Mário J. Franca

Analysis of lock-exchange gravity currents over smooth and rough beds

Gravity currents produced by full-depth lock-release of saline water into a fresh water tank are studied focusing on the influence of the initial density of the saline mixture in the lock and the bed roughness on gravity current kinematics. Temporal evolution of the current front position and front velocity are analysed and related to different phases of the current. Time–space evolution of current depth-averaged density and current height are assessed as well. Roughness of the channel bed plays an important role in the current kinematics, particularly in decreasing the front velocity due to extra drag at the bed. The analysis of Froude numbers, estimated with the initial and local reduced gravity and established with different length scales of the current, allow for the definition of the important variables and current dynamics of each phase of the current development.

Eliminating velocity aliasing in acoustic Doppler velocity profiler data

We present an efficient dealiasing method for acoustic Doppler velocity measurements which is directly carried out on the Doppler phase shift signal. Corrections are applied to the Doppler frequency measured by the acoustic Doppler velocity profiler receivers, before any transformation into the velocity domain is made. The method does not require knowledge of the flow direction. It consists of a first step in which a threshold analysis determines the need for dealiasing, and a second one in which the Doppler information is corrected. For the second step, an algorithm is developed which follows the complex signal history. It determines the correction to be applied to the instantaneous Doppler frequency in order to make it coherent with the neighbouring space/time points. The efficiency and improvement obtained with this algorithm are demonstrated using field measurement data with our ADVP.

Dynamics of the head of gravity currents

The present work experimentally investigates the dynamics of unsteady gravity currents produced by lock-release of a saline mixture into a fresh water tank. Seven different experimental runs were performed by varying the density of the saline mixture in the lock and the bed roughness. Experiments were conducted in a Perspex flume, of horizontal bed and rectangular cross section, and recorded with a CCD camera. An image analysis technique was applied to visualize and characterize the current allowing thus the understanding of its general dynamics and, more specifically, of the current head dynamics. The temporal evolution of both head length and mass shows repeated stretching and breaking cycles: during the stretching phase, the head length and mass grow until reaching a limit, then the head becomes unstable and breaks. In the instants of break, the head aspect ratio shows a limit of 0.2 and the mass of the head is of the order of the initial mass in the lock. The average period of the herein called breaking events is seen to increase with bed roughness and the spatial periodicity of these events is seen to be approximately constant between runs. The rate of growth of the mass at the head is taken as a measure to assess entrainment and it is observed to occur at all stages of the current development. Entrainment rate at the head decreases in time suggesting this as a phenomenon ruled by local buoyancy and the similarity between runs shows independence from the initial reduced gravity and bed roughness.

Image analysis technique applied to lock-exchange gravity currents

An image analysis technique is used to estimate the two-dimensional instantaneous density field of unsteady gravity currents produced by full-depth lock-release of saline water. An experiment reproducing a gravity current was performed in a 3.0 m long, 0.20 m wide and 0.30 m deep Perspex flume with horizontal smooth bed and recorded with a 25 Hz CCD video camera under controlled light conditions. Using dye concentration as a tracer, a calibration procedure was established for each pixel in the image relating the amount of dye uniformly distributed in the tank and the greyscale values in the corresponding images. The results are evaluated and corrected by applying the mass conservation principle within the experimental tank. The procedure is a simple way to assess the time-varying density distribution within the gravity current, allowing the investigation of gravity current dynamics and mixing processes.

A computational model of rockfill dam breaching caused by overtopping (RoDaB)

The outflow hydrograph from a dam failure is a boundary condition of a dam break flood model used on the downstream valleys risk management. Limited research has been made on the rockfill dams breaching process and there are no specific models to this type of structures yet. This paper describes a lumped model for the compulation of the outflow hydrograph due to a Rockfill Dam Breaking named RoDaB. The methodology is based on the governing equations of reservoir routing and depletion, and breach erosion. Results obtained f r om experimental tests performed in a laboratory flume were considered to fulfil the phenomenological aspects to which does not exist any analytical approach so far. The erosion process from the dam breach is modelled as a function of two erosion parameters and of the breach final geometry dimensions obtained from the experiments. Finally, the model RoDaB is applied to a rockfill dam, with characteristics typical of this type of structure, and the results are compared with the ones fro...

The terms of turbulent kinetic energy budget within random arrays of emergent cylinders

This article is aimed at quantifying and discussing the relative magnitude of key terms of the equation of conservation of turbulent kinetic energy (TKE) in the inter-stem space of a flow within arrays of vertical cylinders simulating plant stems of emergent and rigid vegetation. The spatial distribution of turbulent quantities and mean flow variables are influenced by two fundamental space scales, the diameter of the stems and the local stem areal number-density. Both may vary considerably since the areal distribution of plant stems in natural systems is generally not homogeneous; they are often arranged in alternating sparse and dense patches. The magnitude of the terms of the budget of TKE in the inter-stem space has seldom been quantified experimentally and is currently not well known. This work addresses this research need. New databases, consisting of three-component LDA velocity series and two-component PIV velocity maps, obtained in carefully controlled laboratory conditions, were used to calculate the terms of the TKE budget. The physical system comprises random arrays of rigid and emergent cylinders with longitudinally varying areal number-density. It is verified that the main source of TKE is vortex shedding from individual cylinders. The rates of production and dissipation are not in equilibrium. Regions with negative production, a previously unreported feature, are identified. Turbulent transport is particularly important along the von Karman vortex street. Convective rate of change of TKE and pressure diffusion are most relevant in the vicinity of the cylinders.

