Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 119-126). Available also in a digital version from Dissertation Abstracts.

Thesis (M. Phil.)--University of Hong Kong, 1974. / Mimeographed.

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

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published_or_final_version / Physiology / Master / Master of Philosophy

No description available.

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A method called SRICOS-EFA is presented in this dissertation for scour prediction. The method is based on the calculation of two basic parameters: the maximum depth of scour and the initial rate of scour. The maximum depth of scour is based on an equation obtained from flume tests and the initial rate is based on an equation giving the initial shear stress obtained from numerical simulations. The initial scour rate is then read on the Erosion Function Apparatus (EFA) erosion function curve at the corresponding value of the calculated shear stress. A hyperbola is used to connect the initial scour rate to the maximum scour depth and describes the complete scour depth vs. time curve. The erodibility function curve can be measured in the EFA. As the results show, the SRICOS-EFA method can handle the multi-flood hydrograph and multilayer soil system. It can be used to solve the complex pier and contraction scour alone; it can also handle the superposition of complex pier scour and contraction scour. A simplified SRICOS-EFA method was developed based on the case histories for contraction scour. EFA tests were performed to investigate the influence of different pH values and different levels of salinity on the soil erodibility. An attempt was made to find the correlation between the critical shear stress, and the initial slope of the erodibility function on the one hand and some geotechnical parameters on the other. A solution for future hydrograph prediction was developed in this dissertation. The prediction consists of using a past hydrograph, preparing the frequency distribution plot for the daily stream flows, sampling the distribution randomly and preparing a future hydrograph, which has the same mean and standard deviation as the measured hydrograph. A frequency distribution plot of scour depths can be used to quote a scour depth with a corresponding probability of occurrence and risk level based on future hydrographs. In the verification process, 10 bridge case histories and 3 scour databases were used to check whether the method is good enough to provide sound results in real cases.

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