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  • Open access
  • 38 Reads
Stochastic assessment of the influence of reservoir operation in hydrological dam safety through risk indexes
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

Hydrological dam safety assessment methods traditionally assume that the reservoir is full while it receives the design flood. In practice, reservoir management strategy determines the probability distribution of reservoir levels at the beginning of flood episodes. In this study we present a method to economically assess the influence of reservoir management strategy on hydrological dam safety and downstream flood risk. The method was applied to a gated-spillway dam located in the Tagus river basin. A set of 100000 inflow hydrographs was generated through a Monte Carlo procedure, reproducing the observed statistics of main hydrograph characteristics: peak flow, volume and duration. The set of 100000 hydrographs was routed through the reservoir applying the Volumetric Operation Method as flood control strategy. Three different scenarios were studied: initial reservoir level equal to maximum normal level, equal to a maximum conservation level and following the probability distribution of initial reservoir levels. In order to evaluate economically the influence of initial variable reservoir level and compare the three scenarios, a global risk index was applied. The index combines the hydrological risk for the dam, linked to the maximum water level experienced in the reservoir while the flood is routed, and the flood risk in the downstream river reach, linked to the discharge releases from the dam. The results highlighted the importance of considering the fluctuation of initial reservoir level for assessing the risk related to hydrological dam safety. Within the case study, the global risk index reduced its value up to 93 % if variable initial reservoir level is accounted, from 1445.6 x103 to 83 x103 euros.

  • Open access
  • 22 Reads
Effort and performance of the management of water for agriculture under climate change in Southern Europe
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

We evaluate alternatives for the management of water for agriculture under climate change in six representative basins of Southern Europe: Duero-Douro, Ebro, Guadalquivir, Po, Maritsa-Evros and Struma. Management objective is maximizing water availability, understood as the maximum demand that can be satisfied with a given reliability. We focus on water availability for agriculture. For simplification we are assuming only two types of demands: urban and irrigation. Water is first allocated to urban demands following the established priority and the remaining resources are allocated to agriculture. If water availability is not enough to satisfy all irrigation demands, management measures are applied with the goal of achieving a balance between resources and demands. We present an analysis of three possible management measures to face water scarcity in the long term scenario: increasing reservoir storage, improving efficiency of urban water use and modifying water allocation to environmental flows. These management measures are globally evaluated for the selected basins in three representative climate scenarios, comparing their possible range and effectiveness. While in some basins, like Ebro or Struma, measures can significantly increase water availability and compensate for a fraction of water scarcity due to climate change, in other basins, like Guadalquivir, water availability cannot be enhanced with management measures and irrigation water use will have to be reduced.

  • Open access
  • 24 Reads
Energy dissipation structures: Influence of aereation in supecritical flows
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

Adequate design of energy dissipation structures is essential for effective flood control. Theoretical analysis of these structures is complex due to the turbulent nature of supercritical flow and is further complicated by air entrainment. The effect of aeration on water flow has been one of most analysed phenomena during the last decades due to its influence on the behaviour of hydraulic structures. The objective of this study is to characterize the influence of aeration on the boundary friction in supercritical conditions and fully turbulent flows. Our analysis is based on a physical model designed to reproduce these phenomena in the Hydraulics Laboratory of CEDEX (Spain), funded by a research project of the Spanish Ministry of Economy.

The structure consists of a spillway chute 6.5 m high, 0.5 m wide and slope of 75%, followed by a 10 m horizontal channel where the hydraulic jump is confined. Water and air are supplied by a pump and compressors and controlled at the entrance by several valves and flowmeters. This device has a pressurized intake, with 10 m head of additional pressure, which generates the mixture to reproduce different scenarios of emulsified flow, with rates up to 300 l/s of water and 3000 l/min of air. Under these conditions, velocity ranges from 6 to 12 m/s with Froude number between 4 and 11.

Currently, the channel is monitored to measure the velocity profile and air concentration in the flow. The aim of our research is to analyse the relation between air concentration and energy dissipation by friction, in order to link the air concentration rate to the Manning roughness number and other methods of friction quantification. This characterization is based on the velocity and concentration profiles along the vertical axis. We collected data of 12 different scenarios to analyse the relationship between aeration rate and roughness coefficient. We found that greater air entrainment implies acceleration of the flow. Since friction is the main energy dissipation mechanism in the chute, hydraulic structures should be designed to minimize air entrainment and thus enhance energy dissipation by friction.

