Over the past decades, severe environmental problems have been caused by the inefficient water re... more Over the past decades, severe environmental problems have been caused by the inefficient water resources management by humans; in rivers, diversions, dam constructions and water abstractions mainly for irrigation purposes lead to flow-regime alterations and threat river ecosystems mainly during the dry period when the flow is dramatically reduced. To prevent fish extinction and ensure the good ecological status of aquatic ecosystems, the EU Water Framework Directive proposes the definition of a minimum flow rate, commonly called ''Ecological Flow'', in rivers. In this study, we compare two different approaches for the determination of the Ecological Flow in Sperchios river, Central Greece, for the case of Squalius vardarensis which is the most important species inhabiting this river. The first approach is the Hydraulic-Habitat model PHABSIM which is an easy-to-use engineering tool utilizing onedimensional calculations both for hydraulic and habitat modeling. We defin...
We pose the following research question, “what are (i) the minimum required computation grid and ... more We pose the following research question, “what are (i) the minimum required computation grid and (ii) the required form of hydrodynamic equations, i.e., shallow water equations (SWE) or diffusion wave equations (DWE), in 2D modeling to minimize the computational time while maintaining an acceptable level of error in the prediction of water depths and the extent of flood inundated areas?”. To answer this question, we apply the HEC-RAS 1D/2D model to simulate a disastrous flash flood in the town of Mandra, in Attica, Greece, in November 2017. HEC-RAS 1D/2D combines 1D modeling in the cross-sections of the two main streams of Mandra with 2D modeling in the rest of the potentially flooded area of the computational domain which has an area equal to 18.36 km2. We perform calculations for 8 scenarios that combined various grid sizes (with approximately 44,000–95,000 control volumes) with the use of the SWE or DWE. We derive the following conclusions: (i) calculated maximum water depths usi...
A mathematical model is applied to the design of two-storey final settling tanks. Computations sh... more 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.
Ecohydraulic models have commonly used the flow velocity, water depth, and substrate type (i3 mod... more Ecohydraulic models have commonly used the flow velocity, water depth, and substrate type (i3 models) as the three fundamental determinants of the distribution of freshwater biota, but a fourth determinant has largely been neglected: stream vegetation. In this study, we provide the hydraulic and habitat information required to develop vegetation-adapted ecohydraulic models (i4 models) in streams. We calculated drag forces and Manning’s roughness coefficients (nV) for nine types of submerged, emergent, and overhanging stream vegetation. In addition, we developed habitat suitability curves (HSCs) for benthic macroinvertebrates for these stream vegetation types. Hydraulic modules can now be upgraded to simulate stream vegetation by including the vegetation-adapted nV values within an additive approach in which nV is added to the n value of the inorganic substrate to which the vegetation is rooted. Habitat modules can also be upgraded to include macroinvertebrate HSCs for stream vegetat...
What would happen in Mediterranean rivers and streams if warming but not drying occurred? We exam... more 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 d...
Generic habitat suitability criteria (HC) are often developed from spatially and temporally varia... more 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 develo...
ABSTRACT Experimental measurements of concentrations were performed for vertical dense jets disch... more ABSTRACT 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, in terms of geometrical parameters and the densimetric Froude number.
Two Integrated Surface water - Groundwater flow Models (ISGMs) have been developed at the Nationa... more Two Integrated Surface water - Groundwater flow Models (ISGMs) have been developed at the National Technical University of Athens (NTUA), Greece and Cardiff University (CU), UK to investigate surface water-groundwater interactions. The models are based on physical processes and are capable of describing more accurately the recharge and discharge flow paths between surface and ground waters. The NTUA ISGM consists of a 3-D surface water flow sub-model (FLOW-3DL) and a 3-D saturated groundwater flow sub-model. The CU ISGM is based on the 2-D surface water model DIVAST, which has been extended to include 2-D saturated groundwater flow. Both models use the finite difference method and orthogonal grids. The momentum and mass conservation equations are the governing equations for both surface and groundwater flows. The ISGMs have been applied to two simple cases and their results have been compared to computations using only surface water models (FLOW-3DL and DIVAST) to demonstrate the ne...
The Plastiras dam was constructed in the late 1950s mainly for electric power production, but it ... more The Plastiras dam was constructed in the late 1950s mainly for electric power production, but it has also partially covered irrigation needs and water supply of the plain of Thessaly. Later, the site has been designated as an environment conservation zone because of ecological and landscape values, while tourist activities have been developed around the reservoir. Irrigation of agricultural land, hydroelectric production, drinking water supply, tourism, ecosystem water quality and scenery conservation have evidently been conflicting targets for many years. Good management would require a multi-criteria decision making. Historical data show that the irregular water release has resulted in a great annual fluctuation of the reservoir water level. This situation could be improved by a rational management of abstractions. Apparently, higher release leads simultaneously to more power production and to irrigation of a larger agricultural land. Moreover, demands for electricity and for irri...
One of the most favorable and cost-saving methods to counteract water quality deterioration in a ... more One of the most favorable and cost-saving methods to counteract water quality deterioration in a harbor basin is the use of flushing culverts, which reduce flushing times and avoid areas of low flow and eddies formation. In the present paper, results of experiments carried out in a 2-D facility in order to investigate wave and dissolved oxygen transmission in harbor basins through flushing culverts are presented and analyzed. Incident wave and transmitted wave heights were determined using wave gauges for various combinations of wave characteristics and geometric characteristics of the flushing culverts. An empirical equation using regression analysis was derived, which correlates the wave transmission coefficient with the characteristics of the waves and the geometrical characteristics of the flushing culvert. Further the oxygen transmission coefficient through flushing culverts, was expressed as a function of the characteristics of waves and the dimensions of the flushing culvert....
