The U.S. airbase and military supply port located in the Azores is a vital link in the military supply chain. In 2001, the 1,975-ft-long breakwater that protects the port had deteriorated to the point that the harbor facilities were in peril. In response, the Naval Facilities Engineering Command (NAVFAC) retained the services of a private contractor to provide technical guidance and input on the coastal engineering and construction aspects for the rehabilitation of the breakwater.
The Azores Islands are strategically located in the North Atlantic Ocean about 2,600-mi east of New York City, N.Y. The nine-island archipelago is one of two autonomous regions that, together with mainland Portugal, form the Portuguese Republic. About 1,000-mi west of the mainland, the Azores have been at the crossroads of navigation in the North Atlantic since the 15th century: Christopher Columbus stopped in the Azores on his first return trip from the New World in 1493.
In 1943, the British invoked a 500-year-old treaty with the Portuguese to establish an airbase at Lajes Field on Terceira Island. The U.S. was given permission to assist the British in setting up the base and within two weeks the U.S. Navy arrived to start construction of unloading facilities and a fuel pipeline in Praia Bay, where a permanent military port was established. Lajes Field, leased from the Portuguese government by the U.S. Air Force, now provides support for Department of Defense, allied nation and other authorized aircraft.
The breakwater protecting the military port was constructed in the early 1960s with stone from a local quarry. Armor stone sizes were 5-T to 9-T and 9-T to 13-T on a slope of 2:1. Construction reports at the time noted that the armor stone was undersized and the construction was modified by utilizing 15-T concrete blocks as armor protection. Since completion of construction, the breakwater was subjected to decades of damaging storms and had deteriorated significantly. In hindsight, the damage was predictable; however, at the time of construction, the ability to predict wave heights and design storms was very limited.
An Emergency Repair
In December 2001, a storm with a wave height approaching the 100-year return period breached the breakwater and threatened the fuel unloading pier. The breach—and the associated accelerated damage to the breakwater—put the fuel pier in jeopardy due to possible structural damage to the pier itself and an increase in the hazards associated with the transfer of fuel to the shore-based facilities. A major part of the Air Force’s support mission relies on its ability to provide refueling services to U.S. and allied nation planes. The breach required emergency repair.
Emergency repairs to close the breach in this area were completed by placing 20-T concrete antifer cubes as armor protection on new core and under-layer stone. These materials were selected because they were readily available on the island and there was limited time to design and implement a more sophisticated repair prior to winter. It was recognized that this emergency repair would not last long and attention was directed to a more substantial repair to be undertaken in 2003 while studies were developed to determine the most cost-effective long-term rehabilitation of the breakwater.
The U.S. Air Force is responsible for the military port and breakwater on Terceira. In the Azores, civil works for the Air Force are undertaken by NAVFAC Atlantic. W.F. Baird & Assoc. Ltd. was chosen to provide technical guidance and input on the coastal engineering design aspects of the repair project and to coordinate the model studies with the U.S. Army Engineering and Research Development Center (ERDC) in Vicksburg, Miss.
Physical model studies played an extremely important role in the development of the design solution. It was clear that a replacement for the undersized concrete armor units was going to be needed. Initial calculations determined the most cost-effective concrete armor unit would be the ERDC-developed COR-LOC armor unit. For the temporary repair, it was decided to utilize an approach that stabilized the underwater slope with the previously used antifers and armor the breakwater crest with 32-T COR-LOC units. This temporary repair was undertaken and completed in the 2003 construction season.
A Permanent Solution
As the temporary repair was ongoing, a permanent design concept was being developed. There were two significant storms at the end of 2003 and beginning of 2004. From observation by the professional staff during these initial repair efforts, it was evident that the whole breakwater was rapidly deteriorating. Among the several major issues to be considered in the permanent design were long-term degradation of the breakwater over its entire length; how to stabilize any repaired section within the damaged structure during the winter storm season; and the limited funding stream for repairs, which necessitated a phase approach.
Concurrent with the temporary repairs, Baird issued three reports that described the wave climate at the site that were used to support the development of the designs for the permanent breakwater repairs:
- The first report, Final Wave Data Report, June 2003, presented wave data used to support the physical model studies performed by the ERDC Coastal and Hydraulic Laboratories (CHL).
- The second report, Wave Conditions Occurring in October 2003, dated January 2004, presents the results of a wave hindcast undertaken by Baird to predict the wave conditions that occurred in fall 2003. A storm that occurred on Oct. 26, 2003, produced relatively large wave conditions at the breakwater. Sections of the breakwater that had not been temporarily repaired in the 2003 construction season were damaged; however, the section that had been repaired with the 32-T COR-LOC units was not damaged. The hindcast was done to determine the magnitude of the waves during the storm and to provide an improved understanding of both the stability of the temporary repair and the stability of the unrepaired sections of the breakwater.
- The third report, Wave Climate Study, May 2004, supplements the first report by presenting statistics that describe the variability in the wave climate throughout the year. The statistics were intended to assist the contractor in developing a methodology for breakwater repair.
A review of the condition of the structure as it existed at the end of 2003 showed the breakwater was deteriorating rapidly. The undersized concrete armor units from the 1960s continued to be damaged every year. Progressive damage had reduced the return period for destruction of the existing armor layer to about five years. Two-dimensional flume testing of the temporary repair predicted the antifer toe protection would be damaged by storms with a recurrence interval of 10 years to 15 years. Given the high probability that the breakwater could be breached in the subsequent five years, it was recommended that a consistent repair section be implemented along the entire length of the breakwater.
Many repair concepts were considered and four were chosen for further investigation in the development of the design for the repair of the breakwater. These ranged from no additional work, to conventional design with concrete blocks, to protecting the breakwater with a wide berm of stone. In addition, a “hybrid” solution was tested with 32-T COR-LOC concrete armor units protecting the slope above water and a berm of quarried stone protecting the toe of the COR-LOC units. This last concept took advantage of the existing relatively stable S profile of the breakwater—enhancing it rather than removing it.
A program of physical model studies was completed at ERDC. Various refinements to the design concepts were tested. The physical modeling allowed a trial-and-error approach to breakwater design whereby different concepts were evaluated. The test results showed the hybrid solution consisting of 32-T COR-LOC units protecting the crest of the breakwater and a berm of 5-T to 20-T stone protecting the toe of the COR-LOC was the best repair option for the conditions at the military port. A preliminary set of construction documents was issued in May 2004, and work was completed by a Portuguese contractor in April 2009.