Peter Ashmore

Monitoring River-Channel Change Using Terrestrial Oblique Digital Imagery and Automated Digital Photogrammetry

Imagery acquired using a high-resolution digital camera and ground survey has been used to monitor changes in bed topography and plan form, and to obtain synoptic water-surface and flow-depth information in the braided, gravel-bed Sunwapta River in the Canadian Rockies. Digital images were obtained during daily low flows during the summer meltwater season to maximize the exposed bed area and to map the water surface on the days with the highest flows. Images were acquired from a cliff-top 125 m above and at a distance of 235 m from the riverbed and used to generate high-resolution orthophotos and digital elevation models (DEMs) at a ground resolution of 0.2 m, within an area 80 × 125 m. The creation of DEMs from oblique and nonmetric imagery using automated digital photogrammetry can be difficult, but a solution based on rotation of coordinates is described here. Independent field verification demonstrated that root mean square accuracies of 0.045 m in elevation were achieved. The ground survey data representing riverbed topography were merged with photogrammetric DEMs of the exposed bars. The high-flow water surface could not be surveyed directly because wading was dangerous but was derived by ground survey of selected accessible points and photogrammetry. The DEMs and depth map provide high-resolution, continuous data on the channel morphology and will be the basis for subsequent two-dimensional flow-modeling of velocity and shear stress fields. The experience of using digital photogrammetry for monitoring river-channel change allows the authors to identify other potential benefits of using this technique for fluvial research and beyond.

Measurements in a Braided River Chute and Lobe: 1. Flow Pattern, Sediment Transport and Channel Change

This paper and its companion (Ashworth et al., this issue) discuss measurements of channel change and associated flow and sediment transport processes in a representative chute-and-bar reach within a proglacial gravel-bed river, the Sunwapta River in the Canadian Rockies. During a week in which water discharge through the reach increased then decreased, a sediment wave passed into and partly along the reach. At first the chute aggraded, then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries. Measurements of velocity, shear stress, and gravel transport rate revealed day-to-day changes in the divergent pattern of flow and sediment transport. The off-center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport, but as the lobe grew, the flow and transport gradually switched away to the other distributary. Width-averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill; the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations. By the end of the study the new lobe had almost accreted onto the original bar head, supporting the idea that most braid bars are of compound origin. Temporal and spatial patterns in grain size distributions of the bed, bed load, and deposited sediment are discussed in the companion paper.

Measurements in a Braided River chute and lobe: 2. Sorting of bed load during entrainment, transport, and deposition

Grain size distributions of bed material, bed load, and bar head deposition were sampled during the evolution of a chute and lobe in the braided gravel bed Sunwapta River, Alberta. Although bed shear stress and total bed load transport rate varied substantially within the reach and from day to day (Ferguson et al., this issue), the median diameter D50 of bed load was remarkably constant. The bed initially fined from chute to bar head but became more uniform. Sediment deposited on the bar head also had a near constant D50, similar to that of the bed load truncated at the same lower size limit. Both load and deposition were finer on average than the bed, suggesting overall selective transport despite little sign of local sorting. In contrast to median diameters, maximum sizes (Dmax) of sediment in motion and deposited on the bar head varied substantially and showed a weak but significant dependence on shear stress. Inferring selective entrainment from this evidence alone would be dubious because Dmax was also found to increase systematically with the mass of sediment sampled. However, two different methods of analysis of fractional transport rates throughout the size range also showed size-selective entrainment and transport. In the marginal transport conditions of this study, size sorting appears to occur but only weakly and mainly in the coarser fractions.

