Panayiotis Diplas

The role of impulse on the initiation of particle movement under turbulent flow conditions.

Fundamental to our understanding of erosional and transport phenomena in earth-surface dynamics and engineering is knowledge of the conditions under which sediment motion will begin when subjected to turbulent flow. The onset criterion currently in use emphasizes the time-averaged boundary shear stress and therefore is incapable of accounting for the fluctuating forces encountered in turbulent flows. We have validated through laboratory experiments and analytical formulation of the problem a criterion based upon the impulse imparted to a sediment grain. We demonstrate that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grains threshold of motion, and that their product, or impulse, is better suited for specifying such conditions.

An experimental study of flow through rigid vegetation

[1] Better understanding of the role of vegetation in the transport of fluid and pollutants requires improved knowledge of the detailed flow structure within the vegetation. Instead of spatial averaging, this study uses discrete measurements at multiple locations within the canopy to develop velocity and turbulence intensity profiles and observe the changes in the flow characteristics as water travels through a vegetation array simulated by rigid dowels. Velocity data was collected with a one dimensional laser Doppler velocimeter under emergent and submerged flow conditions. The effects of dowel arrangement, density, and roughness were also examined. The results show that the velocity within the vegetation array is constant with depth and the velocity profile is logarithmic above it, however the boundaries are marked by inflection points. The strongest vortices and turbulence intensities can be found there, especially in the region immediately downstream of a dowel. These results support the idea that the flow in the region near the bed and at the top of the dowel array is very unstable leading to the formation of coherent structures and are areas of significant mass and momentum exchange.

Turbulent Flow through Idealized Emergent Vegetation

This paper presents results of several large-eddy simulations (LES) of turbulent flow in an open channel through staggered arrays of rigid, emergent cylinders, which can be regarded as idealized vegetation. In this study, two cylinder Reynolds numbers, RD =1,340 and RD =500 , and three vegetation densities are considered. The LES of the lowest density and at RD =1,340 corresponds to a recently completed laboratory experiment, the data of which is used to validate the simulations. Fairly good agreement between calculated and measured first- and second-order statistics along measurement profiles is found, confirming the accuracy of the simulations. The high resolution of the simulations enables an explicit calculation of drag forces, decomposed into pressure and friction drag, that are exerted on the cylinders. The effect of the cylinder Reynolds number and the cylinder density on the drag and hence on the flow resistance is quantified and in agreement with previous experimental studies. Turbulence structur...

Role of instantaneous force magnitude and duration on particle entrainment

[1] A new criterion for the onset of entrainment of coarse sediment grains is presented here. It is hypothesized that not only the magnitude, but also the duration of energetic near bed turbulent events is relevant in predicting grain removal from the bed surface. It is therefore proposed that the product of force and its duration, or impulse, is a more appropriate and universal criterion for identifying conditions for particle dislodgement. This conjecture is investigated utilizing two theoretical models, representative of two modes of entrainment: saltation and rolling. In these models, instantaneous, highly fluctuating turbulent forces are simulated as short‐lived pulses of characteristic magnitude and duration, which transfer adequate fluid momentum to the particle, to trigger its entrainment. The analytical solution of the respective equations of motion is employed in deriving representations of threshold conditions in terms of the impulse characteristics. It is shown that hydrodynamic forces of sufficiently high magnitude are capable of entraining a particle only when they last long enough so that their impulse exceeds a critical value. To illustrate further the validity of the critical impulse concept, as well as extend and generalize its application to different entrainment levels of an individual grain, a novel experimental setup is utilized. This setup facilitates observations of angular displacement of a steel mobile particle in air due to electromagnetic pulses of different magnitude and duration. The experimentally obtained conditions for partial or complete entrainment support the concept of a critical impulse.

Impulse and particle dislodgement under turbulent flow conditions

In this study, we investigated the role of turbulence fluctuations on the entrainment of a fully exposed grain near threshold flow conditions. Experiments were carried out to measure synchronously the near bed flow velocity and the particle movement for a range of flow conditions and resulting particle entrainment frequencies. We used a simplified bed geometry consisted of spherical particles to reduce the complexities associated with the variations in the bed and flow details in an effort to identify the underlying dominant physical mechanism. An analysis was performed based on common force approximations using near bed flow velocity. Turbulence fluctuations were treated as impulses, which are products of magnitude and duration of applied force. It is demonstrated that besides the magnitude of the instantaneous forces applied on a sediment grain, their duration is important as well in determining whether a particle will be entrained by a turbulent flow event. Frequency of particle entrainment varied rema...

