RECONNAISSANCE FOLLOWING THE SEPTEMBER 29, 2009
TSUNAMI IN SAMOA
Ian N. Robertson Lyle Carden H. Ronald Riggs Solomon Yim Yin Lu Young Krystian Paczkowski Devin Witt
UNIVERSITY OF HAWAII COLLEGE OF ENGINEERING
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
Research Report UHM/CEE/10-01 January 20, 2010
Reconnaissance Following the September 29th, 2009 Tsunami in Samoa
Ian N. Robertson Lyle Carden H. Ronald Riggs Solomon Yim Yin Lu Young Krystian Paczkowski Devin Witt
Research Report UHM/CEE/10‐01 January 20, 2010 Samoa Tsunami Reconnaissance January 20th, 2010
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Executive Summary On Tuesday, September 29, 2009, a magnitude 8.0 earthquake shook the islands of Samoa at 6:48 AM local time (17:48 UTC). Inhabitants of both American Samoa and Samoa (formerly known as Western Samoa) report feeling significant ground shaking for up to three minutes. USGS reported the earthquake depth at 18 km with epicenter at 15.509o South and 172.034o West. This indicates that the earthquake resulted from movement along the subduction zone between the Pacific plate and the Australian plate. The epicenter was approximately 190 km due South of Apia, capital of Samoa on the island of Upolu, and 195 km Southwest of Pago Pago, capital of American Samoa on the island of Tutuila. The earthquake generated a series of tsunami waves that reached the Samoan islands between 15 to 20 minutes after the earthquake. Tutuila Island in American Samoa suffered significant damage to coastal buildings and infrastructure on almost all shorelines. Preliminary runup measurements indicate maximum runup of 12 m (39 ft) in Poloa and Fagasa. Upolu island in Samoa suffered severe damage to coastal buildings and infrastructure along the southeast and eastern shores, while the rest of the island coastline was relatively unaffected. In American Samoa there were 32 deaths and 2 missing people attributed to the tsunami inundation. In Samoa, there were 137 deaths attributed to the tsunami inundation and 310 injuries, while 6 people are still missing. Anecdotal evidence indicated that the number of deaths was significantly reduced by public awareness of the connection between earthquakes and tsunamis. It appeared that many residents had moved to high ground after feeling the earthquake, while others who remained in coastal areas were on the lookout for abnormal ocean activity. On seeing the ocean withdrawal that preceded the first tsunami inundation, most coastal occupants headed for high ground, aware that this indicated the likelihood of a tsunami wave. The reconnaissance survey reported here was performed from November 2nd to 5th to document the effect of the tsunami on coastal structures and buildings on Tutuila Island, American Samoa, and Upolu Island, Samoa. The results of this survey will be compared with observations made during a similar survey by the authors of the Mississippi coastline after hurricane Katrina. In addition, effects of the tsunami and debris on near‐shore coral reefs around Tutuila Island were investigated using a remote operated vehicle (ROV). The results of this survey indicate that most timber and masonry residential structures subjected to tsunami loads suffered significant damage or complete destruction. However, traditional open‐walled fales and elevated homes fared much better, unless impacted by large floating debris. Engineered structures such as commercial buildings, schools and churches generally performed much better structurally than neighboring residential buildings. Churches in particular are often built slightly elevated above the surrounding land. In addition, it is likely that greater care and craftsmanship were exercised during construction of these structures because of the importance they held for the surrounding community. Significant structural damage to a new ferry pier in Malaela, Samoa, and the main drydock in Pago Pago, American Samoa, may have significant economic consequences. Damage to three coastal bridges on the only road around Tutuila Island had the potential to cut residents off from aid immediately after the disaster. Rapid reconstruction and repairs by the local communities
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allowed these bridges to open within days of the event. Scour depths up to 1.2 meters (4 ft) were observed around some residential foundations, though general widespread sediment transport, either onshore or offshore, was not observed on either of the islands surveyed. An underwater Remote Operated Vehicle (ROV) was used to survey damage to coral reefs at three locations around Tutuila Island, American Samoa. Video images from the ROV were used to assess damage to various types of coral and to record the types and quantity of debris deposited offshore by the tsunami drawdown. In general, more debris and greater damage to the coral reef was observed as the water depth decreased closer to shore. Offshore from Leone, A.S., debris could be found in depths up to 20 meters (65 ft), while the majority of the coral damage occurred in water depths less than 12 meters (39 ft). Offshore from Poloa, A.S., debris was found in depths up to 30 meters (98 ft), while coral damage occurred in water depths up to 17 meters (56 ft). A majority of the debris found offshore from both Leone and Poloa consisted of sheet metal roofing from damaged residences, and other debris such as clothing, wooden window frames, and tires. Damage to the coral reefs ranged from broken pieces of coral to entire coral heads broken off at the base and flipped upside down on the ocean floor.
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Acknowledgements The authors wish to acknowledge the assistance of a number of organizations and individuals in the collection of data for this report. Jeanne Johnston and Janet Wheatcroft of FEMA provided transportation on American Samoa. They also provided valuable first‐hand accounts and photographs taken shortly after the tsunami and assisted in arranging a dive boat for the coral reef survey. Joey Cummings of 93KHJ radio recounted his experiences on the second floor of Pago Plaza during the tsunami inundation. He also provided copies of numerous videos that he had collected of the tsunami inundation and aftermath. Karl Raynar helped to organize the meeting with Joey and accompanied us during the first day of survey on American Samoa. He also contributed numerous photographs taken immediately after the tsunami. Steven Baldridge and Hermann Fritz contributed photographs taken during their visit to American Samoa shortly after the tsunami. Sai Mauia served as guide and translator during our survey of American Samoa. He also assisted with travel arrangements for Samoa. The authors would also like to thank Prof. Guy Meadows and Dave Parsons of the Marine Hydrodynamic Laboratory at the University of Michigan for the generous loan of the ROV used in the underwater survey, and for providing instruction on its operation. We are also grateful to Paul Hillman and Dr. John Lindsay of the NOAA Media Center for providing high‐definition underwater images and video during the coral reef survey. Funding for this reconnaissance trip was provided by the National Science Foundation under grant number CMMI‐1005740. The authors would also like to acknowledge the general financial contribution from the Dept. of Naval Architecture and Marine Engineering at University of Michigan. This support is gratefully acknowledged.
