, once formed, could last over 50 mins. The investigation demonstrated that the drag coefficient of the seedling trees jam was higher than that of the dowels jam. More importantly, a spike in the drag force was also observed irrespective of whether the jams were formed by dowels or seedling trees. Detailed investigation of the flow field around the debris jam and pier provided insight into the mechanics of debris jams. Three half-cone-shaped debris jams of the same dimensions were designed and built. The three jams were fabricated using: a) 20 cm long dowels, b) 30 cm long dowels, or c) a 3D printer. For each jam, four sections were measured using an Acoustic Doppler Velocimeter (ADV). The results indicated that the flow fields around the 20 cm length dowel jam and the 30 cm length dowel jam were similar. In addition, the section behind the pier and debris jam showed divided zones termed herein as the accelerated high-velocity zone, the high shear transition zone, and the wake dead zone. As for the drag coefficient, the 20 cm length dowels jam and 30 cm length dowels jam shared a very close magnitude of 1.7, but the drag coefficient of the 3D printer debris jam was only 0.88 which indicated the debris jam built by individual pieces behaved differently than the block jam.

Yes / River dynamics and sediment transport play an important role in river bed morphology. Building a bridge pier along the river alters the cross-section of the river and causes the change in flow processes. These changes are mainly responsible for pier scour. In this paper, the usage of collars to reduce scour around circular piers has been investigated. The collars with different diameters and depth positions have been studied using previous data and additional data collected in the present study to assess their effectiveness in reducing scour. Using a wide range of measured data, an empirical equation to compute the maximum scour depth around the circular piers in the presence of collars has been proposed. The proposed equation has been validated and proven to be applicable to a wide range of pier layouts. It has been found that the maximum efficiency can be achieved by fixing the collar at bed level and adopting a collar diameter 1.5–2.5 times of pier diameter.

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Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The scour process of unlined spillways is an important research topic of value in engineering practice. In South Africa numerous unlined spillway dams have experienced severe erosion. This led, in some cases, to the costly concrete lining of spillways for erosion protection. On the other hand, the erosion of unlined spillways can lead to damage to, and even failure of dams and consequently can affect public safety, properties, infrastructure and the environment. In this regard, methods to predict erosion of unlined spillways are therefore needed as tools in the risk management and design of existing unlined spillways as well as future spillway structures. The prediction of the rock scouring process is challenging and empirical formulas have been established to predict incipient conditions for scour. These empirical methods however do not predict the rate of scour or the ultimate equilibrium rock scour. The key objective in this study was to investigate the applicability of a non-cohesive two-dimensional (2D) sediment transport hydrodynamic mathematical model to simulate unlined spillway scour. A physical model flume test was set up to simulate rock scour represented by uniformly sized polyethylene cubes. The flume slope and discharge were varied in the different test scenarios. The 2D mathematical model correctly predicted the extent and location of scour as observed in the laboratory tests. Temporal changes in the scour formation were also predicted with reliability. This was achieved by only calibrating the hydraulic roughness of the 2D model, and by specifying the ”rock ” particle settling velocity and material density. The simulation results were satisfactory, providing an accurate and detailed erosion prediction. From this, the mathematical modelling was validated by using a field case study. The results obtained with the mathematical model were promising for non-cohesive cases and could be applied to field prototype cases if the rock joint structure is known. This would typically apply in fault zones, where the joints would give an idea of the rock size to be used in the mathematical model. In general, where jointed rock is more massive and acts ”cohesive ”, rock parameters describing critical scour conditions for the rock in terms of stream power are required to be built into the mathematical models. / AFRIKAANSE OPSOMMING: Die uitskuringsproses van onbelynde oorlope is ’n belangrike navorsingsonderwerp in die ingenieurspraktyk. In Suid-Afrika word ernstige erosie in baie onbelynde oorlope van damme ondervind. In sommige gevalle het dit gelei tot die duur belyning van oorlope met beton, om die oorlope te beskerm. Aan die anderkant kan die erosie van onbelynde oorlope lei tot groot skade en selfs tot die faling van ’n dam. Dit kan weer lei tot skade aan eiendom, infrastruktuur en die omgewing, en die publiek in die gevaar stel. Daarom is dit nodig dat daar besin word oor metodes om erosie in onbelynde oorlope te voorspel, sodat die risiko bestuur kan word en om te sorg vir die beter ontwerp van onbelynde oorlope in die toekoms. Dit is moeilik om die uitskuringsproses te voorspel, maar empiriese formules is bestaan om die aanvang van uitskuring te voorspel. Hierdie empiriese metodes voorspel egter nie die snelheid waarteen die uitskuring sal plaasvind of die uiteindelike mate waartoe dit sal gebeur nie. Die hoofdoelwit van hierdie studie was om die toepasbaarheid van ’n nie- kohesie, twee-dimensionele (2D) hidrodinamiese wiskundige model te ondersoek, om sodoende die uitskuring van onbelynde oorlope te simuleer. ’n Fisiese model om die uitskuring van rots te simuleer is ook gebou. Die rots is deur polietileen blokkies van dieselfde grootte gemodelleer. Verskillende kanaalhellings en deurstromings is in verskillende toetse gebruik. Die 2D wiskundige model het volgens die waarnemings in laboratorium toetse, die mate en ligging van die uitskuring korrek voorspel. Veranderinge wat met verloop van tyd in die uitskuring formasie plaasgevind het, is ook betroubaar voorspel. Dit is gedoen deur die hidrouliese ruheid van die 2D model te kalibreer en deur te spesifiseer hoe vinnig die ”rots ” deeljies afsak en wat die digtheid van die materiaal is. Die uitslag van die simulasie was bevredigend en het die erosie akkuraat en in detail voorspel. Die wiskundige modellering is gevalideer deur middel van ’n gevallestudie. Die uitslae wat met die wiskundige model verkry is, is belowend, en geld vir nie-kohesie gevalle. Dit kan op prototipe gevalle in die veld toegepas word as die rots se naatstruktuur bekend is. Dit kan toegepas word in foutsones waar die nate ’n aanduiding sal gee van die grootte van ”rotse ” wat in die wiskundige model gebruik moet word. Maar as die rots baie groot is en die kohesie goed is, is dit nodig om meer parameters betreffende uitskuringstoestande gekoppel aan stroomdrywing, in die wiskundige model te gebruik.

