Saving the Guajataca Dam Spillway: Emergency Actions and Interim Risk Reduction Measures (IRRMs)

Guajataca Dam is located in the northwest quadrant of Puerto Rico and is owned and operated by Puerto Rico Electric Power Authority (PREPA). The dam system consists of an earthen embankment
(approximately 1,000 ft long by 120 ft high), with an intake tower and outlet tunnel along the right abutment, and an uncontrolled concrete spillway on the left abutment. The approximately 30,000 cfs capacity spillway has a semicircular concrete weir, with 9 inch concrete slabs in the upper half or so, and 4 to 6 six inch thick lightly reinforced slabs in the lower half (pre-Maria). The original dam was designed and built approximately between 1923 and 1928, with significant modifications performed in the early 1980s. The outlet works discharge water to the west to a water supply canal, as well as to the Guajataca River to the northwest, with an estimated system discharge capacity of about 650 cfs.

The dam was designed and built primarily for irrigation and water supply purposes by the Isabela Irrigation Service (IIS), a state or municipal agency no longer in existence. The meager documentation available, dating back to a 1918 War Department report, indicates that two types of dams were initially considered at the time of inception, a concrete dam with incorporated spillway, and an earthen dam with a spillway on its left abutment. A 1923 US Bureau of Reclamation report indicated that the concrete dam option was dropped over concerns for potential movement and/or foundation conditions, based on limited geological information, and both reports recommended additional geologic studies. The first geologic mapping of Puerto Rico (by USGS) was not published until the 1960s and has been updated only once since. Today we know that the valley in which the dam was founded is landslide territory. From its beginning, there were troubles with movement of the dam as it was being built, as well as issues with cracking of the concrete spillway slabs, leading to design changes and retrofits over its 90+ years of life.