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  • 1. Admass, M.
    et al.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    3D numerical modelling of flow and sediment transport in rivers2007Inngår i: International Journal of Sediment Research, ISSN 1001-6279, Vol. 22, nr 2, s. 168-174Artikkel i tidsskrift (Fagfellevurdert)
  • 2.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik.
    Controlling floodwater mosquitoes by river regulation2019Inngår i: Water and Environment JournalArtikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Floodwater mosquitoes cause serious problems especially the spread of mosquito-borne diseases. The main challenge is to implement acceptable general mitigation methods. In recent years, encouraging steps have been taken in favour of developing more sustainable measures such as modern biological methods. The novelty of the present study is in suggesting an alternative green method that can eliminate the use of any secondary biological or chemical agents. The method is based on establishing an alternative river regulation scheme that limits flooding of high-risk lowland areas. A combined hydrodynamic modelling and optimization method gave effective river regulation schemes. The main idea is to maintain the total power production for a modified inflow hydrograph. The approach was applied to a 30-km reach of the Österdalälven River located in central Sweden. It was found that with the specific constraints of the site a reduction of about 8% of lowland flood areas is possible if the inflow hydrograph is moderately modified by 20%. Higher reductions are possible by further modification of the hydrograph. The proposed method has a general character controlled by the specific site requirements of a critical flow or a water surface elevation. Based on the foregoing information, the method can be applied to similar sites.

  • 3.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Hållbarhet och miljöteknik.
    Dynamics of vortical structures in the Baltic Sea2019Inngår i: Dynamics of atmospheres and oceans (Print), ISSN 0377-0265, E-ISSN 1872-6879, Vol. 88, artikkel-id UNSP 101117Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A fully calibrated and validated 3D hydrodynamic model is applied to the Baltic Sea to investigate the vortical structures in the Baltic Sea in the period 2000-2009. The novelty of the study is the detection and dynamics of large scale mesoscale vortices both in horizontal and vertical planes by using the instantaneous velocity vectors. This study provides some new insights into the type and dynamics of vortical structures in the Baltic Sea. In the horizontal plane, vortices with typical length and velocity scale of 5-12 km and 5-10 cm/s are present. They are of open type three-dimensional structures having two important features. They extend to 90% of the flow depth and have a clear periodicity of one to seven days. The vortices rotate both in clockwise and anti-clockwise directions. The baroclinic Rossby deformation radii are in the same range. The maximum numbers of mesoscale vortices are within the 15-m thick surface layer that diminishes towards the bottom layer. In the vertical planes, there is a multi-layered structure with the layer numbers varying from 2 to 4. Each layer is characterized by a set of distinct vortices. Taylor-Gortler type vortices appear within the surface layers. The middle flow layers are featured by distorted large-scale vortices. Boundary trapped vortices and large local circulation patterns feature the deeper bottom flow layers. There are two underlying mechanisms of vortex generation: one is shear layer instability of the Kelvin Helmholtz type and the other Taylor-Gortler. A conceptual general vortex model is proposed that applies to all basins of the Baltic Sea in vertical planes. Three types of structures are possible in the mixed layer, pycnocline, and the deep layer. In these are Taylor-Gortler vortices, mesoscale coherent structures, and trapped boundary vortices, respectively.

  • 4.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Experimental study and 3D numerical simulations for a free-overflow spillway2006Inngår i: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 132, nr 9, s. 899-907Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The main objectives of the present work were to investigate the flow field over a spillway and to simulate the flow by means of a three-dimensional (3D) numerical model. Depending on the wall curvature, the boundary layer parameters decreased or increased with increasing distance along the spillway. The growth of the boundary layer along the spillway is better described as a function of Reynolds number than the normalized streamwise length. A simplified form of the 3D momentum equation can be used to obtain a rough estimate of the skin friction. The velocity profile in the boundary layer along the spillway is described by a velocity-defect relationship. Numerical models provide a cost-effective means of simulating spillway flows. In this study, the water surface profiles and the discharge coefficients for a laboratory spillway were predicted within an accuracy range of 1.5-2.9%. The simulations were sensitive to the choice of the wall function, grid spacing, and Reynolds number. A nonequilibrium wall function with a grid spacing equal to a distance of 30 wall units gave good results.

