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Flash floods are one of the most severe natural hazards worldwide; they can occur within a few minutes or hours, and can move at very high flow velocities, striking with violence and little warning. Early warning of flash floods is extremely important for vital risk mitigation and requires the knowledge of the critical rainfall producing flooding that is typically considered as “warning index”. The small spatial and temporal scales at which flash floods occur make the prediction of critical rainfall challenging, particularly in data-poor environments, where high-resolution weather models and advanced monitoring networks may not be available.In this paper that is based on the Master thesis performed by the first of the authors, we present a methodology to estimate the critical rainfall for flash flooding based on an integrated hydrologic-hydrodynamic model. The model is applied in the area of interest for various combinations of rainfall - soil moisture conditions to (1) determine input hydrographs that are used as boundary conditions for the hydrodynamic model, and (2) to calculate the distribution of the “critical hazard” in the cells of the 2D computational domain. In the present work, we define critical hazard combining the main hydrodynamic characteristics that are water depth and flow velocity. We import all calculated “critical hazard” values into a GIS-based database. We apply the integrated model in the town of Mandra in Greece, which was stricken by a flash flood on the 15th of November 2017 that was the third most disastrous “November” flood in Attica, for a relatively large number of scenarios-combinations of rainfall - soil moisture conditions to determine critical rainfall values and to compare them with values found in the literature from similar catchment areas.

What would happen in Mediterranean rivers and streams if warming but not drying occurred? We examined whether the delivery of environmental flows within a warming climate can maintain suitable macroinvertebrate habitats despite warming. A two-dimensional ecohydraulic model was used to (1) simulate the influence of water temperature and flow on macroinvertebrates by calculating habitat suitability for 12 climate change scenarios and (2) identify the mechanism by which macroinvertebrate assemblages respond to warming. The results suggest that not all watersheds will be equally influenced by warming. The impact of warming depends on the habitat conditions before warming occurs. Watersheds can, thus, be categorized as losing (those in which warming will degrade current optimal thermal habitat conditions) and winning ones (those in which warming will optimize current sub-optimal thermal habitat conditions, until a given thermal limit). Our models indicate that in losing watersheds, the delivery of environmental flows can maintain suitable habitats (and, thus, healthy macroinvertebrate assemblages) for up to 1.8–2.5 °C of warming. In winning watersheds, environmental flows can maintain suitable habitats when thermal conditions are optimal. Environmental flows could, thus, be used as a proactive strategy/tool to mitigate the ecological impacts of warming before more expensive reactive measures within a changing climate become necessary.

We present a teaching methodology to answer the question ‘How can we link teaching with research in our engineering courses?’ Due to the combination of hybrid Problem Based Learning with Research Tutored/Oriented Teaching, the methodology enables the students to acquire the skills of ‘problem solving’, ‘working in teams’, ‘analytic’, ‘written communication’, ‘planning my own work’, ‘organization’, ‘learning more deeply’, ‘tackling with unfamiliar problems’ and ‘innovation’; the last two are directly related to the Scientific Research Process and are considered important for modern engineers. We apply the methodology in the course ‘Ecological Modelling of Surface Waters’ that we teach in TUM and NTUA following the teaching sequence ‘Scope of modelling – Background knowledge – Simple model – Tutorial – Real case’. Due to its simplicity, the proposed methodology can be easily applied by University researchers, who are usually not familiar with the theoretical aspects of learning.

We propose a systematic procedure that combines computational fluid dynamics (CFD) modeling and experimental work to answer two research questions that are usually posed by researchers and managers of water treatment plants: ‘Is the effect of wind on settling tanks important?’ and ‘How can we determine this effect in our settling tanks?’ We apply this procedure in the water treatment plant of Aharnes, Athens to derive the following conclusions. (1) The effect of wind increases with increasing co-current wind velocity, increasing settling velocity and decreasing flow rate. (2) In windy steady-state flow conditions, the degree of complexity and three-dimensionality of the flow field that is observed in calm conditions is reduced and the removal efficiency decreases from 85.1 in calm conditions to 82.0%. Predicted efficiencies for constant and variable inlet solids' concentrations compare favorably with measurements. (3) In windy, transient flow conditions, field data show that the effect of wind on the tank's efficiency can be very pronounced and within the first half hour of the windy period the efficiency decreases to approximately 55%; the present model does not capture this effect, because it cannot simulate the sludge layer and the subsequent re-suspension of the settled solids.

Arachthos River is the largest river in Epirus and the 8th largest in Greece; it is 110 km long and its drainage area is 2209 km2. After emanating from Pindus mountains (near Metsovo), it enters into the Pournari Reservoir in Arta, passes through Arta and discharges into the Ambracian Gulf near Kommeno. Arachthos River prevents flooding of the city of Arta and supplies water to most of Epirus.The design of flood protection works in Arachthos River is currently in progress; it is performed by a consortium of Greek Consulting Firms for the Ministry of Infrastructure and Transportation. In the present work, we examine the effect of Flood Retention Ponds on the inundation area and the subsequent flood risk for the city of Arta. The Flood Retention Ponds are constructed immediately downstream of the Pournari Reservoir and 5600 m upstream of the historic Bridge of Arta; their exact locations were identified after a preliminary study and field surveys. Firstly, we performed the design of the Flood Retention Ponds, based on international standards and specifications found in the international literature; then, we performed hydrodynamic calculations using the Hydrologic Engineering Center's-River Analysis System (HEC-RAS) 1D/2D with and without the Flood Retention Ponds. Thirdly, we compared the calculations and the corresponding inundation areas and derive conclusions on the effect of Flood Retention Ponds.