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Table of Contents Executive Summary ...... 3 Acknowledgements ...... 5 Table of Contents ...... 7 Introduction ...... 9 The Samoan Islands ...... 9 Earthquake and tsunami characteristics ...... 10 Fatalities and injuries...... 13 Economic consequences ...... 14 Survey objectives ...... 14 Tsunami Characteristics ...... 15 Tsunami bore ...... 15 Run‐up data ...... 16 Buildings ...... 19 Engineered Buildings ...... 19 Pago Plaza – Two‐story steel framed office building, Pago Pago, A.S...... 19 Single‐story metal frame building in Pago Plaza ...... 22 Pre‐manufactured metal building, Pago Pago, A.S...... 25 Timber Framed Warehouse, Pago Pago, A.S...... 28 Matatula school buildings, Tula, A.S...... 30 Poloa school buildings, Poloa, A.S...... 31 Church buildings ...... 39 Non‐Engineered Residential Structures ...... 57 Reinforced Masonry Houses ...... 59 Timber Framed Houses ...... 75 Open Walled Fales ...... 80 Elevated Houses ...... 94 Bridges and Roadways ...... 101 Bridge abutment collapse, Leone, A.S...... 101 Bridge pier scour/settlement, Amanave, A.S...... 105 Coastal road damage, Fagasa, A.S...... 107
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Culvert roadway repair, Alega, A.S...... 109 Port Facilities ...... 113 Foundation/uplift failure of new ferry pier, Malaela, I.S...... 113 H‐pile wall collapse, Pago Pago, A.S...... 118 Barge uplift damage to wharf, Pago Pago, A.S...... 120 Small boat docks, Pago Pago, A.S...... 121 Ship mooring line failure, Pago Pago, A.S...... 124 Foundation Scour ...... 127 Tsunami Effect on Coral Reef ...... 131 References ...... 143 Appendix A – Survey Team ...... 145
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Introduction
The Samoan Islands The Samoan islands are located in the South Pacific about halfway between Hawaii and New Zealand. They are divided into two geopolitical entities, namely American Samoa (A.S.) and the Independent State of Samoa (I.S.), formerly known as Western Samoa. American Samoa is an unincorporated territory of the United States of America. There are 7 inhabited islands in American Samoa, with Tutuila as the largest and most populated. Based on the 2000 census released in June 2003, American Samoa had a population of around 57,000 (U.S. Census bureau, 2003). A current estimate of the population by the CIA World Factbook is 66,000 (C.I.A., 2009). Samoa gained its independence from New Zealand in 1962. It is made up of 10 islands of which Savai’i and Upolu are the largest. The majority of the population lives on Upolu where the capital, Apia, is located. The population based on a 2006 census was 179,000, while the current estimated population of Samoa is around 220,000 (C.I.A. World Factbook, 2009). Construction practices throughout the Samoan Islands have a lot of similarities. Experience gained from cyclones Ofa in 1990 and Val in 1991 lead to significant improvements in the wind resistant design and construction of residential and commercial buildings. Both states have tsunami warning signs along the coastlines (Figure 1) and in some hotels (Figure 2). Tsunami education in September as part of National Preparedness Month had warned residents about the potential of tsunamis following earthquakes. Figure 1: Tsunami Hazard Zone sign, A.S.
Figure 2: Warning notice on door of hotel rooms in Aggie Grey’s Hotel, Apia, I.S. 9 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Earthquake and tsunami characteristics On Tuesday, September 29, 2009, a Mw 8.0 earthquake shook the islands of Samoa at 6:48 AM local time (17:48 UTC). Inhabitants of both American Samoa and Samoa report feeling significant ground shaking for up to three minutes. USGS reported the earthquake depth at 18 km with epicenter at 15.509o South and 172.034o West. This indicates that the earthquake resulted from movement along the subduction zone between the Pacific plate and the Australian plate. The epicenter was approximately 190 km due South of Apia, capital of Samoa on the island of Upolu, and 195 km Southwest of Pago Pago (pronounced Pango Pango), capital of American Samoa on the island of Tutuila (Figure 3).
Figure 3: Mw 8.0 Earthquake epicenter location (USGS Image) The earthquake generated a series of tsunami waves that reached the Samoan islands between 15 to 20 minutes after the earthquake. DART buoys, located north of the Samoan islands and East of Tonga, only recorded the first tsunami wave one hour after the earthquake, and were therefore not helpful in providing a warning to the inhabitants of the Samoan islands (Figure 4 and Figure 5). However, the DART buoy readings were instrumental in allowing the cancellation of the tsunami watch for the majority of the Pacific basin including Hawaii and the US west coast.
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Figure 4: DART buoy locations near Samoan Islands
Figure 5: DART buoy records showing MOST model comparison (PMEL, 2009) 11 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
The tsunami waves were also recorded by a single tidal gage at the entrance to the Pago Pago harbor. Figure 6 shows the location of this tidal gage on the American Samoa Island of Tutuila, along with maximum wave heights predicted by PMEL using the MOST model. Figure 7 shows the wave height record at this tidal gage along with the PMEL MOST model predictions. Local residents confirmed the leading negative wave arrival time approximately 15 to 20 minutes after the earthquake. The maximum measured wave height of 2.5m appears consistent with the MOST model predictions, and with observed damage at that location on the south shore of the Pago Pago harbor entrance. The low predicted wave heights on the southeast shore were confirmed by the general lack of tsunami damage along this shoreline (Figure 6). This was particularly fortunate at the coastal airport runway which is the only point of entry for rapid delivery of off‐island relief supplies and emergency responders.
Figure 6: American Samoa Island of Tutuila showing MOST prediction of maximum wave heights (PMEL, 2009)
Figure 7: Pago Pago tidal gage record with MOST model prediction (PMEL, 2009)
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No tide gage data were available for Upolu Island, Samoa, but our survey found extensive inundation and damage along the east and southeast coastlines shown in Figure 8. There was limited to no inundation or damage along the southwest, west, and north coastlines as indicated in Figure 8.
Figure 8: Upolu Island, Samoa, indicating extent of tsunami damage
Fatalities and injuries In American Samoa there were 32 deaths and 2 missing people attributed to the tsunami inundation. The deaths reported by the LBJ Tropical Medical Center included 4 children, 16 adults (20 to 60 years) and 13 seniors over 60 years of age, one of which was attributed to a heart attack after the tsunami event (Johnson, 2009). Some of the deaths occurred when people were caught in their vehicles by the incoming waves. Because many of the injured were treated in place, and not transported to hospitals, there is no accurate count of injuries caused by the tsunami. In Samoa, there were 137 deaths attributed to the tsunami inundation and 310 injuries, while 6 people are still missing. About 3200 people (640 families) have been left homeless (OCHA, 2009). The majority of deaths occurred along the East and Southeast shorelines where high ground is relatively far inland, making escape more difficult for those who did not respond immediately after feeling the earthquake. Anecdotal evidence indicated that the number of deaths was significantly reduced by public awareness of the connection between earthquakes and tsunamis. It appeared that many residents had moved to high ground after feeling the earthquake, while others who remained in coastal areas were on the lookout for abnormal ocean activity. On seeing the ocean withdrawal
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that preceded the first tsunami inundation, most coastal occupants on American Samoa headed for high ground, aware that this indicated the likelihood of a tsunami wave. September 2009 was National Preparedness Month in the USA. In American Samoa, the Department of Homeland Security created a powerpoint presentation that was shown at schools and government agencies to train them in preparedness. After feeling the earthquake, teachers and staff at the Poloa school gathered the school children and walked along the coastal road for 500 meters to get to high ground just before the first tsunami wave struck and virtually destroyed their school buildings. A number of residents reported successfully seeking refuge in two‐story buildings. This includes Joey Cummings, a local radio and television personality, who was on the air from the second floor of Pago Plaza at the time of the earthquake and tsunami inundation of the first floor of the building. Lima Coulter in Leone reported that her grandchildren survived by seeking refuge on the second floor of a two‐story building.
