1,721,093 research outputs found
Use of the Lorenz curve to quantify statistical nonuniformity of sediment transport rate
No full textThe Lorenz curve and the Gini index are statistical tools that have been widely used to quantify nonuniformity in economics, ecology, and medicine, but have apparently not been previously applied in hydraulic engineering. In this work, the Lorenz curve was applied to the analysis of sediment transport data from several experiments with natural and lightweight solid particles. The distributions of instantaneous sediment transport rate data have been analyzed, leading to some interesting observations: (1) the values are quite nonuniformly distributed, with a significant proportion of zero values at the lowest sediment transport rates; and (2) the distributions of the values considering only samples with nonzero values are still quite nonuniform; yet the variability of the Gini index with the average sediment transport rate is much smaller than that obtained for the complete data samples, indicating a major role of intermittency in determining the nonuniformity of sediment transport
Experimental characterization of bed-load dunes in a closed conduit: size, period, and migration celerity
No full textResults are presented for dune development and migration in a closed conduit with rectangular cross section. While dunes were not clearly formed with the lowest flow rates (exceeding only slightly the threshold flow rate for sediment transport), these bed-forms regularly developed for higher transport intensities. Furthermore, the development of dunes required a certain length. Spatial and temporal measurements of the bed elevation enabled several statistical analyses to be exploited: Cumulative Frequency Distributions, auto- and cross-correlation functions, and spectra were used to characterize the properties of the bed-load dunes. The dune height was an increasing function of the flow rate and, in turn, of the transport conditions. The same held for the dune celerity, while the integral scale of auto-correlation and the dune period were decreasing functions of the flow rate. The dune length did not show any evident trend with the transport conditions. The results for the present dunes in a closed conduit are tentatively compared to predictors for dunes in open channels
Discussion of “Protecting vertical-wall abutments with riprap mattresses” by A.H. Cardoso and C.M.S. Fael
No full textAn experimental investigation of sediment kinematics and multi‐scale propagation for laboratory bed‐load dunes
Full text linkThe dynamics of river bedforms is still not well understood and detailed
experimental investigation may provide significant insight into the process
mechanisms. In this study, results are presented for a laboratory experiment
that returned long time series of sediment kinematics for bed-load dunes.
The run was performed in a closed conduit but, in the light of prior literature,
the results are also representative of dunes under open-channel flows.
A flow rate of 1.4 times the threshold for sediment transport was used, and
the experiment lasted long enough to measure the sediment kinematics for
four full dunes. This enabled a stability of mean values to be obtained for
the key properties; concentration of moving sediment, sediment velocity and
sediment transport rate, that were measured over a Eulerian grid. At a measuring
location, all the properties present an oscillation pattern that resembles
the passage of the dunes, also including higher-frequency oscillations
(due to the flow turbulence) superimposed onto the low-frequency ones
related to the dunes. The time evolution of the sediment transport rate is
more similar to that of the sediment concentration than to that of the sediment
velocity, which varies comparatively less. A multi-scale propagation is
demonstrated considering the propagation of the dunes, of the sediment
grains, and of quick ‘sediment gusts’ triggered by the flow field. Taking
advantage of a Taylor-like hypothesis, that is assumed to be valid as a firstorder
approximation, a mean temporal evolution of the sediment–kinematics
properties is determined. Although limited to a single hydro-dynamic condition,
and thus in need of further testing, the present results are of interest
for a number of applications, including bedform analysis, live-bed scour processes,
and quantification of sediment transport rates by measuring surrogate
quantities
Fluctuations and time scales for bed-load sediment motion over a smooth bed
No full textResults are presented for experiments of bed-load sediment transport over a plane, smooth bed. The smooth-bed configuration, though not adequate for mimicking natural streams, enables the effects of bed roughness to be filtered out, thus, highlighting the role of flow turbulence for particle dynamics. Sediments were individually tracked along their paths, measuring position and velocity of the individual grains. A number of analyses were then applied to the data: probability density function, auto-correlation, and spectra of the grain velocity. Several Lagrangian time scales of particle motion were obtained and compared to available data for the turbulent flow field to determine a phenomenological interpretation of the process
Celerity and Height of Aggradation Fronts in Gravel-Bed Laboratory Channel
No full textWe present the results of laboratory experiments specifically designed to quantify the height and migration rate of an aggrading
gravel front. The experiments were performed in sediment feed mode with constant water and sediment supply. Particular care was put into
the experimental procedure and the methods to determine the quantities of interest to ensure reproducibility of the results. The celerity and
height of an aggradation front were modeled as functions of the Froude number of the flow for the initial bed profile and of a load ratio defined
as the ratio of the sediment feed rate to the transport capacity of the flow for the initial bed. The two control parameters (Froude number and
load ratio) also determined the translational or dispersive nature of a sediment front. Two predictors were provided to estimate the dimensionless
height and celerity of an aggradation front. The former was an increasing function of the load ratio and a decreasing function of the
Froude number, and the latter was roughly proportional to the squared Froude number and had no evident relationship with the load ratio. The
present results are of interest for scholars and practitioners needing to determine the key properties of swift gravel fronts as those developing,
for example, during flash floods
Sulla modellazione idraulico-morfologica dei corsi d’acqua: il torrente Mallero e la propagazione dell’incertezza legata all’alimentazione solida
No full text
Roughening elements as abutment scour countermeasures
Full text linkThis paper presents the results of clear-water scour experiments with a vertical-wall abutment. The upstream face of the latter was provided with roughening elements as devices to intercept the downflow responsible for the formation of the principal vortex. The time evolution of the scour depth at several locations around the abutment was measured for different hydrodynamic and geometric configurations.
The countermeasure performance was quantified using an appropriate dimensionless framework, and a best arrangement of the roughening
elements was proposed within the range of the experimental conditions investigated. The full database is provided as supplemental material
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