The region of Attica is characterized by a relatively large number of floods over a long period of time. The flash flood in Mandra on the 15th of November 2017 was the third most disastrous “November” flood in Attica; most of the population was affected by the flood (23 deaths and 24 people injured), while basements and ground floors of buildings in the town were seriously impacted.The two main streams that pass through the town of Mandra are Soures and Agia Aikaterini, whose catchment area is equal to 23.0 and 22.0 km2, respectively. These streams are characterized by significant morphological changes due to the intensive construction activities in the greater area that resulted in a dramatic decrease of their available cross-sectional areas and the occurrence of floods even at low flow rates.We applied the HEC-RAS 1D/2D to model the flash flood using a high resolution Digital Surface Model (DSM) and topographic survey data, to obtain the most accurate representation of the area of Mandra. Moreover, we imported to the model all technical works, such as culverts and bridges that affect the flow. For the model calibration, we employed (a) videos, photographs, information from the local population and satellite images to determine the inundation area and (b) in situ measurements of the flood water depth, in various locations within the town of Mandra. The results of the model were compared with calculations performed within a previous Thesis in 2018 using TELEMAC-2D.The present paper is based on the Master Thesis of the first author; it was performed within the project “National Network on Climate Change and its Impacts (CLIMPACT)” of the General Secretariat of Research and Technology.

Nowadays, the aquatic biodiversity is highly under pressure due to anthropogenic changes of the rivers such hydraulic structures changing the diversity of flow and aquatic fauna as well as sediment continuity. This can have severe consequences on the fish population in the river reach. Fish are strongly depending on a certain substrate composition throughout all their life stages. Juveniles for example are depending on a certain availability of shelter in the substrate in order to survive this stage.Therefore, we investigate the effects of changes in the sediment composition at a hydropower plant in Switzerland on the availability of potential shelter for juvenile fish. By utilizing the observed correlation between parameters describing the fine tail of a riverbed’s grain size distribution and shelter abundance for juvenile Atlantic salmon, we predict the available shelter in a river reach by using a 3D hydrodynamic numerical model directly coupled to a morphodynamic model. The initial substrate composition was assumed to be spatially uniform, its parameters based on a grain size distribution curve derived from collected sediment samples.This model can now be used for habitat improvement scenario modeling. Based on the assumption that a specific mixture of sediment coming from upstream travelling through the river reach will positively influence the potential shelter availability, different scenarios can be investigated. The baseline for comparison was the simulation of the bed changes without any sediment supply from upstream. The baseline discharge was set to 100 m3 /s and was applied for 24 hours. The resulting bed changes create a map of the potential shelter availability of this grain size mixture. Then, two scenarios with sediment inflow from the upstream boundary were simulated. One coarse and one fine mixture of sediment were chosen as inputs, with the goal of investigating their impact on shelter abundance. The former designed to have a positive effect while the latter expected to reduce interstitial voids in the substrate and have a negative effect on available shelter.The investigation is conducted as part of the EU Horizon 2020 funded project FIThydro (funded under 727830)

AbstractThe flash flood in Mandra on the 15th of November 2017 was the third most disastrous “November” flood in Attica; it was characterized by heavy sediment and debris transport that can be easily observed in Figure 1.We applied the Hydrologic Engineering Center's-River Analysis System (HEC-RAS) to model sediment transport using the Ackers-White sediment transport equation that is engraved in HEC-RAS to analyze sediment transport characteristics. The required input data were based on a limited number of available studies, which mainly include a survey performed by the Hellenic Centre for Marine Research in the coastal area of the Elefsis Bay where sediments were deposited after the catastrophic event. We compared the results of the model with calculations performed within a previous Thesis in 2018 using TELEMAC-2D and SISYPHE.The present paper is based on the Diploma Thesis of the first author; it was performed within the project “National Network on Climate Change and its Impacts (CLIMPACT)” of the General Secretariat of Research and Technology. Figure 1. The greater area of Mandra (a) before and (b) after the flood event

In the present work, we verified a 3D computational fluid dynamics model for vertical slot fish-passes (VSFs) that employs the renormalization-group k-epsilon turbulence model (RNG KE) and the volume of fluid (VOF) method. We compared model calculations with experiments in two pool designs T1 and T2 of an experimental VSF and with 2D calculations using the shallow water equations (SWE) and the standard k-epsilon (2D SKE) model. Calculations of the 3D model showed (1) good agreement with experiments and 2D calculations in predicting mean flow velocities, (2) better performance in the determination of the water surface in the VSF, which is attributed to the accurate VOF method, (3) superior prediction of turbulence characteristics than the 2D model, which is due to the 3D RNG KE model that overcomes the problem of turbulence overestimation of the 2D SKE model, and to the fact that the 3D model takes into account the 3D features of the flow in the fish pass. Moreover, the present 3D calculations showed that the common assumptions in VSFs that (1) the flow is 2D, and (2) the simulation of 4 pools of a VSF is sufficient to obtain satisfactory results, are not always valid. Flow can be considered as 2D only in pool design T2 and for certain geometries and flow characteristics in pool design T1; while, eventually, all the pools of a fish pass need to be modeled to ensure accurate results. Finally, the present work illustrates the need to perform fish experiments simultaneously with flow experiments.