Economic consequences In October, 2009, the Government of Samoa estimated the direct impacts of the tsunami were in the region of SAT$380 million ($147 million). Damage to the single power generation plant on American Samoa, located at sea level in Pago Pago harbor, resulted in lack of electrical power for all of Tutuila Island until FEMA generators were flown in and installed a week after the tsunami. Power was not lost on Upolu Island, Samoa, because power generation plants are located on the North side of the island which did not experience damaging tsunami inundation. A number of tourist resorts on the south coast of Upolu Island, Samoa, have had to close for several months while repairs are made to structures damaged by the tsunami. A new ferry pier in Malaela, Samoa, intended to improve ferry transportation between Samoa and American Samoa, was badly damaged and will likely have to be demolished and replaced before ferry service can start. Damage to dry‐docks in Pago Pago harbor has resulted in delays of over a month for repair of larger ships, primarily fishing boats. Commercial fishing and tuna canning are major economic activities on American Samoa.
Survey objectives The reconnaissance survey reported here was performed primarily to document structural effects of the tsunami on coastal structures and buildings on Tutuila Island, American Samoa, and Upolu Island, Samoa. The results of this survey will be compared with observations made during a similar survey by the authors after hurricane Katrina (Robertson, et al, 2006 & 2007). In addition, effects of the tsunami and debris on near‐shore coral reefs were investigated using a remote operated vehicle (ROV). The survey was performed 4 weeks after the tsunami to allow for the initial emergency response to be completed and give residents time to start recovering from the tragedy. The survey of Tutuila Island, American Samoa, was performed on Monday and Tuesday, November 2nd and 3rd, and the survey of Upolu Island, Samoa, was performed on Wednesday
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and Thursday, November 4th and 5th. The coral reef surveys were performed off the coast of Tutuila Island on November 4th and 5th. Although much of the debris had already been cleared, especially from the roadways, damaged buildings and other structures were still available for inspection during the survey. In addition, buildings that survived the tsunami were inspected to determine the reasons for their improved performance compared with other structures in the same area. Tsunami Characteristics
Tsunami bore Video footage of the initial tsunami inundation was captured by an FBI security camera located on the roof of the two‐story Pago Plaza building in Pago Pago (Figure 9). The camera was focused on the parking lot located on the shore side of the building (Figure 10). Figure 11 shows snapshots of the incoming wave as it picked up vehicles parked in the lot and washed them into a metal framed building at the bottom left of the camera image. This building was severely damaged during the tsunami and was demolished and removed before our survey.
Figure 9: Aerial view of Pago Pago harbor after tsunami (Courtesy of Hermann Fritz)
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Figure 10: Location of FBI security camera on roof of Pago Plaza building
Figure 11: Video capture images of incoming tsunami wave from FBI security camera By measuring the parking lot markings relative to this metal building, it was possible to estimate the velocity of the incoming wave at 5 m/s (18 kph, 11 mph).
Figure 12: Measurements used for tsunami wave velocity estimate
Run‐up data A number of run‐up surveys were performed shortly after the tsunami in an effort to record the extent of inundation before evidence was removed in the cleanup effort. The maximum run‐up 16 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
elevation was measured at 12m near Poloa and Fagasa on American Samoa. Figure 13 shows preliminary run‐up measurements around American Samoa. Run‐up measurements around Upolu Island, Samoa, were not yet available at the time this report was published.
Figure 13: Preliminary run‐up data (courtesy Vasily Titov)
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Buildings The performance of buildings subjected to tsunami wave loading varies greatly depending on the type of construction and level of engineering involved in the building design. In the following sections, buildings are separated into “engineered” and “non‐engineered”. Although design drawings were not available for any of the surveyed buildings, a judgment was made based on the type of construction and level of detailing as to whether engineering expertise had been involved at the time of design and construction of the building.
Engineered Buildings Buildings considered to be engineered are multistory commercial buildings, large‐span warehouses, school buildings and churches. A number of engineered buildings in both Samoa and American Samoa were impacted by the tsunami inundation. The following examples illustrate the performance of these buildings.
Pago Plaza – Two‐story steel framed office building, Pago Pago, A.S. Pago Plaza is a two‐story steel framed office building housing a number of retail stores at the ground level and commercial offices on the second level, including an FBI office and the 93KHJ radio station and NBC television affiliate (Figure 14). The building was inundated to approximately 2.3 m (7.5 feet) resulting in virtually complete destruction of all non‐structural components at the first floor level (Figure 15). At the time of our visit, all debris had been removed from the first floor level (Figure 16). The building structure consists of a steel frame with infill masonry walls, assumed to be reinforced and grouted (Figure 16). The steel frame consists of 10” square hollow steel sections as columns supporting wide flange girders and joists at the second level. The base of these columns is covered by the slab‐on‐grade at the first floor level, but is assumed to be bolted to the foundations below the slab‐on‐grade. The second floor is a concrete on metal deck slab supported on the steel joists. Diagonal bracing is provided in the plane of the second floor even though the concrete deck is usually assumed to serve this function. All connections between structural steel members appeared to be well‐designed and constructed (Figure 17). No damage was noted to any of the structural steel members. The infill masonry walls were also undamaged, though windows and doors had all failed, allowing water access to the ground floor. Because of their superior performance compared with other masonry walls, particularly in non‐engineered buildings, it was assumed that these walls were well reinforced and solid grouted. In addition, the walls span between the ground floor slab and the second floor diaphragm (approximately 4 m (13 ft)), giving them greater resistance to lateral loads than walls that cantilever from the ground floor only (Figure 18). In addition to hydrodynamic loads, this building was likely subjected to a number of impacts from floating debris, including cars from the parking lot and timber debris from adjacent buildings. There was no apparent damage from these debris impacts.
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Figure 14: Pago Plaza commercial building
Figure 15: Joey Cummings indicating inundation level in Pago Plaza
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Figure 16: Interior of ground floor of Pago Plaza after tsunami debris removal
Figure 17: Welded and bolted connections between steel framing members in Pago Plaza 21 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 18: CMU exterior wall spanning from ground floor to second floor of Pago Plaza
Single‐story metal frame building in Pago Plaza The single‐story metal building seen at the bottom left of the FBI video sequence (Figure 11) was struck by the incoming tsunami wave carrying a number of cars from the parking area adjacent to Pago Plaza. This building was completely destroyed by the resulting forces as seen in the video capture image taken the day after the tsunami (Figure 19). Inspection of the remaining foundation slab (Figure 20) and anchor plates (Figure 21) indicate that the metal building was framed with 100 mm diameter pipe columns. These columns failed at the weld between the pipe and the base plate. Fully grouted reinforced masonry walls forming part of building were also destroyed by the tsunami flow (Figure 22). Unlike the masonry walls of the adjacent Pago Plaza building, these walls would have had no lateral support at the roof framing, particularly after failure of the metal frame structure. They were unable to resist the hydrodynamic and debris impact loads while acting as cantilever walls, resulting in bending failure at the base of the wall (Figure 22).
