THE IMPACT OF SOIL EROSION IN THE UPPER BLUE NILE ON DOWNSTREAM RESERVOIR SEDIMENTATION THE IMPACT OF SOIL EROSION IN THE UPPER BLUE NILE ON DOWNSTREAM RESERVOIR SEDIMENTATION DISSERTATION Submitted in fulfilment of the requirements of the Board for Doctorates of Delft University of Technology and of the Academic Board of the UNESCO-IHE Institute for Water Education for the Degree of DOCTOR to be defended in public on 28 th October 2014 in Delft, The Netherlands by YASIR SALIH AHMED ALI born in Wad Medani, Sudan BSc Civil Engineering, University of Khartoum, Khartoum, Sudan MSc Water Management, Water Management and Irrigation Institute, University of Gezira, Wad Medani, Sudan MSc Water Science and Engineering ‘Hydraulic Engineering and River Basin Development’, UNESCO-IHE, Delft, the Netherlands This dissertation has been approved by the supervisors: Prof.dr.ir. A.E. Mynett Prof.dr. N.G. Wright Composition of the Doctoral Committee: Chairman: Rector Magnificus, DelftUniversity of Technology Vice-chairman: Rector, UNESCO-IHE Prof.dr.ir. A.E. Mynett UNESCO-IHE/ TU Delft, Promotor Prof.dr. N.G. Wright UNESCO-IHE/ University of Leeds, Promotor Prof.dr.ir. N.C. van de Giesen Delft University of Technology Dr.ir. A. Crosato UNESCO-IHE/ TU Delft Dr.ir. A.J.F. Hoitink Wageningen University Prof.dr. Y.A. Mohamed HRC, Sudan/UNESCO-IHE Prof.dr.ir. W.Uijttewaal Delft University of Technology, reserve member CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2014, Yasir Salih Ahmed Ali All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publishers. Although all care is taken to ensure the integrity and quality of this publication and information herein, no responsibility is assumed by the publishers or the author for any damage to property or persons as a result of the operation or use of this publication and or the information contained herein. Published by: CRC Press/Balkema PO Box 11320, 2301 EH Leiden, The Netherlands e-mail: [email protected] www.crcpress.com – www.taylorandfrancis.co.uk ISBN 978-1-138-02742-8 (Taylor & Francis Group) To the memory of my lovely sister, Nada SUMMARY Population growth in the upper Blue Nile Basin led to fast land-use changes from natural forest to agricultural land, which resulted in speeding up the soil erosion processes. Soil erosion undesirably reduces soil fertility and hence the agricultural productivity upstream. Eroded sediment is transported to the lower Blue Nile Basin, where sedimentation occurs at many locations. In the reservoirs, sedimentation leads to serious reduction in storage capacity, causing hydropower generation problems and negative impacts on the socio-economic, environmental and ecological system. The sediment settling inside irrigation canals leads to water shortage and management difficulties. Sediment deposition in the main channel of the river raises the bed level and enhances flood risks. The Blue Nile River Basin is currently experiencing new developments, both in Ethiopia and Sudan. The Grand Ethiopia Renaissance Dam (GERD) is under construction about 30 km upstream of the Ethiopian-Sudanese border. Recently, the Roseires Dam located 110 km downstream the Ethiopian-Sudanese border has been heightened by 10 m, increasing the storage capacity of the reservoir by additional 3700 million m 3. Some dams are planned in Ethiopia for hydropower production. These developments will strongly affect the water resources and sediment deposition in the lower Blue Nile Basin. Sedimentation in the new reservoirs and in irrigation canals will depend on the operation of these dams, but the only effective solution to reduce the sedimentation problems is reducing the sediment input. This can be achieved by means of erosion control practices in the upper basin. For this, given the vastness of the upper basin, it is important to identify the areas where the largest amounts of sediment are produced. The main objective of this research is to identify these areas and quantify the amounts of sediment involved. This research investigates also the effects of the new developments on the sediment processes. Several knowledge gaps have been filled in by this research in order to fulfill the goals. Missing information comprised: Bathymetric and morphological data including river cross-sections, as well as river bed and bank material along the main river and tributaries. Hydrological data including flow discharges and sediment concentration. Water and sediment transport distribution along the river network at all flow conditions. History of sedimentation (including amounts) in the basin. Origin of the deposited sediment in the lower basin. Relation