This analysis is an expansion of research done by Rollin Hotchkiss during his Ph.D work. The research uses fluid flow, sediment transport, and perturbation theory to predict where scour will occur in a variable-width channel. The resulting equations also determine equilibrium scour depth based upon the stream bed elevation derived from a dimensionless bed slope equation. Hotchkiss perturbed the width of the channel using a second order Taylor Series perturbation but neglected second order terms. The present work follows the same procedures as Hotchkiss but maintains the second order terms. The primary purpose is to examine how the additional terms impact the final equilibrium scour depth and location results. The results of this research show a slight variation from the previous work. With respect to a hypothetical case, there was not a significant amount of change, thereby verifying that scour migrates downstream with an increase in discharge. Interestingly, the comparison shows a slight increase in sediment discharge through the test reach analyzed. Supplementary to previous research, values of scour depth and location in terms of distance from the start of channel-width perturbation are provided; at the lowest discharge maximum scour occurs 4% of a wavelength upstream of the narrowest portion, and at the highest discharge maximum scour occurs at the narrowest point. Additionally, a one-dimensional HEC-RAS sediment transport model and a two- dimensional SRH flow model were compared to the analytical results. Results show that the model output of the HEC-RAS model and the SRH model adequately approximate the analytical model studied. Specifically, the results verify that maximum scour depth transitions downstream as discharge increases.

The undermining of bridge foundations can lead to either costly repairs or a bridge collapse. These foundations must be designed to counter the effects of scour. Current practice does not allow for accurate estimates of scour in erodible rock. Scour in rock can be related to geotechnical and hydraulic properties. A field study of eleven bridge sites provided samples of the bedrock where the abrasive resistance of the rock was determined and hydraulic properties of the channel were calculated. Laboratory abrasion resistance values from a modified slake durability test and hydraulic variables such as stream power were compared to recent and past stream channel cross-sections. A preliminary model has been proposed wherein the degradation of the stream channel is related to the abrasive resistance of the bedrock and the area under the daily stream power. This method provides an estimate of the degradation of the stream bed due to abrasion by bedload and flood events, not necessarily local or contraction scour. / Graduation date: 1998