  • 5.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Flow characteristics of bottom outlets with moving gates2010Inngår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 48, nr 4, s. 476-482Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study investigates the discharge characteristics of a bottom outlet with a moving gate by Flow3D. Experimental results for a scale model outlet of the Aswan Dam, Egypt, were used. Two different flow features were found. Pressurized flow established if the flume was filled and then the gate was slowly opened. However, a free surface flow occurred if the gate was fully opened and the entire flume was slowly flooded with water. The numerical simulations successfully captured the two flow patterns as well as the discharges and water surface profiles. The discharges were predicted with sufficient accuracy using the first-order momentum advection scheme. In comparison with the k-epsilon turbulence model, the Re-Normalization Group model yields the best agreement with the experiments. The model performed with similar accuracy for both model and prototype cases.

  • 6.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik (flyttat 20130630).
    Mitigation of sedimentation problems in the lower reach of the River Klaralven2008Inngår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 46, nr 2, s. 224-236Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The study focuses on the sedimentation problems found in the lower reach of the River Klaralven, where the river bifurcates into a west and an east channels at the city of Karlstad. During a period of thirty years, the sediment transport capacity of the west-channel has gradually diminished. This has caused a growing concern about the risk of flooding in the city. The study has aimed to find relevant methods for both increasing the hydraulic capacity of the west river channel, and mitigation of sedimentation problems. The main approach was numerical flow and sediment transport modelling. A two-dimensional depth-averaged model was used to study the influence of various river training measures. The model was found effective to address the sedimentation problem and the reduced hydraulic capacity in the river. To mitigate these problems, two effective engineering methods were controlling the water levels by partial gates and the use of groynes or vanes. The results suggested that a carefully calibrated and verified 2-D depth-averaged model can be used in supporting river restoration works and flood alleviation schemes.

  • 7.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik (flyttat 20130630), Vattendragsteknik (flyttat 20130630).
    Reservoir sedimentation2012Inngår i: Encyclopedia of Earth Sciences Series, Springer Netherlands, 2012, s. 628-649Kapittel i bok, del av antologi (Fagfellevurdert)
  • 8.
    Dargahi, Bijan
    KTH, Tidigare Institutioner                               , Mark- och vattenteknik.
    Scour development downstream of a spillway2003Inngår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 41, nr 4, s. 417-426Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The scouring process downstream of spillways is an important research topic of value in engineering practice. The objectives of the present experimental study were to examine the similarity development of scour profiles, the controlling scour mechanism and predictions of the scour geometry. No experimental evidence was found in support of the similarity assumption for the scouring process. Scouring downstream of a spillway is controlled by large secondary flows and a vortex system induced by a hydraulic jump. The main dimensionless parameters controlling the scouring process are the relative operating head, the relative sediment size, and the relative roughness of the scour protective plate. The maximum scour depth and the rate of sediment transport can be estimated by using simple, power-type equations that relate the scour geometry to the controlling scour parameters.

  • 9.
    Dargahi, Bijan
    KTH, Tidigare Institutioner, Mark- och vattenteknik.
    Three-dimensional flow modelling and sediment transport in the River Klaralven2004Inngår i: Earth Surface Processes and Landforms, ISSN 0197-9337, E-ISSN 1096-9837, Vol. 29, nr 7, s. 821-852Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A three-dimensional flow model that uses the RNG k-epsilon turbulence model and a non-equilibrium wall function was applied to the River Klaralven in the southwest part of Sweden. The objectives were to study the nature of the flow in the river bifurcation and to investigate the short-term sediment transport patterns in the river. The effectiveness of three-dimensional flow models depends upon: (1) how well the river geometry and it surface roughness are modelled; and (2) the choice of the closure model. Improvements were obtained by modelling the river in two parts: the entire river reach, and a selected part. Composite Manning coefficients were used to account for roughness properties. The method requires a calibration process that ensures the water surface profiles match the field data. The k-epsilon model under-predicted both the extent of flow separation zones and the number of secondary flow regions having a spiral motion, in comparison with the RNG k-epsilon model. The 3-D model could predict with good accuracy both the general and secondary flow fields in the river. The results agreed well with the 3-D velocity measurements using an acoustic Doppler current profiler. A conceptual model was developed that accounts for the development of secondary flows in a river bifurcation having two bends. The main flow feature in the river cross-sections was the existence of multiple counter-rotating spiral motions. The number of spiral motions increased as the river bends were approached. The river bends also caused vorticity intensification and increased the vertical velocities. The application of the 3-D flow model was extended by solving the sediment continuity equation. The sediment transport patterns were related to the secondary flow fields in the river. The sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank.

