On validating predictions of plant motion in coupled biomechanical-flow models

Recent developments in integrated biomechanical-flow models have enabled the prediction of the influence of vegetation on the flow field and associated feedback processes. However, to date, such models have only been validated on the hydraulic predictions and/or mean plant position. Here we introduce an approach where dynamic surrogate plant motion, measured directly in flume experiments, is used to allow a validation approach capable of assessing the accuracy of time-dependent flow–vegetation interaction within a numerical model. We use this method to demonstrate the accuracy of an existing Euler–Bernoulli beam model in predicting both mean and dynamic plant position through time and space

The hydraulic description of vegetated river channels: the weaknesses of existing formulations and emerging alternatives

Currently, many of the methods used to predict the effect of vegetation on river flow suffer from one or both of the following problems: (1) a strong dependence on parameters that have a poor physical basis and which are only readily determined using empirical means; and (2) a poor conceptual basis, in terms of the way they represent the effects of vegetation on the flow, especially in higher dimensionality numerical models. This limits their contribution to problems that extend beyond basic hydraulic prediction (e.g., of water levels) to ecosystem understanding. In this study, we show how use of coupled biomechanical–hydraulic models may lead to a much-improved representation of a range of open-channel flow processes. Preliminary experiments over hypothetical vegetation canopies are producing very encouraging results and may provide the means for an improved representation of vegetation in higher dimensionality numerical models that may result in a better justification and more reliable identification of the conveyance parameters needed for both flood identification and the characterization of habitat

Evaluating the success of public participation in integrated catchment management

Recognition of the need to manage the water environment in more holistic ways has resulted in the global growth of Integrated Catchment Management (ICM). ICM is characterised by horizontal integration, encouraging interdisciplinary working between traditionally disparate management sectors, alongside vertical integration, characterised by the engagement of communities; central is the promotion of participatory governance and management decision-making. ICM has been translated into policy through, for example, the EU Water Framework Directive and at a national level by policies such as the Catchment Based Approach in England. Research exploring the implementation of these policies has reported success at a catchment level, but further research is required to explore practices of management at local level within catchments. This paper presents the findings of participatory research undertaken with a catchment partnership in the northeast of England to explore the integration of top-down policy translation with how local communities interact with management agencies at sub-catchment scale (a bottom-up perspective). The research found that supra-catchment scale drivers dominate the vertical interplay between management systems at more local levels. These drivers embed traditional practices of management, which establishes public participation as a barrier to delivery of top-down management objectives, resulting in practices that exclude communities and participatory movements at the local level. Although collaboration between agencies at the partnership scale offers a potential solution to overcoming these obstacles, the paper recommends changes to supra-catchment governance structures to encourage flexibility in developing local participatory movements as assets. Further research is necessary to develop new practices of management to integrate local people more effectively into the management process

Does the canopy mixing layer model apply to highly flexible aquatic vegetation? Insights from numerical modelling

Vegetation is a characteristic feature of shallow aquatic flows such as rivers, lakes and coastal waters. Flow through and above aquatic vegetation canopies is commonly described using a canopy mixing layer analogy which provides a canonical framework for assessing key hydraulic characteristics such as velocity profiles, large-scale coherent turbulent structures and mixing and transport processes for solutes and sediments. This theory is well developed for the case of semi-rigid terrestrial vegetation and has more recently been applied to the case of aquatic vegetation. However, aquatic vegetation often displays key differences in morphology and biomechanics to terrestrial vegetation due to the different environment it inhabits. Here we investigate the effect of plant morphology and biomechanical properties on flow–vegetation interactions through the application of a coupled LES-biomechanical model. We present results from two simulations of aquatic vegetated flows: one assuming a semi-rigid canopy and the other a highly flexible canopy and provide a comparison of the associated flow regimes. Our results show that while both cases display canopy mixing layers, there are also clear differences in the shear layer characteristics and turbulent processes between the two, suggesting that the semi-rigid approximation may not provide a complete representation of flow–vegetation interactions

Coherent flow structures in a depth-limited flow over a gravel surface : the role of near-bed turbulance and influence of Reynolds number

In gravel bed rivers, the microtopography of the bed exerts a significant effect on the generation of turbulent flow structures. Although field and laboratory measurements have indicated that flows over gravel beds contain coherent macroturbulent flow structures, the origin of these phenomena, and their relationship to the ensemble of individual roughness elements forming the bed, is not quantitatively well understood. Here we report upon a flume experiment in which flow over a gravel surface is quantified through the application of digital particle imaging velocimetry, which allows study of the downstream and vertical components of velocity over the entire flow field. The results indicate that as the Reynolds number increases (1) the visual distinctiveness of the coherent flow structures becomes more defined, (2) the upstream slope of the structures increases, and (3) the turbulence intensity of the structures increases. Analysis of the mean velocity components, the turbulence intensity, and the flow structure using quadrant analysis demonstrates that these large-scale turbulent structures originate from flow interactions with the bed topography. Detection of the dominant temporal length scales through wavelet analysis enables calculation of mean separation zone lengths associated with the gravel roughness through standard scaling laws. The calculated separation zone lengths demonstrate that wake flapping is a dominant mechanism in the production of large-scale coherent flow structures in gravel bed rivers. Thus, we show that coherent flow structures over gravels owe their origin to bed-generated turbulence and that large-scale outer layer structures are the result of flow-topography interactions in the near-bed region associated with wake flapping

