ALBERT

Description: Numerical morphological modelling of braided rivers, using a physics-based approach, is increasingly used as a technique to explore controls on river pattern and, from an applied perspective, to simulate the impact of channel modifications. This paper assesses a depth averaged non-uniform sediment model (Delft3D) to predict the morphodynamics of a 2.5 km long reach of the braided Rees River, New Zealand, during a single high-flow event. Evaluation of model performance primarily focused upon using high-resolution Digital Elevation Models (DEMs) of Difference, derived from a fusion of terrestrial laser scanning and optical empirical bathymetric mapping, to compare observed and predicted patterns of erosion and deposition, and reach scale sediment budgets. For the calibrated model, this was supplemented with planform metrics (e.g. braiding intensity). Extensive sensitivity analysis of model functions and parameters was executed, including consideration of numerical scheme for bedload component calculations, hydraulics, bed composition, bedload transport and bed slope effects, bank erosion and frequency of calculations. Total predicted volumes of erosion and deposition corresponded well to those observed. The difference between predicted and observed volumes of erosion was less than the factor of two that characterises the accuracy of the Gaeuman et al. bedload transport formula. Grain size distributions were best represented using two-phi intervals. For unsteady flows, results were sensitive to the morphological time scale factor. The approach of comparing observed and predicted morphological sediment budgets shows the value of using natural experiment datasets for model testing. Sensitivity results are transferable to guide Delft3D applications to other rivers. This article is protected by copyright. All rights reserved.

Description: Abstract Invasive nonnative species acting as “ecosystem engineers” or “geomorphic agents” can represent a major landscape disturbance. Quantification of their biogeomorphic impacts remains a key knowledge gap, and aquatic‐terrestrial transition zones may be particularly exposed to impacts. We demonstrate how burrowing invasive species represent a potentially significant but unquantified erosion risk at aquatic margins. We reveal a lack of quantitative research on geophysical impacts, despite increasing concerns over threats to waterways and flood defense infrastructure. We explore example animals of global interest, comprising crustaceans, fish, reptiles, and mammals and reveal the global nature of the issue: over 100 countries, states, or territories where at least one example species is established, and over 20 with 3‐6 species present. We present a conceptual model for the impacts of burrows on stability and erosion at aquatic margins using established models of geotechnical, hydrological, and hydraulic drivers. Burrows are hypothesized to (i) alter failure plane position, decrease failure plane length, and increase failure plane angle (thereby decreasing bank shear strength); (ii) modify the spatial distribution of porewater pressure, thereby increasing subsurface flow (seepage), reducing cohesion, and increasing the likelihood of slip failures at the bank face; (iii) increase turbulence and sediment entrainment at burrow entrances; and (iv) alter flow resistance at the bank face. Most effects are expected to increase bank instability/erosion with the exception of (iv) which has the potential to offer protection from fluvial action. We call for further research in these areas to quantify impacts for different environments and different invasive species.

Description: This paper provides novel observations linking the connections between spatially-distributed bedload transport pathways, hydraulic patterns and morphological change in a shallow, gravel-bed, braided river. These observations shed light on the mechanics of braiding processes and illustrate the potential to quantify coupled material fluxes using remotely sensed methods. The paper focuses upon a 300 m long segment of the Rees River, New Zealand, and utilises spatially dense observations from a mobile acoustic Doppler current profiler (aDcp) to map depth, velocity and channel topography through a sequence of high-flow events. Apparent bedload velocity is estimated from the bias in aDcp bottom tracking and mapped to indicate bedload transport pathways. Terrestrial Laser Scanning (TLS) of exposed bar surfaces is fused with the aDcp surveys to generate spatially continuous Digital Elevation Models (DEMs), which quantify morphological change through the sequence of events. Results map spatially distributed bedload pathways that were likely to link zones of erosion and deposition. The coherence between the channel thalweg, zone of maximum hydraulic forcing and maximum apparent bedload pathways varied. This suggests that, in places, local sediment supply sources exerted a strong control on the distribution of bedload, distinct from hydraulic forcing. The principal braiding mechanisms observed were channel choking leading to subsequent bifurcation. Results show the connection between sediment sources, pathways and sinks, and their influence on channel morphology and flow path directions. The methodology of coupling spatially dense aDcp surveys with TLS has considerable potential to understand connections between processes and morphological change in dynamic fluvial settings.

Description: Recent advances in technology have revolutionized the acquisition of topographic data, offering new perspectives on the structure and morphology of the Earth's surface. These developments have had a profound impact on the practice of river science, creating a step change in the dimensionality, resolution, and precision of fluvial terrain models. The emergence of “hyperscale” survey methods, including structure from motion photogrammetry and terrestrial laser scanning (TLS), now presents the opportunity to acquire 3-D point cloud data that capture grain-scale detail over reach-scale extents. Translating these data into geomorphologically relevant products is, however, not straightforward. Unlike traditional survey methods, TLS acquires observations rapidly and automatically, but unselectively. This results in considerable “noise” associated with backscatter from vegetation and other artifacts. Moreover, the large data volumes are difficult to visualize; require very high capacity storage; and are not incorporated readily into GIS and simulation models. In this paper we analyze the geomorphological integrity of multiscale terrain models rendered from a TLS survey of the braided River Feshie, Scotland. These raster terrain models are generated using a new, computationally efficient geospatial toolkit: the topographic point cloud analysis toolkit (ToPCAT). This performs an intelligent decimation of point cloud data into a set of 2.5-D terrain models that retain information on the high-frequency subgrid topography, as the moments of the locally detrended elevation distribution. The results quantify the degree of terrain generalization inherent in conventional fluvial DEMs and illustrate how subgrid topographic statistics can be used to map the spatial pattern of particle size, grain roughness, and sedimentary facies at the reach scale.

