Angel N. Menéndez

Mathematical simulation of pollutant dispersion

Abstract A numerical technique based on the computation of contaminant dispersion in a localized area is presented. It allows a two-dimensional description of the plume when the dispersion scale is much smaller than the hydrodynamic one. The technique was implemented in the computational system MANCHAS, developed at the LHA, and was applied to the plume generated by the sewage discharge of Buenos Aires city in the Rio de la Plata. The velocity and depth fields were achieved by the computational system HIDROBID II, also developed at the LHA. The physical parameters involved in the phenomenon, such as dispersion coefficients and time of decay, were estimated from field measurements. Some numerical tests were performed for different tides and wind velocities. It can be concluded from the results that the wind, as a generator of drift currents, is the relevant mechanism of transport of the pollutant.

Water quality assessment for a coastal zone through numerical modeling

Water quality assessment for the Southern Coastal Fringe (SCF) of the Inner Plata River is performed based on numerical modeling. A hydrodynamic model of the whole Inner Plata River is implemented and validated based on water level and velocity measurements. It drives a water quality model for the SCF, adjacent to Buenos Aires Metropolitan Region (BAMR), from which many pollutant coastal loads emerge. This model represents the dynamics and kinetics of biological, physical, and chemical parameters, and is calibrated based on concentration data. Aptitude criteria for different water uses, in terms of limiting concentrations for modeled parameters, are established. They are applied to the model results in order to determine limited water use zones, which constitute a diagnosis for the present situation. A project scenario, with the implementation of the Remediation Master Plan formulated by the Water Company for the BAMR (which includes construction of a new outfall and extension of the present one), is eval...

Effect of climate change on navigation channel dredging of the Parana River

This paper presents an analysis of the effect of climate change on modifying the dredging cost to maintain the navigation channel at the actual capacity of the Parana waterway (Argentina). The Parana−Paraguay Rivers system is one of the most important inner navigation waterways in the world, where approximately 100 million tons of cargo are transported per year. Maintenance of the navigation channel requires continuous dredging by Hidrovía SA (limited liability company), which is responsible for ensuring the minimum water depth for navigation. A failure event occurred during January 2012 when a bulk cargo carrier ran aground, interrupting fluvial trading for 10 days. Numerical models were applied to simulate hydro-sedimentation processes at the Lower Parana River to estimate dredging costs for a given flow discharge. The resulting function relates the sedimentation rate (i.e. the dredging effort required to keep the present depth for vessel draft) to forcing hydrology conditions. This function and the statistical evaluation of climate scenarios were used to calculate the probability of failure for navigation and the associated cost of channel maintenance. The most appropriate dredging effort was estimated by detecting the minimum total cost (i.e. dredging plus failure) to varying the yearly average discharge and by analysing the sensitivity of the total cost to different degrees of economic impact.

The asymptotic wave form for a space-limited perturbation in open channels

Through a spectral analysis of the linearized Saint Venant equations, it is shown that any space-limited perturbation of an otherwise steady flow evolves into a gaussian form which attenuates smoothly with time, being the result of the contribution of the longest forward spectral components. It is also shown that the shortest forward and backward spectral component contributions, relevant for intermediate times, preserve the initial wave form but attenuate exponentially. Nonlinear effects are briefly discussed. It is postulated that, due to the filtering behaviour of open channels, it is expected that, in addition to a gaussian wave form, only steps and sinusoids should be observed asymptotically.

An Integrated Hydrodynamic-Sedimentologic-Morphologic Model for the Evolution of Alluvial Channels Cross-Sections

Abstract A practical implementation of an integrated hydrodynamic-sedimentologic-morphologic model for the evolution of the cross section of an alluvial channel is presented. It is shown that the driving hydrodynamics can be solved through a series of concatenated 1D hydrodynamic models, which reduces the mathematical complexity to its minimum. A mechanistic model for bed load transport is presented, which can be used readily by practitioners. As part of the morphologic model, a sliding algorithm is proposed to limit the lateral slope which, in particular, efficiently deals with non-cohesive bank erosion. The obtained integrated model is validated by comparing its results with experimental data and previous calculations for evolving channel cross sections. The comparison shows good agreement between these data and the numerical results. To illustrate the model capabilities, numerical experiments are performed on open channels with different radii of curvature to attain stabilized cross-section forms.

Impact of Climate Change on sediment yield from the Upper Plata Basin

In a Climate Change context, it is relevant to assess the potential impacts of variations of climate drivers on sediment yield from basins with high soil exportation rates. This paper develops a methodology, based on the erosion potential method, which was applied to the upper basins of Bermejo and Pilcomayo Rivers (in the Upper Plata Basin), to estimate future trends in sediment yield. Projections for climate drivers (rainfall and temperature) were obtained through four regional climate models, with boundary conditions provided by three global climate models, for the period 2011–2100. Satellite imagery and gridded climate data were used to implement the sediment yield baseline scenario (1961–1990), which was calibrated based on solid discharge data. The model was used to determine projections of sediment yields, as time series of mean annual values for each basin. The analysis was performed on average for three-decadal periods, namely, the scenarios of ‘near future’ (2011–2040), for which current yields are essentially maintained; ‘intermediate future’ (2041–2070), where a reduction trend is detected; and ‘far future’ (2071–2100), with a recovery of yields, which even increase above the baseline scenario rate in the case of the Bermejo Upper Basin.