The relation between particle path length distributions and channel morphology in gravel-bed streams: a synthesis

Abstract The path length (downstream displacement over a given time period) of individual bed particles in gravel-bed rivers is central to morphological methods for measuring bed load transport rate and is also fundamental to understanding the bed load transport process and the development of channel morphology. Previous studies of particle movement using tracers report predominantly strongly positively skewed frequency distributions of path length with modes close to the point of entrainment. However, gravel-bed rivers often have regularly spaced erosion (scour pools) and deposition (channel bars) sites that are several channel widths apart and it is reasonable to expect that particle path length would reflect this morphological scale, at least during flows large enough to create and modify the morphology. Here, we synthesize and re-analyze results from published bed load tracing experiments in gravel-bed rivers to identify the variety of possible path length distributions for differing channel morphology, channel dimensions, bed particle size, and particle mobility (i.e. flow magnitude) and to look for occurrences of path length coinciding with the length scale of the morphology. The results show that path length distributions may be positively skewed, symmetrical, and uni-, bi-, or multi-modal and may include modes that coincide with known or expected pool–bar spacing. Primary path length modes equivalent to possible pool–bar spacing are more probable at higher non-dimensional bed shear stress, from which it is inferred that both particle mobility and channel morphology exert an influence on particle path lengths and that particle movement is unlikely to be stochastic except at relatively low particle mobility. Existing data are inadequate for more than a preliminary analysis of this problem consequently there is a need for new data collected explicitly and systematically to confirm these preliminary results, isolate the effect of the several variables that influence the characteristics of path length frequency distributions and identify the conditions under which path length coincides with the length scale of the dominant morphology.

Channel Adjustment and a Test of Rational Regime Theory in a Proglacial Braided Stream

The upstream reach of the Sunwapta River, Alberta, provides a useful quasi-experimental field case of channel adjustment in a proglacial stream. Historically, the formation of a proglacial lake deprived the river of its coarse sediment supply for several decades and lead to a dramatic decrease in braiding intensity close to the lake while braiding intensity increased further downstream. This response to the reduction of gravel input is consistent with previous experimental results. Subsequent construction activity and channelization close to the lake have contributed to the continuation of these temporal and spatial trends in channel pattern. The current state of adjustment of the river morphology can be explained, in the context of these historical changes, using rational regime equations. The study reach has no tributaries and bed material size decreases twofold along the reach while width and braiding intensity increase, yet channel slope decreases by only 10%. The absence of any significant change in discharge downstream along the reach allows testing of regime equations under conditions in which discharge is held constant. The current downstream trends in slope and fluctuations in width are predicted reliably from rational regime equations, but not by the existing empirical hydraulic geometry relations. The rational equations incorporate the effect of grain size and slope on channel width and the effect of width and grain size on channel slope. The regime equations are successful even though they were devised for single channel gravel streams. The small (10%) decrease in slope along the reach, despite a halving of median grain size, is attributed to the counteracting (positive) effect on slope of the downstream increase in braiding intensity and width. The downstream increase in braiding intensity must be largely the result of decreasing grain size. This confirms the influence of grain size on channel pattern thresholds and demonstrates, using spatial transitions in channel pattern, that channel pattern predictions based on stream power alone are inadequate.

Sediment yield mapping in a large river basin: the Upper Yangtze, China

Abstract A number of studies have mapped sediment yield at global or regional scales using sediment load measurements from rivers. However, the suitability of the limited mapping methods has not been fully addressed, particularly for large river basins where the sediment load data were obtained from a hierarchical river network. This study examines some of the issues related to the mapping approaches using long-term sediment load data obtained in the Upper Yangtze basin, China. The sediment yield data are treated as point values and interpolated using the kriging function in Arc/Info GIS. Barriers have been incorporated into the interpolation procedure to confine the interpolation points to the same major flow systems. The incorporation of barriers causes sharp changes of the interpolated values along the barrier lines, and significantly increases interpolation time. Scaling ratios relative to a standard size of drainage area have been developed for major drainage basins to remove effects of drainage sizes on sediment yield. By incorporating the scaling ratios the sediment load data from various sizes of drainage areas can be adjusted to what they would be if the drainage areas were the same and a less biased sediment yield map can be obtained as compared to using the original dataset.

