Dam Failure Case Study: Guajataca Dam (Puerto Rico, 2017)

Guajataca dam is located in northwest Puerto Rico and is a hydraulic fill core embankment dam designed and constructed in the 1920s to provide irrigation and municipal water to the region. In September 2017, about two weeks after Hurricane Irma passed to the north, the island was directly impacted by Hurricane Maria. The precipitation resulted in discharge flows through the spillway that exploited design flaws and structural defects causing undermining and removal of spillway slabs; head-cutting; partial erosion of the stabilization buttress; and failure of portions of the downstream concrete chute leaving a large scour hole in the lower half of the spillway.

The initiation of erosion was attributed primarily to the poor alignment of the spillway with respect to the river that resulted in flows spilling off the lower left side in an area with no scour protection. The flows caused erosion and head cutting of the weak underlying foundation materials that subsequently undermined the spillway chute and caused the lower concrete slabs to fail. Most of the project was constructed on a large, active, translational landslide complex that has caused significant, ongoing distress to the dam and spillway structure since initial construction. The damaged condition of the spillway and eroded embankment and stability berm raised concerns that continued spillway flows would further erode the foundation and enlarge the scour hole resulting and head-cutting creating a direct connection with the pool and/or cause removal of the stabilization buttress that could accelerate the foundation landslide and jeopardize the integrity and safety of the dam.

The US Army Corps of Engineers (USACE) was engaged to support the dam owner and provide emergency measures to stabilize the failed spillway; restore the downstream stabilization buttress; protect the dam from further erosion; and develop Interim Risk Reduction Measures (IRRM*) to mitigate the failed structure and compromised buttress.

*Note: although IRRM’s may represent significant structural repairs, they are not considered by USACE to be a long-term permanent fix that mitigates all spillway or dam potential failure modes that may be actionable at this dam.