  • 10.
    Dargahi, Bijan
    KTH, Tidigare Institutioner                               , Mark- och vattenteknik.
    Three-dimensional modelling of ship-induced flow and erosion2003Inngår i: Proceedings of the Institution of Civil Engineers: Water and maritime engineering, ISSN 1472-4561, E-ISSN 1753-7800, Vol. 156, nr 2, s. 193-204Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ship-induced erosion by propeller jets causes considerable damage in canals, harbours, locks and waterways. jet velocity reaching the bed could be as high as 15 m/s depending on the ship and the machinery. The objectives of the present study were to investigate the nature of the propeller jet flow and to estimate the rate of erosion caused by the jet. Three-dimensional numerical models can be used to simulate ship-induced propeller flows provided the propeller can be replaced with a simpler geometry. The study modelled the propeller flow by one lower and one upper jet flow. The turbulent characteristics, such as the turbulence intensities and the energy balance of the lower jet, are similar to a flat wall boundary layer flow. The application of the three-dimensional flow model was extended by numerically solving the sediment continuity equation. The computations were based on the lower part of the jet velocity profiles. The method enables the scour cavity induced by a stationary ship with a running engine to be calculated. The computed scour cavity agreed well with the field data.

  • 11.
    Dargahi, Bijan
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Mark- och vattenteknik.
    Cvetkovic, Vladimir
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Mark- och vattenteknik.
    Hydrodynamic and Transport Characterization of theBaltic Sea 2000-20092014Rapport (Annet vitenskapelig)
  • 12.
    Dargahi, Bijan
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattenvårdsteknik.
    Cvetkovic, Vladimir
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattenvårdsteknik.
    Hydrodynamic and Transport Properties of Saltsjo Bay in the Inner Stockholm Archipelago2011Inngår i: Journal of Coastal Research, ISSN 0749-0208, E-ISSN 1551-5036, Vol. 27, nr 3, s. 572-584Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    DARGAHI, B. and CVETKOVIC, V., 2011. Hydrodynamic and transport properties of Saltsjo Bay in the inner Stockholm archipelago, Sweden. Journal of Coastal Research, 27(3), 572-584. West Palm Beach (Florida), ISSN 0749-0208. A three-dimensional hydrodynamic model was successfully calibrated and validated for Saltsjo Bay, located in the inner Stockholm archipelago. The work aims to obtain a scientific understanding of specific hydrodynamic characteristics of the bay. The focus is on the influence of the freshwater inflow on the hydrodynamic characteristics of the bay, which shares common features with other relatively small bays and estuaries. The model was used for investigating the flow structure, stratification, exchange process, flushing time, and oxygen content. The predicted water levels, temperature, salinity, and dissolved oxygen (DO) profiles were in good agreement with the measurements. The flow structure in the bay is characterised by the existence of large secondary flow regions and multilayer flows. The principal cause of the large secondary flow regions is the interaction of prevailing two-layer flows that have opposite directions. The stratification can be characterised by two long winter and summer stratification periods and two short overturn periods. In the absence of the freshwater, the two-layer flow changed to a three-layer flow but the flow stratification remained unaltered. The flushing time in Saltsjo Bay (2-29 days) is similar to that found in smaller water bodies. The flushing time increased to 40 days when the freshwater inflows were removed from the model boundaries. The period from 1 September to 1 November is characterised by bottom oxygen deficiency when the DO concentrations fall below 5 mg/L. At the open boundary, 3% to 25% of the total volume of the bay is exchanged daily. The work should be of interest and relevant to other bays of comparable size that have similar hydrodynamic characteristics and are subject to freshwater inflows.