Gravel-Bed Rivers: Processes, Tools, Environments

Each chapter in the book has been specifically commissioned to represent areas in which recent progress has been made in the field. The topics covered also represent a coherent progression through the principal areas of the subject (hydraulics; sediment transport; river morphology; tools and methods; applications of science). EDITORS MICHAEL CHURCH Department of Geography, The University of British Columbia, Vancouver, British Columbia, Canada

The relationship between particle travel distance and channel morphology: Results from physical models of braided rivers

Channel form and sediment transport are closely linked in alluvial rivers, and as such the development of a conceptual framework for the downstream controls on particle mobility and likely deposition sites has immense value in terms of the way we understand and predictively model rivers. Despite the development of conceptual models which frame flood-scale particle transport distance (termed path length) as a function of channel bar locations, an understanding of the controls on such path lengths in braided rivers remains especially elusive, in large part due to the difficulty in explicitly linking morphology and particle transport distances in the field. Here we utilize a series of laboratory flume experiments to link path length distances with channel morphology. Our morphologic characterization is based on ultrahigh-resolution digital elevation models and bar classifications derived from structure-from-motion topography, while we simultaneously capture particle path lengths using fluorescent tracer particles over the course of five physical model simulations. Our findings underscore the importance of channel bars in acting as deposition sites for particles in transport; 81% of recovered tracers were found in association with compound, point, lateral, or diagonal bars. Bar heads (29%) and bar margins (41%) were the most common bar-related deposition surfaces for recovered tracers. Peaks in particle deposition frequency corresponding to channel bars were often noted on path length distributions from tracer data; most tracers were deposited in areas that had experienced shallow (Δz= 0.002 m) deposition. Average path length distance (2.5 m) was closely related to average confluence-diffluence spacing (2.3 m) across all runs. The transferability of this understanding to braided streams has important implications for the development of simplified morphodynamic models which seek to predict braided channel evolution across multiflood timescales.

Recent (1995-1998) Canadian research on contemporary processes of river erosion and sedimentation, and river mechanics.

Canadian research on contemporary erosion and sedimentation processes covers a wide range of scales, processes, approaches and environmental problems. This review of recent research focuses on the themes of sediment yield, land-use impact, fine-sediment transport, bed material transport and river morphology and numerical modelling of fluvial landscape development. Research on sediment yield and denudation has confirmed that Canadian rivers are often dominated by riparian sediment sources. Studies of the effects of forestry on erosion, in-stream sedimentation and habitat are prominent, including major field experimental studies in coastal and central British Columbia. Studies of fine-sediment transport mechanisms have focused on the composition of particles and the dynamics of flocculation. In fluvial dynamics there have been important contributions to problems of turbulence-scale flow structure and entrainment processes, and the characteristics of bedload transport in gravel-bed rivers. Although much of the work has been empirical and field-based, results of numerical modelling of denudational processes and landscape development also have begun to appear. The nature of research in Canada is driven by the progress of the science internationally, but also by the nature of the Canadian landscape, its history and resource exploitation. Yet knowledge of Canadian rivers is still limited, and problems of, for example, large pristine rivers or rivers in cold climates, remain unexplored. Research on larger scale issues of sediment transfer or the effects of hydrological change is now hampered by reductions in national monitoring programmes. This also will make it difficult to test theory and assess modelling results. Monitoring has been replaced by project- and issues-based research, which has yielded some valuable information on river system processes and opened opportunities for fluvial scientists. However, future contributions will depend on our ability to continue with fundamental fluvial science while fulfilling the management agenda.

Geometry and grain-size characteristics of the basal surface of a braided river deposit

Description of the three-dimensional geometry and grain-size patterns of fluvial deposits and their relationship to the morphodynamics of the river has been an elusive objective in sedimentology, hindered by limited exposure and the time scale of fluvial dynamics. In small-scale physical models the time scales are short enough to map river morphology and sedimentology during significant (and continuous) morphological development of the river and its deposits, especially for braided rivers. Using close-range digital photogrammetry, the dynamics of the river morphology, and resulting deposit geometry and sedimentology, were captured using digital elevation model differencing techniques, combined with automated grain-size mapping from image texture analysis. Using these novel methods we show the temporal development and characteristics of braided river deposit geometry, and, for the first time, map the characteristics and development of the basal surface of braided river deposits in relation to river morphodynamics and formative processes. The basal surface has considerable relief, wide variation in grain size similar to that of the river as a whole, develops progressively over time by switching of channels producing adjacent patches and ribbons of basal incision of different ages, and is only partially related to bed scour associated with laterally migrating channel confluences.