Sedimentological and Hydrological Investigation of Mosul Dam Reservoir

Sammanfattning: Reservoir sedimentation is the main problem that directly affects the performance of dams due to the reduction in the storage capacity of their reservoirs. Monitoring the storage capacity of reservoirs is an important issue for the planners, designers and operators of the dams. Iraq mainly depends on the rivers Tigris and Euphrates for its water resources. Until the 1970s, Iraq was regarded as a rich country with regard to its water resources, due to the presence of the Tigris and Euphrates rivers. Recently, its water resources have decreased significantly due to an increased water demand and global climate changes. In view of this situation, it became necessary to know about Iraq’s water resource trends to adopt prudent water resource management strategies. Among these strategies is the assessment of the sedimentation rates in its reservoirs to determine their actual storage capacities and reduction rates of storage capacity through time. Mosul Dam Reservoir (MDR) is the biggest and one of the most important strategic projects in Iraq. It is a multipurpose project constructed to store water and to handle flood control and hydropower generation but the main purpose was to provide water for three irrigation projects that cover 2,500 km2 of agricultural areas. The dam is located on the River Tigris in northern Iraq, 60 km north-west of the city of Mosul. The project was designed to store 11.11 km3 with water surface area of about 380 km2 at the maximum operation level 330 m a.s.l. It is noteworthy to mention that MDR has operated since 1986, and no detailed studies have been carried out to determine the sedimentation characteristics in its reservoir since that time. In the present work, the storage capacity, sedimentation rates, area-storage capacity (ASC) curves, sediment nature and their grain size distribution and bottom morphology of its reservoir were studied. Direct and indirect methods were used to achieve these goals. In the direct methods, two topographic maps for MDR’s area were established in a triangular irregular network (TIN) format using Arc/GIS software. One of them before dam construction from pre-construction topographic map scale 1:50000 and other from bathymetric survey that was conducted in 2011. These maps were used; to calculate the volume of deposited sediment, to develop and evaluate ASC curves, to determine the bed morphology and to estimate the useful life of MDR. The results of the two surveys indicated that 1.143 km3 of sediment were deposited in MDR during the 25 years of its operation. This implies that the reduction in its original storage capacity was 10.29% with an annual reduction rate of 0.441% which is less than the sedimentation rates in the worldwide and Middle East. Furthermore, the results showed that 0.563 km3 and 0.58 km3 of sediment were deposited in live storage and dead storage zones respectively. This indicates that the live and dead storage zones lost 6.9% and 19.66% of their storage capacity till 2011 respectively. According to these results, MDR’s useful life willbe about 127 years or 121.5 years based on the depletion of its dead storage or 50% of its maximum storage capacity respectively. Likewise, the survey suggested new ASC curves for MDR. Comparison of the two TIN maps showed most of the sediment was deposited in the upper part of MDR, where the River Tigris enters MDR and it gradually reduced towards the dam site. There were erosion areas within the reservoir and mainly close to the dam body which might be due to dissolving the gypsum. In addition, fifty six sediment samples were collected from the bottom of its reservoir to study the nature of sediment deposited using the Van-Veen grab sampler. The samples were covering most of the reservoir area. The results revealed that the sediments were comprised of gravel, sand, silt and clay in the ratios 3.8%, 15%, 55.5% and 25.7% respectively. The distribution of these sediments indicates that the silt portion represents the highest or 77% of the bottom sediment of this reservoir followed by clay 13.5% and then sandwith gravel 9.5%. However, sand percentages are the highest in the northern zone of the reservoir where the River Tigris enters the reservoir and decrease gradually towards the dam site. In the meantime, silt percentage decreases towards the dam site whilst the finer fraction (i.e. clay) increases. The sediment is poorly sorted, nearly symmetrical in skewness and leptokurtic, very leptokurtic, to mesocratic. Indirect approaches using several empirical and semi-empirical techniques to determine sedimentation characteristics in MDR were used. Three empirical approaches based on the particle grain size of the sediment deposited showed that MDR’s useful life ranged from 122.5 to 132 years. Six different empirical methods to determine the sediment trap efficiency (TE) of MDR were adopted. These methods were based on the residence time principle (water retention time). The methods were used to determine monthly TE and long-term TE for MDR for the period 1986 to 2011. The TE results with sediment entering MDR wereused to calculate the amount of sediment deposited during its operational period. The comparison of the results with the bathymetric survey showed that all techniques gave good agreement, especially those depending on monthly TE, but the method that was proposed for large dams gave a more accurate result with 0.350% percentage error. To find out the sediment deposited depth at the Mosul dam site and to establish and predict changes in its ASC curves, four empirical and semi-empirical techniques were used for MDR. The results obtained were evaluated using the bathymetric survey data. The comparison of the results for establishing the ASC curves showed that one method agreed with bathymetric results whilst two methods gave good results for the sedimentation depth at the dam site. Furthermore, three of these methods were modified to predict the future changes in the ASC curves with sedimentation, based on the data from 11 reservoirs in the USA. The modified approaches were applied to MDR to predict the futureASC curves for 50, 75, 100 and 125 years. The curves predicted by these methods demonstrated compliance with the method adopted by the U.S. Bureau of Reclamation ‘area reduction method’. Finally, the indirect approaches can help decision makers, planners and designers, to monitor sedimentation characteristics and to implement prudent strategies for management of MDR in the future and to know the most suitable technique to adopt for MDR in the future.