A.P. Nicholas

A new method for evaluating errors in high‐frequency ADV measurements

The acoustic Doppler velocimeter (ADV) measures three-dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single-point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright

Sediment slugs: large-scale fluctuations in fluvial sediment transport rates and storage volumes

Variations in fluvial sediment transport rates and storage volumes have been described previously as sediment waves or pulses. These features have been identified over a wide range of temporal and spatial scales and have been categorized using existing bedform classifications. Here we describe the factors controlling the generation and propagation of what we term sediment slugs. These can be defined as bodies of clastic material associated with disequilibrium conditions in fluvial systems over time periods above the event scale. Slugs range in magnitude from unit bars (Smith, 1974) up to sedimentary features generated by basin-scale sediment supply disturbances (Trimble, 1981). At lower slug magnitudes, perturbations in sediment transport are generated by local riverbank and/or bed erosion. Larger-scale features result from the occurrence of rare high- magnitude geomorphic events, and the impacts on water and sediment production of tectonics, glaciation, climate change and anthropogenic influences. Simple sediment routing functions are presented which may be used to describe the propagation of sediment slugs in fluvial systems. Attention is drawn to components of the fluvial system where future research is urgently required to improve our quantitative understanding of drainage-basin sediment dynamics.

MODELLING FLOOD HYDRAULICS AND OVERBANK DEPOSITION ON RIVER FLOODPLAINS

This paper outlines a numerical model for the prediction of floodplain inundation sequences, overbank deposition rates and deposit grain size distributions. The model has two main components: first, a simplified hydraulic scheme which predicts floodwater flow depths and velocities, and second, a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by convective and diffusive processes and sediment deposition as a function of particle settling rates. These relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains. The model is applied to a 600 m reach of the River Culm, Devon, U.K. using data derived from a range of field and laboratory techniques. Continuous records of river stage and suspended sediment concentration provide the models upstream boundary input requirements. These are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. The models sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period. The model is shown to predict complicated floodwater inundation sequences and patterns of suspended sediment dispersion and deposition, which are largely a product of the complex topography of the floodplain. These results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry.

Modelling bedload yield in braided gravel bed rivers

Abstract This paper outlines an approach for estimating the annual bedload yield of a braided channel. This procedure is based on the extension of theory of flow and sediment transport in braided rivers recently presented by Paola (1996) . The revised approach accounts explicitly for the relationship between increasing discharge and varying channel hydraulics, and is suitable for use in obtaining bedload transport rate estimates over a range of discharges. Integration of such estimates using flow-duration data allows annual bedload yield to be determined. Model parameterisation is achieved using topographic survey data for the Waimakariri River, New Zealand. Comparison of modelled bedload yield with values estimated from repeated topographic surveys indicates that the model is able to accurately predict both the medium-term (c. 30 years) mean annual bedload yield of the Waimakariri at Crossbank (the section 17.8 km upstream of the river mouth), and also short-term fluctuations in bedload yield associated with varying annual flow statistics. Streamwise patterns of volumetric erosion and deposition determined for a 45-km length of the Waimakariri using the model are also in broad agreement with trends identified in topographic survey data for the period 1961–1997. However, significant deviations between modelled and surveyed volumes of cut and fill are evident at some locations. Comparison of model performance with conventional applications of bedload transport equations, which tend to underestimate transport rates for braided channels, suggests that the approach presented here may represent a significant improvement. This is the case because the model quantifies the relationship between braid intensity and spatial variability in flow hydraulics at a cross-section. Output from the model suggests that braided rivers may transport a significant proportion of their annual bedload at lower discharges than those indicated by earlier theoretical approaches. Results also provide quantitative support for the argument that increased intensity of braiding may promote higher rates of bedload transport in gravel bed rivers.