  • Open access
  • 44 Reads
Temporal and elevation trend detection of rainfall erosivity density in Greece
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

This paper presents certain characteristics of trends in rainfall erosivity density (ED), that have not been so far investigated in depth in the current literature. Global warming is expected to increase the intensity of rainfall in Europe and consequently to increase the soil erosion rates. This potential may have a significant impact, especially on Greece, which is inflicted by the phenomenon of desertification. Raw pluviograph data were acquired from the Greek National Bank of Hydrological and Meteorological Information for 108 stations. Precipitation time series values were cleared from noise and errors and the ratio of missing values was computed. Erosive rainfalls were identified, their return period was determined using Intensity–Duration–Frequency (IDF) curves and erosivity values were computed. A Monte Carlo method was utilized to assess the impact of missing values ratio to the computation of annual erosivity (R) and ED values. It was found that the R values are underestimated in a linear way, while ED is more robust against the presence of missing precipitation values. Indicatively, the R values are underestimated by 49%, when only 50% of the erosive rainfall events are used while at the same time the estimation error of ED is 20%. Using predefined quality criteria for coverage and time length a subset of 18 stations was selected. Their annual ED values, as well as the samples' autocorrelation and partial autocorrelation functions were computed, in order to investigate the presence of stochastic trends. Subsequently, Kendall's Tau was used in order to yield a measure of the monotonic relationship between annual ED values and time. Finally, the hypothesis that ED values are affected by elevation was tested. In conclusion: a) it is suggested to compute ED for the assessment of erosivity in Greece instead of the direct computation of R, b) stationarity of ED was found for the majority of the selected stations, in contrast to reported precipitation trends for the same time period and c) the hypothesis that ED values are not correlated to elevation could not be rejected.

  • Open access
  • 38 Reads
The effect of sample size on bivariate rainfall frequency analysis of extreme precipitation
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

and volume and duration should be taken into account in a multivariate framework. The primary goal of this study is to compare univariate and joint bivariate return periods of extreme precipitation that all rely on different probability concepts in selected meteorological stations of Cyprus. Pairs of maximum rainfall depths with corresponding durations are estimated and compared using annual maximum series (AMS) for the complete period of the analysis and 30-year subsets for selected data periods. Marginal distributions of extreme precipitation are examined and used for the estimation of typical design periods. The dependence between extreme rainfall and duration is then assessed by an exploratory data analysis using K-plots and Chi-plots, and the consistency of their relationship is quantified by Kendall’s correlation coefficient. Copulas from Archimedean, Elliptical and Extreme Value families are fitted using a pseudo-likelihood estimation method, evaluated according to the corrected Akaike Information Criterion and verified using both graphical approaches and a goodness-of-fit test based on the Cramér-von Mises statistic. The selected copula functions and the corresponding conditional and joint return periods are calculated and the results are compared with the marginal univariate estimations of each variable. Results highlight the effect of sample size on univariate and bivariate rainfall frequency analysis for hydraulic engineering design practices.

  • Open access
  • 31 Reads
A regional sensitivity analysis of a multi-variable hydrological model: A case study of a Greek catchment
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

The importance of climate data in hydrological process simulation is widely recognized. The evaluation of the hydrological budget response to climate variability is required especially in water resource management. The present paper illustrates a case study of sensitivity analysis for the hydrological model SWAT (Soil and Water Assessment Tool) using climate data from the Havrias river basin in north Greece. The ERA-Interim reanalysis daily climate data, were used as input data to drive the SWAT model. The SWAT model was calibrated for the period from 1981 to 2000. The sensitivity of the hydrological parameters to the alteration of the climate data was analyzed by using eleven hypothetical scenarios. These scenarios regard different combinations of temperature, wind speed, precipitation and relative humidity. The results show that the changes of precipitation temperature and relative humidity have significant influence in evapotranspiration and percolation (and consequently recharge) in the study region. On the contrary, the wind speed negligibly affects on the hydrological components. Overall, the Havrias river basin hydrological budget is sensitive to shifts in climate data and the utilization of reliable and accurate climate models outputs is necessary in order water managers to be able to build scenarios providing sustainability against the potential future climate change impacts.

  • Open access
  • 24 Reads
Copula bias correction for extreme precipitation in re-analysis data over a Greek catchment
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

The projection of extreme precipitation events with higher accuracy and reliability, which engender severe socioeconomic impacts more frequently, is considered a priority research topic in the scientific community. Although large scale initiatives for monitoring meteorological and hydrological variables exist, the lack of data is still evident particularly in regions with complex topographic characteristics. The latter results in the use of reanalysis data or data derived from Regional Climate Models, however both datasets are biased to the observations resulting in non-accurate results in hydrological studies. The current research presents a newly developed statistical method for the bias correction of the maximum rainfall amount at watershed scale. In particular, the proposed approach necessitates the coupling of a spatial distribution method, namely Thiessen polygons, with a multivariate probabilistic distribution method, namely copulas, for the bias correction of the maximum precipitation. The case study area is the Nestos river basin where the several extreme episodes that have been recorded have direct impacts to the regional agricultural economy. Thus, using daily data by three monitoring stations and daily reanalysis precipitation values from the grids closest to these stations, the results demonstrated that the bias corrected maximum precipitation totals (greater than 90%) is much closer to the real max precipitation totals, while the respective reanalysis value underestimates the real precipitation totals. The overall improvement of the outputs, shows that the proposed Thiessen-copula method could constitute a significant asset to hydrologic simulations.