Although various methods are currently available for modelling the habitat preferences of aquatic biota, studies comparing the performance of data-driven habitat models are limited. In this study, ...

Hydraulic-habitat and biological data were integrated within a twofold-purpose study: (i) to investigate the response of freshwater macroinvertebrates to a rainfall-induced high flow event of moderate magnitude and (ii) to derive hydroecological relationships between habitat variability and macroinvertebrate microdistribution. 142 microhabitats (unique combinations of flow velocity, water depth and substrate type) allocated in four sites of no or very minor anthropogenic influence were sampled and analyzed, before and after the event. Freshwater macroinvertebrates were additionally collected and specific community metrics were derived. To identify possible pre- and post- impact benthic community differences, independent sample t-tests were applied, while Boosted Regression Tree models were developed to quantify the response of macroinvertebrates to flow alteration. Macroinvertebrate abundance, taxonomic richness, EPT richness and diversity decreased significantly by 90%, 60%, 50% and 25% respectively between the pre- and post- impact microhabitats. The relative abundance of macroinvertebrate predators and passive filter feeders increased after the event, mainly in specific substrate types (boulders and large stones), which served as flow refugia, maintaining less degraded (compared to finer substrates), still heavily impacted, benthic communities. According to the hydroecological analysis, the high flow event exerted the strongest impact on all macroinvertebrate metrics. Optimal (suitable) ranges of the hydraulic-habitat variables for benthic macroinvertebrates were identified (optimal flow velocity from 0.3 m/s to 0.7 m/s, optimal water depth at 0.2 m), while boulders and large stones were the most suitable substrate types. The aforementioned data provide valuable information for the provisioning of biologically-derived environmental flows and an essential input of hydrodynamic habitat models to facilitate the selection of the optimal environmental flow scenario towards ensuring the integrity of aquatic ecosystems downstream of anthropogenic activities provoking hydrological alteration.

Fishway research has long focused on post-establishment assessments to optimize fish passage efficiency. Model-based optimization with the use of hydrodynamic-habitat models (HHMs) has rarely been employed to investigate passage suitability based on key hydraulic variables. In this study, a vertical slot fishway was designed, the hydraulic properties of twelve types that varied by slope, slot width and pool size were simulated using a two-dimensional HHM, and the suitability of macroinvertebrates and of seven fish groups, encompassing various guilds and size classes, was assessed for each fishway type at seven discharge scenarios, ranging from 0.1 m3/s to 1.3 m3/s. Suitability was primarily influenced by water depth. At optimal depths, suitability was fine-tuned by flow velocity to different optima per biotic group based on fixed interplays between discharge, slope and slot width: water depths were higher at high discharges, low slopes and narrow slots; flow velocities were higher at high discharges, high slopes and wide slots. Our ecologically optimal, multispecies, vertical slot fishway, had 5% slope and 30-cm wide slots, with discharges shifting between 0.15 m3/s and 0.6 m3/s to balance the needs of all species and life stages studied. Yet, even this optimal fishway could not guarantee 100% suitability for all species. The overall suitability was moderate (approx. 50–60%), higher at the pools and lower at the slots, and critically influenced by the type of substrate. We conclude that technical fishways cannot be simultaneously ideal for all species and life stages of a river reach/basin, due to the often-contrasting habitat preferences that need to be adapted to fixed interplays of hydraulic conditions. Within these limitations, however, model-based ecological pre-optimization is crucial for optimizing and deploying fishways that will guarantee suitable passage routes for a wide range of species and life stages, ultimately restoring the hydromorphological and ecological connectivity of dammed rivers.

An integrated model is presented for the calculation of the characteristics in the intermediate field region of brine discharges from reverse osmosis desalination plants into unstratified stagnant coastal waters. The model consists of the near field model Modified CorJet Model and the far field model, which are interconnected via a coupling algorithm. This algorithm has been developed to simulate the flow and concentration characteristics of negatively buoyant jets (NBJ) after their impingement on the bottom. The coupling method was developed to be active according to literature, however further work and investigation is needed to be applicable for NBJ discharged into other ambient environments and especially in cases where the background values of ambient flow and concentrations affect the NF values and vice versa. The integrated model was validated with data from the literature as well as with data from experiments conducted in this study showing a good agreement. The coupling algorithm was also compared to other coupling techniques used in the literature for NBJ discharges showing better estimations of the experimental data.

A 2-D CFD model is applied to investigate the effect of co-current and counter-current wind velocities of up to 7.5 m/s in the settling tanks of EYDAP Water Treatment Plant of Aharnes, in Greece, and the following conclusions are drawn: (1) without wind, the flow field is characterized by a large recirculation region with significant short-circuiting, while for windy conditions a two-layer flow is observed, in which the surface layer follows the wind direction. The suspended solids’ concentration fields strongly depend on the corresponding flow fields. (2) Calculated local removal efficiencies, without wind, near water surface show a satisfactory agreement with measurements. When the sludge removal mechanism is operating, the calculated efficiency without wind is equal to 83.1%; this value is lower than the experimental value (86.0 ± 1.0%) and the predicted value by a 3-D model (85.7%). When co-current wind velocity increases, short-circuiting increases and the efficiency decreases; however, not n...