Bed load fluctuations in a steep channel

Abstract: Bedload transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge. The observed bedload transport rate pulses are periodic and a consequence of grain sorting. Moreover, the presence of large, relatively immobile boulders, such as erratic stones, which are often present in mountain streams, has an impact on flow conditions. The detailed analysis of a 13 hours laboratory experiment is presented in this paper. Boulders were randomly placed in a flume with a steep slope (6.7%), and water and sediment were constantly supplied to the flume. Along with the sediment transport and bulk mean flow velocity, the boulder protrusion, boulder surface, and number of hydraulic jumps, which are indicators of the channel morphology, were measured regularly during the experiment. Periodic bedload transport rate pulses are clearly visible in the data collected during this longduration experiment, along with correlated fluctuations in the flow velocity and bed morphology. The links among the bulk velocity, the time evolution of the morphology variables, and the bedload transport rate are analyzed via correlational analysis, showing that the fluctuations are strongly related. A phase analysis of all observed variables is performed, and the average shapes of the time cycles of the fluctuations are shown. Observations indicate that the detected periodic fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, which varies in time and space, clearly influences these bedload transport rate pulses. Finally, known bedload transport rate formulae are tested, showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations.

Bulk velocity measurements by video analysis of dye tracer in a macro-rough channel

Steep mountain rivers have hydraulic and morphodynamic characteristics that hinder velocity measurements. The high spatial variability of hydraulic parameters, such as water depth (WD), river width and flow velocity, makes the choice of a representative cross-section to measure the velocity in detail challenging. Additionally, sediment transport and rapidly changing bed morphology exclude the utilization of standard and often intrusive velocity measurement techniques. The limited technical choices are further reduced in the presence of macro-roughness elements, such as large, relatively immobile boulders. Tracer tracking techniques are among the few reliable methods that can be used under these conditions to evaluate the mean flow velocity. However, most tracer tracking techniques calculate bulk flow velocities between two or more fixed cross-sections. In the presence of intense sediment transport resulting in an important temporal variability of the bed morphology, dead water zones may appear in the few selected measurement sections. Thus a technique based on the analysis of an entire channel reach is needed in this study. A dye tracer measurement technique in which a single camcorder visualizes a long flume reach is described and developed. This allows us to overcome the problem of the presence of dead water zones. To validate this video analysis technique, velocity measurements were carried out on a laboratory flume simulating a torrent, with a relatively gentle slope of 1.97% and without sediment transport, using several commonly used velocity measurement instruments. In the absence of boulders, salt injections, WD and ultrasonic velocity profiler measurements were carried out, along with dye injection technique. When boulders were present, dye tracer technique was validated only by comparison with salt tracer. Several video analysis techniques used to infer velocities were developed and compared, showing that dye tracking is a valid technique for bulk velocity measurements. RGB Euclidean distance was identified as being the best measure of the average flow velocity.

Opportunity and Economic Feasibility of Inline Microhydropower Units in Water Supply Networks

AbstractSmall-scale hydropower is emerging as a decentralized source to satisfy local demand for electricity. In water supply systems, microhydropower can be used for energy recovery associated with excessive pressure control. However, there is a lack of specific solutions for applications within networks where discharges are highly variable and there are limitations of pressure. An arrangement of microturbines specially conceived for water supply networks is proposed, based on a recently tested microturbine for inline installation in pipes. The installation of up to four turbine units is possible within a buried chamber created around an existing pipe. The location of the chambers is analyzed using an optimization algorithm that considers two objective functions: energy production and economic value. The feasibility of the proposed arrangement for a microhydropower plant was assessed through a case study of a subgrid of the water supply network of Lausanne, Switzerland. A detailed analysis of the cost br...

Turbulence in Rivers

The study of turbulence has always been a challenge for scientists working on geophysical flows. Turbulent flows are common in nature and have an important role in geophysical disciplines such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work to advance the understanding of river processes. Nevertheless, despite more than a century of attempts to correctly formalize turbulent flows, much still remains to be done by researchers and engineers working in hydraulics and fluid mechanics. In this contribution we introduce a general framework for the analysis of river turbulence. We revisit some findings and theoretical frameworks and provide a critical analysis of where the study of turbulence is important and how to include detailed information of this in the analysis of fluvial processes. We also provide a perspective of some general aspects that are essential for researchers/practitioners addressing the subject for the first time. Furthermore, we show some results of interest to scientists and engineers working on river flows.