Brett C. Eaton

Effects of large floods on sediment transport and reach morphology in the cobble-bed Sainte Marguerite River

Abstract Sediment transport rates were estimated for two flood events on the cobble-bed Sainte Marguerite River in the Saguenay region, Canada. Morphologic methods were used to derive one set of estimates, and a combination of the Meyer-Peter and Muller equation with a dimensionless sediment transport ratio (after Dietrich et al. [Nature 340 (1989) 215]) was used to derive another set of estimates. Both sets of estimates give consistent results for the first event (which had a decade-scale return period), and for the second event (which was the largest flood on record and had a century-scale return period). The transport occurring during the second event was an order of magnitude greater than that occurring during the first event: despite this disparity in the transport intensity of the two events, the channel morphology remained qualitatively similar. The observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s.

Large wood transport and jam formation in a series of flume experiments

Large wood has historically been removed from streams, resulting in the depletion of in-stream wood in waterways worldwide. As wood increases morphological and hydraulic complexity, the addition of large wood is commonly employed as a means to rehabilitate in-stream habitat. At present, however, the scientific understanding of wood mobilization and transport is incomplete. This paper presents results from a series of four flume experiments in which wood was added to a reach to investigate the piece and reach characteristics that determine wood stability and transport, as well as the time scale required for newly recruited wood to self-organize into stable jams. Our results show that wood transitions from a randomly distributed newly recruited state to a self-organized, or jam-stabilized state, over the course of a single bankfull flow event. Statistical analyses of piece mobility during this transitional period indicate that piece irregularities, especially rootwads, dictate the stability of individual wood pieces; rootwad presence or absence accounts for up to 80% of the variance explained by linear regression models for transport distance. Furthermore, small pieces containing rootwads are especially stable. Large ramped pieces provide nuclei for the formation of persistent wood jams, and the frequency of these pieces in the reach impacts the travel distance of mobile wood. This research shows that the simulation of realistic wood dynamics is possible using a simplified physical model, and also has management implications, as it suggests that randomly added wood may organize into persistent, stable jams, and characterizes the time scale for this transition.

NSERC's HydroNet: A National Research Network to Promote Sustainable Hydropower and Healthy Aquatic Ecosystems

Abstract NSERCs HydroNet is a collaborative national five-year research program initiated in 2010 involving academic, government, and industry partners. The overarching goal of HydroNet is to improve the understanding of the effects of hydropower operations on aquatic ecosystems, and to provide scientifically defensible and transparent tools to improve the decision-making process associated with hydropower operations. Multiple projects are imbedded under three themes: 1) Ecosystemic analysis of productive capacity offish habitats (PCFH) in rivers, 2) Mesoscale modelling of the productive capacity offish habitats in lakes and reservoirs, and 3) Predicting the entrainment risk of fish in hydropower reservoirs relative to power generation operations by combining behavioral ecology and hydraulic engineering. The knowledge generated by HydroNet is essential to balance the competing demands for limited water resources and to ensure that hydropower is sustainable, maintains healthy aquatic ecosystems and a vibr...

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

Detecting the timing of morphologic change using stage-discharge regressions: a case study at Fishtrap Creek, British Columbia, Canada.

Nine submersible pressure transducers were installed at various locations in a study reach of Fishtrap Creek during the 2006 freshet. The channel morphology of the reach underwent a moderate change in 2006, due to the effects of the McLure forest fire which, in 2003, killed all of the riparian vegetation in the study reach and burned about 62% of the Fishtrap Creek watershed. By examining the changes in the rating relations between the water stage recorded by the pressure transducers and the discharge measured at a Water Survey of Canada gauging station located just downstream of the study reach, we were able to determine the timing of morphologic changes in the stream over the course of the freshet. Shifts in the rating curve were identified by first graphically analysing the stage-discharge relations, and then constructing stage discharge regressions for that part of the record for which the rating relation appeared to be stable. The residuals associated with the calculated rating equations were then computed for the entire period of record, and used to assess the temporal pattern of changes in the rating relation. The analysis shows that the largest changes in the rating relation occurred in the vicinity of the largest morphologic changes (as determined by analysis of repeated cross-sectional surveys) and that, in areas where the morphology was nearly stable, the rating relation remained relatively stable. The sensitivity of the rating relations to changes in the channel morphology suggests that deployment of submersible pressure transducers may be a suitable and cost-effective means for monitoring channel stability near existing gauging stations or stable control cross-sections where the rating relation is unlikely to change.

Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

While the stabilizing function of large grains in step-pool streams has long been recognized, the role they play in gravel-bed streams is less clear. Most researchers have ignored the role of large grains in gravel-bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d50 values (1.6 mm); however the d90 of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well-developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first-order control on channel stability. Copyright

Simulating riparian disturbance: Reach scale impacts on aquatic habitat in gravel bed streams

Large wood governs channel morphology, as well as the availability of in-stream habitat, in many forested streams. In this paper, we use a stochastic, physically based model to simulate wood recruitment and in-stream geomorphic processes, in order to explore the influence of disturbance history on the availability of aquatic habitat. Specifically, we consider the effects of fire on a range of stream sizes by varying the rate of tree toppling over time in a simulated forest characterized by a tree height of 30 m. We also consider the effects of forest harvesting with various riparian buffer sizes, by limiting the lateral extent of the riparian stand. Our results show that pulsed inputs of wood increase the availability and variability of physical habitat in the postfire period; reach-averaged pool area and deposit area double in small streams, while side channels increase by over 50% in intermediate-sized channels. By contrast, forest harvesting reduces the availability of habitat within the reach, though the effects diminish with increasing buffer size or stream width; in laterally stable streams the effects are minimal so long as buffer width is large enough for key pieces to be recruited to the reach. This research emphasizes the importance of natural disturbance in creating and maintaining habitat heterogeneity and shows that scenario-based numerical modeling provides a useful tool for assessing the historical range of variability associated with natural disturbance, as well as changes in habitat relevant to fish. It can be also used to inform forest harvesting and management.

Assessing Erosion Hazards due to Floods on Fans: Physical Modeling and Application to Engineering Challenges

AbstractExperiments using a 1∶30 scale physical model show that channel degradation on alluvial fans is dominated by lateral channel migration rather than vertical incision. The results are used to...

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.

Recent Canadian Research on Fluvial Sediment Transport and Morphology, 2003-2007

Review of recent research on river dynamics in Canada reveals a diverse, active and growing research community engaged in fundamental research at all scales and motivated by a concern for understanding the Canadian landscape, the impacts of development and resource activities and concerns for environmental quality. Several research groups and collaborative programs (national and international), and many individual researchers have made significant progress in areas such as fluvial turbulence, sand bedforms, eco-geomorphology, gravel-bed river morpho-dynamics, regime analysis, numerical modelling of fluvial dynamics and fine-sediment erosion and transport, all supported by new developments in instrumentation and remote sensing applications.