  • 13.
    Dargahi, Bijan
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Kolluru, V.
    Cvetkovic, Vladimir
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Mark- och vattenteknik.
    Multi-layered stratification in the Baltic Sea: Insight from a modeling study with reference to environmental conditions2017Inngår i: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 5, nr 1, artikkel-id 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The hydrodynamic and transport characteristics of the Baltic Sea in the period 2000-2009 were studied using a fully calibrated and validated 3D hydrodynamic model with a horizontal resolution of 4.8 km. This study provided new insight into the type and dynamics of vertical structure in the Baltic Sea, not considered in previous studies. Thermal and salinity stratification are both addressed, with a focus on the structural properties of the layers. The detection of cooler regions (dicothermal) within the layer structure is an important finding. The detailed investigation of thermal stratification for a 10-year period (i.e., 2000-2009) revealed some new features. A multilayered structure that contains several thermocline and dicothermal layers was identified from this study. Statistical analysis of the simulation results made it possible to derive the mean thermal stratification properties, expressed as mean temperatures and the normalized layer thicknesses. The three-layered model proposed by previous investigators appears to be valid only during the winter periods; for other periods, a multi-layered structure with more than five layers has been identified during this investigation. This study provides detailed insight into thermal and salinity stratification in the Baltic Sea during a recent decade that can be used as a basis for diverse environmental assessments. It extends previous studies on stratification in the Baltic Sea regarding both the extent and the nature of stratification.

  • 14.
    Dargahi, Bijan
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Setegn, Shimelis Gebriye
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Combined 3D Hydrodynamic and Watershed Modelling of Lake Tana, Ethiopia2011Inngår i: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 398, nr 1-2, s. 44-64Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

     The growing high demand for lake Tana water portends a disturbing future. The main objective of this paper is to make a contribution to the development of a sustainable use of the water of Lake Tana. A fully three-dimensional hydrodynamic model was combined with a watershed model and together, these models were successfully validated for the year 2006. The flow structure is characterized by large recirculation and secondary flow regions. Secondary flows are induced by hydrodynamic instabilities occurring at the interfaces of layers with a velocity gradient and the interaction with the irregularities of the bed. The weak stratification process in Lake Tana is characterized by a classic summer profile, which is more pronounced during January-February. Mixing processes in the lake are controlled by wind, the mixing energy induced by both river inflows and the lake outlet, and convective mixing due to the negative buoyancy. An alarming fall of the water levels in Lake Tana was found in response to the planned water withdrawal. The long flushing time (19 months) will not allow a fast decay of contaminated materials released into the lake. The flow structure will not be significantly modified by the planned water withdrawal but the flushing time will decrease. The hydrodynamics of Lake Tana resemble a closed system similar to a shallow reservoir with an overflow type outlet. The implication is that the lake is vulnerable to changes in external conditions and sustainable use of the water resource of the lake will require awareness of this vulnerability. The combined watershed and hydrodynamic models would be effective tools to achieve this awareness. It is also necessary to address the impact of climate change on the fate of the lake. These are all difficult challenges that need to be addressed to safeguard the sensitive eco-system of the area.

  • 15.
    Ferdos, Farzad
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part I: numerical validation2016Inngår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 54, nr 6, s. 663-677Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Analyses of turbulent flows through the downstream slopes of embankment dams are important for dam safety assessments, especially considering high-risk scenarios such as a sudden release of water due to internal erosion. Flow prediction is difficult in such situations due to coarseness of construction materials and high Reynolds numbers. The present study addresses this issue through comprehensive numerical modelling. The novelty of the proposed approach lies in a combination of large-scale experiments and three-dimensional numerical simulations, leading to a fully calibrated and validated model that is applicable to flows through cobble-sized materials (100–160 mm in diameter) at high Reynolds numbers (>104). Comparing the results of the standard turbulence models to data from the large-sale experiments, the renormalization group theory-based model yielded the smallest relative errors based on the hydraulic gradients. Considering the flow field, the turbulent shear stress increased by a factor of 17, and the time-averaged vorticities intensified by factors of 2, 6 and 10 for vorticities in the x-, y- and z-directions, respectively, due to the presence of cobbles.

  • 16.
    Ferdos, Farzad
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part II: flow physics2016Inngår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 54, nr 6, s. 678-691Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evaluation of hydraulic performance of coarse porous media at high Reynolds numbers is of significant importance for dam safety assessment. To address this task, the present study uses a numerical approach based on a rigorous theoretical framework. The novelty of the study lies in the application of a fully calibrated and validated numerical three-dimensional model and a set of modified equations for momentum transfer and the associated coefficients for flow in porous media. A Lagrangian particle tracking model was used to estimate the lengths of the flow channels that developed in the porous media. Gamma distributions were fitted to the normalized channel lengths, and the scale and shape parameters of the gamma distribution were found to be Reynolds number dependent. These shape parameters can be estimated from the suggested polynomial equations. The proposed normalized length parameter can be used to evaluate permeability, energy dissipation, induced forces, and diffusion. It was found that shear forces exerted on the coarse particles depend on the inertial forces of the flow and can be estimated using the proposed equation for the developed turbulent flows in porous media.

