Robert G. Millar

Influence of bank vegetation on alluvial channel patterns

Numerous studies have suggested the importance of bank vegetation as a control of channel patterns; however, to date there is no conclusive evidence that vegetation does represent a significant control. An analytical model is developed in order to assess the influence of bank vegetation on channel patterns of alluvial gravel-bed rivers. Three channel types are considered: meandering, wandering, and braided. Bank vegetation effects are quantified in terms of a friction angle ϕ′. A new theoretical meandering-braiding transition criterion is formulated that includes ϕ′, median grain diameter D50, and bank-full discharge Q. The theoretical relation is tested against field data from 137 rivers and successfully discriminates between meandering and braided rivers. Wandering rivers show greater scatter. It is concluded that bank vegetation, as expressed in terms of ϕ′, does exert significant and quantifiable control on alluvial channel patterns. A simple planform stability diagram is developed to determine the sensitivity of gravel-bed rivers to changes in bank vegetation.

Grain and form resistance in gravel-bed rivers Résistances de grain et de forme dans les rivières à graviers

Grain and form resistances for bankfuli and near banicfull flows have been determined for 176 gravel reaches using data compiled from several published sources. Partitioning grain and form resistan...

Predicting gravel bed river response to environmental change: the strengths and limitations of a regime‐based approach

Rivers respond to environmental changes such as climate shifts, land use changes and the construction of hydro-power dams in a variety of ways. Often there are multiple potential responses to any given change. Traditionally, potential stream channel response has been assessed using simple, qualitative frameworks based largely on professional judgement and field experience, or using some form of regime theory. Regime theory represents an attempt to use a physically based approach to predict the configuration of stable channels that can transport the imposed sediment supply with the available discharge. We review the development of regime theory, and then present a specific regime model that we have created as a stand-alone computer program, called the UBC Regime Model (UBCRM). UBCRM differs from other regime models in that it constrains its predictions using a bank stability criterion, as well as a pattern stability criterion; it predicts both the stable channel cross-sectional dimensions as well as the number of anabranches that the stream must have in order to establish a stable channel pattern. UBCRM also differs from other models in that it can be used in a stochastic modelling mode that translates uncertainty in the input variables into uncertainty in the predicted channel characteristics. However, since regime models are fundamentally based on the concept of grade, there are circumstances in which the model does not perform well. We explore the strengths and weaknesses of the UBCRM in this paper, and we attempt to illustrate how the UBCRM can be used to augment the existing qualitative frameworks, and to help guide professionals in their assessments. Copyright

Ballast Requirements for LWD Habitat Structures

A theoretical analysis is presented that forms the basis for determining ballast requirements for LWD habitat structures. Limited field testing of the analysis indicates good agreement with the theory. Tentative design curves have been developed. A monitoring and assessment program is currently underway to test the stability of LWD structures, and to verify the proposed design curves under a range of field conditions.

Bank Vegetation, Bank Strength, and Application of the University of British Columbia Regime Model to Stream Restoration

The University of British Columbia Regime Model (UBCRM) is based on rational regime theory. A feature of the model is that it quantifies the effect of bank vegetation and its effect on channel geometry. Three bank vegetation models can be applied to gravel bed rivers with either noncohesive, cohesive, or composite banks. Simplified dimensionless equations for width and slope derived using the UBCRM are applied to a site on the Coldwater River, British Columbia. Between 1953 and 2003, there were significant land use changes that included riparian and floodplain clearing. The observed widening and steepening can be explained by a reduction in bank strength and that changes in the sediment load, discharge, or grain size do not appear to be significant. Applied correctly, the UBCRM can provide qualitative and quantitative insight into the primary causes of historic disturbance and can serve as an aid in restoration design. Because of the physically based nature of the parameters in the UBCRM, analysis and design are directly linked to fluvial processes including flow resistance, sediment transport, and bank stability.

An optimization model for the development and response of alluvial river channels

In this thesis an optimization model has been developed to calculate the equilibrium geometry of alluvial gravel-bed rivers for a given set of independent variables. The independent variables are the discharges, both the magnitude and duration which are represented by a flow-duration curve; the mean annual load, both volume and grain size distribution, which is imposed on to the channel reach from upstream; and the geotechnical properties of the bank sediment. The unknown dependent or decision variables to be solved for include the channel width, depth, bank angle, roughness, and grain size distribution of the bed surface. The dependent variables adjust subject to the constraints of discharge, bedload, bank stability, and valley slope, to determine a channel geometry which is optimal as defined by a maximization of , which is the coefficient of sediment transport efficiency. The work in this thesis is an extension of earlier models that have predicted the geometry of sand and gravel rivers with reasonable success, however the degree of scatter associated with these models limited their application to quantitative engineering applications. The advances in this thesis over the earlier optimization models are the inclusion of the bank stability analyses, modelling using the full flow-duration data, and calculating the grain size distribution of the bed-surface. The formulation presented in this thesis is specific to gravel-bed rivers, however it can be reformulated for sand-bed rivers.