ALBERT

Description: Previous research into sediment cover in bedrock-alluvial channels has focussed on total sediment cover, rather than the spatial distribution of cover within the channel. The latter is important because it determines the bedrock areas that are protected from erosion, and the start and end of sediment transport pathways. We use a 1:10 Froude-scaled model of an 18 by 9 m reach of a bedrock-alluvial channel to study the production and erosion of sediment patches, and hence the spatial relationships between flow, bed topography and sediment dynamics. The hydraulics over this bed are presented in the companion paper. In these experiments specified volumes of sediment were supplied at the upstream edge of the model reach as single inputs, at each of a range of discharges. This sediment formed patches and, once these stabilised, flow was steadily increased to erode the patches. In summary: 1) patches tend to initiate in the lowest areas of the bed, but areas of topographically-induced high flow velocity can inhibit patch development; 2) at low sediment inputs the extent of sediment patches is determined by the bed topography and can be insensitive to the exact volume of sediment supplied; and, 3) at higher sediment inputs more extensive patches are produced, stabilised by grain-grain and grain-flow interactions, and less influenced by the bed topography. Bedrock topography can therefore be an important constraint on sediment patch dynamics, and topographic metrics are required that incorporate its within-reach variability. The magnitude and timing of sediment input events controls reach-scale sediment cover.

Description: The controls on hydraulics in bedrock-alluvial rivers are relatively poorly understood, despite the importance of the flow in determining rates and patterns of sediment transport and consequent erosion. To measure hydraulics within a bedrock-alluvial channel, we developed a 1:10 Froude-scaled laboratory model of an 18 x 9 m bedrock-alluvial river reach using terrestrial laser scanning and 3D printing. In the reported experiments, water depth and velocity were recorded at 18 locations within the channel at each of 5 different discharges. Additional data from runs with sediment cover in the flume were used to evaluate the hydraulic impact of sediment cover; the deposition and erosion of sediment patches in these runs is analysed in the companion paper. In our data: 1) spatial variation in both flow velocity and Froude number increases with discharge; 2) bulk flow resistance and Froude number become independent of discharge at higher discharges; 3) local flow velocity and Reynolds stress are correlated to the range of local bed topography at some, but not most, discharges; 4) at lower discharges, local topography induces vertical flow structures and slower velocities, but these effects decrease at higher discharges and, 5) there is a relationship between the linear combination of bed and sediment roughness and local flow velocity. These results demonstrate the control that bedrock topography exerts over both local and reach-scale flow conditions, but spatially distributed hydraulic data from bedrock-alluvial channels with different topographies are needed to generalise these findings.

Description: Energy and power system models can be categorized into four levels based on the complexity captured in terms of planning and operation. In the past, power system planning was based on meeting the load duration curve at minimum cost. The increasing share of variable generation (VG) makes operational constraints more important in the planning problem, and there is more and more interest in considering aspects such as sufficient ramping capability, sufficient reserve procurement, power system stability, storage behavior, and the integration of other energy sectors often through demand response assets. In VG integration studies, several methods have been applied to combine the planning and operational timescales. We present a four‐level categorization for the modeling methods, in order of increasing complexity: (1a) investment model only, (1b) operational model only, (2) unidirectionally soft‐linked investment and operational models, (3a) bidirectionally soft‐linked investment and operational models, (3b) operational model with an investment update algorithm, and (4) co‐optimization of investments and operation. The review shows that using a low temporal resolution or only few representative days will not suffice in order to determine the optimal generation portfolio. In addition, considering operational effects proves to be important in order to get a more optimal generation portfolio and more realistic estimations of system costs. However, operational details appear to be less significant than the temporal representation. Furthermore, the benefits and impacts of more advanced modeling techniques on the resulting generation capacity mix significantly depend on the system properties. Thus, the choice of the model should depend on the purpose of the study as well as on system characteristics. This article is categorized under: Wind Power 〉 Systems and Infrastructure Energy Systems Analysis 〉 Economics and Policy Energy Policy and Planning 〉 Economics and Policy

