Summaries and pointers to some primary sources can be found on the Wikipedia page I linked, which at a high-level answers these questions:
A September 2006 report by the United States Army Corps of Engineers noted, “In terms of internal erosion potential of the foundation, Mosul Dam is the most dangerous dam in the world.” The report further outlined a worst-case scenario, in which a sudden collapse of the dam would flood Mosul under 65 feet (20 m) of water and Baghdad, a city of 7 million, to 15 feet (4.6 m), with an estimated death toll of 500,000.[17] A report on 30 October 2007 by the US Special Inspector General for Iraq Reconstruction (SIGIR) said that the dam’s foundations could give way at any moment.[18]
According to The Economist, “One study says that if the dam collapses, Mosul would be submerged within hours. Another warns that half a million Iraqis could be killed by floodwaters, and more than a million forced from their homes. Disease and looting as the floodwaters raced through Baiji, Tikrit, Samarra, and even parts of Baghdad would complete that dreadful scenario.”[19] Nadhir al-Ansari, an engineer involved in the building of the dam who is currently Professor of Engineering at the Luleå University of Technology, Sweden, said that the floodwaters would take four hours to reach Mosul and 45 hours to reach Baghdad, and that more than a million people would be killed if a “good evacuation plan” were not in place.[20]
In 2004, dam manager Abdulkhalik Thanoon Ayoub ordered the dam’s water level, which can reach 330 metres (1,083 ft) above sea level, to have a maximum of 319 metres (1,047 ft), thus reducing the pressure on the structure. Nevertheless, Iraqi officials maintain that the U.S. government is overstating the risk. The Army Corps of Engineers has proposed that the Badush Dam downstream be completed to serve its purpose of obstructing the large wave that would result if the Mosul Dam collapsed. This has been resisted by Iraqi officials, who note that the current plan for the Badush Dam is US$300 million to provide hydroelectric power and help irrigation while the proposed expansion would cost $10 billion.[17]
In 2007, the U.S. Army Corps of Engineers developed and executed a US$27 million plan to help continue maintenance and repairs on the dam in the short-term. The Iraq Government was also recommended a long-term solution that includes the construction of 67 m (220 ft) deep walls around the dam foundation. The project would cost $4 billion and take approximately four to five years to complete.
If people have questions about the estimates, I recommend directly contacting and asking the authors of primary sources, if they’re available (and letting us know what you learn). I will add this Wikipedia information into the post for legibility.
None of those studies mention 1.5 million deaths. One of the studies estimating 500,000 deaths is cited as outlining a worst-case scenario. The most pessimistic estimate of 1 million deaths is conditional on no good evacuation plan being in place. More importantly, all of these studies were based on the dam’s condition before extensive repairs were carried out in 2016–2017—repairs that were undertaken to address just those safety concerns. As the Wikipedia article notes, “In May 2017, Iraqi Minister of Water Resources Hassan Janabi stated that no danger to the dam remained and it was going back to normal operation. Carlo Crippa, the project manager, said the dam structure now showed no significant signs of distress.”
Summaries and pointers to some primary sources can be found on the Wikipedia page I linked, which at a high-level answers these questions:
If people have questions about the estimates, I recommend directly contacting and asking the authors of primary sources, if they’re available (and letting us know what you learn). I will add this Wikipedia information into the post for legibility.
None of those studies mention 1.5 million deaths. One of the studies estimating 500,000 deaths is cited as outlining a worst-case scenario. The most pessimistic estimate of 1 million deaths is conditional on no good evacuation plan being in place. More importantly, all of these studies were based on the dam’s condition before extensive repairs were carried out in 2016–2017—repairs that were undertaken to address just those safety concerns. As the Wikipedia article notes, “In May 2017, Iraqi Minister of Water Resources Hassan Janabi stated that no danger to the dam remained and it was going back to normal operation. Carlo Crippa, the project manager, said the dam structure now showed no significant signs of distress.”