  • 17.
    Ferdos, Farzad
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Solari, Luca
    Mechanism of concentrated leak erosion phenomenon in embankment dams2016Manuskript (preprint) (Annet vitenskapelig)
  • 18.
    Ferdos, Farzad
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Solari, Luca
    Mechanism of suffusion erosion phenomenon in embankment dams2016Manuskript (preprint) (Annet vitenskapelig)
  • 19.
    Mekonnen, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    3D flow and sediment transport modeling in curved channels2007Inngår i: 3rd IASTED International Conference on Environmental Modelling and Simulation, EMS 2007: Honolulu, HI: 20 August 2007 through 22 August 2007, 2007, s. 82-87Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A 3D hydrodynamic model coupled with a 2D bed load transport model is used to predict flow and sediment transport in a curved channel. The 3D hydrodynamic model is part of ECOMSED (open source code). ECOMSED has a long history of successful applications to oceanic, coastal, and estuarine waters. However, curved channel applications of the code are scarce. Improvements in the advection scheme of momentum and turbulence, and shear stress partitioning were necessary to reproduce realistic and comparable results in a curved channel. To account for the dynamics of the mobile bed boundary, a model for the bed load transport was included in the code. The model reproduced measured secondary currents, bed shear stress distribution, and erosion-deposition patterns on a curved channel.

  • 20.
    Mekonnen, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    ECOMSED - Three dimensional modeling in rivers2006Inngår i: River Flow 2006, Vols 1 and 2 / [ed] Ferreira, RML; Alves, CTL; Leal, GAB; Cardoso, AH, 2006, s. 1259-1267Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The 3-D hydrodynamic and sediment transport model ECOMSED was adapted to simulate flow and sediment transport in rivers. A bed load transport model was included in the code. Improvements in treatment of river roughness parameterization, bed form effects, and automatic update of flow depth due to bed evolution were also made. The model was applied to I-km long reach of the River Klaralven, located in the county of Varmland, Sweden. The model successfully predicted the general flow patterns and sediment transport characteristics of the river reach.

  • 21.
    Mekonnen,, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Modelling exchange processes between two basins in Stockholm archipelago2007Inngår i: Proceedings of the American Water Resource Association 2007 Annual Conference, American Water Works Association, 2007Konferansepaper (Annet vitenskapelig)
  • 22.
    Mekonnen, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Three dimensional numerical modelling of flow and sediment transport in rivers2007Inngår i: International Journal of Sediment Research, ISSN 1001-6279, Vol. 22, nr 3, s. 188-198Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The 3D numerical model, ECOMSED (open source code), was used to simulate flow and sediment transport in rivers. The model has a long history of successful applications to oceanic, coastal and estuarine waters. Improvements in the advection scheme, treatment of river roughness parameterization and shear stress partitioning were necessary to reproduce realistic and comparable results in a river application. To account for the dynamics of the mobile bed boundary, a model for the bed load transport was included in the code. The model reproduced observed secondary currents, bed shear stress distribution and erosion-deposition patterns on a curved channel. The model also successfully predicted the general flow patterns and sediment transport characteristics of a 1-km long reach of the River Klaralven, located in the north of the county of Varmland, Sweden.

  • 23.
    Mekonnen, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Wörman, Anders
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Mixing and exchange processes between two estuaries in Stockholm Archipelago, Sweden2008Inngår i: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    ECOM, a three-dimensional time dependant hydrodynamic and thermodynamic model, was used to investigate the mixing and exchange processes between Farstaviken and Baggensfjärden estu-aries in Stockholm archipelago during April to November 1997. Possible causes of bottom oxy-gen deficiency in the smaller estuary of Farstaviken were also investigated. The model with an additional boundary condition for the sediment-water heat exchange and modifications to reduce the pressure gradient error successfully produced the observed temperature profiles by allowing a time variable reduction in the magnitude of the observed wind speed. In Farstaviken (during stratification periods) the epilimnion region occupies 2.5%-17.5% of the flow depth. The ex-change process between the two smaller and larger estuaries is different from that observed be-tween larger basins where density driven currents dominate. Here, the process is primarily con-trolled by wind action and tides. On average 5% of the total water volume of Farstaviken is exchanged with Baggensfjärden on daily basis. The bottom oxygen deficiency in Farstaviken during April to November 1997 is partially related to the topography of the basin and the wind sheltering effect. To investigate this issue as well as the sensitivity of mixing to the changes in topography, additional simulations were performed using a modified bathymetry. The original bathymetry was smoothed using the 2D Gaussian filter. The results showed that both the intensi-ties and penetration depths of eddies were increased in comparison with the original bathymetry. The enhanced circulation can also contribute to increase the content of dissolved oxygen near the bottom of Farstaviken.

