How Large Immobile Particles Impact Sediment Transport and Bed Morphology in Gravel Bed Rivers

sediment transport;bed morphology;gravel-bed;keystone

McKie, Christopheraffiliation1:Faculty of Engineering ; advisor:Annable, William ; Annable, William ;

University of Waterloo

关键词: bed morphology;    sediment transport;    gravel-bed;    Master Thesis;    keystone;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/14731/3/McKie_Christopher.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Large particles can be deposited in natural stream channels as a result of failed erosionprotection measures or geological deposits. The impacts these large particles have on thenatural systems have been studied, however the previous literature that has been completedeither has a very narrow scope applicable only to alpine rivers or are simplified and do notfully capture the processes that occur in a natural channel system. Additionally, the resultsoften contradict each other, and give an unclear understanding of the effects these largeparticles have on bed morphology and sediment transport.This thesis utilizes a laboratory experiment to evaluate the effects that varying densitiesof large immobile particles in a gravel-bed channel have on sediment transport and bedmorphology. The objective of this study is to gain further understanding and to consolidateexisting literature to provide a more holistic overview of the effects of these large particleson a channel bed. It was expected that large immobile particles would cause an increasein channel roughness, and that the impacts to sediment transport and bed morphologywould reflect this.The laboratory experiment consisted of 5 test cases with varying densities of largeimmobile particles, and one base case with no large particles present. In each case, theflume bed was composed of a poorly sorted gravel mixture with a bi-modal distributionof sand and gravel meant to be representative of a natural gravel-bed channel. The largeparticles were sized to be representative of common engineering principles by applying afactor of safety to a minimum stable particle size. Each experimental case consisted of asingle hydrograph with continuous sediment input scaled to the flow rate.The results of the test cases and the base case proved that relating the large particledensity to an increase in channel roughness was too simplistic to explain the trends foundwithin this study. At low densities of large immobile particles, the transported material andthe bed material both became coarser. At medium densities of large immobile particles,the bed material size and erosion reached a maximum, and the system also approachedequal mobility. Finally, at high densities of large immobile particles, the size of transportedmaterial and bed material sizes were similar to that of the base case, and the sedimenttransport also had the strongest clockwise hysteresis trend. These results indicate thedifficult of relating large immobile particle density to channel roughness to explain theeffects on sediment transport and bed morphology.In an effort to provide a more holistic explanation, and to consolidate the existing lit-erature, a more complex explanation was developed using the findings of previous researchand relating it to the results found within this study. This complex model is made up of3 main points:1. Isolated large immobile particles create localized areas of increased erosive forces,and localized protected areas (Brayshaw et al., 1983).2. At a narrow range of large immobile particle spacings, flow structures build uponeach other and amplify their erosive forces (Tan and Curran, 2012).3. Densely spaced large immobile particles causes high energy skimming flow that isable to create powerful eddies in gaps between the large particles (Hassan and Reid,1990).This complex model explains the trends and results found within this study. Addi-tionally, the results of this research were used to form the framework for predicting orunderstanding the impacts to a natural channel system caused by the introduction of largeimmobile material. Finally, the results of this study can be used to further research anddevelop design criteria for engineered in-channel structures to remedy imbalanced channelprocesses.