Description: Sediment grains in a bedrock-alluvial river will be deposited within or adjacent to a sediment patch, or as isolated grains on the bedrock surface. Previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress. However, this analysis neglected potential interactions between the sediment patches, local hydraulics and grain entrainment. We present a series of flume experiments that measure the influence of sediment patches on grain entrainment. The flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains. In each experiment sediment was added either as individual grains or as a single sediment pulse. Flow was then increased until the sediment was entrained. Analysis of the experiments demonstrates that: 1) for individual grains, coarse grains are entrained at a higher discharge than fine grains; 2) once sediment patches are present, the different in entrainment discharge between coarse and fine grains is greatly reduced; 3) the sheltering effect of patches also increases the entrainment discharge of isolated grains; 4) entire sediment patches break-up and are eroded quickly, rather than through progressive grain-by-grain erosion, and 5) as discharge increases there is some tendency for patches to become more elongate and flow-aligned, and more randomly distributed across the bed. One implication of this research is that the critical shear stress in bedrock-alluvial channels will be a function of the extent of the sediment cover. Another is that the influence of sediment patches equalises critical shear stresses between different grain sizes and grain locations, meaning that these factors may not need to be accounted for. Further research is needed to quantify interactions between sediment patches, grain entrainment and local hydraulics on rougher bedrock surfaces, and under different types of sediment supply. This article is protected by copyright. All rights reserved.

Description: Abstract Many models of incision by bedrock rivers predict water depth and shear stress from discharge; conversely, palaeoflood discharge is sometimes reconstructed from flow depth markers in rock gorges. In both cases assumptions are made about flow resistance. The depth‐discharge relation in a bedrock river must depend on at least two roughness length scales (exposed rock and sediment cover) and possibly a third (sidewalls). A conceptually attractive way to model the depth‐discharge relation in such situations is to partition the total shear stress and friction factor, but it is not obvious how to quantify the friction factor for rough walls in a way that can be used in incision process models. We show that a single flow resistance calculation using a spatially‐averaged roughness length scale closely approximates the partitioning of stress between sediment and rock, and between bed and walls, in idealised scenarios. Both approaches give closer fits to the measured depth‐discharge relations in two small bedrock reaches than can be achieved using a fixed value of Manning's n or the Chézy friction factor. Sidewalls that are substantially rougher or smoother than the bed have a significant effect on the partitioning of shear stress between bed and sidewalls. More research is needed on how best to estimate roughness length scales from observable or measurable channel characteristics.

Description: Abstract Sediment transport equations typically produce transport rates that are biased by orders of magnitude. A causal component of this inaccuracy is the inability to represent complex grain‐scale interactions controlling entrainment. Grain‐scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two‐dimensional (2D) force or moment balances. However, this approach neglects many complexities of real grains, including grain shape, cohesion and the angle of entrainment relative to flow direction. To better represent this complexity, we develop the first vector‐based, fully three‐dimensional (3D) grain rotation entrainment model that can be used to resolve any entrainment formulation in 3D, and which also includes the effect of matrix cohesion. To apply this model we use X‐ray computed tomography to quantify the 3D structure of water‐worked river grains. We compare our 3D model results with those derived from application of a 2D entrainment model. We find that the 2D approach produces estimates of dimensionless critical shear stress ( ) that are an order of magnitude lower than our 3D model. We demonstrate that it is more appropriate to use the c‐axis when calculating 2D projections, which increases values of to more closely match our 3D estimates. The 3D model reveals that the main controls on critical shear stress in our samples are projection of grains, cohesive effects from a fine‐grained matrix, and bearing angle for the plane of rotation (the lateral angle of departure from downstream flow that, in part, defines the grain's direction of pivot about an axis formed by two contact points in 3D). The structural precision of our 3D model demonstrates sources of geometric error inherent in 2D models. By improving flow properties to better replicate local hydraulics in our 3D model, entrainment modelling of scanned riverbed grains has the potential for benchmarking 2D model enhancements. This article is protected by copyright. All rights reserved.