  • 24.
    Mekonnen, Muluneh Admass
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Wörman, Anders
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Gebeyehu, Admasu
    SWECO Environment AB.
    Hydrological modelling of Ethiopian catchments using limited data2009Inngår i: Hydrological Processes, ISSN 0885-6087, E-ISSN 1099-1085, Vol. 23, nr 23, s. 3401-3408Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    The hydrological component of Soil and Water Assessment Tool (SWAT) model is adapted for Ethiopian catchments based on primary knowledge of the coherence spectrum between dis-charge and runoff. The implication is that only periods longer than about 50 days can be reliably represented in the model based on the available data. An improved method reflecting soil water retention in terms of cumulative evapotranspiration, so that its value is less dependent on soil storage and more dependent on antecedent climate, is used. The improved method is attractive for Ethiopian conditions due to limited soil data availability and the fact that the time-scale of cumulative evaporation can be evaluated over periods longer than 50 days. The spectrum analysis was done on the available nearby climatic data in three watersheds in Ethiopia to analyze the effects of data limitation on the temporal and spatial scales suitable to account for in comparta-mentalized runoff models. The time scales of SWAT for the surface runoff and groundwater flow response were constrained so as to be consistent with the results of the spectrum analysis. The performance of the SWAT model to predict daily stream flow response was compared to the Seasonal Model (SM) and the Original Linear Purturbation Model (OLPM) both of which need previous seasonal behavior of the stream flow.

  • 25. Setegn, S. G.
    et al.
    Melesse, A. M.
    Rayner, D.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Mark- och vattenteknik.
    Climate change impact on water resources and adaptation strategies in the Blue Nile River Basin2014Inngår i: Nile River Basin: Ecohydrological Challenges, Climate Change and Hydropolitics, Springer International Publishing , 2014, s. 389-404Kapittel i bok, del av antologi (Annet vitenskapelig)
    Abstract [en]

    We compared projected changes in precipitation and temperature across global climate models (GCMs) for two future periods to get an indication of the consistency of the projected changes in the Lake Tana subbasin of the Blue Nile basin. We found that the models projected temperature increases of around 2∈°C to 5∈°C for 2080-2100, depending on the model and emission scenario. The interquartile ranges of the projected temperature increases for 2070-2100 for the three emission scenarios show 2.0-4.4∈°C in the wet season and 2.2-4.9∈°C in the dry season. The ensemble of GCMs we examined includes models that project increases and decreases in seasonal precipitation. The interquartile ranges of the projected rainfall changes for 2070-2100 for the three emission scenarios show -∈13 to +∈12∈% in the wet season and -∈14 to +∈16∈% in the dry season. The study investigated how changes in temperature and precipitation might translate into changes in streamflows and other hydrological components using downscaled outputs from different climate models. The direction of streamflow changes followed the direction of changes in rainfall. The responses of evapotranspiration, soil moisture (SW), and groundwater (GW) were also examined, and it was found that changes in GW flow may be a significant component of the changes in streamflow. The effect of climate change has the potential to cause agricultural drought, unless there is ample water available for irrigation. However, a reduction in rainfall may cause reduced GW recharge, which would significantly reduce its contribution to streamflow. Lake Tana is highly sensitive to variations in rainfall, as well as in river inflows and evaporation.