River restoration with the use of in-stream structures has been widely implemented to maintain/improve physical habitats. However, the response of aquatic biota has often been too weak to justify the high costs of restoration projects. The ecological effectiveness of river restoration has thus been much debated over claims that large-scale environmental drivers often overshadow the potential positive ecological effects of locally placed in-stream structures. In this study, we used a two-dimensional hydrodynamic-habitat model to evaluate the ecological effectiveness of habitat restoration with the use of in-stream structures in various water discharges, ranging from near-dry to environmental flows. The habitat suitability of benthic macroinvertebrates and of three cyprinid fish species was simulated for six restoration schemes and at four discharge scenarios, and was compared with a reference model, without in-stream structures. We found that the ecological response to habitat restoration varied by species and life stages, it strongly depended on the reach-scale flow conditions, it was often negative at near-environmental flows, and when positive, mostly at near-dry flows, it was too low to justify the high costs of river restoration. Flow variation was the major environmental driver that our local habitat restoration schemes attempted -but mostly failed-to fine-tune. We conclude that traditional river restoration, based on trial and error, will likely fail and should be ecologically pre-optimized before field implementation. Widespread use of in-stream structures for ecological restoration is not recommended. However, at near-dry flows, the response of all biotic elements except for macroinvertebrates, was positive. In combination with the small habitat-suitability differences observed among structure types and densities, we suggest that sparse/moderate in-stream structure placement can be used for cost-effective river restoration, but it will only be ecologically effective -thus justifying the high implementation costs-when linked to very specific purposes: (i) to conserve endangered species and (ii) to increase/improve habitat availability/suitability during dry periods, thus proactively preventing/reducing the current and future ecological impacts of climate change.

Generic habitat suitability criteria (HC) are often developed from spatially and temporally variable hydroecological datasets to increase generality, cost-effectiveness, and time-efficiency of habitat models. For benthic macroinvertebrates (BMIs), however, there is no prior knowledge on the spatiotemporal variation in their habitat preferences and how this may be reflected in the final environmental flow (e-flow) predictions. In this study, we used a large, spatiotemporally variable BMI-hydroecological dataset and developed generic, local, and season-specific subsets of HC for three seasons and two river types within various data pre-treatment options. Each subset was used to train a fuzzy habitat model, predict the habitat suitability in two hydrodynamically-simulated river reaches, and develop/compare model-based e-flow scenarios. We found that BMIs shift their habitat preferences among seasons and river types, consequently, spatiotemporally variable e-flow predictions were developed, with the seasonal variation being greater than the typological one. Within this variation, however, we found that with proper data pre-treatment, the minimum-acceptable e-flows from the generic models mostly (65&ndash, 90%) lay within the acceptable e-flows predicted by the local and season-specific models. We conclude that, within specific limitations, generic BMI-HC can be used for geographically extended, cost-effective e-flow assessments, compensating for the within-limits loss of predictive accuracy.

[EN] We propose the novel integrated modelling procedure 3H-EMC for the determination of the environmental flow in rivers and streams; 3H-EMC combines Hydrological, Hydrodynamic and Habitat modelling with the use of the Environmental Management Classes (EMCs) that are defined by the Global Environmental Flow Calculator. We apply 3H-EMC in the Sperchios River in Central Greece, in which water abstractions for irrigation cause significant environmental impacts. Calculations of the hydrodynamic-habitat model, in which the large and the small chub are the main fish species, suggest discharge values that range from 1.0 m3/s to 4.0 m3/s. However, hydrological modelling indicates that it is practically difficult to achieve discharges that are higher than approximately 1.0-1.5 m3/s. Furthermore, legislation suggests significantly lower values (0.4-0.5 m3/s) that are unacceptable from the ecological point of view. This behaviour shows that a non-integrated approach, which is based only on hydrodynamic-habitat modelling does not necessarily result in realistic environmental flows, and thus an integrated approach is required. We propose the value of 1.0 m3/s as the "optimum" environmental flow for Sperchios River, because (a) it satisfies the habitat requirements, as expressed by the values of weighted useable area that are equal to 2180 and 1964 m2 for the large and small chub, respectively, and correspond to 82 and 95% of their respective maximum values, (b) it is consistent with the requirements of Environmental Classes A and B, whose percentiles are higher than 75% for discharge (77.2%) and for habitat availability (>83.5% for the large chub and >85.0% for the small chub), (c) it is practically achievable from the hydrological point of view, and (d) it is higher than the value proposed by the Greek legislation. The proposed modelling approach can be applied to any river or stream using the same or similar modelling tools, which should be linked via suitable coupling algorithms., Hydraulic field measurements were performed within the framework of a research project by the Hellenic Centre of Marine Research (HCMR) entitled KRIPIS “Development of an integrated management system for river basin, coastal and marine zones” (http://imbriw.hcmr.gr/en/). Fish habitat data were collected within the framework of the ECOFLOW research project (www. ecoflow.gr). A part of the modelling work has been performed, while the first author was a visiting professor at the Technical University of Munich (TUM); thanks are due to the Bavarian State Ministry of Education and Cultural Affairs and the German Academic Exchange Service (DAAD).