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Figure 19: Video capture of metal frame building in Pago Plaza parking area (courtesy Joey Cummings)
Figure 20: Foundation of metal frame building in Pago Plaza parking area
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Figure 21: Failed weld connection between 100 mm diameter pipe column and base plate
Figure 22: Base of reinforced grouted CMU
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Pre‐manufactured metal building, Pago Pago, A.S. The community center building located adjacent to Pago Plaza was constructed using concrete masonry units (CMU) for the front section of the building and a pre‐manufactured long‐span metal frame structure for the back of the building (Figure 23). The pre‐manufactured metal frame was struck by a number of forms of floating debris, including a small boat and a shipping container (Figure 24). The metal frame members suffered significant damage due to failure of the base connections of two of the columns. Two 25 mm (1 in) diameter L‐shaped anchor bolts connecting the column base plate to the concrete pedestal broke free (Figure 25), probably because of failure of the pedestal (Figure 26). There is no evidence of steel reinforcing ties (hoops) in the pedestal, which would have helped to prevent the splitting failure that allowed the anchor bolts to break free. The resulting lateral displacement of the base of the column lead to lateral flexural failure of the roof beams supported by these columns (Figure 27).
Figure 23: Community center in Pago Plaza showing damage to pre‐manufactured metal framing at rear of building
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Figure 24: Small boat and shipping container impact damage to metal frame
Figure 25: Anchor bolts broken out of concrete pedestal
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Figure 26: Failed foundation pedestal (left) and undamaged pedestal (right)
Figure 27: Lateral bending failure of roof beam in pre‐manufactured metal building
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Timber Framed Warehouse, Pago Pago, A.S. A large timber framed warehouse adjacent to Pago Plaza suffered primarily non‐structural damage during the tsunami (Figure 28). The CMU walls around the base of the building were able to resist the tsunami loads without failure, while the timber framed low roof and walls above these CMU walls were damaged (Figure 29). The good performance of the CMU wall may in part be attributed to the numerous buttresses and wide cap beam connecting these buttresses around the perimeter of the warehouse. The heavy timber trusses supporting the roof were undamaged, however the non‐structural ceiling suspended from these trusses had partially collapsed. A second‐story wood framed office wing at the front of the building failed when it was lifted by the water either due to restrained flow uplift, or buoyancy. Debris caught between the first floor CMU walls and the second floor framing confirms the extent of the uplift (Figure 30).
Figure 28: Timber framed warehouse with damage to low roof and ceiling
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Figure 29: Damaged low roof around timber framed warehouse
Figure 30: Uplift damage to second story office wing
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Matatula school buildings, Tula, A.S. The school buildings in Tula, American Samoa, are constructed of reinforced concrete beam column frames with reinforced CMU infill walls with window openings (Figure 31). The buildings were subjected to approximately 1.5 m (5 ft) inundation during the tsunami. Adjacent residential CMU wall buildings experienced significant damage (Figure 85 to Figure 90) while damage to the school building was limited to CMU cracking below the window opening (Figure 32). The louver windows were also damaged during the tsunami. Had the windows been designed as breakaway elements, the damage to the surrounding CMU may have been prevented. By the time of our survey, the CMU walls had been repaired and new window frames installed and painted in preparation for re‐opening of the school (Figure 33).
Figure 31: Matatula school building viewed from shoreline
Figure 32: Damaged CMU and louver windows
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Figure 33: Repaired CMU walls and window frames
Poloa school buildings, Poloa, A.S. The small coastal village of Poloa was devastated by the tsunami inundation. Some of the highest runup measurements on American Samoa were made in and near Poloa. In addition to destroying almost every home in the village, the tsunami also caused extensive damage to the Poloa school buildings. Fortunately, teachers and staff at the school gathered the 120 plus school children and walked along the coastal road for 500 meters to get to high ground just before the first tsunami wave struck (Figure 34). The main school building is a single story open frame building with an 80 foot square floor plan (Figure 35). The main support structure consisting of 24”x24” reinforced concrete columns and 10”x30” reinforced concrete beams was undamaged by the tsunami (Figure 36 and Figure 37). However, all non‐structural walls and furnishings in the school were destroyed. In addition, wave uplift loads caused almost total failure of the lower roof structure around the perimeter of the building (Figure 38 and Figure 39). Although it probably would not have made any difference to the roof performance during this tsunami, it was noted that the weak link in the roof cladding attachment was between the purlin and roof rafter. Figure 40 shows the high quality attachment between the corrugated roof cladding and the supporting timber purlins. This connection is made with hex‐head screws and large washers to prevent pull‐through of the screw heads, located at every other ridge in the corrugated sheeting. However, the timber purlins were only connected to the supporting rafters using widely spaced nails, which pulled out due to the hydrodynamic uplift loads (Figure 41).
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The water level was sufficient to cause uplift to the central high roof (Figure 42), resulting in pullout of lag screws connecting the supporting posts to the roof beams (Figure 43) and horizontal splitting along the two side beams (Figure 44).
Figure 34: Evacuation route taken by Poloa school staff and students
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Figure 35: Poloa school buildings looking northeast
Figure 36: Typical reinforced concrete column in main school building
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Figure 37: Typical reinforced concrete beam in main school building
Figure 38: Damage to low roof on south side of main school building
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Figure 39: Damage to low roof on north side of main school building
Figure 40: Roof cladding attachment to supporting timber purlins
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Figure 41: Nails used to attach purlin to supporting roof rafter
Figure 42: Uplift damage to high roof on Poloa school building
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Figure 43: Main roof uplift led to failure of lag screw connection at top of supporting posts.
Figure 44: Horizontal cracks in roof beams caused by main roof uplift A second school building was completely destroyed by the tsunami (Figure 45). This building appeared to have been built for high wind loads as evidenced by the steel plate connectors at column bases and beam to column connections (Figure 46). Failure at these connections had typically occurred in the timber member at, or close to, the connection (Figure 47). In addition, wall base connections to the concrete slab‐on‐grade consisted of well‐anchored bolts at relatively close spacing (Figure 48). However, these high wind design features were unable to resist the hydrodynamic loads applied by the tsunami, resulting in complete structural failure.
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Figure 45: Complete destruction of school office building
Figure 46: Galvanized metal connectors for high wind design
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Figure 47: Failure of timber members at or near connections
Figure 48: Closely spaced wall hold‐down bolts unable to resist tsunami loads
Church buildings Church buildings in both Samoa and American Samoa seemed to perform significantly better than surrounding residential buildings, even though they were presumably subjected to the same tsunami wave inundation. Most churches are constructed of reinforced concrete beam‐ column frames, with reinforced concrete masonry unit (CMU) infill walls (Figure 49). They are often built slightly elevated above the surrounding land. In addition, it is likely that greater care
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and craftsmanship were exercised during construction of the churches because of the importance they held for the surrounding community.
