Fixing the Most Dangerous Dam in the World

TL;DR

Mosul Dam earned its 'most dangerous' title because its gypsum foundation dissolves in water, requiring a $500M grouting overhaul to prevent catastrophic collapse.

Key Points

• 1.Mosul Dam was built on fundamentally flawed geology. Its foundation contains gypsum, which is 200 times more soluble in water than limestone, creating a positive feedback loop of dissolution, voids, and increased seepage from the moment the reservoir filled.
• 2.Seepage and sinkholes confirmed the danger immediately. Within one year of filling, seepage reached 800 liters (200 gallons) per second — enough to fill an Olympic pool hourly — and sinkholes began appearing downstream as underground caverns collapsed.
• 3.The US Army Corps of Engineers declared it 'the most dangerous dam in the world' in 2006. Risk modeling using even the Iraqi minister's optimistic 1-in-1000 annual failure probability placed expected fatalities at 500–1,500 per year, far into the 'unacceptable' red zone on dam risk charts.
• 4.ISIS seized the dam in August 2014, halting the critical 24/7 grouting operations for 8 days. Equipment was looted, the workforce was disrupted, and cement supplies were cut off, causing voids to go untreated and reigniting fears of imminent collapse before Kurdish and Iraqi forces recaptured it.
• 5.A $500M Italian-led rehabilitation project ran from 2016 to 2019. Crews drilled 5,000+ boreholes totaling 400 km, injected 41,000 cubic meters of grout, and achieved 98% of permeability tests below 3 Lugeons — conducted near active ISIS front lines with military security throughout.
• 6.The grouting is a half-billion-dollar bandaid, not a permanent fix. Gypsum dissolution continues as long as the reservoir exists, so Iraqi staff were trained to maintain ongoing grouting operations indefinitely after the project concluded.
• 7.Two permanent solutions exist but remain unaffordable. Completing the downstream Badush Dam (a backup flood-capture structure halted in the late 1980s) or installing a deep cutoff wall — estimated at $3–5 billion in 2018 — are both on the table but financially unresolved.