A computational fluid dynamics model is presented for the calculation of the flow, suspended solids, and tracer concentration fields in the settling tanks of the water treatment plant of Aharnes, an important component of the water supply system of the greater area of Athens, Greece. The model is applied to investigate the expected negative effect of the wind on the hydraulic and settling performance of the tanks and to evaluate the improvement resulting from the installation of one and two baffles; the wind is modeled using a simple and very conservative approach that involves the setting of a constant horizontal flow velocity on the free surface. The model is calibrated and verified with field turbidity measurements. Calculations show that the effect of wind on the flow field and the hydraulic efficiency is strong, with the creation of massive re-circulation areas with intense mixing and high short circuiting; however, the effect of wind on the settling performance of the tanks is not pronounced. The removal efficiency of the tanks, which is 72.48% in calm conditions, is reduced to 68.07% for windy conditions; moreover, it increases to 70.00 and 71.04%, when one or two baffles are installed, respectively.

Experimental measurements of concentrations were performed for vertical dense jets discharged downwards to a horizontal or a sloping bottom, aiming at: (a) determining the dilution at the impingement point on a solid bed and assess the influence of the boundary, and (b) evaluating the additional dilution achieved within the density current, which forms in the vicinity of the impingement point. It is found that the presence of the solid boundary affects the axial dilution of the approaching jet up to a distance of hD, which is well correlated to the source height and the length scale LM. The dilution at the impingement point on the boundary is reduced appreciably compared to that of a boundary-free jet at the same location. The additional dilution within the density current on a sloping bottom is well correlated to the dimensionless distance from the impingement point. Dimensionless empirical equations are proposed for the dilution at the impingement point and in the density current near the bed, i...

A simple modification is introduced into the integral model (IM) CorJet in an effort to predict better the characteristics of negatively buoyant jets (NBJ) discharged in a stationary ambient. Although this modification was developed for the CorJet model, it can be applied to every IM which employs the entrainment hypothesis. The detrainment of fluid from the main flow is taken into account by inserting a coefficient “p” into the conservation equations of volume, buoyancy and tracer mass flux. This coefficient expresses the ratio of the specific mass flux of the detrained fluid to the net specific mass flux entrained to the NBJ. The value of p is assumed constant along the jet trajectory and up to the maximum jet height, becoming zero thereafter. Results show that the modified CorJet model (MCM) predicts reasonably well experimental data from the literature and data from experiments performed in this work. The optimal value of p and therefore the detrained fluid from the main NBJ flow was found to decrease as the jet initial densimetric Froude number increases.

A methodology is presented and applied to assess the oil contamination probability in the Gulf of Patras and the environmental impacts on the environmentally sensitive area of Mesolongi – Aitoliko coastal lagoons, and to examine the effectiveness of response systems. The procedure consists of the following steps: (1) Determination of the computational domain and the main areas of interest, (2) determination of the drilling sites and oil release characteristics, (3) selection of the simulation periods and collection of environmental data, (4) identification of the species of interest and their characteristics, (5) performance of stochastic calculations and oil contamination probability analysis, (6) determination of the worst-cases, (7) determination of the characteristics of response systems, (8) performance of deterministic calculations, and (9) assessment of the impact of oil spill in the areas of interest. Stochastic calculations that were performed for three typical seasonal weather variations of the year 2015, three oil release sites and specific oil characteristics, showed that there is a considerable probability of oil pollution that reaches 30% in the Mesolongi – Aitoliko lagoons. Based on a simplified approach regarding the characteristic of the sensitive birds and fish in the lagoons, deterministic calculations showed that 78–90% of the bird population and 2–4% of the fish population are expected to be contaminated in the case of an oil spill without any intervention. The use of dispersants reduced the amount of stranded oil by approximately 16–21% and the contaminated bird population of the lagoons to approximately 70%; however, the affected fish population increased to 6–8.5% due to the higher oil concentration in the water column. Mechanical recovery with skimmers “cleaned” almost 10% of the released oil quantity, but it did not have any noticeable effect on the stranded oil and the impacted bird and fish populations.

An integrated model is presented for the calculation of the discharge of thermal effluents from power plants into coastal waters; the model consists of the near field model CorJet and the far field model FLOW-3DL that are interconnected via an active coupling algorithm. Firstly, the model is validated using experimental data; moreover, calculations are compared with passive coupling simulations to identify the dominant differences among these methods. Then, the model is applied to simulate the single-port thermal discharge originating from a thermal power plant to the non-stratified coastal waters in the region of Mantoudi in Evia, Greece. Model predictions are compared with CORMIX far field estimations and calculations employing passive coupling. Calculations verify the need for the application of an integrated active model. The detailed information for the coupling algorithm that is contained in this paper, including its difficulties and their resolution, permits its implementation to any active coupling between practically any near field with any far field model.

This paper presents a methodology for the optimization of the operation of an alternating oxidation ditch system, performing carbon oxidation–nitrification, denitrification and settling, through the use of a mathematical model and pilot testing. Measurements have been made in a pilot plant located in the Sanitary Engineering Research and Development Center of E.Y.D.A.P at Metamorphosis-Athens. Also, a mathematical model consisting of 1-dimensional mass balance (convection–dispersion) equations, based on the IAWPRC activated sludge model No. 1 was used. After the calibration and verification of the model with the experimental data, the optimization of the system’s performance was achieved by testing operational cycles with different time lengths. Since carbon removal is way under the 91/271 E.U. directive’s upper limits and since the ditch system has not been designed for phosphorous removal, the limits for nitrogen removal have been used for comparison. In this work’s example, from the eight oper...