Figure 49: Church under construction in Malaeloa, A.S. The church in Poloa is a good example of a structure that survived while all buildings around it were destroyed (Figure 50). The highest runup in Poloa was measured at 10.7m (35 feet) directly behind the church building as indicated in Figure 51. The presence of a small wall and grade level change in front of the church may have reduced the wave loading (Figure 52). Nevertheless, the water level was sufficient to damage the roof eave and all windows in the church, including the upper stained glass windows (Figure 53). However, there was no apparent damage to the structural frame and CMU walls. The main windows had been replaced and furnishings returned to the church at the time of our visit (Figure 54).
Figure 50: Poloa church shortly after tsunami (courtesy Hermann Fritz)
40 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Highest runup in Poloa (10.7m)
Figure 51: Poloa church shortly after tsunami (courtesy Hermann Fritz)
Figure 52: Stairs and small wall in front of church building in Poloa
41 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 53: Damage to eave and upper windows on Poloa church building
Figure 54: Poloa church one month after the tsunami showing estimated inundation level The Church of the Latter Day Saints (LDS Church) in Malaela, on the East coast of Samoa, was completely gutted by the tsunami wave. However, the structural frame remained relatively unscathed. The LDS church is located close to the shoreline and only a few feet above sea level (Figure 55). Most residential structures in the area were completely destroyed or extensively damaged. The structural frame of the church sanctuary was composed of a reinforced concrete frame and
42 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
CMU walls. The front wall of the sanctuary, facing the shoreline, consisted of a series of CMU walls that appeared to serve as buttresses to resist the tsunami loads (Figure 56). The front wall of the adjacent office/classroom building was a flat CMU wall, but strengthened by perpendicular interior CMU walls bracing the front wall (Figure 57). These features, and the likely superior quality of the reinforced masonry, may have contributed to the improved performance of these CMU walls compared with residential CMU walls that failed under similar tsunami loading. The tsunami flow lifted the concrete slab‐on‐grade at the church entrance, resulting in impact damage to some of the roof support columns (Figure 58). The remainder of the structural framing was virtually undamaged by the tsunami waves (Figure 59). All windows, including frames, had broken out of the side walls of the church sanctuary, leaving a relatively open frame, thus reducing the overall hydrodynamic loads on the structure (Figure 60). These window frames appeared to have been installed with only three fasteners on each jamb (Figure 61). This unintentionally resulted in the windows behaving as “breakaway” non‐structural elements. It is possible that this reduction in load on the structural posts between the windows reduced the potential structural damage. Replacement of the windows will be much less expensive than reconstruction of the entire building had there been extensive structural damage.
Figure 55: Location of LDS Church relative to ocean, Malaela, I.S.
43 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 56: Front CMU wall on LDS church sanctuary, Malaela, I.S.
Figure 57: Front CMU wall on office/classroom building at LDS church, Malaela, I.S.
44 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 58: Slab‐on‐grade and porch column damage due to tsunami flow
Figure 59: Church hall structure intact, though all windows washed out
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Figure 60: Interior of LDS church hall showing open frame resulting from breakaway of all windows
Figure 61: Typical window jamb showing only three fasteners securing window frame
46 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
The office/classroom building adjoining the LDS church sanctuary also suffered non‐structural damage during the tsunami. Primary damage was in the form of window failure as in the sanctuary. In addition, water pressure separated the rear CMU wall from the supporting steel columns (Figure 62). As with the window failure, this CMU wall also acted as an unintentional “breakaway” wall. Because the ties between the steel columns and CMU wall failed, the large hydrodynamic load on the wall was not able to transfer to the columns. The steel columns were therefore undamaged, and still able to support the roof structure above. Replacement of the CMU wall will be much less expensive than the potential damage to the roof if the steel columns had failed.
Figure 62: Rear masonry wall broken away from structural steel columns. A large timber pole had washed into the center of this office/classroom wing of the LDS church (Figure 63). There was no evidence of impact damage on the structural elements of the building, but the pole must have entered via a window or door opening and may have contributed to the destruction of the non‐structural partitions in the interior of the office/classroom wing. Behind the LDS church buildings, a steel framed tower supported a water tank while an adjacent steel post supported a satellite antenna (Figure 64). Neither of these structures had suffered any damage during the tsunami, though the water tank had been replaced by a temporary tank at ground level.
47 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 63: Timber pole debris inside LDS church classroom/office wing.
Figure 64: Water tank support tower and satellite antenna undamaged by the tsunami flow. A church building in Amanave survived while timber framed buildings in the neighborhood were destroyed or heavily damaged (Figure 65). The church is somewhat elevated above the surrounding grade (Figure 66), but damage to the roof eave shows that the tsunami flow depth at this location was still substantial (Figure 67). The building survived with only non‐structural damage to windows and doors and building contents (Figure 68). A nearby church building in Amanave suffered structural damage due to internal water pressing outwards on the CMU walls (Figure 69). The North wall leaned outwards but remained intact (Figure 70) while the South wall collapsed resulting in partial roof collapse (Figure 71). The lack of diaphragm action in the roof system meant that the CMU walls acted as cantilever walls, rather than spanning from ground level to the roof. This church was being demolished during our survey (Figure 72).
48 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
A small church chapel and Mother Mary statue in Lepa, I.S., also survived the tsunami relatively unscathed while adjacent buildings were destroyed or heavily damaged (Figure 73).
Figure 65: Debris from timber framed homes around surviving church in Amanave, A.S.
Figure 66: Undamaged church in Amanave, A.S. (Note damaged church in background ‐ Figure 69)
49 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Figure 67: Damage to eave indicates level of inundation.
Figure 68: Non‐structural damage to front door and windows.
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Figure 69: Damaged church building in Amanave, A.S. (Photo: www.americansamoa.gov)
Figure 70: Outward water pressure resulting in failure of CMU walls cantilevered from foundation (Courtesy Jeanne Johnston)
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Figure 71: Outward pressure on CMU walls causing collapse of walls and roof (Courtesy Jeanne Johnston).
Figure 72: Demolition of damaged Amanave church building, A.S.
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Figure 73: Surviving church building and Mother Mary statue in Lepa, south coast of I.S. A church building in Poutasi, I.S., suffered damage to the entryway facing the coastline. It appeared that collapse of the flat roof over the entryway allowed the roof sheeting to block the window and door openings on the front of the building (Figure 74), resulting in increased damming loads. The reinforced concrete columns and header beam framing the entry failed due to inward hydrodynamic pressure (Figure 75). The lack of lateral bracing in the form of ceiling or roof diaphragm meant that the header beam was forced to span the full width of the building under lateral bending, and the columns acted as cantilevers from the ground floor. This is a relatively weak configuration that was never designed for the lateral loads induced by the tsunami flow, particularly enhanced by the damming effect of the entryway roof sheeting. The rest of the building was relatively undamaged except for non‐structural elements and contents (Figure 76).
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Figure 74: Church building front entry roof collapse, Poutasi, I.S.
Figure 75: Damaged columns and header beam framing church entry.
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Figure 76: Louver and stained glass window damage in church building, Poutasi, I.S.
