Yvonne E. Martin

Evaluation of bed load transport formulae using field evidence from the Vedder River, British Columbia

Abstract Bed load transport formulae have not been tested extensively over medium spatial and temporal scales. The lack of such testing is undoubtedly due to a paucity of field data available for testing at these scales. Extensive transport, morphologic, and discharge data, available for a 10-year period along an 8-km study reach of the Vedder River, British Columbia, are used to test the predictive capabilities of bed load transport formulae at medium scales. The transport data were obtained using the morphologic approach, which is based on the premise that changes in channel morphology reflect the transfer of sediment. Such transport estimates can be made over larger spatial and temporal scales than is possible using traditional measurement techniques. This study focuses on evaluating the original and revised versions of the Bagnold stream power formula, the Meyer-Peter and Muller formula and a stream power correlation. Overall, the formulae are found to underpredict gravel transport rates for the Vedder River. The discrepancy is particularly pronounced in 1982–1983 and 1987–1990, which are periods during which significant dredging occurred. Dredging activity may have loosened the bed structure or resulted in a disequilibrium morphology, thereby increasing sediment movement. Analysis is undertaken to assess how the use of mean daily discharge vs. 15-min increment discharge series affects transport predictions. Results show that only the Meyer-Peter and Muller formula is particularly sensitive to which discharge series is used in calculations. Modification of the Shields parameter is found to significantly affect transport predictions, although it cannot explain alone the discrepancies between field data and calculated results. Predictions of total deposition in the study reach are well within an order-of-magnitude for all equations. The Bagnold-type formulae and the Meyer-Peter and Muller formula do not capture the relatively even distribution of deposition along the study reach. Surprisingly, the simple stream power correlation captured this downstream pattern of deposition. Cumulative distributions of predicted bed load transport are found to be most realistic for the original and revised Bagnold formulae. Results of this study do not suggest that one particular formula is consistently preferred.

High spatial resolution satellite imagery, DEM derivatives, and image segmentation for the detection of mass wasting processes

An automated approach to identifying landslides using a combination of high-resolution satellite imagery and digital elevation derivatives is offered as an alternative to aerial photographic interpretation. Previous research has demonstrated that per pixel spectral response patterns are ineffective in discriminating mass movements. This technique utilizes image segmentation and digital elevation data in order to identify mass movements based not only on their reflectance but also on their shape properties and their geomorphic context. Dividing the classification by process into debris slides, debris flows, and rock slides makes the method far more useful than methods that group all mass movements together. A hierarchical classification scheme is utilized to eliminate areas that are not of interest and to identify areas where mass movements are probable. A supervised classification is then conducted using spectral, shape, and textural properties to identify failures that were greater than 1 ha in area. The resulting accuracy was 90 percent for debris slides, 60 percent for debris flows, and 80 percent for rock slides.

Modelling hillslope evolution: linear and nonlinear transport relations

Many recent models of landscape evolution have used a diffusion relation to simulate hillslope transport. In this study, a linear diffusion equation for slow, quasi-continuous mass movement (e.g., creep), which is based on a large data compilation, is adopted in the hillslope model. Transport relations for rapid, episodic mass movements are based on an extensive data set covering a 40-yr period from the Queen Charlotte Islands, British Columbia. A hyperbolic tangent relation, in which transport increases nonlinearly with gradient above some threshold gradient, provided the best fit to the data. Model runs were undertaken for typical hillslope profiles found in small drainage basins in the Queen Charlotte Islands. Results, based on linear diffusivity values defined in the present study, are compared to results based on diffusivities used in earlier studies. Linear diffusivities, adopted in several earlier studies, generally did not provide adequate approximations of hillslope evolution. The nonlinear transport relation was tested and found to provide acceptable simulations of hillslope evolution. Weathering is introduced into the final set of model runs. The incorporation of weathering into the model decreases the rate of hillslope change when theoretical rates of sediment transport exceed sediment supply. The incorporation of weathering into the model is essential to ensuring that transport rates at high gradients obtained in the model reasonably replicate conditions observed in real landscapes. An outline of landscape progression is proposed based on model results. Hillslope change initially occurs at a rapid rate following events that result in oversteepened gradients (e.g., tectonic forcing, glaciation, fluvial undercutting). Steep gradients are eventually eliminated and hillslope transport is reduced significantly.

