Ana Maria Ferreira da Silva

On the quantification of the bed development time of alluvial meandering streams.

This paper concerns the quantitative evaluation of the time of bed development of alluvial meandering streams. In agreement with the prevailing approach, it is assumed that the stream centerline follows a sine-generated curve; the banks are rigid. The flow is turbulent and subcritical, and the flow width is much larger than the flow depth. The movable bed is flat at time t=0 ; at t= Tb , the bed reaches its equilibrium or developed state. With the aid of dimensional and physical considerations, an expression is found for the duration of bed development Tb . According to this expression, Tb is proportional to the square of the flow width B and inversely proportional to the specific volumetric bed-load rate corresponding to the channel-averaged flow. The proportionality factor is found to be a function of the initial deflection angle θ0 alone. The form of this function is investigated on the basis of a series of experimental runs carried out by the writers in a sine-generated channel having an intermediate ...

Regions of bars, meandering and braiding in da Silva and Yalin's plan

The existence region plan of alternate bars, multiple bars, meandering and braiding, introduced by da Silva in 1991 and presented in its most recent form by Yalin and da Silva in the 2001 IAHR monograph Fluvial Processes, is updated and revised. For this purpose, the plan is extended by adding available laboratory and field data from the recent literature, as well as earlier data that only recently became available to the authors. Data from two series of laboratory runs conducted by the authors are also included. These were conducted to fill in a gap in the existing data, pertinent to the definition of the existence region of alternate bars. This work led to a revision to the location of the boundary lines of the existence regions of both alternate bars and meanders. An attempt was also made to reveal the existence regions of different types of multiple bars.

Application of continuous wavelet transform to the study of large-scale coherent structures

It is generally recognized that large-scale turbulent coherent structures play an important role in the transport of sediment and contaminants in rivers. They are also believed to be related to the origin and development of a variety of fluvial bed and plan forms. While intensive laboratory and field research has been devoted in recent years to the study of large-scale vertical coherent structures, no such efforts have yet been directed to the study of large-scale horizontal coherent structures. This paper is intended as a contribution to address the existing lack of information on the latter structures. Its objective is to report an application of continuous wavelet transform (CWT) to the detection and establishment of the length and time scales of the largest coherent structures existing in a shallow open channel flow (width to depth ratio equal to 25), focusing primarily on the horizontal structures. The analysis is based on measurements of instantaneous flow velocity previously carried out in a 21 m long, 1 m wide flume. These include 306 single point velocity measurements collected throughout the flow field at a constant distance from the bed surface, the duration of each measurement being 120 s; and 20 min long measurements carried out at selected locations. The velocity was measured with the aid of a 2D Micro Acoustic Doppler Velocity meter. Large-scale horizontal coherent structures could be identified in all of the velocity records, and appeared as quasi-cyclic, sustained features in the flow. The intervals of time where such structures could not be detected were invariably short in comparison to the measurement time. CWT was found to be particularly well suited to determine the average time and length scales of the structures, two quantities of special significance in river morphodynamics. The average time scale of the large-scale horizontal structures for the investigated flow was found to be equal to 22.3 s, which implies a length scale of five times the flow width. Individual horizontal coherent structures with characteristic times approximately twice larger than the value of average time scale could be identified in the flow. However, these were infrequent occurrences in the flow.

Recent Advances from Research on Meandering and Directions for Future Work

This paper presents a review of the present understanding of the kinematics of meandering flow, and its relationship to bed deformation as well as downstream migration and lateral expansion of meander loops. Taking into account the conditions prevailing in natural, low-land alluvial meandering rivers, the paper focuses primarily on the behaviour of streams having “large” values of width-to-depth ratio. The present review is preceded by a brief description of meandering defining geometric characteristics, as these are invoked throughout this manuscript. The paper is also used as an opportunity to outline future directions for research. These involve matters related to the topics under consideration that remain obscure and which, in the writer’s view, constitute subjects particularly worthwhile as future research topics for their scientific as well as practical significance. More specifically, these concern the nature and analytical formulation of meander wavelength; the value of width-to-depth ratio beyond which the effect of cross-circulation becomes of secondary importance where the meandering bed deformation is concerned; and, finally, the unification of present methods of determination of meandering planimetric evolution with the principle of self-formation of alluvial streams as expressed by regime theory.

A cement-based method for fixing sand in laboratory channels

A method to fix movable beds in river-related laboratory research or physical model studies is proposed and investigated. The motivation was to develop a method that would: (1) not involve the use of harsh chemicals and (2) ensure that the granular roughness of the original movable bed was maintained. The method involves coating the surface with a mixture of sand and Portland cement. Laboratory tests were conducted to determine the amount of cement, and investigate the best method of application, effect on roughness, shear strength and durability. It is found that the method can be used to fix both flat and deformed beds. The method is particularly effective for sands up to 1.1 mm, when it is possible to ensure that the granular roughness of the fixed bed is comparable to that of the movable bed by merely using in the coating mixture a coarser sand than the original one.

Meandering Morphodynamics: Insights from Laboratory and Numerical Experiments and Beyond

AbstractThis paper, written to mark the 60th anniversary of the Journal of Hydraulic Engineering, focuses on the nature of meandering flow and its coupling to bed and bank deformation. An outline o...

A Study of Large-Scale Horizontal Turbulence in Alluvial Streams, with a View Towards Its Morphological Consequences

As is well known, the origin of alternate bars and meandering remains a matter of debate in the literature. This chapter concerns the hypothesis previously raised by a number of authors that large-scale turbulence, and more specifically large-scale horizontal coherent structures (LSHCS’s), are the reason for their occurrence, or at least a contributing factor. In order to properly assess the validity of such hypothesis, it is necessary to first develop an understanding of the dynamics of LSHCS’s occurring in open-channel and river flows, and establish their characteristic scales. This work is motivated by this need. The work is to be viewed as an extension of recent experimental studies on LSHCS’s in shallow flows to the case of deep flows. Accordingly, a laboratory flow having width-to-depth ratio of 7.1, and conveyed in a 1 m wide and 21 m long straight channel, is investigated. The flat channel bed was formed by a coarse silica sand. Flow velocity measurements were carried out with a 2D SonTekTM Micro ADV. Several techniques are used to treat the velocity signals, including continuous wavelet transform and quadrant and spectral density analyses. Special attention is paid to the time and length scales of the LSHCS’s, the effect of superimposition of the LSHCS’s on the mean flow, and the variation of turbulence structure over the flow depth. To the best knowledge of the writers, this is the first work where LSHCS’s have been detected in a deep open-channel flow. The work is used also to shed light on why the initiation of meandering in alluvial streams is not always preceded by the occurrence of alternate bars.

Closure to “Regions of bars, meandering and braiding in da Silva and Yalin's plan”

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