Iran Eduardo Lima Neto

Sediment redistribution due to a dense reservoir network in a large semi-arid Brazilian basin

Abstract This paper investigates for a 25-year period the sediment distribution in a semi-arid Brazilian basin (2 × 104 km2) with a network containing more than 4000 surface reservoirs. The methodology is based on rating curves and fitted parameters derived from field data. The results showed that suspended load corresponded to 70% of the total sediment yield (148 t km-2 year-1). The relatively low contribution of the suspended load (compared with other semi-arid regions) was attributed to the impact of the numerous upstream reservoirs, which retained 235 t km-2 year-1. The micro (<1 hm3), small (1–10 hm3), medium-sized (10–50 hm3), and large or strategic (>50 hm3) reservoirs responded to, respectively, 5, 17, 30 and 48% of the total sediment retention by the reservoir network. This indicates that retention in the non-strategic reservoirs has a positive impact on water availability, since siltation of the strategic reservoirs would be expected to more than double if only such reservoirs existed. Citation Lima Neto, I. E., Wiegand, M. C. & de Araújo, J. C. (2011) Sediment redistribution due to a dense reservoir network in a large semi-arid Brazilian basin. Hydrol. Sci. J. 56(2), 319–333.

Maximum suction lift of water jet pumps

This paper describes an experimental study on water jet pumps with different diameters and nozzle-to-throat area ratios. The results revealed that the area ratio was an important parameter to characterize the maximum suction lift of the jet pumps, while their diameters had a negligible effect. All jet pumps reached the cavitation regime at a suction lift of about 8 mH2O. In the non-cavitating region, it was found that the higher the area ratio, the higher the maximum suction lift for the same motive pressure head. However, the lower the area ratio, the higher the resistance to enter the cavitation regime. A dimensionless correlation was obtained for the non-cavitating region to describe the maximum suction lift as a function of a modified Thoma number and the area ratio. Curve fitting of experimental data also provided a dimensionless correlation to predict the onset of cavitation. Finally, applications of the results are presented.

Bubble plume modelling with new functional relationships

In this study, a new parameterization is proposed to be incorporated into the classical integral model for bubble plumes. The virtual origin of the flow, the entrainment coefficient and the momentum amplification factor were obtained by adjusting the model to a wide range of experimental data. This allowed to generate functional relationships based on the gas discharge and the water depth. Model simulations using these novel relationships resulted in a better agreement with experimental data than other approaches for bubble plumes, notably for tests under shallow to intermediate water depths. Therefore, the present model is proposed as a simpler approach to predict the flow induced by bubble plumes for a large range of operational conditions.In this study, a new parameterization is proposed to be incorporated into the classical integral model for bubble plumes. The virtual origin of the flow, the entrainment coefficient and the momentum amplification factor were obtained by adjusting the model to a wide range of experimental data. This allowed to generate functional relationships based on the gas discharge and the water depth. Model simulations using these novel relationships resulted in a better agreement with experimental data than other approaches for bubble plumes, notably for tests under shallow to intermediate water depths. Therefore, the present model is proposed as a simpler approach to predict the flow induced by bubble plumes for a large range of operational conditions.

Trade-off between reservoir yield and evaporation losses as a function of lake morphology in semi-arid Brazil

This study investigates the relationships between yield and evaporation as a function of lake morphology in semi-arid Brazil. First, a new methodology was proposed to classify the morphology of 40 reservoirs in the Ceará State, with storage capacities ranging from approximately 5 to 4500 hm3. Then, Monte Carlo simulations were conducted to study the effect of reservoir morphology (including real and simplified conical forms) on the water storage process at different reliability levels. The reservoirs were categorized as convex (60.0%), slightly convex (27.5%) or linear (12.5%). When the conical approximation was used instead of the real lake form, a trade-off occurred between reservoir yield and evaporation losses, with different trends for the convex, slightly convex and linear reservoirs. Using the conical approximation, the water yield prediction errors reached approximately 5% of the mean annual inflow, which is negligible for large reservoirs. However, for smaller reservoirs, this error became important. Therefore, this paper presents a new procedure for correcting the yield-evaporation relationships that were obtained by assuming a conical approximation rather than the real reservoir morphology. The combination of this correction with the Regulation Triangle Diagram is useful for rapidly and objectively predicting reservoir yield and evaporation losses in semi-arid environments.

Tamanho de bolhas de ar formadas por difusores não porosos na água

Experiments are conducted in this study to investigate the size of air bubbles formed from orifice diffusers in a water tank. The size of individual bubbles and the Sauter diameter are evaluated using an image processing technique. The results confirm that the air flow rate and the nozzle diameter are important parameters controlling the bubble size distribution and Sauter diameter. Dimensional analysis is performed to provide a general dimensionless relationship for the bubble Sauter diameter by using an appropriate length scale, which is based on the air flow rate, and the Reynolds number at the nozzle exit. This relationship fits well the experimental data from this study and previous investigations available in the literature, also including experiments for multiple orifice diffusers. Finally, potential applications of the dimensionless relationship obtained here are presented.