River Training and Ecological Enhancement Potential Using In-Stream Structures

The use of in-stream flow training structures for channel stabilization has become increasingly popular due to its potential cost effectiveness and ecological benefits. When properly designed and maintained, these structures help to protect the channel from erosion and lateral migration and may also provide grade control. Additionally, in-stream structures may improve fish and macroinvertebrate habitat and increase hyporheic exchange. However, a large number of these projects fail due to inadequate design guidelines. In this study, various types of in-stream sill and deflector structures are described. A literature review including case studies, journal articles, and standards developed by various agencies suggests design guidelines which are currently available but do not meet the rigorous engineering-based hydraulic design criteria necessary for success. A practitioner opinion survey of personnel from state agencies, federal agencies, and private firms indicates that these structures are being used extensively in at least 76% of the USGS physiographic provinces. In general, respondents indicated that in the areas of cost, performance, maintenance, and environmental enhancement in-stream structures are preferable to the most common alternative: riprap revetment. Information from 39 case studies suggests that successful projects involve multiple structures and are located in rivers with relatively high aspect ratios.

Bed load sediment transport in ephemeral and perennial gravel bed streams

Perennial gravel streams are usually dominated by a bed material stratified in terms of grain size, with a coarse surface layer overlying a finer subsurface (Figure 1a). However, field observations from some ephemeral gravel streams in the Middle East and other locations exhibit the opposite stratification (Figure 1b). Recent studies have found that under the same range of flow strengths, such an ephemeral stream has a much higher transport efficiency of bed load sediment (the movement of particles by rolling and sliding on the bed, or by making short hops of a few grain diameters in distance), which suggests a completely different behavior from perennial streams [e.g., Laronne and Reid, 1993; Reid and Laronne, 1995].

Hydraulic Complexity Metrics for Evaluating In-Stream Brook Trout Habitat

A two-dimensional hydraulic model (River2D) was used to investigate the significance of flow complexity on habitat preferences of brook trout (Salvelinus fontinalis) in the high-gradient Staunton River in Shenandoah National Park, Virginia. Two 100-m reaches were modeled where detailed brook trout surveys (10–30-m resolution) have been conducted annually since 1997. Spatial hydraulic complexity metrics including area-weighted circulation and kinetic energy gradients (KEG) were calculated based on modeled velocity distributions. These metrics were compared to fish density in individual habitat complexes (10–30-m subreaches) to evaluate relationships between fish location and average flow complexity. In addition, the fish density was compared to additional habitat variables including percent cascade (CS), pool (PL) and riffle, and in-stream ( ISCN ) and riparian cover. There were negative correlations between modeled mean velocity (VEL) and maximum depth (MAXD) and fish density; however, there were no stati...

Modelling of fine and coarse sediment interaction over alternate bars

Abstract Flume experiments demonstrated an ability to model bed features, such as pavement and the alternate bar structure, that are commonly encountered in natural gravel-bed streams. The effect of pool-and-riffle topography on the infiltration of fines into, and their removal from, the channel bed was examined. Fines were deposited first and removed last from the downstream side of the bar and from the pool area. When there is no bed motion, the flow is capable of completely removing the fines from the pavement but not from the subpavement. When the bed is active, both pavement and subpavement can be cleaned.

Untangling boulder dislodgement in storms and tsunamis: Is it possible with simple theories?

Boulders can move during storms and tsunamis. It is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field. In this contribution, we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of Newtons Second Law of Motion in polar coordinates and defining a critical position for boulder dislodgement. We find that the boulder dislodgement is not only a function of the causative wave, but also of the roughness in the vicinity of the boulder and the slope angle. We employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis. As the main result of our numerical experiments, we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values. This allows us to conclude that simple theories are applicable to answer the questions asked in the title, but we argue only if they contain a critical dislodgement condition like the one presented here.