  • Open access
  • 60 Reads
Hydrometeorological extremes in a warmer climate. A local scale assessment for the island of Crete
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

A future warmer atmosphere indicates that precipitation will increase as a consequence of the higher humidity concentrations. According to the Clausius-Clapeyron relationship precipitation increase by a factor of 7% per degree of warming. However, recent studies have shown that increase in precipitation extremes can exceed this scaling rate. In this regard we focus on the flash flood prone area of Crete by analysing high resolution precipitation records form a dense network of meteorological stations to see if the relationship of precipitation and dew point temperature lies within the Clausius-Clapeyron theory. We then use simulation outputs of a “present day event” from a set of very high resolution (about 2 km grid spacing) convective permitting regional climate models (CPRCM) to see if the models are able to capture intense convection and thus accurately simulate extreme precipitation events over Crete. A second set of simulations for the present day event, but with a perturbation of +2oC, was used to examine intensity changes and to see what similar events might look like in a future weather. We finally focus on a high impact flash flood event occurred on 17 October 2006 and we study changes in hydrological impacts. The developed information can advance local scale knowledge in the context of climate change adaptation and appropriate risk management.

  • Open access
  • 141 Reads
Quantification of the expected changes in annual maximum daily precipitation quantiles under climate change in the Iberian Peninsula
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

Climate model outputs can be used to assess the expected behaviour of extreme precipitations in the future due to climate change. In Europe, the EURO-CORDEX project provides precipitation projections in the future under various representative concentration pathways (RCP), proposed by the Fifth Assessment Report (AR5) of the IPCC. The EURO-CORDEX project regionalised Global Climate Model (GCM) outputs in Europe through a set of Regional Climate Models (RCM).

In this work, 12 combinations of GCM and RCM under two scenarios (RCP 4.5 and RCP 8.5) supplied by the EURO-CORDEX programme are analysed in the Iberian Peninsula. Annual maximum daily precipitation series are considered. Precipitation quantiles for a set of probabilities of exceedance are estimated by using the three-parameter Generalized Extreme Value distribution and L-moments. Precipitation quantiles expected in the future are compared with the precipitation quantiles in the control period, for each climate model. Delta changes in precipitation quantiles are obtained as thresholds exceeded by a percentage of climate models: 50%, 68% and 90%. Three periods in the future are considered: 2011-2040, 2041-2070 and 2071-2100.

An approach based on Monte Carlo simulations is developed, in order to assess the uncertainty from the climate model projections. Expected changes in the future are compared with the sampling uncertainty in the control period. Thus, statistical significant changes are identified. The higher the significance threshold, the fewer cells with significant changes are identified. Consequently, a set of maps are obtained for various thresholds, in order to assist the decision making process in subsequent climate change studies.

The results of this study are currently being applied to implement the Floods Directive in Spain, regarding the preliminary assessment of flood risks.

  • Open access
  • 18 Reads
Vulnerability analysis of flood & waterlogging disaster bearing bodies in highly urbanized areas of China
Published: 15 November 2018 by MDPI AG in 3rd International Electronic Conference on Water Sciences (ECWS-3) session Submission

China is in a period of rapid urbanization, which is forming a number of highly urbanized regional clusters, such as the Pearl River Delta region and the Yangtze River Delta region. The urban development has also profoundly changed the local environment, including local climate conditions, underlying topography, municipal & water conservancy facilities, which increases the risk of flood & waterlogging disasters. Due to the high concentration of population and industries in these areas, the loss of urban flood and waterlogging has also increased at an unprecedented rate. Therefore, it is of great significance to strengthen the analysis and evaluation of the characteristics and impact of flood & waterlogging disasters in highly urbanized areas. Based on the comprehensive analysis of domestic and foreign related studies, this study quantitatively analyzed the vulnerability of flood & waterlogging disaster-bearing bodies in highly urbanized areas of China, considering the characteristics of different disaster-bearing bodies, such as industry and commerce, surrounding agriculture, housing, traffic, lifeline facilities (like water, gas and electricity), etc. The disaster-causing mechanism and response of different disaster-bearing bodies to flood & waterlogging events were analyzed, and the mechanism curve of flood & waterlogging loss rate and submergence degree was established. The study on the quantitative method of vulnerability of flood disaster-bearing bodies in highly urbanized areas will provide theoretical support for dynamic assessment of flood & waterlogging disaster losses and it could also provide technical support for scientific flood control and disaster reduction in China for the future.

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