Numerical simulation of three-dimensional flow hydraulics in a braided channel

Results are presented from a numerical simulation of three-dimensional flow hydraulics around a mid-channel bar carried out using the FLUENT/UNS computational fluid dynamics (CFD) software package. FLUENT/UNS solves the three-dimensional Reynolds-averaged form of the Navier–Stokes equations. Turbulence closure is achieved using a RNG k–ϵ model. Simulated flow velocities are compared with measured two-dimensional velocities (downstream and cross-stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the flow structures observed in the field. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r=0·88) and a mean difference of 0·09 m s−1. Deviations between simulated and measured velocities may be identified that are both random and systematic. The former may reflect a number of factors including subgrid-scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh configuration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities. These results illustrate the potential for using CFD software to simulate flow hydraulics in natural channels with complex configurations. They also highlight the need for detailed spatially distributed datasets of three-dimensional flow variables to establish the accuracy and applicability of CFD software. Copyright

Simulation of braided river flow using a new cellular routing scheme

Abstract Results are presented from a numerical simulation of two-dimensional flow patterns in a braided river using a simple cellular routing scheme. The results of the routing scheme are compared with field measurements of discharge per unit width obtained within the study reach at low flow and, for higher flows, with the predictions of a more sophisticated hydraulic model that solves the two-dimensional shallow water form of the Navier–Stokes equations. An assessment is made of the sensitivity of the routing scheme to variations in the values of its main parameters, and appropriate values are determined based on the physical characteristics of the study site and available flow measurements. It is shown that despite the simple approach adopted by the cellular routing scheme to simulate processes of water redistribution, it is able to replicate accurately both the field data and the results of the more sophisticated hydraulic model. These results indicate that the routing scheme outlined here is able to overcome some of the limitations of previous simple cellular automata models and may be suitable for use in modelling bedload transport and channel change in complex fluvial environments. As such this research represents a small and ongoing contribution to the field of numerical simulation of braided river processes.

The significance of particle aggregation in the overbank deposition of suspended sediment on river floodplains

Abstract The effects of suspended sediment aggregation on rates of overbank deposition and on the grain size composition of deposited sediment were investigated using a combined field measurement and numerical modelling approach. Measurements of the in situ grain size characteristics of the suspended sediment load (termed the effective size distribution) of the River Culm, Devon, were obtained using a custom-built water elutriation system. Samples of suspended sediment and of sediment deposited on the floodplain were analysed in the laboratory to determine the size distributions of their constituent discrete mineral particles (termed the ultimate size distribution). Interpretation of these field data was aided by the development of a two-dimensional finite difference model of flood hydraulics and suspended sediment dispersion and deposition. Field measurements and model predictions show that particle aggregation results in higher relative contributions of fine size fractions to overbank deposits. Aggregation may also provide a mechanism for explaining the poor agreement between theoretical and observed trends in relationships between mean deposit grain size and distance from the main channel. Suspended sediment aggregation is shown to cause significant increases in rates of floodplain deposition. Increases in deposition rates resulting from aggregation may, however, be less than expected because of the effects of aggregate shape and density. The latter may offset increases in particle size so that the settling velocities of the largest aggregates do not increase appreciably with particle diameter.

Numerical modelling of floodplain hydraulics and suspended sediment transport and deposition

A numerical model is presented for the prediction of floodplain inundation extents, flow depths and velocities, and patterns of suspended sediment dispersion and deposition. The model employs a simplified and novel treatment of fluid flow and a conventional convection–diffusion mass balance relationship for suspended sediment transport and deposition. These relationships are solved over a finite difference grid with a relatively fine nodal spacing, which enables a detailed representation of complex floodplain topography. The model is applied to a 600 m reach of the River Culm, Devon, UK. Continuous records of river stage and suspended sediment concentration monitored at the upstream boundary of the study reach provide the data needed to operate the model. These data are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. Model predictions of overbank flow depths and velocities and patterns of suspended sediment concentrations and deposition amounts are presented. Predicted patterns of overbank deposition are described and compared with field measurements obtained with the aid of astroturf sedimentation traps. Grain size distributions obtained for samples of deposited sediment retrieved from the sedimentation traps are also examined and compared with model predictions. The results of both the field measurement programme and modelling work are shown to have significant implications for the representation of floodplain topography and suspended sediment particle size and settling properties in numerical simulations of overbank processes.

Modeling alluvial landform change in the absence of external environmental forcing

Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feedbacks between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change.

Measuring and defining bimodal sediments: Problems and implications

Gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions. These can be quantified in several ways, giving wide differences in derived statistics and calculations. Problems associated with measuring, summarizing, and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition. No single index of bimodality serves all purposes; alternatives are suggested. Measured near-bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume; this has implications for the abrupt downstream transition often found from gravel to sand bed.