Based on the experience of the authors and the literature, a learning procedure is presented that provides the required knowledge on water quality mathematical modeling in a educational, critical and practical manner; it consists of three steps: (1) brief presentation of the basic modeling theory that is applied using a simple problem, which can be selected from a series of general water quality problems contained in this work, (2) construction of a simple computer program in EXCEL to solve, simulate and examine this problem interactively, and (3) brief presentation of the basic Computational Fluid Dynamics theory and its application in a real life problem; the present study focuses on the first two steps. The proposed procedure also includes 25 questions and answers on scientific, educational and practical matters aiming at making the presentation interactive and interesting. It is currently being applied in two courses at the National Technical University of Athens and the Technical University of Munich; it can be easily modified to be used as a teaching–learning tool in a specific standard or e-learning course.

Complex spatio-temporal environmental problems call for an integrated assessment of environmental systems due to their interdisciplinary nature. Within this context, the approach of integrated water resource management (IWRM) at the catchment scale, which is at the heart of advanced European environmental policy such as the water framework directive (WFD), argues for integrated modelling tools and approaches. This paper examines recent technological developments that facilitate integrated modelling and focuses on a novel approach, the open modelling interface (OpenMI), which allows models built by different developers to communicate at runtime. The individual models, when made compliant with the OpenMI standard, can be considered as ‘components' of an integrated model and seamlessly exchange data. Two OpenMI-enabled integrated modelling case studies are presented and their results critically discussed. The paper concludes with opportunities and challenges towards the design of flexible, component-based models supporting integrated environmental policy in general and the WFD implementation in particular.

A three-dimensional integrated mathematical model, ModEling aquacultures-Sea environment Interactions (MESI), is presented to describe the interactions between aquacultures and the sea environment and to evaluate the environmental effects on the coastal waters due mainly to the non-utilized fish food originating from the aqua- cultures. The model consists of three sub-models: (1) a fish growth and nitrogen and phosphorus transformation sub-model for the calculation of the growth of the fishes and the generated loads of pollutants, (2) a hydrodynamic sub-model for the determination of the flow field in the coastal region and (3) a water quality model to determine the concentration contours of the water quality variables in the coastal region. The application of the model to a coastal area of simple geometry containing an aquaculture demonstrates its ability to evaluate the effects of the aquaculture on the coastal waters.

Summary Coupled modelling of surface and subsurface systems is a valuable tool for quantifying surface water–groundwater interactions. In the present paper, the 3-D non-steady state Navier–Stokes equations, after Reynolds averaging and with the assumption of a hydrostatic pressure distribution, are for the first time coupled to the 3-D saturated groundwater flow equations in an Integrated suRface watEr–grouNdwater modEl (IRENE). A finite-difference method is used for the solution of the governing equations of IRENE. A semi-implicit scheme is used for the discretisation of the surface water flow equations and a fully implicit scheme for the discretisation of the groundwater flow equations. The two sets of equations are coupled at the common interface of the surface water and groundwater bodies, where water exchange takes place, using Darcy’s law. A new approach is proposed for the solution of the coupled surface water and groundwater equations in a simultaneous manner, in such a fashion that gives computational efficiency at low computational cost. IRENE is verified against three analytical solutions of surface water–groundwater interaction, which are chosen so that different components of the model can be tested. The model closely reproduces the results of the analytical solutions and can therefore be used for analysing and predicting surface water–groundwater interactions in real-world cases.

A computational fluid dynamics (CFD) model is presented and applied in the design of the secondary settling tanks of Psyttalia Wastewater Treatment Plant in Athens, Europe’s largest sewage treatment facility. The tanks are of the Gould Type II consisting of the following regions: an inlet-flocculation chamber with an inlet baffle, two zones of settling separated by an intermediate baffle, an outlet region, and a sludge collection region. The number of tanks and their dimensions were determined with an empirical design procedure. Then, theoretical considerations, information from similar existing tanks, and preliminary CFD calculations were combined to determine the dimensions of the main regions and the positions of the baffles. Finally, detailed CFD calculations were performed to examine the performance of the tanks for various design conditions. Computations showed that the flow in the inlet-flocculation region was completely mixed; while in the settling regions a “three-layer” structure with relatively...

The hydraulic efficiency of water process tanks can be significantly improved by performing simple modifications in the initial geometries of the tanks. This concept was applied in the main tanks of the water supply network of Athens, which are used for disinfection. Various modified geometries were formulated by “inserting” guiding walls in these tanks, a very common, simple and inexpensive modification. The hydraulic efficiency of the tanks was assessed with a CFD model, which calculated the flow fields and the flow through curves (FTCs) for the initial and the modified geometries. From the comparison of the results the following conclusions were drawn: (1) the flow for the initial geometries was characterized by high levels of short-circuiting, large re-circulation regions and high degrees of mixing, and (2) the use of guiding walls created flow fields in the tanks with significant volumes of “plug flow”, reduced short-circuiting, smaller re-circulation regions and thus less mixing. The modified geometries showing the best hydraulic efficiency were proposed for construction. This procedure, which can be applied in the all types of water tanks, resulted in significant cost savings, because the modifications were examined with the CFD model, prior to their actual implementation in the real tanks.

