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Roughness Elements (roughness + element)
Selected AbstractsFlow-retarding effects of vegetation and geotextiles on soil detachment during concentrated flowHYDROLOGICAL PROCESSES, Issue 17 2009A. Knapen Abstract Roughness elements at the soil surface (e.g. crop residues, rock fragments, vegetation, geotextiles) strongly reduce the erosivity of overland flow (both interrill and concentrated overland flow) and hence soil detachment rates. Common methods for shear stress partitioning that allow the calculation of effective flow shear stress in the presence of roughness elements originate from river hydraulics but seem invalid for overland flow. An alternative method to estimate the effective flow shear stress in the presence of a soil surface cover has been presented and tested for soil detachment by concentrated runoff on soil surfaces covered by crop residues by Knapen et al., (2008). In this method, the estimation of effective flow shear stress is based on the recalculation of the hydraulic radius for covered soil surfaces using flow hydraulics on uncovered surfaces. However, the applicability of this method for roughness elements different from crop residues and under field conditions needs to be tested to confirm its validity. Therefore, runoff data from three experimental studies (conducted on geotextile and grassed-covered soil surfaces) are analysed in this study. The results show that effective flow shear stress, calculated using this method, is not only a good soil detachment predictor for soil surfaces covered with crop residues but also for the tested soil surfaces with a vegetation or geotextile cover. However, the geotextile experiments point to one of the shortcomings of the method. At high flow shear stress levels, vortex erosion due to flow turbulence is reported for the thickest geotextiles. These flow turbulences are not accounted for since the method is based on average flow characteristics. Copyright © 2009 John Wiley & Sons, Ltd. [source] Computational fluid dynamics modelling of boundary roughness in gravel-bed rivers: an investigation of the effects of random variability in bed elevationEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2001A.P. Nicholas Abstract Results from a series of numerical simulations of two-dimensional open-channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open-channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra-grid-scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h,=,0·1,0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra-grid-scale roughness also allows a reduction in the height of the near-bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near-bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel-bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh-independent solutions. Copyright © 2001 John Wiley & Sons, Ltd. [source] Flow-retarding effects of vegetation and geotextiles on soil detachment during concentrated flowHYDROLOGICAL PROCESSES, Issue 17 2009A. Knapen Abstract Roughness elements at the soil surface (e.g. crop residues, rock fragments, vegetation, geotextiles) strongly reduce the erosivity of overland flow (both interrill and concentrated overland flow) and hence soil detachment rates. Common methods for shear stress partitioning that allow the calculation of effective flow shear stress in the presence of roughness elements originate from river hydraulics but seem invalid for overland flow. An alternative method to estimate the effective flow shear stress in the presence of a soil surface cover has been presented and tested for soil detachment by concentrated runoff on soil surfaces covered by crop residues by Knapen et al., (2008). In this method, the estimation of effective flow shear stress is based on the recalculation of the hydraulic radius for covered soil surfaces using flow hydraulics on uncovered surfaces. However, the applicability of this method for roughness elements different from crop residues and under field conditions needs to be tested to confirm its validity. Therefore, runoff data from three experimental studies (conducted on geotextile and grassed-covered soil surfaces) are analysed in this study. The results show that effective flow shear stress, calculated using this method, is not only a good soil detachment predictor for soil surfaces covered with crop residues but also for the tested soil surfaces with a vegetation or geotextile cover. However, the geotextile experiments point to one of the shortcomings of the method. At high flow shear stress levels, vortex erosion due to flow turbulence is reported for the thickest geotextiles. These flow turbulences are not accounted for since the method is based on average flow characteristics. Copyright © 2009 John Wiley & Sons, Ltd. [source] Hyporheic and total transient storage in small, sand-bed streamsHYDROLOGICAL PROCESSES, Issue 12 2008John M. Stofleth Abstract Key processes in stream ecosystems are linked to hydraulic retention, which is the departure of stream flow from ideal ,plug flow', and reflects fluid movement through surface and hyporheic storage zones. Most existing information about hyporheic exchange is based on flume studies or field measurements in relatively steep streams with beds coarser than sand. Stream tracer studies may be used to quantify overall hydraulic retention, but disaggregation of surface and hyporheic retention remains difficult. A stream tracer approach was used to compute the rates at which stream water is exchanged with water in storage zones (total storage) in short reaches of two small, sand-bed streams under free and obstructed flow conditions. Tracer curves were fit to the one-dimensional transport with inflow storage model OTIS-P. Networks of piezometers were used to measure specific discharge between the stream and the groundwater. In the sand-bed streams studied, parameters describing total retention were in the upper 50% of data compiled from the literature, most of which represented streams with beds coarser than sand. However, hyporheic storage was an insignificant component of total hydraulic retention, representing only 0·01,0·49% of total exchange, and this fraction did not increase after installation of flow obstructions. Total retention did not vary systematically with bed material size, but increased 50,100% following flow obstruction. Removal of roughness elements, such as large wood and debris dams, is detrimental to processes dependent upon transient storage in small, sand-bed streams. Copyright © 2007 John Wiley & Sons, Ltd. [source] Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat islandINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2003A. John Arnfield Abstract Progress in urban climatology over the two decades since the first publication of the International Journal of Climatology is reviewed. It is emphasized that urban climatology during this period has benefited from conceptual advances made in microclimatology and boundary-layer climatology in general. The role of scale, heterogeneity, dynamic source areas for turbulent fluxes and the complexity introduced by the roughness sublayer over the tall, rigid roughness elements of cities is described. The diversity of urban heat islands, depending on the medium sensed and the sensing technique, is explained. The review focuses on two areas within urban climatology. First, it assesses advances in the study of selected urban climatic processes relating to urban atmospheric turbulence (including surface roughness) and exchange processes for energy and water, at scales of consideration ranging from individual facets of the urban environment, through streets and city blocks to neighbourhoods. Second, it explores the literature on the urban temperature field. The state of knowledge about urban heat islands around 1980 is described and work since then is assessed in terms of similarities to and contrasts with that situation. Finally, the main advances are summarized and recommendations for urban climate work in the future are made. Copyright © 2003 Royal Meteorological Society. [source] The influence of large convective eddies on the surface-layer turbulenceTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006S. S. Zilitinkevich Abstract Close to the surface large coherent eddies consisting of plumes and downdraughts cause convergent winds blowing towards the plume axes, which in turn cause wind shears and generation of turbulence. This mechanism strongly enhances the convective heat/mass transfer at the surface and, in contrast to the classical formulation, implies an important role of the surface roughness. In this context we introduce the stability-dependence of the roughness length. The latter is important over very rough surfaces, when the height of the roughness elements becomes comparable with the large-eddy Monin,Obukhov length. A consistent theoretical model covering convective regimes over all types of natural surfaces, from the smooth still sea to the very rough city of Athens, is developed; it is also comprehensively validated against data from measurements at different sites and also through the convective boundary layer. Good correspondence between model results, field observations and large-eddy simulation is achieved over a wide range of surface roughness lengths and convective boundary-layer heights. Copyright © 2007 Royal Meteorological Society [source] | ||||||||||