Numerical modelling of landscape evolution: geomorphological perspectives

A resurgence of interest in landscape evolution has occurred as computational technology has made possible spatially and temporally extended numerical modelling. We review elements of a structured approach to model development and testing. It is argued that natural breaks in landscape process and morphology define appropriate spatial domains for the study of landscape evolution. The concept of virtual velocity is used to define appropriate timescales for the study of landscape change. Process specification in numerical modelling requires that the detail incorporated into equations be commensurable with the particular scale being considered. This may entail a mechanistic approach at small (spatial) scales, whereas a generalized approach to process definition may be preferred in large-scale studies. The distinction is illustrated by parameterizations for hillslope and fluvial transport processes based on scale considerations. Issues relevant to model implementation, including validation, verification, calibration and confirmation, are discussed. Finally, key developments and characteristics associated with three approaches to the study of landscape modelling:(i) conceptual; (ii) quasi-mechanistic; and (iii) generalized physics, are reviewed.

Re‐examination of Bagnold's empirical bedload ­formulae

Bagnold developed his formula for bedload transport over several decades, with the final form of the relation given in his 1980 paper. In this formula, bedload transport rate is a function of stream power above some threshold value, depth and grain size. In 1986, he presented a graph which illustrated the strength of his relation. A double-log graph of bedload transport rate, adjusted for depth and grain size, versus excess stream power was shown to collapse along a line having a slope of 1·5. However, Bagnold based his analyses on limited data. In this paper, the formula is re-examined using a large data set in order to define the most consistent empirical representation, and dimensional analysis is performed to seek a rationalization of the formula. Functional analysis is performed for the final version of the equation defined by Bagnold to determine if the slope of 1·5 is preserved and to assess the strength of the relation. Finally, relations between excess stream power and bedload transport are examined for a fixed slope of 1·5 to assess the performance of various depth and grain size adjustment factors. The rational scaling is found to provide the best result. Copyright

The influence of geomorphic processes on plant distribution and abundance as reflected in plant tolerance curves

Ecologists describe plant distribution using direct gradient analysis, by which a tolerance curve of species abundance is described along an environmental gradient (any environmental variable that affects plant distribution). Soil moisture is generally the gradient in low-relief areas that explains the most variation. Traditional direct gradient analyses have used terrain structure (i.e., transects up or down hillslopes) as a correlate to soil moisture. Here we use a numerical tectonic and geomorphic process-based landscape development model to create two landscapes with different geomorphic characteristics: (1) to demonstrate the influence of geomorphic processes on soil moisture patterns and plant distribution and (2) to evaluate the effectiveness of transects in describing moisture gradients and tolerance curves on landscapes dominated by creep or overland flow. We use a topographic index to approximate the distribution of soil moisture as it is determined by the shape of these different landscapes. Tr...

Biogeosciences survey Studying interactions of the biosphere with the lithosphere, hydrosphere and atmosphere

The biogeosciences are a rapidly expanding field, and for this reason the full scope of possible topics falling under this heading is not always recognized. The biogeosciences cover all fields of the biological sciences and their interactions with the relevant Earth spheres (i.e. atmosphere, hydrosphere, lithosphere), and are studied over a wide range of temporal and spatial scales. While interdisciplinary work has been recognized for many years, it is recommended that all biogeosciences studies should ultimately strive to understand process operation and feedbacks, and in doing so a common ground to approaches of study can be defined. The notion of multidisciplinary versus interdisciplinary research is considered herein. It is by following an approach of explanation-based science that the complex interplay of biological and environmental processes can be understood best. Understanding of system behaviour and functioning should be a core goal of biogeosciences research. This review offers a proposed classification and summary of the full range of topics falling under the umbrella of the biogeosciences, and in doing so sets the stage for a future series of progress reports focusing on recent developments in the biogeosciences.

Linking Soil Moisture Variation and Abundance of Plants to Geomorphic Processes: A Generalized Model for Erosion‐Uplifting Landscapes

Author(s): Ding, J; Johnson, EA; Martin, YE | Abstract: ©2018. American Geophysical Union. All Rights Reserved. The diffusive and advective erosion-created landscapes have similar structure (hillslopes and channels) across different scales regardless of variations in drivers and controls. The relative magnitude of diffusive erosion to advective erosion (D/K ratio) in a landscape development model controls hillslope length, shape, and drainage density, which regulate soil moisture variation, one of the critical resources of plants, through the contributing area (A) and local slope (S) represented by a topographic index (TI). Here we explore the theoretical relation between geomorphic processes, TI, and the abundance and distribution of plants. We derived an analytical model that expresses the TI with D, K, and A. This gives us the relation between soil moisture variation and geomorphic processes. Plant tolerance curves are used to link plant performance to soil moisture. Using the hypothetical tolerance curves of three plants, we show that the abundance and distribution of xeric, mesic, and hydric plants on the landscape are regulated by the D/K ratio. Where diffusive erosion is the major erosion process (large D/K ratio), mesic plants have higher abundance relative to xeric and hydric plants and the landscape has longer and convex-upward hillslope and low channel density. Increasing the dominance of advective erosion increases relative abundance of xeric and hydric plants dominance, and the landscape has short and concave hillslope and high channel density.