High-resolution numerical modelling of flow-vegetation interactions

In this paper, we present and apply a new three-dimensional model for the prediction of canopy-flow and turbulence dynamics in open-channel flow. The approach uses a dynamic immersed boundary technique that is coupled in a sequentially staggered manner to a large eddy simulation. Two different biomechanical models are developed depending on whether the vegetation is dominated by bending or tensile forces. For bending plants, a model structured on the Euler–Bernoulli beam equation has been developed, whilst for tensile plants, an N-pendula model has been developed. Validation against flume data shows good agreement and demonstrates that for a given stem density, the models are able to simulate the extraction of energy from the mean flow at the stem-scale which leads to the drag discontinuity and associated mixing layer

Reconstruction of subgrid scale topographic variability and its effect upon the spatial structure of three dimensional river flow

A new approach to describing the associated topography at different scales in computational fluid dynamic applications to gravel bed rivers was developed. Surveyed topographic data were interpolated, using geostatistical methods, into different spatial discretizations, and grain-size data were used with fractal methods to reconstruct the microtopography at scales finer than the measurement (subgrid) scale. The combination of both scales of topography was then used to construct the spatial discretization of a three-dimensional finite volume Computational Fluid Dynamics (CFD) scheme where the topography was included using a mass flux scaling approach. The method was applied and tested on a 15 m stretch of Solfatara Creek, Wyoming, United States, using spatially distributed elevation and grain-size data. Model runs were undertaken for each topography using a steady state solution. This paper evaluates the impact of the model spatial discretization and additional reconstructed-variability upon the spatial structure of predicted three-dimensional flow. The paper shows how microtopography modifies the spatial structure of predicted flow at scales finer than measurement scale in terms of variability whereas the characteristic scale of predicted flow is determined by the CFD scale. Changes in microtopography modify the predicted mean velocity value by 3.6% for a mesh resolution of 5 cm whereas a change in the computational scale modifies model results by 60%. The paper also points out how the spatial variability of predicted velocities is determined by the topographic complexity at different scales of the input topographic model

Stability for Borell-Brascamp-Lieb inequalities

We study stability issues for the so-called Borell-Brascamp-Lieb inequalities, proving that when near equality is realized, the involved functions must be $L^1$-close to be $p$-concave and to coincide up to homotheties of their graphs.Comment: to appear in GAFA Seminar Note

Equivalent thermo-mechanical parameters for perfect crystals

Thermo-elastic behavior of perfect single crystal is considered. The crystal is represented as a set of interacting particles (atoms). The approach for determination of equivalent continuum values for the discrete system is proposed. Averaging of equations of particles' motion and long wave approximation are used in order to make link between the discrete system and equivalent continuum. Basic balance equations for equivalent continuum are derived from microscopic equations. Macroscopic values such as Piola and Cauchy stress tensors and heat flux are represented via microscopic parameters. Connection between the heat flux and temperature is discussed. Equation of state in Mie-Gruneisen form connecting Cauchy stress tensor with deformation gradient and thermal energy is obtained from microscopic considerations.Comment: To be published in proceedings of IUTAM Simposium on "Vibration Analysis of Structures with Uncertainties", 2009; 14 pages

S and D Wave Mixing in High TcT_c Superconductors

For a tight binding model with nearest neighbour attraction and a small orthorhombic distortion, we find a phase diagram for the gap at zero temperature which includes three distinct regions as a function of filling. In the first, the gap is a mixture of mainly $d$-wave with a smaller extended $s$-wave part. This is followed by a region in which there is a rapid increase in the $s$-wave part accompanied by a rapid increase in relative phase between $s$ and $d$ from 0 to $\pi$. Finally, there is a region of dominant $s$ with a mixture of $d$ and zero phase. In the mixed region with a finite phase, the $s$-wave part of the gap can show a sudden increase with decreasing temperature accompanied with a rapid increase in phase which shows many of the characteristics measured in the angular resolved photoemission experiments of Ma {\em et al.} in $\rm Bi_2Sr_2CaCu_2O_8$Comment: 12 pages, RevTeX 3.0, 3 PostScript figures uuencoded and compresse