Pollutant dispersion in water currents under wind action

A closed expression for the dimensionless dispersion coefficient due to differential advection in the presence of wind is obtained, through integration of the associated velocity and concentration vertical profiles. Tsuruyas model is used for the velocity distribution, while the concentration profile is obtained from a simplified pollutant transport equation explicitly deduced. The expression reduces to Elders value (≈5.9) for the particular case of no wind. Higher values arise for winds acting in the same direction as the flow; and vice versa, at least for a practical range of wind velocities. For the case in which the wind acts along a direction non-coplanar with the mean flow direction, it is proposed to apply the analysis independently to the coplanar direction and its normal, where the mean current is, then, null. For this normal direction, the dimensionless dispersion coefficient value is around 24.9. In this way, the theory can be applied to 2D-horizontal pollutant transport modeling.

An experimental study on the continuous breaking of a dam

An experimental investigation of the flow resulting from the continuous breaking of a dam is presented. Based on these experimental results, a coherent and consistent picture is built about the flow characteristics associated with the finite removal time. Plausible speculations about the flow behaviour for the small and large removal time limits, where no experimental data could be obtained, are offered. This constitutes a solid theoretical basis for future mathematical models. A practical method for calculating discharge and water depth hydrographs at the dam site is presented, which can be used to predict the propagation of the resulting wave.

Numerical modeling to define remediation actions for water quality in streams

Numerical modeling can play a key role in establishing an adequate strategy to achieve specific goals regarding water quality in streams. This is illustrated through an application to a big water basin with both rural and urbanized zones, and with degraded water quality. One-dimensional unsteady hydrodynamic modeling, linked to a hydrological model, is used. It is shown that the identification of the main pollutant sources, the estimation of their associated loads, and the assessment of non-point contributions, constitute a fundamental step in the model buildup. Only the relevant mechanisms are included in the water quality model. A challenging statistical criterion is proposed in order to calibrate the model. This model constituted the backbone for the formulation of the integral strategy adopted to remediate the water quality of the river, including the reduction in biochemical oxygen demand loads from big industries and water treatment plants, and the location and design parameters of aeration stations.

Interaction Between Hydraulic and Numerical Models for the Design of Hydraulic Structures

The design of complex hydraulic structures requires its testing through hydraulic models (i.e., reduced scale physical representations). The main practical limitation of hydraulic models are the so called ‘scale effects’, i.e., the fact that only the primary physical mechanisms can be correctly represented, while the secondary ones are distorted. In particular, for free surface flows the gravitational driving forces – primary mechanism – must be correctly scaled in relation to inertia (Froude scaling), leading to an incorrect representation of viscous forces (no Reynolds scaling) – usually the leading secondary mechanism – as the fluid in the hydraulic model is the same as in the prototype (water). Though for most applications Reynolds number effects introduce only small quantitative deviations, which can be readily absorbed within the margin of safety assumed for design, this is not always the case. In fact, they can for example accumulate, in such a way that the effects compete with those arising from the primary mechanism. In those cases, being the Reynolds effects distorted in the hydraulic model, the observed response deviates from the one corresponding to the prototype, thus needing some empirical correction. Numerical modeling is the appropriate tool to help solving in a rigorous way this type of difficulty. A sound numerical model should be able to correctly represent both the primary and secondary mechanisms, i.e., it is not subject to ‘scale effects’. Its main limitations might arise from insufficient resolution, or from inaccurate representation of turbulence effects. The first limitation could be overcome by reducing the spatial step of the numerical grid; the second one, by resorting to more elaborated theoretical approaches. Based on these observations, the following strategy is proposed: (i) the flow in the hydraulic model is numerically simulated, i.e., the dimensions of the hydraulic model are used (thus accounting for the ‘spurious’ scale effects); this constitutes a way of validating the theoretical model; eventually, adjustments in the representation (higher resolution, more elaborated theoretical approaches) are introduced in order to improve the comparison; (ii) the flow in the prototype is numerically simulated, by introducing the dimensions of the prototype in the validated numerical model (i.e., distortion of secondary mechanisms is now avoided); this constitutes the adequate way of extrapolating the results to the prototype dimensions. Two problems (with quite different levels of complexity) are presented as case studies in order to illustrate the proposed approach, both of them associated to the design of the Third Set of Locks of the Panama Canal (communicating the Atlantic and Pacific Oceans), for which the present authors were responsible: (a) the determination of the time for water level