Bijan Dargahi

Hydrological Modelling in the Lake Tana Basin, Ethiopia Using SWAT Model

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 stream- flow in the Lake Tana Basin. The model was calibrated and validated on four tributaries of Lake Tana; Gumera, GilgelA- bay, Megech and Ribb rivers using SUFI-2, GLUE and ParaSol algorithms. The sensitivity analysis of the model to sub- basin 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.

Impact of climate change on the hydroclimatology of Lake Tana Basin, Ethiopia

[1] 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.

Scour development downstream of a spillway

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.

Flow characteristics of bottom outlets with moving gates

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-ϵ 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.

Climate Change Impact on Agricultural Water Resources Variability in the Northern Highlands of Ethiopia

Climate Change impact on Agricultural Water Resources Variability in the Northern Highlands of Ethiopia

Climate Change Impact on Water Resources and Adaptation Strategies in the Blue Nile River Basin

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.

A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part I: numerical validation

ABSTRACT 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.

Hydrodynamic and Transport Properties of Saltsjö Bay in the Inner Stockholm Archipelago

Abstract A three-dimensional hydrodynamic model was successfully calibrated and validated for Saltsjö 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 Saltsjö 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.

Mitigation of sedimentation problems in the lower reach of the River Klarälven

The study focuses on the sedimentation problems found in the lower reach of the River Klarälven, 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 modelwas used to study the influence of various river training measures. The modelwas 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.

A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part II: flow physics

ABSTRACT 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.