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56 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Non‐Engineered Residential Structures There are three main types of residential structures found in the villages of Samoa and American Samoa: Enclosed reinforced masonry houses. Enclosed timber framed houses. Open walled “fales” with wood or concrete posts. Based on the 2000 census data, American Samoa had around 10,000 residential housing units, around 78% of which were detached single family homes. The interior of the houses are typically modest with minimal interior partitions and open floor plans. Interior fixtures and furnishings are also generally minimal. About 60% of the houses have complete kitchen facilities and complete plumbing facilities. Some of the houses have detached bathroom facilities outside the house, following local custom. Similar statistics for housing in Samoa were not available, although observations suggest a similar housing stock, particularly in the villages around the coastline. Although the Samoan word “fale” refers to all homes, it is commonly used to refer to the open walled buildings typically used as guest houses and family meeting places but historically used as the primary residences. Many fales have recently been used to accommodate those with damaged and destroyed houses. While the wall types for the structures have three different variations, the roofs and foundations for each type are similar. The structures are typically founded on a reinforced concrete slab on grade with thickenings at the slab edges for anchorage of vertical wall reinforcing or anchor bolts, as illustrated by the bare slabs in Figure 77. The roofs typically consist of wood trusses spaced 24 to 36 inches apart and 2x4 inch purlins spaced 24 inches apart. Roof cladding is most commonly corrugated metal or, less commonly, standing seam roof (Figure 78). The roofing is nailed to the purlins through the ridges in the metal sheeting as shown in Figure 79. There is generally no plywood or other sheathing to provide diaphragm action in the roof system. The roof framing is therefore adequate to resist vertical gravity loads, but is not able to provide lateral support for the top of walls, or transfer lateral loads to more rigid elements.
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Figure 77: Typical concrete foundation slabs for houses in Poloa, A.S.
Figure 78: Damaged roof framing in Tula, A.S., showing standing seam metal roofing
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Figure 79: Typical corrugated roof sheathing and nail fasteners The housing construction practices are observed to reflect lessons learned from past cyclones, particular the two major cyclones in 1990 and 1991 (Chock, 2004). The major changes made after these events are reflected in the roof construction, with galvanized spiral nails or hex‐head screws and oversized washers being more prevalent (Figure 79). In addition, standing seam roofs started to replace corrugated metal roofing. The typical roof truss spacing has decreased from the 36 inches to 24 inches. Hurricane ties, typically H2.5, became common for attaching the trusses and rafters to the wall top plates. There is also increased anchorage between the roof trusses and interior walls. Unfortunately, the roof hardening for future cyclones was not observed to have had much benefit during the tsunami. This is not unexpected because of the significantly different loading applied by tsunami bores compared with high wind conditions during cyclones. The performance during the tsunami of each of these three different types of residential structures is discussed below.
Reinforced Masonry Houses Reinforced masonry houses are the most prevalent form of residential construction observed in Samoa and American Samoa. A typical undamaged structure is shown in Figure 80. These structures are typically constructed with partially grouted 6 inch thick concrete masonry blocks around the outside perimeter of the house, as exemplified in Figure 81. They are typically reinforced with vertical ½” diameter vertical reinforcing bars spaced at 24 inches. There were typically no horizontal reinforcing bars observed except in a concrete bond beam at the top of the wall that typically also serves as a header for window and doorway openings. The roofs of these types of houses almost universally had corrugated metal roof sheeting attached to 2x4 inch purlins spaced 2 feet apart supported by wood trusses spaced 2 to 4 feet apart.
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Figure 80: Undamaged masonry house in Fagaitua, A.S.
Figure 81: Typical 6 inch concrete masonry block with ½” reinforcing at 24 inches on center This roof system did not appear to be effective as a roof diaphragm, and therefore it provided minimal support to the top of the masonry walls. As a result, the primary lateral resistance in each wall was as a vertical cantilever from the concrete slab. In some cases additional lateral 60 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
strength is provided by the bond beam at the top of a wall spanning between perpendicular masonry walls. Therefore, in some cases the walls had sufficient strength to resist the tsunami inundation forces, while in other cases they collapsed. The performance was dependent on the depth of the tsunami flow, the length of the horizontal span of the wall between perpendicular walls, the number and size of openings in the wall, and the quality of the horizontal bond beam at the top of the wall. On the eastern side of Tutuila in the village of Tula many masonry houses were partially destroyed by the tsunami. For the house shown in Figure 82 through Figure 86, the estimated maximum tsunami flow depth was 1.8 to 2.1 m (6 to 7 ft) above the slab, just below the ceiling level of the house. The slab was estimated to be 4 m (13 ft) above mean sea level. The front of the house is located around 40 m (130 ft) from the shoreline. The middle span of the front (Figure 82) and back (Figure 83 and Figure 84) walls of the house were destroyed, while the end portions of the walls remained largely undamaged. The front and back walls were braced by perpendicular walls at the two ends and two intermediate locations, which provided considerable strength and helped to support the end portions of the walls. There was some damage to the ceiling where the walls collapsed, but the roof was generally intact (Figure 84 and Figure 85). One interior wall perpendicular to the direction of wave inundation also failed as shown in Figure 86. Figure 83 and Figure 84 show a segment of a wall hanging by the horizontal reinforcing in the bond beam, which also acts as a header beam for the windows and doors. Two pairs of horizontal reinforcing are present, one at the top of the bond beam and a second at the bottom of the bond beam. This is typical of the type of bond beam observed in the masonry houses and did help to distribute lateral loads at the top of the walls to the perpendicular walls. In this house the length of the central wall segment was too large to resist the lateral loads. Figure 87 shows a bond beam above one of the doorways. The vertical ties in the beam have 90 degree hooks at the bottom. The bottom layer of horizontal reinforcing was torn out of the beam. The use of 180 degree hooks might have helped to hold the bottom layer of reinforcing in place and add lateral strength to the doorway header.
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Figure 82: Front side of damaged masonry house in Tula, A.S.
Figure 83: Failed back wall but intact roof on house in Tula, A.S.
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Figure 84: Back wall of house from different angle, Tula, A.S.
Figure 85: Partially damaged ceiling in house in Tula, A.S.
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Figure 86: Precarious interior wall in house in Tula, A.S.
Figure 87: Typical bond beam above a doorway in green house in Tula, A.S.
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Figure 88: Damaged front wall in asphalt shingle roof house in Tula, A.S. A neighboring house in Tula, shown in Figure 88, had a heavily damaged masonry wall on the front (coastal) side of the house. The remainder of the house was largely intact. Unlike the typical houses on the island, this house had a plywood roof diaphragm with asphalt shingle roofing. The roof remained largely intact and did not appear to be impacted by the tsunami. Another adjacent masonry house also had substantial damage to the front of the house although the majority of the house remained largely intact (Figure 89). This house, similar to those previously described, was typical of the level of damage sustained to masonry houses along this section of coastline in the village. Similar levels of damage were observed in Alao, to the south of Tula, as shown in Figure 90. The masonry house on the left side of the photo had slight structural damage to the front wall but the remainder of the house was structurally intact. The windows and doors were non‐existent and presumably had been removed due to damage.
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Figure 89: Another damaged masonry house in Tula, A.S.