Flood damping by reservoirs: proposition of a graphical parametric method

Flood reduction constitutes an important characteristic of surface reservoir design. Thus, to ensure that minimum safety standards are met, the Safety Dams Act was approved in 2010. Many multipurpose surface reservoirs are aging and getting out of the current technical standards. In Ceará State, Brazil, there are several dams built for many decades, with spillways sized by empirical formulae which are currently outdated. This article contributes to improve the understanding of the morphological and hydrological factors involved in the process of damping floods in reservoirs and the development of a simple application method that estimates the damping, from such factors. To do so, hydrological simulations of several scenarios representative of reservoir and hydrographic basins configurations were performed. As a result, the impact of each parameter on flood damping was determined. A graphical method was proposed in which all the morphological parameters of the basin and the reservoir were aggregated into a single dimensionless parameter the Reservoir Damping Index (Φ). With the value of Φ calculated, it is possible to estimate the damping capacity of the reservoir. This method can be used for the design of new reservoirs or for the verification of reservoir spillways already constructed with other methodologies.

ERRO TIPO I E “PODER” DE TESTES DE ADERÊNCIA PARA POSTOS PLUVIOMÉTRICOS NO CEARÁ

RESUMO Devido a curta extensao das series historicas de precipitacao e vazao disponiveis, utilizam-se testes de aderencia para verificar o ajuste de distribuicoes de probabilidade a estas series. Quando aderidas, as distribuicoes podem estimar os valores criticos dos fenomenos e as suas ocorrencias. Sendo assim, realizou-se a analise de quatro testes de aderencia – Kolmogorov-Smirnov, Qui-Quadrado, Cramer-von Mises e Anderson-Darling –, quanto ao Erro Tipo I e ao “Poder”, determinou-se o comportamento de cada teste no ajuste de quatro distribuicoes de probabilidade: Gama, Normal, Lognormal e Gumbel. Tal estudo foi feito via Simulacao Monte Carlo. Os resultados das analises dos testes foram utilizados na definicao de qual distribuicao de probabilidade se ajustava melhor as series historicas de 24 postos pluviometricos no Estado do Ceara, Brasil. Verificou-se que, para o coeficiente de variacao e a extensao dos dados dos postos, o teste com Erro Tipo I mais proximo do valor esperado e com maior “Poder” e o Qui-Quadrado. Palavras-chave: series historicas, fenomenos hidrologicos, series sinteticas, simulacao Monte Carlo. TYPE 1 ERROR AND “POWER” OF ADHERENCE TESTS FOR PLUVIOMETRIC GAUGES IN CEARA ABSTRACT Due to the short extent of available historical time series of rainfall and flow, adherence tests are carried out to verify the fitting of different probability distributions to such time series. When there is adherence, the distributions can estimate the critical values of the phenomena and their occurrence. Thus, the analysis of four adherence tests was conducted including - Kolmogorov-Smirnov, Chi-Square, Cramer-von Mises and Anderson-Darling – considering Type 1 Error and “Power”, in order to determine the behavior of each test regarding the fitting of four probability distributions: Gamma, Normal, Lognormal and Gumbel. Such study has been done via Monte Carlo simulation. The results of the analysis of the tests were used in the definition of which probability distribution was a better fit for the historical time series of 24 pluviometric gauges in the State of Ceara, Brasil. It was verified that, considering the coefficient of variation and the extent of data of the gauges, the Type 1 Error test that is closer to the expected values and has greater “power” is the Chi-Square. Keywords: historical time series, hydrological phenomena, synthetic series, Monte Carlo Simulation

Influence of mass transfer on bubble plume hydrodynamics

This paper presents an integral model to evaluate the impact of gas transfer on the hydrodynamics of bubble plumes. The model is based on the Gaussian type self-similarity and functional relationships for the entrainment coefficient and factor of momentum amplification due to turbulence. The impact of mass transfer on bubble plume hydrodynamics is investigated considering different bubble sizes, gas flow rates and water depths. The results revealed a relevant impact when fine bubbles are considered, even for moderate water depths. Additionally, model simulations indicate that for weak bubble plumes (i.e., with relatively low flow rates and large depths and slip velocities), both dissolution and turbulence can affect plume hydrodynamics, which demonstrates the importance of taking the momentum amplification factor relationship into account. For deeper water conditions, simulations of bubble dissolution/decompression using the present model and classical models available in the literature resulted in a very good agreement for both aeration and oxygenation processes. Sensitivity analysis showed that the water depth, followed by the bubble size and the flow rate are the most important parameters that affect plume hydrodynamics. Lastly, dimensionless correlations are proposed to assess the impact of mass transfer on plume hydrodynamics, including both the aeration and oxygenation modes.

Closure to “Bubble plume modelling with new functional relationships” by IRAN E. LIMA NETO, J. Hydraulic Res. 50(1), 2012, 134–137.

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