  • 26.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Texas A & M University, Spatial Science Laboratory.
    Sediment Yield Modelling from Anjeni Gauged Watershed, Ethiopia using SWAT2005 Model2008Inngår i: The Open Hydrology Journal, ISSN 1874-3781Artikkel i tidsskrift (Annet vitenskapelig)
  • 27.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Melesse, Assefa M.
    Modeling of Sediment Yield From Anjeni-Gauged Watershed, Ethiopia Using SWAT Model2010Inngår i: Journal of the American Water Resources Association, ISSN 1093-474X, E-ISSN 1752-1688, Vol. 46, nr 3, s. 514-526Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Soil and Water Assessment Tool (SWAT) was tested for prediction of sediment yield in Anjeni-gauged watershed, Ethiopia. Soil erosion and land degradation is a major problem on the Ethiopian highlands. The objectives of this study were to evaluate the performance and applicability of SWAT model in predicting monthly sediment yield and assess the impacts of subbasin delineation and slope discretization on the prediction of sediment yield. Ten years monthly meteorological, flow and sediment data were used for model calibration and validation. The annual average measured sediment yield was 24.6 tonnes/ha. The annual average simulated sediment yield was 27.8 and 29.5 tones/ha for calibration and validation periods, respectively. The study found that the observed values showed good agreement with the simulated sediment yield with Nash-Sutcliffe efficiency (NSE) = 0.81, percent bias (PBIAS) = 28%, RMSE-observations standard deviation ratio (RSR) = 0.23, and coefficient of determination (R superset of) = 0.86 for calibration and NSE = 0.79, PBIAS = 30%, RSR = 0.29, and R superset of = 0.84 for validation periods. The model can be used for further analysis of different management scenarios that could help different stakeholders to plan and implement appropriate soil and water conservation strategies.

  • 28.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Melesse, A. M.
    Abbaspour, K.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, R.
    Effect of Downscaling Methods on the Assessment of the Impact of Climate Change in Watershed Scale Management Models2009Inngår i: Proceedings of the American Water Resources Association - AWRA 2009: Spring Specialty Conference: Managing Water Resources Development in a Changing Climate/TPS-09-1, 2009Konferansepaper (Fagfellevurdert)
  • 29.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Melesse, A. M.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, R.
    Water Resources Variability as a Result of Changing Climate: A Case Study in the Lake Tana Basin, Blue Nile Ethiopia2009Inngår i: Proceedings of the American Water Resources Association (AWRA) 2009: Spring Specialty Conference: Managing Water Resources Development in a Changing Climate /TPS-09-1, 2009Konferansepaper (Fagfellevurdert)
  • 30.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Rayner, David
    Melesse, Assefa M.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Impact of Changing Climate on Water Resources Variability in the Lake Tana Basin, EthiopiaManuskript (preprint) (Annet vitenskapelig)
  • 31. Setegn, Shimelis Gebriye
    et al.
    Rayner, David
    Melesse, Assefa M.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Raghavan
    Impact of climate change on the hydroclimatology of Lake Tana Basin, Ethiopia2011Inngår i: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 47, s. W04511-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change has the potential to reduce water resource availability in the Nile Basin countries in the forthcoming decades. We investigated the sensitivity of water resources to climate change in the Lake Tana Basin, Ethiopia, using outputs from global climate models (GCMs). First, we compiled projected changes in monthly precipitation and temperature in the basin from 15 GCMs. Although the GCMs uniformly suggest increases in temperature, the rainfall projections are not consistent. Second, we investigated how changes in daily temperature and precipitation might translate into changes in streamflow and other hydrological components. For this, we generated daily climate projections by modifying the historical data sets to represent the changes in the GCM climatologies and calculated hydrological changes using the Soil and Water Assessment Tool (SWAT). The SWAT model itself was calibrated and validated using the flows from four tributaries of Lake Tana. For the Special Report on Emissions Scenarios A2 scenario, four of the nine GCMs investigated showed statistically significant declines in annual streamflow for the 2080-2100 period. We interpret our results to mean that anthropogenic climate changes may indeed alter the water balance in the Lake Tana Basin during the next century but that the direction of change cannot be determined with confidence using the current generation of GCMs.

  • 32.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Texas A & M University, Spatial Science Laboratory.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Hydrological Modelling in the Lake Tana Basin, Ethiopia using SWAT model2008Inngår i: The Open Hydrology Journal, ISSN 1874-3781, Vol. 2, s. 49-62Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The SWAT2005 model was applied to the Lake Tana Basin for modeling of the hydrological water balance. The main objective of this study was to test the performance and feasibility of the SWAT model for prediction of streamflow in the Lake Tana Basin. The model was calibrated and validated on four tributaries of Lake Tana; Gumera, GilgelAbay, Megech and Ribb rivers using SUFI-2, GLUE and ParaSol algorithms. The sensitivity analysis of the model to subbasin delineation and HRU definition thresholds showed that the flow is more sensitive to the HRU definition thresholds than subbasin discretization effect. SUFI-2 and GLUE gave good result. All sources of uncertainties were captured by bracketing more than 60% of the observed river discharge. Baseflow (40% - 60%) is an important component of the total discharge within the study area that contributes more than the surface runoff. The calibrated model can be used for further analysis of the effect of climate and land use change as well as other different management scenarios on streamflow and soil erosion.