Figure 90: Relatively new masonry house surrounded by devastated wood framed houses in Alao, A.S. There was little other damage to masonry houses on the southeastern side of Tutuila. On the west side, however, there was extensive damage with the worst cases observed in Poloa. As described earlier (Figure 77) most of the residential buildings in Poloa were reduced to concrete 66 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
slabs, with what remained of the houses having been removed at the time of our survey. Figure 77 was taken from the church looking north, while Figure 91 shows the same area looking south towards the church. Figure 91 was taken soon after the tsunami, confirming that little was left of the structures. In Poloa it was estimated that the typical house slab elevation was around 15 ft above mean sea level, therefore at a higher elevation than the houses in Tula. However, the wave runup was estimated at up to 35 feet above sea level, therefore above the ceiling levels of most houses. The additional depth of water was significant and resulted in complete destruction of the masonry houses. Due to the lack of remaining structural elements, it was difficult to assess the original condition of the houses in Poloa. Some of the structures appeared to be open walled fales as evidenced by the column reinforcing and absence of wall reinforcing. Other houses were enclosed with continuous wall reinforcing. One residence in Poloa that did survive is shown in Figure 92. This house was at a higher elevation than the typical houses, estimated at 30ft above mean sea level. It only had inundation around the base of the building, with most of the louver window panes unbroken.
Figure 91: The village of Poloa soon after the tsunami (Photo courtesy of Hermann Fritz)
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Figure 92: A surviving masonry house in Poloa Further damage to residential structures was observed on the southwest coast of Tutuila in Amanave (Figure 93). Several masonry structures were damaged during the tsunami. The slab shown in Figure 94 is what remains of a masonry house. This house is located directly next to a stream outlet where the tsunami inundation and drawdown were concentrated. Unlike most masonry houses of this type, there is no apparent reinforcing extending out of the walls, which may explain the extent of damage. This is not considered typical of the construction practices observed in Samoa and American Samoa. Interestingly, the house in the background in Figure 94 appeared to have lost its windows and doors but did not exhibit major structural damage.
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Figure 93: View of Amanave, A.S.
Figure 94: Slab remaining after tsunami destroyed a masonry residence in Amanave, A.S. Nearby, in the village of Leone further tsunami damage was observed. The remains of a masonry house are shown in Figure 95. The front of the house appears to be an open patio area
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as indicated by discrete pairs of column reinforcing. The rear of the house has continuous walls indicated by the distributed vertical reinforcing. In between some of the columns at the front of the house, the tile pattern indicates that there were some, at least partial height, walls between the individual columns.
Figure 95: Reinforced masonry residence consisting of individual columns and wall segments in Amanave, A.S. A large number of masonry houses were damaged by the tsunami on the island of Upolu in Samoa. The damage was concentrated on the east and southeast coastlines of the island. The flat plains along these coastlines, where a large proportion of the houses were concentrated, were generally larger than similar coastal plains on Tutuila (Figure 96). This corresponded to a larger number of houses inundated and potentially damaged or destroyed by the tsunami.
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Figure 96: Typical widespread damage to residential buildings along the east coast of Upolu, I.S. Figure 97 shows a masonry house on the coastline in Samusu that was completely destroyed. The slab of the house was estimated at 6 feet above mean sea level, therefore at a lower elevation than the typical houses on Tutuila. Like those observed on American Samoa the house was built with 6 inch wide masonry blocks. The blocks were fully grouted and reinforced with 1/2 inch diameter reinforcing bars spaced 16 inches apart, as shown in Figure 98 though Figure 100. There was also horizontal wall reinforcing observed at the corners and along the length of the walls (Figure 101). The horizontal reinforcing was not only in bond beams at the top of the walls but also at intermediate locations. Therefore, this house was very well built and robust, but clearly the level of tsunami flow was too great for this and other similar houses in the area.
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Figure 97: Destroyed masonry house in Samusu, I.S.
Figure 98: Failed front wall of house in Samusu, I.S.
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Figure 99: Typical masonry block walls in house in Samusu, I.S.
Figure 100: Vertical reinforcing fracture in front wall of house in Samusu, I.S.
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Figure 101: Corner wall with horizontal reinforcing in house in Samusu, I.S. Scour was observed around the slab on grade foundations of many houses along this coastline. This is exemplified by the corner scour around the house in Figure 102 where 1.2 m (4 ft) of scour was observed. The open masonry superstructure of the house was intact, but the roof and all non‐structural items were destroyed.
Figure 102: Corner scour around house in Satitoa, I.S. 74 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
Timber Framed Houses Some residential structures on Samoa and American Samoa consist of wood framed stud walls with predominantly corrugated metal roofs supported by wood trusses spaced 24 inches to 36 inches apart. The walls are generally sheathed with 1x6 shiplap siding or T1‐11 plywood siding. The windows are generally horizontal glass louver windows. A selection of typical wood framed houses is shown in Figure 103 through Figure 106.
Figure 103: Typical wood frame house in Fagaitua A.S.
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Figure 104: Wood frame houses in Fagaitua A.S.
Figure 105: Typical wood frame house in Fagamalo A.S.
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Figure 106: Rebuilt wood framed house in Samusu, I.S. These enclosed timber framed houses are less common than masonry walled structures in both Samoa and American Samoa, but they were the most vulnerable form of residential structure to tsunami inundation. Where wood framed houses were inundated by the tsunami they were almost universally destroyed. Any portions of the structures that may have survived were demolished and cleaned up by the villagers at the time of our survey. Figure 107 shows a typical bare concrete slab remaining from a timber framed residence in Tula, A.S. This building is located almost directly behind the masonry house shown in Figure 82 through Figure 87 and at a similar elevation, demonstrating the relative performance of wood framed and masonry houses. This building was estimated to have experienced a flow depth of around 1.8 to 2.1 m (6 to 7 ft) above the concrete slab based on reports from residents. Similar complete or extensive damage to wood framed houses was common in this and other areas with tsunami inundation.
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Figure 107: Bare concrete slab‐on‐grade with single remaining bottom plate with anchor bolts and FEMA temporary shelters in Tula, A.S. The timber framed structures typically had 5/8 inch diameter anchor bolts spaced 32 inches apart connecting the exterior perimeter wall bottom plate to the concrete slab, as shown in Figure 107. There was typically no evidence of holddown anchors at the ends of the walls, therefore in‐plane flexural resistance of the walls relied on the regularly spaced anchor bolts. The addition of holddowns may have help to increase the strength of walls oriented parallel to the tsunami inundation flow, but would not be expected to provide significant strength for walls perpendicular to the direction of water flow. The wall top plate was typically connected to the roof framing with steel hurricane clips, which appears to have been as a result of experience from recent cyclone events, unfortunately these provided little additional resistance to the tsunami. Figure 108 shows a timber framed house (painted blue) in Alao, A.S., that structurally performed well in the tsunami while other timber framed houses behind this one were completely destroyed (Figure 109). This house appears to be built in the style of the FEMA cyclone resistant houses with robust corner shear walls as described by Chock (2004) after the 1990 and 1991 cyclones. The corner CMU shear walls remained undamaged while other portions of the walls broke away. The roof structure managed to stay relatively intact. In comparison, the masonry structure beside this house performed better than the timber framed house.