A computational fluid dynamics (CFD) model was used to evaluate the thermal comfort conditions in the indoor stadium of the Galatsi Arena, which hosted the sports of rhythmic gymnastics and table tennis during the Olympic Games “Athens 2004”. The CFD code CFX was applied to calculate the 3D airflow and temperature fields in the Arena for various values of temperatures (Tin) of conditioned inlet air. Calculated mean velocities and temperatures were used to determine the thermal comfort indices predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) and to evaluate the thermal conditions in the various regions of the Arena. Calculated PMV and PPD values showed that thermal conditions in the Galatsi Arena were very satisfactory for Tin = 16 °C; only a small percentage (

A 3-D CFD model based on the Volume of Fluid method to calculate the free surface was used to study supercritical flow in gradual open channel expansions. The model was first applied to a modified Rouse expansion, for which extensive experimental data are available. A detailed comparison between the numerical and the experimental results concerning surface profiles, velocities, free surface uniformity, and shock front location showed satisfactory agreement, especially for lower supercritical Froude numbers. The model was subsequently applied to optimize and finalize the design of a compound expansion structure with variable bottom elevation, piers, and inner walls. CFD calculations showed that flow conditions in this structure are satisfactory, provided that bottom ramps are used to ensure a uniform variation of the bottom elevation

Thermal comfort conditions in the amphitheater of the MPC (Main Press Center), which was used in the Olympic Games of Athens 2004, are evaluated with Computational Fluid Dynamics (CFD) modeling. A CFD code is applied to calculate three-dimensional flow velocities and temperatures in the amphitheater for various values of temperatures of conditioned inlet air. Flow velocities and temperatures are used to determine the values of the comfort indices: PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied). Calculated PMV and PPD values show that thermal conditions in the amphitheater of the MPC are very satisfactory for temperature of conditioned inlet air equal to 16.5 °C; only a small percentage (less than 8.5%) of the occupants is expected to be slightly uncomfortable.

The Athens 2004 Olympic Games were completed successfully in open and indoor stadiums in Greece. Prior to the Games, the Ministry of Culture launched a study on the evaluation and improvement of the indoor conditions safety, air quality, and comfort in the newly developed sports arena complexes and stadiums for Athens 2004. This study was performed with the use of Computational Fluid Dynamics CFD models. CFD models have been increasingly used in the last 15–20 years to evaluate comfort conditions thermal and draught , smoke conditions, and indoor air quality. There is a considerable amount of research work dealing with the application of CFD models in indoor environments, mainly offices and residential areas. However, there are only a few published studies in indoor stadiums with either CFD models or experimental techniques. For example, Nishioka et al. 2000 performed measurements to evaluate the indoor thermal environments in a large domed stadium in Japan, which was built mainly for professional baseball games. CFD modeling was performed in the Yoyogi National Stadium in Tokyo FLUENT 2001 and the Olympic ice hockey stadium in Turin Chown et al. 2006 . The Yoyogi National Stadium was constructed in Tokyo for the Olympic Games of 1964. Since the Olympics, the stadium has continued to be used for swimming competitions, but has also been used as a concert hall. A CFD evaluation of the stadium interior was performed to better understand the quality of the air-conditioning system for both modes of stadium operation. In the Olympic ice hockey stadium in Turin, a CFD study was performed to evaluate fire hazards in the stadium by reviewing smoke conditions in the arena for various scenarios and uses, i.e. Olympic or concert modes. In the present work, a CFD model is used to calculate airflow velocities and temperatures in Nikea Indoor Stadium, which hosted weightlifting during the Athens Games. The calculated ve

A hydrodynamic model was applied to calculate the flow field and the renewal time in Salman Bay, a semi-enclosed coastal water body of particular environmental significance in the region of Jeddah, Saudi Arabia. Initially the model was calibrated with field data. It was then applied to assess the effect of a series of engineering modifications of Salman Bay, involving enlarging of the cross-section of a link channel to the Red Sea, dredging of the bay, and reclamation of islands inside the bay, on the flow characteristics and the renewal of waters. Calculations showed that the channel upgrading would significantly improve the renewal of the bay waters. For tidal calculations the renewal time was reduced from 48·6 days to 12·7 days, whereas for a wind-driven regime it was reduced from extremely large values to 13–34 days.

A water quality model (MERES) is applied in the multi-purpose reservoir of Plastiras in Greece. The model was calibrated with limited field data using the standard trial and error procedure; next, it was applied for four scenarios of reservoir operation, which correspond to four values of minimum water levels (MWL). Model results were combined with landscape aesthetics to establish a “safe” MWL in the reservoir. MWL was proposed to be +784 m, a value that ensures a very good water quality by retaining the reservoir in the status of “a little beyond oligotrophic” and a very good quality of landscape with an acceptable area of dead zones; it also ensures an annual release of 127.5 hm3 of water within a reliability level of 90%. This quantity can be considered as acceptable from the water supply point of view.

