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Impact of flood by a possible failure of the Mosul dam

Abstract: 
The dam of Mosul on the Tigris river in northern Iraq is the largest in the country and holds around 11 km3 of water. Built on water permeable rock, the inherent instability of the Mosul dam has been known since it was built and this was mitigated by continuous grouting. In recent years adequate maintenance has not taken place, leading to concerns about the safety of the dam. In 2003 the government decided to lower the maximum water level from 330 to 319 metres. During the recent conflict, maintenance has again been disrupted and signs at the dam have triggered renewed concerns, highlighted in the Schnabel report. As a result, the government of Iraq have contracted the Italian engineering company Trevi to undertake remedial work on the dam. This work is scheduled to start later in the Spring of 2016. In late February 2016 the Iraqi government advised the residents of Mosul to move at least 5 km away from the river in case of breach. A UN mission of UNDAC experts left for Iraq in early April to assess the situation, supported by an associated expert from the European Union Civil Protection Mechanism. A number of studies have been devoted to the possible effects of a failure of the dam. In this study we perform a number of medium-resolution (180m) dynamic hydraulic simulations starting from various constant percentages of destruction of the dam and allowing the corresponding quantity of water (supposing the lake to be at its highest level) to flow downslope for periods of 6 and 12 days. Compared to previous studies we provide in addition a complete timescale of the water flow progression, detailed maps of the water depth and extent in the affected cities’ areas and focus more on the numbers of population affected by various water depths. The main scenario analysed in this study, where the dam is 26% destroyed, the level of the lake is at its maximum value of 330m, and most of the lake’s water is allowed to flow out fast, results in a very high wave of water (in places 25m high, mean height around 12m) arriving at Mosul city after 1h40min. The capital Baghdad is reached after about 3.5 days with a max water height of 8m and a mean of around 2m. The simulations suggest that in the above scenario a total of more than 6 million people (close to one sixth of the country’s population) will be affected by floodwaters, with two million of them facing water of more than 2m. Water heights of more than 10m would inundate an area with 270 000 people, most of them in Mosul city and its surroundings, whose lives, houses and infrastructure would risk complete destruction. A number of other scenarios involving lower initial levels of the lake are also analysed, considering levels of 319m (the current one), and 315, 309, 307, 305 and 300m. Significant reductions in the affected populations and later arrival times are seen for lower initial lake levels. The differences are quite prominent in Mosul as well as in Baghdad, where we also provide detailed maps of the inundation in the various cases. The scenario of part of the flood wave being diverted to lake Tharthar by an existing channel near Samarra is also examined in detail. These results are a first-order approximation, favouring speed of calculation over detail for a rapid assessment; it is planned to refine them later using higher resolution ground elevation data (that take much longer to run) and more detailed, time-dependent scenarios of dam failure.