Gareth Pender

Macroturbulent structure of open-channel flow over gravel beds

The turbulent structure of open-channel flow over a mobile gravel bed was investigated in an 8 m long, 0.3 m wide, and 0.3 m deep tilting flume. A flow visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed. The experiments reveal that turbulent flow consists of a sequence of large-scale eddies with a vertical size close to the flow depth, an average length equal to four to five depths, and a width of about two depths. The downstream motion of these eddies causes quasiperiodic fluctuations of the local flow velocity components. The development of longitudinal troughs and ridges on the mobile bed and preferential transport of bed particles along troughs are related to the effect of the eddies. The experimental results indicate that the depth-scale eddies are an important turbulence mechanism contributing to sediment transport.

Flume study of the effect of relative depth on the incipient motion of coarse uniform sediments

An experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0.3 m wide tilting flume. The concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport. The experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter. A revised Shields diagram relating critical stress, grain Reynolds number, and depth to grain size ratio has been derived. This has implications for the calculation of sediment transport rates, design of stable alluvial channels, and physical hydraulic modeling.

Critical shear stress for incipient motion of sand/gravel streambeds

Results of an experimental study of the incipient motion of streambeds composed of sand/gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments. The experiments were conducted in an 8 m long by 0.30 m wide glass-walled tilting flume and an 18 m long by 0.80–1.10 m wide trapezoidal concrete channel. A reference transport method is used to define the beginning of bed material movement. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size (intergranular effects), mixture standard deviation (effect of the shape of grain-size distribution), absolute value of median size (absolute size effect), and bed slope (effect of relative depth on overall flow resistance). The shear stress at incipient motion of median-sized grains in mixtures is found to be the same as for uniform sediment of this size. The present findings are consistent with available flume and field data. A technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal/weakly bimodal sediment mixtures is proposed.

Dam-break flows over mobile beds: Experiments and benchmark tests for numerical models

In this paper, the results of a benchmark test launched within the framework of the NSF–PIRE project “Modelling of Flood Hazards and Geomorphic Impacts of Levee Breach and Dam Failure” are presented. Experiments of two-dimensional dam-break flows over a sand bed were conducted at Université catholique de Louvain, Belgium. The water level evolution at eight gauging points was measured as well as the final bed topography. Intense scour occurred close to the failed dam, while significant deposition was observed further downstream. From these experiments, a benchmark was proposed to the scientific community, consisting of blind test simulations, that is, without any prior knowledge of the measurements. Twelve different teams of modellers from eight countries participated in the study. Here, the numerical models used in this test are briefly presented. The results are commented upon, in view of evaluating the modelling capabilities and identifying the challenges that may open pathways for further research.

Selective bedload transport during the degradation of a well sorted graded sediment bed

The paper presents an analysis of the composition of bedload transport and changes to bed structure and topography during three graded sediment degradation experiments. The analysis suggests that variations in channel hydraulics and active layer composition alone may not explain the observed reductions in sediment transport. Further, the experiments appear to cover a crucial range of mean bed shear stresses for armouring studies, ranging between a condition of passive winnowing, to one of more active armour development in which the coarse grains play a role in determining bed structure. This indicates that the active layer concept, commonly applied in computer models of graded sediment transport, may be limited in its application.

Use of computer models of flood inundation to facilitate communication in flood risk management

Abstract It is now widely recognised that good communication between multi-disciplinary stakeholders is central to effective flood risk management. Recent developments in Geographical Information Systems, increased availability of accurate digital terrain models from remotely sensed data sources and improved graphical computer interfaces have made the outputs from computer models of flood inundation easily accessible to the stakeholder community. As a consequence, predictions from such models are now being used routinely as a means of communication between engineers and other stakeholders in flood risk management. This paper provides a review of the modelling methods most appropriate for flood risk communication. These are one-dimensional models which are suitable for simulating flood risk at a catchment or sub-catchment scale and appropriate for communicating the impact of strategic flood management decisions and two-dimensional models which can be applied across a range of scales but, are now being regularly applied at the relatively small scale, less than 10 km2, where they have the potential to inform and communicate disaster management decisions. The role of such models in communicating between modellers and non-modellers by providing a means for immediate visualisation of “the future” is discussed and illustrated by application to two case studies.

A double layer-averaged model for dam-break flows over mobile bed

Dam-break flows over mobile bed are often sharply stratified, comprising a bedload sediment-laden layer and an upper clear-water layer. Double layer-averaged (DL) models are attractive for modelling such flows due to the balance between the computing cost and the ability to represent stratification. However, existing DL models are oversimplified as sediment concentration in the sediment-laden layer is presumed constant, which is not generally justified. Here a new DL model is presented, explicitly incorporating the sediment mass conservation law in lieu of the assumption of constant sediment concentration. The two hyperbolic systems of the governing equations for the two layers are solved separately and simultaneously. The new model is demonstrated to agree with the experimental measurements of instant and progressive dam-break floods better than a simplified double layer-averaged model and a single layer-averaged model. It shows promise for applications to sharply stratified sediment-laden flows over mobile bed.

A non‐Fickian, particle‐tracking diffusion model based on fractional Brownian motion

SUMMARY The work is motivated by the recent discovery that ocean surface drifter trajectories contain fractal properties. This suggests that the dispersion of pollutants in coastal waters may also be described using fractal statistics. The paper describes the development of a fractional Brownian motion model for simulating pollutant dispersion using particle tracking. Numerical test cases are used to compare this new model with the results obtained from a traditional Gaussian particle-tracking model. The results seems to be significantly different, which may have implications for pollution modelling in the coastal zone. # 1997 John Wiley & Sons, Ltd.

Multiple Time Scales of Fluvial Processes with Bed Load Sediment and Implications for Mathematical Modeling

Fluvial bed load transport is often considered to assume a capacity regime exclusively determined by local flow conditions, but its applicability in naturally occurring unsteady flows remains to be theoretically justified. In addition, mathematical river models are often decoupled, being based on simplified conservation equations and ignoring the feedback impacts of bed deformation to a certain extent. So far whether the decoupling could have considerable impacts on the fluvial processes with bed load transport remains poorly understood. This paper presents a theoretical investigation of both issues. The multiple time scales of fluvial processes with bed load sediment are evaluated to examine the applicability of bed load transport capacity and decoupled models. Numerical case studies involving active bed load transport by highly unsteady flows complement the analysis of the time scales. It is found that bed load transport can sufficiently rapidly adapt to capacity in line with local flow because sediment...

Modelling roll waves with shallow water equations and turbulent closure

ABSTRACT A physically enhanced model is proposed for roll waves based on the shallow water equations and k−ϵ turbulence closure along with a modification component. It is tested against measured data on periodic permanent roll waves, and the impact of turbulence is demonstrated to be essential. It is revealed that a regular inlet perturbation may lead to periodic permanent or natural roll waves, when its period is shorter or longer than a critical value inherent to a specified normal flow. While a larger amplitude or shorter period of a regular inlet perturbation is conducive to the formation of periodic permanent roll waves, their period remains the same as that of the perturbation, while their amplitude increases with the perturbation period and is independent of the perturbation amplitude. An irregular inlet perturbation favours the formation of natural roll waves, so does a larger amplitude of the perturbation.