Description: Understanding the role of external controls on the morphology of braided rivers is currently limited by the dearth of robust metrics to quantify and distinguish the diversity of channel form. Most existing measures are strongly dependent on river stage and unable to account for the three-dimensional complexity that is apparent in digital terrain models of braided rivers. In this paper, we introduce a simple, stage-independent morphological indicator that enables the analysis of reach-scale regime morphology as a function of slope, discharge, sediment size and degree of confinement. The index is derived from the bed elevation frequency distribution and characterizes a statistical width-depth curve averaged longitudinally over multiple channel widths. In this way, we define a “synthetic channel” described by a simple parameter that embeds information about the river morphological complexity. Under the assumption of uniform flow, this approach can be extended to provide estimates of the reach-averaged shear stress distribution, bed load flux and at-a-station-variability of wetted width. We test this approach using data from a wide range of labile channels including 58 flume experiments and three gravel bed braided rivers. Results demonstrate a strong relationship between the unit discharge and the shape of the elevation distribution, which varies between a U-shape for typical single-thread confined channels, to a Y-shape for multi-thread reaches. Finally, we discuss the use of the metric as a diagnostic index of river condition that may be used to support inferences about the river morphological trajectory. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Quantifying the morphology of braided rivers is a key task for understanding braided river behaviour. In the last decade, developments in geomatics technologies and associated data processing methods have transformed the production of precise, reach-scale topographic datasets. Nevertheless, generating accurate Digital Elevation Models (DEMs) remains a demanding task, particularly in fluvial systems. This paper identifies a threefold set of challenges associated with surveying these dynamic landforms: complex relief, inundated shallow channels and high rates of sediment transport, and terms these challenges the “morphological”, “wetted channel” and “mobility” problems respectively. In an attempt to confront these issues directly, this paper presents a novel survey methodology that combines mobile terrestrial laser scanning and non-metric aerial photography with data reduction and surface modelling techniques to render DEMs from the resulting very high resolution datasets. The approach is used to generate and model a precise, dense topographic dataset for a 2.5 km reach of the braided Rees River, New Zealand. Data were acquired rapidly between high flow events and incorporate over 5 x 10 9 raw survey observations with point densities of 1600 pts m -2 on exposed bar and channel surfaces. A detailed error analysis of the resulting sub-metre resolution is described to quantify DEM quality across the entire surface model. This reveals unparalleled low vertical errors for such a large and complex surface model; between 0.03-0.12 m in exposed and inundated areas of the model respectively. This article is protected by copyright. All rights reserved.

Description: [1]  Previous flume-based research on braided channels has revealed four classic mechanisms that produce braiding: central bar development, chute cutoff, lobe dissection, and transverse bar conversion. The importance of these braiding mechanisms relative to other morphodynamic mechanisms in shaping braided rivers has not yet been investigated in the field. Here we exploit repeat topographic surveys of the braided River Feshie (UK) to explore the morphodynamic signatures of different mechanisms of change in sediment storage. Our results indicate that, when combined, the four classic braiding mechanisms do indeed account for the majority of volumetric change in storage in the study reach (61% total). Chute cutoff, traditionally thought of as an erosional braiding mechanism, appears to be the most common braiding mechanism in the study river, but was more the result of deposition during the construction of diagonal bars than it was the erosion of the chute. Three of the four classic mechanisms appeared to be largely net aggradational in nature, whereas secondary mechanisms (including bank erosion, channel incision, and bar sculpting) were primarily net erosional. Although the role of readily erodible banks in facilitating braiding is often conceptualized, we show that bank erosion is as or more important a mechanism in changes in sediment storage than most of the braiding mechanisms, and is the most important ‘secondary’ mechanism (17% of total change). The results of this study provide one of the first field tests of the relative importance of braiding mechanisms observed in flume settings.

Description: ABSTACT Recession of high-mountain glaciers in response to climatic change frequently results in the development of moraine-dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km 2 ) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice-contact moraine-dammed lake to develop. The lake had a maximum volume of 5.5 × 10 −5  m 3 and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 10 5  m 3 of material was removed from the terminal moraine during breach development. Numerical dam-breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine-dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m 3  s −1 . Results from two-dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s −1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine-dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine-dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact. This article is protected by copyright. All rights reserved.

Description: Gravel-bed braided rivers are characterized by shallow, branching flow across low relief, complex and mobile bed topography. These conditions present a major challenge for the application of higher dimensional hydraulic models, the predictions of which are nevertheless vital to inform flood risk and ecosystem management. This paper demonstrates how high-resolution topographic survey and hydraulic monitoring at a density commensurate with model discretization can be used to advance hydrodynamic simulations in braided rivers. Specifically, we detail applications of the shallow water model, Delft3d, to the Rees River, New Zealand, at two nested scales: a 300 m braid bar unit and a 2.5 km reach. In each case, terrestrial laser scanning was used to parameterize the topographic boundary condition at hitherto unprecedented resolution and accuracy. Dense observations of depth and velocity acquired from a mobile acoustic Doppler current profiler (aDcp), along with low-altitude aerial photography, were then used to create a data-rich framework for model calibration and testing at a range of discharges. Calibration focused on the estimation of spatially uniform roughness and horizontal eddy viscosity, ν H , through comparison of predictions with distributed hydraulic data. Results revealed strong sensitivity to ν H , which influenced cross-channel velocity and localization of high shear zones. The high resolution bed topography partially accounts for form resistance and the recovered roughness was found to scale by 1.2-1.4 D 84 grain diameter. Model performance was good for a range of flows, with minimal bias and tight error distributions, suggesting acceptable predictions can be achieved with spatially uniform roughness and ν H .