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

Abstract

Large particles can be deposited in natural stream channels as a result of failed erosion protection measures or geological deposits. The impacts these large particles have on the natural systems have been studied, however the previous literature that has been completed either has a very narrow scope applicable only to alpine rivers or are simplified and do not fully capture the processes that occur in a natural channel system. Additionally, the results often contradict each other, and give an unclear understanding of the effects these large particles have on bed morphology and sediment transport. This thesis utilizes a laboratory experiment to evaluate the effects that varying densities of large immobile particles in a gravel-bed channel have on sediment transport and bed morphology. The objective of this study is to gain further understanding and to consolidate existing literature to provide a more holistic overview of the effects of these large particles on a channel bed. It was expected that large immobile particles would cause an increase in channel roughness, and that the impacts to sediment transport and bed morphology would reflect this. The laboratory experiment consisted of 5 test cases with varying densities of large immobile particles, and one base case with no large particles present. In each case, the flume bed was composed of a poorly sorted gravel mixture with a bi-modal distribution of sand and gravel meant to be representative of a natural gravel-bed channel. The large particles were sized to be representative of common engineering principles by applying a factor of safety to a minimum stable particle size. Each experimental case consisted of a single 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 particle density to an increase in channel roughness was too simplistic to explain the trends found within this study. At low densities of large immobile particles, the transported material and the 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 approached equal mobility. Finally, at high densities of large immobile particles, the size of transported material and bed material sizes were similar to that of the base case, and the sediment transport also had the strongest clockwise hysteresis trend. These results indicate the difficult of relating large immobile particle density to channel roughness to explain the effects 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 research and relating it to the results found within this study. This complex model is made up of 3 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 upon each other and amplify their erosive forces (Tan and Curran, 2012). 3. Densely spaced large immobile particles causes high energy skimming flow that is able 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 or understanding the impacts to a natural channel system caused by the introduction of large immobile material. Finally, the results of this study can be used to further research and develop design criteria for engineered in-channel structures to remedy imbalanced channel processes.