Over the past decades, severe environmental problems have been caused by the inefficient water re... more Over the past decades, severe environmental problems have been caused by the inefficient water resources management by humans; in rivers, diversions, dam constructions and water abstractions mainly for irrigation purposes lead to flow-regime alterations and threat river ecosystems mainly during the dry period when the flow is dramatically reduced. To prevent fish extinction and ensure the good ecological status of aquatic ecosystems, the EU Water Framework Directive proposes the definition of a minimum flow rate, commonly called ''Ecological Flow'', in rivers. In this study, we compare two different approaches for the determination of the Ecological Flow in Sperchios river, Central Greece, for the case of Squalius vardarensis which is the most important species inhabiting this river. The first approach is the Hydraulic-Habitat model PHABSIM which is an easy-to-use engineering tool utilizing onedimensional calculations both for hydraulic and habitat modeling. We defin...
We pose the following research question, “what are (i) the minimum required computation grid and ... more We pose the following research question, “what are (i) the minimum required computation grid and (ii) the required form of hydrodynamic equations, i.e., shallow water equations (SWE) or diffusion wave equations (DWE), in 2D modeling to minimize the computational time while maintaining an acceptable level of error in the prediction of water depths and the extent of flood inundated areas?”. To answer this question, we apply the HEC-RAS 1D/2D model to simulate a disastrous flash flood in the town of Mandra, in Attica, Greece, in November 2017. HEC-RAS 1D/2D combines 1D modeling in the cross-sections of the two main streams of Mandra with 2D modeling in the rest of the potentially flooded area of the computational domain which has an area equal to 18.36 km2. We perform calculations for 8 scenarios that combined various grid sizes (with approximately 44,000–95,000 control volumes) with the use of the SWE or DWE. We derive the following conclusions: (i) calculated maximum water depths usi...
A mathematical model is applied to the design of two-storey final settling tanks. Computations sh... more 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.
Ecohydraulic models have commonly used the flow velocity, water depth, and substrate type (i3 mod... more Ecohydraulic models have commonly used the flow velocity, water depth, and substrate type (i3 models) as the three fundamental determinants of the distribution of freshwater biota, but a fourth determinant has largely been neglected: stream vegetation. In this study, we provide the hydraulic and habitat information required to develop vegetation-adapted ecohydraulic models (i4 models) in streams. We calculated drag forces and Manning’s roughness coefficients (nV) for nine types of submerged, emergent, and overhanging stream vegetation. In addition, we developed habitat suitability curves (HSCs) for benthic macroinvertebrates for these stream vegetation types. Hydraulic modules can now be upgraded to simulate stream vegetation by including the vegetation-adapted nV values within an additive approach in which nV is added to the n value of the inorganic substrate to which the vegetation is rooted. Habitat modules can also be upgraded to include macroinvertebrate HSCs for stream vegetat...
What would happen in Mediterranean rivers and streams if warming but not drying occurred? We exam... more 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 d...
Generic habitat suitability criteria (HC) are often developed from spatially and temporally varia... more 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 develo...
ABSTRACT Experimental measurements of concentrations were performed for vertical dense jets disch... more ABSTRACT 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, in terms of geometrical parameters and the densimetric Froude number.
Two Integrated Surface water - Groundwater flow Models (ISGMs) have been developed at the Nationa... more Two Integrated Surface water - Groundwater flow Models (ISGMs) have been developed at the National Technical University of Athens (NTUA), Greece and Cardiff University (CU), UK to investigate surface water-groundwater interactions. The models are based on physical processes and are capable of describing more accurately the recharge and discharge flow paths between surface and ground waters. The NTUA ISGM consists of a 3-D surface water flow sub-model (FLOW-3DL) and a 3-D saturated groundwater flow sub-model. The CU ISGM is based on the 2-D surface water model DIVAST, which has been extended to include 2-D saturated groundwater flow. Both models use the finite difference method and orthogonal grids. The momentum and mass conservation equations are the governing equations for both surface and groundwater flows. The ISGMs have been applied to two simple cases and their results have been compared to computations using only surface water models (FLOW-3DL and DIVAST) to demonstrate the ne...
The Plastiras dam was constructed in the late 1950s mainly for electric power production, but it ... more The Plastiras dam was constructed in the late 1950s mainly for electric power production, but it has also partially covered irrigation needs and water supply of the plain of Thessaly. Later, the site has been designated as an environment conservation zone because of ecological and landscape values, while tourist activities have been developed around the reservoir. Irrigation of agricultural land, hydroelectric production, drinking water supply, tourism, ecosystem water quality and scenery conservation have evidently been conflicting targets for many years. Good management would require a multi-criteria decision making. Historical data show that the irregular water release has resulted in a great annual fluctuation of the reservoir water level. This situation could be improved by a rational management of abstractions. Apparently, higher release leads simultaneously to more power production and to irrigation of a larger agricultural land. Moreover, demands for electricity and for irri...
One of the most favorable and cost-saving methods to counteract water quality deterioration in a ... more One of the most favorable and cost-saving methods to counteract water quality deterioration in a harbor basin is the use of flushing culverts, which reduce flushing times and avoid areas of low flow and eddies formation. In the present paper, results of experiments carried out in a 2-D facility in order to investigate wave and dissolved oxygen transmission in harbor basins through flushing culverts are presented and analyzed. Incident wave and transmitted wave heights were determined using wave gauges for various combinations of wave characteristics and geometric characteristics of the flushing culverts. An empirical equation using regression analysis was derived, which correlates the wave transmission coefficient with the characteristics of the waves and the geometrical characteristics of the flushing culvert. Further the oxygen transmission coefficient through flushing culverts, was expressed as a function of the characteristics of waves and the dimensions of the flushing culvert....
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