  • 33.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Texas A & M University, Spatial Science Laboratory.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Identification of Erosion Potential Areas in Lake Tana Catchment, Ethiopia2008Konferansepaper (Annet vitenskapelig)
  • 34.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Texas A & M University, Spatial Science Laboratory.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Identification of High Erosion Potential Areas in Lake Tana Watershed, Ethiopia: Using GIS and SWAT Model2008Inngår i: American Water Resources Association Spring Specialty Conference 2008: GIS and Water Resources V, 2008Konferansepaper (Annet vitenskapelig)
  • 35.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Texas A & M University, Spatial Science Laboratory.
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Streamflow Calibration and Validation of SWAT2005/ArcSWAT in Anjeni Gauged Watershed, Northern Highlands of Ethiopia2006Inngår i: Proceedings of the 4th International SWAT Conference, 2006Konferansepaper (Annet vitenskapelig)
  • 36.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Melesse, Assefa M.
    Spatial delineation of soil erosion vulnerability in the Lake Tana Basin, Ethiopia2009Inngår i: Hydrological Processes, ISSN 0885-6087, E-ISSN 1099-1085, Vol. 23, nr 26, s. 3738-3750Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The main objective of this study was to identify the most vulnerable areas to soil erosion in the Lake Tana Basin, Blue Nile, Ethiopia using the Soil and Water Assessment Tool (SWAT), a physically based distributed hydrological model, and a Geographic Information System based decision support system that uses multi-criteria evaluation (NICE). The SWAT model was used to estimate the sediment yield within each sub-basin and identify the most sediment contributing areas in the basin. Using the NICE analysis, an attempt was made to combine a set of factors (land use, soil, slope and river layers) to make a decision according to the stated objective. On the basis of simulated SWAT, sediment yields greater than 30 tons/ha for each of the sub-basin area, 18.4% of the watershed was determined to be high erosion potential area. The NICE results indicated that 12-30.5% of the watershed is high erosion potential area. Both approaches show comparable watershed area with high soil erosion susceptibility. The output of this research can aid policy and decision makers in determining the soil erosion 'hot spots' and the relevant soil and water conservation measures. (C) Copyright 2009 John Wiley & Sons, Ltd.

  • 37.
    Setegn, Shimelis Gebriye
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Srinivasan, Ragahavan
    Melesse, Assefa M
    Dargahi, Bijan
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    SWAT model application and prediction uncertainty analysis in the Lake Tana Basin, Ethiopia2010Inngår i: Hydrological Processes, ISSN 0885-6087, E-ISSN 1099-1085, Vol. 24, nr 3, s. 357-367Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Lake Tana Basin is of significant importance to Ethiopia concerning water resources aspects and the ecological balance of the area. Many years of mismanagement, wetland losses due to urban encroachment and population growth, and droughts are causing its rapid deterioration. The main objective of this study was to assess the performance and applicability of the soil water assessment tool (SWAT) model for prediction of streamflow in the Lake Tana Basin, so that the influence of topography, land use, soil and climatic condition on the hydrology of Lake Tana Basin can be well examined. The physically based SWAT model was calibrated and validated for four tributaries of Lake Tana. Sequential uncertainty fitting (SUFI-2), parameter solution (ParaSol) and generalized likelihood uncertainty estimation (GLUE) calibration and uncertainty analysis methods were compared and used for the set-up of the SWAT model. The model evaluation statistics for streamflows prediction shows that there is a good agreement between the measured and simulated flows that was verified by coefficients of determination and Nash Sutcliffe efficiency greater than 0.5. The hydrological water balance analysis of the basin indicated that baseflow is an important component of the total discharge within the study area that contributes more than the surface runoff. More than 60% of losses in the watershed are through evapotranspiration. Copyright (C) 2009 John Wiley & Sons, Ltd.

1 - 37 of 37
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