Around 11.5 * 106 m3 of rock detached from the eastern slope of the
Santa Cruz valley (San Juan province, Argentina) in the first fortnight
of January 2005. The rockslide?debris avalanche blocked the course,
resulting in the development of a lake with maximum length of around
3.5 km. The increase in the inflow rate from 47,000?74,000 m3/d between
April and October to 304,000 m3/d between late October and the first
fortnight of November, accelerated the growing rate of the lake.
On 12 November 2005 the dam failed, releasing 24.6 * 106 m3 of water.
The resulting outburst flood caused damages mainly on infrastructure,
and affected the facilities of a hydropower dam which was under construction
250 km downstream from the source area. In this work we describe
causes and consequences of the natural dam formation and failure,
and we dynamically model the 2005 rockslide?debris avalanche with
DAN3D. Additionally, as a volume ~ 24 * 106 m3of rocks still remain
unstable in the slope, we use the results of the back analysis to
forecast the formation of a future natural dam. We analyzed two potential
scenarios: a partial slope failure of 6.5 * 106 m3 and a worst case
where all the unstable volume remaining in the slope fails. The spreading
of those potential events shows that a new blockage of the Santa
Cruz River is likely to occur. According to their modeled morphometry
and the contributing watershed upstream the blockage area, as the
one of 2005, the dams would also be unstable. This study shows the
importance of back and forward analysis that can be carried out to
obtain critical information for land use planning, hazards mitigation,
and emergency management.