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"Tsakiris, G. (George)"
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Water quality modelling for rivers and streams
The main objective of the Water Framework Directive in the European countries is to achieve a \"good status\" of all the water bodies, in the integrated management of river basins. In order to assess the impact of improvement measures, water quality models are necessary. During the previous decades the progress in computer technology and computational methods has supported the development of advanced mathematical models for pollutant transport in rivers and streams. This book is intended to provide the fundamental knowledge needed for a deeper understanding of these models.
Book
Drought Risk Assessment and Management
The paper presents an overview of issues related to the estimation of drought severity, the vulnerability of affected systems towards the drought hazards, the assessment of system drought risk and the required preparedness planning against droughts. To face these recurrent and temporary phenomena, a proactive approach is promoted based on technocratic support, systematic organisational and institutional structure and active public participation. Special emphasis is given to the simple and practical approaches, though scientifically sound, for the characterisation of drought episodes and the assessment of drought risk affected systems.
Journal Article
Drought characterisation based on an agriculture-oriented standardised precipitation index
Drought is a major natural hazard with significant effects in the agricultural sector, especially in arid and semi-arid regions. The accurate and timely characterisation of agricultural drought is crucial for devising contingency plans, including the necessary mitigation measures. Many drought indices have been developed during the last decades for drought characterisation and analysis. One of the most widely used indices worldwide is the Standardised Precipitation Index (SPI). Although other comprehensive indices have been introduced over the years, SPI remains the most broadly accepted index due to a number of reasons, the most important of which are its simple structure and the fact that it uses only precipitation data. In this paper, a modified version of SPI is proposed, namely the Agricultural Standardised Precipitation Index (aSPI), based on the substitution of the total precipitation by the effective precipitation, which describes more accurately the amount of water that can be used productively by the plants. Further, the selection of the most suitable reference periods and time steps for agricultural drought identification using aSPI is discussed. This conceptual enhancement of SPI aims at improving the suitability of the index for agricultural drought characterisation, while retaining the advantages of the original index, including its dependence only on precipitation data. The evaluation of the performance of both SPI and aSPI in terms of correlating drought magnitude with crop yield response in four regions of Greece under Mediterranean conditions indicated that aSPI is more robust than the original index in identifying agricultural drought.
Journal Article
Propagating Dam Breach Parametric Uncertainty in a River Reach Using the HEC-RAS Software
Dam break studies consist of two submodels: (a) the dam breach submodel which derives the flood hydrograph and (b) the hydrodynamic submodel which, using the flood hydrograph, derives the flood peaks and maximum water depths in the downstream reaches of the river. In this paper, a thorough investigation of the uncertainty observed in the output of the hydrodynamic model, due to the seven dam breach parameters, is performed in a real-world case study (Papadiana Dam, located at Tavronitis River in Crete, Greece). Three levels of uncertainty are examined (flow peak of the flood hydrograph at the dam location, flow peaks and maximum water depths downstream along the river) with two methods: (a) a Morris-based sensitivity analysis for investigating the influence of each parameter on the final results; (b) a Monte Carlo-based forward uncertainty analysis for defining the distribution of uncertainty band and its statistical characteristics. Among others, it is found that uncertainty of the flow peaks is greater than the uncertainty of the maximum water depths, whereas there is a decreasing trend of uncertainty as we move downstream along the river.
Journal Article
Comparing Various Methods of Building Representation for 2D Flood Modelling In Built-Up Areas
Floods in built-up areas are among the most catastrophic natural hazards mainly due to the high value properties existing in these areas. The most vulnerable areas are the riverine areas with mild terrain which are often encountered in the coastal zone. Due to the mild terrain and the complicated topography caused by buildings, roads and infrastructure, a two-dimensional modelling is required for a realistic simulation of the flood evolvement. In this paper the flood simulation is performed by a recently devised fully dynamic numerical model, the FLOW-R2D, which is based on the two-dimensional Shallow Water Equations solved by the Finite Difference Method and the McCormack numerical scheme. The performance of the model is tested for three alternative representations of the resistance caused by buildings, namely, the reflection boundary, the local elevation rise, and the local increase of the Manning roughness coefficient. The model was run for three different hydrographs and produced time series of water depths and flow velocities in the entire computational domain of the inundated area for each hydrograph. The results of the model for the three alternative building representations and different building alignments were compared with the experimental data available from experiments reported in recent papers. Based on the comparison between numerical and experimental results it was concluded that the reflection boundary method proved to be the most successful building representation for the application of FLOW-R2D. Finally, the data requirements and the required density of the digital terrain model were discussed in relation to the building representation methods.
Journal Article
Implementing Crop Evapotranspiration in RDI for Farm-Level Drought Evaluation and Adaptation under Climate Change Conditions
Agricultural drought is a natural hazard, often leading to significant crop yield losses and jeopardising food security. Climate change is anticipated to increase the duration and the magnitude of drought events, augmenting also their adverse effects. Recent studies, as well as policy initiatives, emphasise the need of proper farm-level management, for efficient mitigation of drought effects and adaptation to climate change. Towards this objective, robust, practical and comprehensible tools should be employed to support decision making process. In this paper, the Crop Reconnaissance Drought Index (CRDI) is introduced, aiming at assisting in agricultural drought analyses, focusing on specific crops. The proposed CRDI is an adjustment of the widely used Reconnaissance Drought Index (RDI), in which the utilised parameter of reference evapotranspiration is replaced by crop evapotranspiration. Along with this amendment, other issues regarding the calculation of CRDI are discussed, such as the selection of appropriate reference periods and methods of crop evapotranspiration assessment. The significance and the advantages of CRDI are illustrated through an application, considering different crops under Mediterranean conditions, in three regions of Greece.
