A strong earthquake of magnitude 8 in Richter scale, occurred in Sichuan Province, China on 12 May 2008, triggered about 257 landslide dams. The erodibility of fresh landslide deposits plays an important role in evaluating the initiation and development of breaching of such landslide dams. In this research, field jet index tests were conducted shortly after the earthquake at 27 locations on the Hongshihe landslide dam and the Libaisi landslide dam. The purpose of these tests was to investigate the erodibility of freshly deposited landslide soils. The landslide deposits are broadly graded. The bulk density increases and the coefficient of erodibility decreases with the depth of deposition. The erodibility of the fresh landslide deposits falls into a moderately resistant category and the fresh deposits are much more erodible than the native geomaterials before the earthquake. The main factors that control soil erodibility are found to be grain-size distribution, void ratio, fines content, and plasticity index. Particularly, the coefficient of erodibility decreases exponentially with the degree of compaction. Two empirical equations are developed for estimating the coefficient of erodibility and critical erosive shear stress of the freshly deposited landslide soils based on their basic soil properties.

Field testing of erodibility of two landslide dams triggered by the 12 May Wenchuan earthquake

Previous analyses have identified the active width of braided rivers, the bed area over which bed load flux and short-term morphological change occurs, as an important element of braiding dynamics and predictions of bed load flux. Here we compare theoretical predictions of active width in gravel-bed braided rivers with observations from Sunwapta River, and from a generic physical model of gravel braided rivers, to provide general observations of the variation in active width, and to develop an understanding of the causes of variation. Bed topography was surveyed daily along a 150 m reach of the pro-glacial Sunwapta River for a total of four weeks during summer when flow was above threshold for morphological activity. In the laboratory, detailed digital elevation models (DEMs) were derived from photogrammetric survey at regular intervals during a constant discharge run. From the field and flume observations there is considerable local and circumstantial variation in active width, but also a general trend in average active width with increasing discharge. There is also a clear relationship of active width with active braiding index (number of active branches in the braided channel network), and with dimensionless stream power, which appears to be consistent across the range of data from field and physical models. Thus there is a link between active width and the river morphology and dynamics, and the possibility of a general relationship for estimating active width from channel pattern properties or reach-scale stream power values, from which approximate bedload flux calculations may be made. The analysis also raises questions about differences between hydraulically-based numerical model computations of instantaneous active width and observation of time-integrated morphological active width. Understanding these differences can give insight into the nature of bedload transport in braided rivers and the relationship to morphological processes of braiding. Copyright © 2011 John Wiley & Sons, Ltd.

Active width of gravel-bed braided rivers

Braided rivers are characterized by multiple, unstable channel and ephemeral bars formed by intense bed-load transport, and a set of very active channel processes. They occur in a range of environments associated with high-energy, coarse-bedded rivers with limited development of riparian vegetation. The occurrence of braiding can be predicted from empirical and rational analysis of regime conditions and in-channel process mechanisms, although exact prediction is made difficult by the gradual transitions between channel pattern types and the limited analysis of vegetation effects. Despite an apparent chaotic appearance, braiding rivers follow hydraulic geometry relations similar in form to those of single-thread channels and appear to have distinctive statistical topographic characteristics. Bar formation, migration, and accretion processes are a major component of river morphology, as are the dynamics of bifurcation, and confluences of individual anabranches. These processes, and the length scale of the associated morphology, account for apparent regularity in the structure of the network of channels. Intensity of braiding also seems to be adjusted to discharge and sediment mobility but only parts of the channel network are active at any one time, even at channel-forming flows. Consequently, the braided channel is formed progressively by channel migration and avulsion, not by simultaneous formation of the entire braided channel network. Large amplitude variations in bedload transport rate occur locally and over short time periods associated with the complex set of channel- and bar-forming processes. Observations, empirical results, rational theory, and numerical models have all contributed to an understanding of braided river processes and are the basis for a more complete theory of the development and morpho-dynamics of braiding.

9.17 Morphology and Dynamics of Braided Rivers

The depositional stratigraphy of within-channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within-channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1AE3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (<1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground-penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grainsize trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object-based models of sandy braided river alluvial architecture

Evolution and sedimentology of a channel fill in the sandy braided South Saskatchewan River and its comparison to the deposits of an adjacent compound bar

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.

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

Recent research on braided river morphology shows that the intensity (number of anabranches) of braiding channels increases with dimensionless discharge and (or) stream power. This variation in intensity reflects the adjustment of total sinuosity of the river to imposed gradient at a given discharge and grain size. Only a subset of channels is active at a given time and this active braiding intensity reflects the limited number of channels that can sustain bed load transport as the flow is divided. This is governed mechanistically by the dynamics of bifurcations and avulsions. Braided channel networks also have a characteristic length scale (or scales) related to the wavelength of the bars from which braiding develops and to the scale of the bars and confluence–bifurcation units within the braided network. The range of scales is limited by the size (and, therefore, number) of the active channels within the network and the width of the entire river.

Intensity and Characteristic Length of Braided Channel Patterns

Fletchers Creek is located in an urbanizing basin near Toronto and has a bed and banks composed primarily of cohesive Halton Till. Critical shear stress and an erodibility coefficient for the till were determined using an in situ jet-tester that directs a submerged jet of water perpendicular to the sediment surface. The results from 10 jet-tests indicate that the till has a relatively low critical shear stress and relatively high erodibility coefficient and could be susceptible to bed scour during flood events. Many other streams in southern Ontario have urbanizing watersheds with cohesive till beds that may also be susceptible to erosion.Key words: critical stress, submerged jet, erodibility, cohesive soils.

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Peter AshmoreP. Ashmore

Papers

Intensity and Characteristic Length of Braided Channel Patterns

In situ jet-testing of the erosional resistance of cohesive streambeds