between land-use changes and sediment yield in the sub basins providing the largest amounts of sediment deposition in the lower basin. Extensive field surveys were conducted both in Ethiopia and Sudan as part of this study to bridge the knowledge gaps. The bed topography was measured at 26 cross sections along the Blue Nile River and major tributaries using an eco-sounder in Ethiopia and an Acoustic Doppler Current Profiler in Sudan. Soil samples were taken from the areas affected by erosion in the upper basin and from the bed and banks of the rivers. Suspended sediment samples were collected at several locations along the Blue Nile River and its tributaries, both in Ethiopia and Sudan. In addition, suspended sediment concentration was sampled on a daily basis near the Ethiopian- Sudanese border during the flood season for 4 years. The collected samples were analyzed at Addis Ababa University laboratory, the Hydraulics Research Station laboratory in Wad Medani and the Technical University of Delft. Annual flow discharge and sediment load balances were obtained by integrating the available and newly measured flow discharges, suspended sediment concentration and the results of numerical models using the Soil and Water Assessment Tool software (SWAT). The yearly sediment balances were estimated at several locations along the main river and the tributaries. Three regression approaches were used to determine the sediment loads from the rating curves derived from the measured data. These were developed using the linear and non-linear log-log regressions, while the statistical bias correction factor was used to improve the linear regression results. The water distribution along the entire river system was assessed in order to quantify the availability of the water resource at all seasons and flow conditions. A one- dimensional hydrodynamic model of the entire river network was developed including all known water uses for irrigation, as well as all major hydraulic structures and their operation rules using the Sobek software. The model was further used to study sediment transport via integration with the water quality module of the Delft3D software. This integrated model (Sobek Rural and Delft3D Delwaq) allowed simulating the morpholological processes along the Blue Nile River, from Lake Tana to Roseires Dam. The model was calibrated and validated based on the Roseires Reservoir historical bathymetric surveys, and the sediment concentrations measured at the Ethiopian-Sudanese border. The model was then run to predict the impact of Roseires Dam heightening and the construction of the Grand Ethiopian Renaissance Dam on sedimentation rates. The history of sedimentation inside Roseires Reservoir, the first sediment trap along the Blue Nile River, was studied by combining historical bathymetric data with the results of a quasi 3D morphodynamic model including vertical sorting (based on the Delft3D software). Selective sedimentation creates soil stratification inside the reservoir allowing for the recognition of specific wet or dry years. The most promising coring locations from where soil samples could be collected were identified by analyzing the results of the model, since the model allowed identifying the areas that were neither subject to net erosion nor to bar migration during the life span of the reservoir. A second measuring campaign took place at these locations to analyze the sediment deposited in the reservoir The origin of the sediment deposited in the reservoir was indentified based on the mineral characteristics of the material. X-Ray Diffraction laboratory analyses allowed assessing the mineral content in the sediment samples that were collected during the field campaigns from the eroded areas in the upper basins and from the deposited soil layers inside Roseires Reservoir. The integration of the results of the X-Ray Diffraction with a cluster analysis allowed identifying the source of the sediments deposited inside Roseires Reservoir. The results showed that the sub basins of Jemma, Didessa and South Gojam constitute the main sediment source areas. The implementation of erosion control practices can therefore start from these sub basins. The land use changes occurred in these sub basins in the last 40 years show that natural forest, woodland, wooded grassland and grassland decreased from more than 70% to less than 25% of the surface area. Instead, the cultivated area increased from 30% to more than 70% of the total surface area. Finally, model results showed that the annual sediment deposition inside the Renaissance Dam (under construction) will vary with time, with maximum