Journal Article
Numerical Model for Two-Dimensional Flood Routing in Complex Terrains
In this paper, a new powerful numerical hydrodynamic in-house model is presented and tested. The model simulates flood routing in two dimensions. It is based on the solution of Shallow Water Equations using the Finite Difference Method according to the explicit McCormack numerical scheme which has shock capturing capability. The innovation of the proposed model lies in the modification of McCormack scheme by incorporating artificial viscosity through a diffusion factor in order to avoid oscillations as proposed by various researchers. Additionally, a threshold of water depth is introduced in order to distinguish the wet and dry cells of the computational domain. The model is capable of producing maps for the inundation extent, water depths and depth-averaged water velocities. Finally, the paper presents extensive testing of the model by comparison with analytical solution, experimental results and with the output of another software package in real world flood simulation studies.
Journal Article
Incorporating aSPI and eRDI in Drought Indices Calculator (DrinC) Software for Agricultural Drought Characterisation and Monitoring
The agricultural sector is vulnerable to extreme phenomena such as droughts, particularly in arid and semi-arid environments and in regions where water infrastructure is limited. Devising preparedness plans, including means for efficient monitoring and timely identification of drought events, is essential for informed decision making on drought mitigation and water management, especially for the water-dependant agricultural sector. This paper presents the incorporation of two new drought indices, designed for agricultural drought identification, in Drought Indices Calculator (DrinC) software. These indices, namely the Agricultural Standardized Precipitation Index (aSPI) and the Effective Reconnaissance Drought Index (eRDI), require commonly available meteorological data, while they employ the concept of effective precipitation, taking into account the amount of water that contributes productively to plant development. The design principles of DrinC software leading to the proper use of the indices for agricultural drought assessment, including the selection of appropriate reference periods, calculation time steps and other related issues, are presented and discussed. The incorporation of aSPI and eRDI in DrinC enhances the applicability of the software towards timely agricultural drought characterisation and analysis, through a straightforward and comprehensible approach, particularly useful for operational purposes.
Journal Article
Analysing Drought Severity and Areal Extent by 2D Archimedean Copulas
Droughts can be considered as multidimensional hazardous phenomena characterised by three attributes: severity, duration and areal extent. Conventionally, drought events are assessed for their severity, using drought indices such as SPI (Standardised Precipitation Index), RDI (Reconnaissance Drought Index), PDSI (Palmer Drought Severity Index) and many others. This approach may be extended to incorporate the modelling of an additional dimension, the duration or the areal extent. Since the marginal distributions describing these dimensions of drought are often different, no simple mixed probability distribution can be used for the bivariate frequency analysis. The copula approach seems to be sufficiently general and suitable for this type of analysis. It is the aim of this paper to analyse droughts as two-dimensional phenomena, including drought severity and areal extent. In this paper, the Gumbel-Hougaard copula from the Archimedean family is used for this two-dimensional frequency analysis. Annual data on historical droughts from Eastern Crete are analysed for their severity and areal extent, producing copula-based probability distributions, incorporating Gumbel marginal probability functions. Useful conclusions are derived for estimating the «OR» return period of drought events related to both severity and areal extent.
Journal Article
Updating IDF Curves Under Climate Change: Impact on Rainfall-Induced Runoff in Urban Basins
The main aims of the present work were to: develop sub-hourly Intensity–Duration–Frequency (IDF) curves considering future climate projections and predict the impact of climate change on rainfall induced runoff in an urban drainage network. The application was undertaken using hourly measured rainfall data from one station. Because data for sub-hourly rainfall durations were not available for the examined station, a relationship between hourly and sub-hourly rainfall depths was established by employing the scale-invariance theory and using 5-min and 10-min measured rainfall data from two nearby stations. IDF curves were developed based on 1-h annual maxima series, using both the Generalized Extreme Value (GEV) and the Gumbel distributions. Prior to the development of the IDF curves, a trend analysis was conducted. Climate change impact on the urban drainage network was assessed based on future IDF curves by employing the Storm Water Management Model (SWMM) for all hydrologic-hydraulic simulations. It was shown that the statistical properties of annual maxima series present a simple scaling property over time scales ranging from 5 min to 1 h. This allowed the development of IDF curves based on 5-min (scaled) annual maxima series using GEV distribution. The results revealed that by year 2100, rainfall intensities of 1-h duration are projected to increase under the mean climate scenario for all return periods examined. Finally, the predicted urban runoff presented significant variability depending on the studied future climate scenario. Peak discharge at the outlet of the urban drainage network ranged from 4.9 m3/s to 8.3 m3/s. With respect to the current situation, the percent change for peak discharge was estimated at -9%, 47% and 69% for the lower, mean and upper climate change scenarios, respectively. In addition, the surface runoff ranged from 22.2 mm to 73.8 mm, according to the examined scenario, with percent difference estimated at 0.02%, 86% and 232% for lower, mean and upper climate change scenarios, respectively.
Journal Article