The SST k–ω based model is applied to calculate air-flow velocities and temperatures in a model office room. Calculations are compared with experiments and with the results of the standard k–e, the RNG k–e model and the laminar model. It is concluded that (a) all the three tested turbulent models predict satisfactorily the main qualitative features of the flow and the layered type of temperature fields and (b) computations with the SST k–ω based model show the best agreement with measurements. The use of this model is proposed combined with a suitable grid.

A mathematical model is applied to the tank of Kipseli in Athens, Greece, which is used for storage, balancing and emergency chlorination. A Flow-Through Curve (FTC) experiment is performed for the initial geometry of the tank. The shape and the characteristics of the FTC show a very poor hydraulic efficiency, with extensive short-circuiting, intense mixing and low detention times. To improve the hydraulic efficiency of the tank the use of four alternative arrangements of guiding walls is examined by the model. Prior to its application, the model is verified by comparing the predicted FTC with the experimental. A satisfactory agreement is observed between the calculated and the experimental curves. Then the model is applied to calculate the flow field and the FTC for the four arrangements. Calculations are compared and the arrangement which shows the highest hydraulic efficiency is proposed for construction.

A mathematical model is applied to the design of two-storey final settling tanks. Computations show that the flow and suspended solids (SS) concentration fields for the upper and bottom tanks are similar. The flow field has the ‘two-layer’ structure, observed in real and laboratory settling tanks, consisting of a bottom current and a free surface return current with approximately equal heights. The SS concentration field is governed by the flow field (and vice versa). The SS concentration profiles are uniform in the major part of the tanks. The hydraulic and SS removal efficiencies improve with decreasing flow rate. In both tanks the outlet SS concentrations are lower than the maximum permissible value (20 mg l−1), with the upper tank showing a better performance than the bottom tank.

A two-phase model for the simulation of sedimentation processes is presented. The model solves the continuity and momentum equations for the pure-clear liquid and the sludge phases, and it is verified against a well-known benchmark problem, for which analytical solutions exist. Numerical simulations of a typical 1-D batch sedimentation process for mono-dispersed particles are carried out and results are found to be in satisfactory agreement with experimental data and model predictions of other researchers. A further expansion of the model to two-dimensions leads to predictions of the dynamic behavior of settling tanks and the eAect of the inclination angle on the sedimentation process. ” 1999 Elsevier Science Inc. All rights reserved.

A general methodology for the study of water quality in rivers is presented. The paper consists of four parts. In the first part the general characteristics of the area of study, which is the Beli Lom river, and its major pollution sources are presented. The effluent of the Razgrad Wastewater Treatment Plant (RWWTP) has been identified as the most significant pollution point source, due to the inadequate performance of the plant. The second part deals with data collection and processing. Four series of data have been collected, including physical, flow and water quality characteristics. In the third part a 1-d, finite-difference, second-order model is presented. In the fourth part, the model is calibrated, for the determination of its main coefficients, and is successfully verified by predicting the BOD and DO concentrations in the Beli Lom river for all series of data. Finally, the model has been applied to determine the maximum BOD and minimum DO effluent concentrations of the RWWTP, so that a minimum DO concentration is maintained throughout the river.

This paper presents a mathematical model for an alternating oxidation ditch system, performing carbon oxidation-nitrification, denitrification and settling. The model consists of 1-dimensional mass balance (convection-dispersion) equations, based on the IAWPRC activated sludge model No 1. The data used for the calibration and the application of the model, are collected from a pilot plant. The present work consists of 3 parts. In the first part a brief description of the pilot plant is made. The second part deals with a short presentation of the basic equations and the boundary conditions of the model. The calibration and application of the model with the corresponding conclusions are contained in the third part. In the calibration, the values of the dispersion coefficient, E, and the correction factors α and β of the equation of the aeration equipment (rotors) have been determined equal to E=170000 m2 day−1, β=0.85 and α=0.3 and 0.825, for the operation of the rotors in the low and high speed, respectively. The application of the model shows that predicted effluent concentration values are in a very good agreement with experiments.

A numerical model for predicting the flow in clarifiers is presented which accounts for density-affected and turbulent flow as well as for the settling of activated sludge. The reliability of the two-dimensional model is demonstrated through verification by means of a laboratory experiment which was specially designed for this purpose. The numerical model is then applied for an evaluation of inlet and outlet arrangements. The inlet is positioned at the bottom and the inlet aperture is varied. An outlet at the top of the end wall is compared with outlets longitudinally extended at the surface. The alternatives are assessed by analysis of streamline patterns, layer charactersitics and the entrainment into the bottom current, i.e. the increase of its flow rate. The study demonstrates how numerical modelling can be used for systematic evaluation of the effect of geometrical configurations on the flow in secondary clarifiers.

A comparative study of coastal circulation by means of two depth-averaged numerical models, a finite difference and a finite element model, is undertaken. From the application of the models to Amvrakikos Gulf, a nearly enclosed coastal water body of particular ecological interest in Western Greece, the following conclusions are drawn. (i) Circulation patterns calculated for the same eddy viscosity coefficient with both models are essentially identical. (ii) Increasing the value of eddy viscosity coefficient results in a significant reduction of the magnitude of the flow velocities with no major changes on the general flow pattern. (iii) Preliminary calculations with the finite difference model show that the incorporation of the k-ε turbulence model has minor effects on the results, but it improves the rate of convergence. (iv) Calculations with both models using alternative conditions in the open sea boundary show only minor differences in the proximity of the boundary.