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Figure 108: Wood framed house with corner CMU shear walls in Alao, A.S.
Figure 109: Devastation behind the blue house in Alao, A.S.
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Open Walled Fales The third main type of residential structure in Samoa and American Samoa are open walled single story buildings with rectangular, oval or circular plan layout. The word “fale” is commonly used to refer to these buildings. They typically have a reinforced concrete slab on grade and a roof supported on multiple columns. There are three main types of vertical structural support for the fales: wood posts embedded in the ground and surrounded at the base by the concrete slab; reinforced concrete posts connected to a reinforced concrete slab by reinforcing dowels; and precast concrete spun pipes filled with concrete and connected to the slab using a single centrally located reinforcing bar. The fale roofs are typically wood framed with either corrugated metal or traditional thatched roofing. Figure 110 is an example of an oval fale with wood posts supporting a traditional thatched roof over the majority of the roof area and corrugated metal sheeting on the flat roof overhang. Figure 111 shows a typical rectangular fale with reinforced concrete posts supporting a corrugated metal roof. Figure 112 through Figure 115 show a large circular traditional fale with spun concrete exterior posts and a large reinforced concrete interior post. Despite the difference in construction materials, the architecture of the fales is consistent, with closely spaced posts around the perimeter of the fales and no supports in the center, except for the large fales such as that shown in Figure 112 through Figure 115 that have a central support column.
Figure 110: Typical oval open walled fale with timber posts and thatched roof, Fagaitua, A.S.
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Figure 111: Typical rectangular fale with corrugated sheeting hip roof on reinforced concrete posts
Figure 112: Partially damaged large circular fale with corrugated metal roof on spun concrete exterior posts in Leone, A.S.
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Figure 113: Fale column constructed of spun concrete pipe as permanent formwork for reinforced concrete column. Note apparent failure of reinforcement splice at base of column. Leone, A.S.
Figure 114: Reinforced concrete post in center of circular fale with spun concrete exterior posts in Leone, A.S.
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Figure 115: Interior roof structure with traditional twine connections for large circular fale, Leone, A.S. Most fales generally performed very well in the tsunami with the water able to flow through the structures and the large number of exterior columns providing enough robustness and redundancy to support the lightweight roofs. Figure 116 shows a very robust wood post fale with masonry corner walls, which would have added to its lateral strength, located in Tula, A.S. The FEMA tent shown next to the fale is located on a concrete slab where a wood framed house was completely destroyed. The fale is located behind the masonry structures shown to be damaged in Figure 82 to Figure 87. Figure 117 shows the interior structure of the fales with the concrete posts connected to a concrete bond beam at the top and concrete slab at the bottom. These columns have been constructed using a spun concrete pipe as permanent formwork as evidenced by the outline of the pipe flanges at the top of the posts. Where damage was observed in the fales it was generally due to debris impact causing local damage at the location of the impact. Figure 118 shows a rectangular fale with reinforced concrete columns. The center column on the ocean side of the fale has been knocked out of place, presumably by debris impact (Figure 119). Because of the substantial reinforced concrete header beam connecting the tops of the columns, and the close spacing of the adjacent columns, this column failure has not affected the overall integrity of the fale, nor resulted in any damage to the roof structure. Replacement of this single column and repair of the damaged header beam will restore the fale to its original condition. Figure 120 shows another example of damage due to a shipping container impact at one corner of the fale. Shipping containers can result in considerable force on a structure, with the magnitude of the force dependent on the velocity of the water flow and the mass of the shipping container including contents. However, the damage to this fale was localized to four columns located directly in the region of the impact (Figure 121). The columns were square
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formed concrete column with a pair of 5/8 inch diameter reinforcing bars (Figure 122). There is no confinement reinforcing in the column. Even if the columns were more heavily reinforced, they would not be expected to resist this impact load without damage. However, the closely spaced columns supporting a fairly substantial header beam meant that there was sufficient redundancy in the structural system so that the remainder of the structure survived (Figure 123).
Figure 116: Concrete column fale with reinforced masonry corner walls located in Tula, A.S.
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Figure 117: Interior structure of fale in Tula, A.S.
Figure 118: Fale with single column missing, Leone, A.S.
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Figure 119: Individual column presumably broken due to debris impact, Leone, A.S.
Figure 120: Fale impacted by shipping container in Leone, A.S.
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Figure 121: Damage to fale columns and header beam due to shipping container impact, Leone, A.S.
Figure 122: Square concrete column in fale damage by shipping container in Leone, A.S.
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Figure 123: Fale partially damaged by shipping container, Leone, A.S. The large circular fale shown in Figure 112 and Figure 115 had a series of exterior columns that failed around about ¼ of the perimeter of the structure (Figure 124). This damage was attributed to debris from a failed timber framed building located between the fale and the shoreline. Despite losing a large number of sequential supports, the fale roof remained essentially intact due to the cantilevered supports from the large central columns and the conical shape of the roof. The structure was able to act like an umbrella to support the roof without the exterior supports. A similar structure on an adjacent lot was reportedly not so fortunate (Figure 125), losing too many exterior supports such that the remains of the structure had to be demolished.
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Figure 124: Circular fale with 10 column failures due to impact by debris from adjacent building, Leone, A.S.
Figure 125: Circular fale with all exterior columns destroyed by debris impact, Leone, A.S. (Courtesy Jeanne Johnston) Figure 126 shows a fale attached to the front of a house as a covered patio in Samusu, I.S. The fale was damaged at one side due to scour around the edge of the concrete slab and base of the wood posts. The posts appeared to have inadequate embedment depth in the ground (Figure 127). Scour around the edge of the concrete slab may have contributed to this failure. Another example where the open nature of the fale was compromised by wall construction was observed in Leone, A.S. (Figure 128). The back side of the fale had been closed by a CMU wall
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with window openings. One side of the CMU wall had relatively large windows, while the other had smaller windows placed higher in the wall at the interior restroom. The fale survived the tsunami flow, but the CMU wall experienced considerably larger loads because of the larger exposed area, particularly the portion with smaller windows. This portion of the wall collapsed leading to loss of support for the roof framing, resulting in damage to the fale roof (Figure 129).
Figure 127
Figure 126: Damaged fale in front of a house in Samusu, I.S.
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Figure 127: Scour around the base of timber posts supporting a fale in Samusu, I.S.
Figure 128: Intact fale with damage to CMU back wall, Leone, A.S.
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Figure 129: Collapsed CMU wall with small windows at back of open fale, Leone, A.S. In a few locations where open walled fales were completely demolished the tsunami runup was as high as the roof of the fale. Examples of this level of damage were observed in Poloa. The slab in the foreground of Figure 77 is an example of such damage. Figure 130 shows the severed column reinforcing with three vertical reinforcing bars into concrete columns. Figure 131 shows the remains of a small fale in Poloa that was destroyed by the excessive tsunami flow depth.
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Figure 130: Column outlines and ruptured reinforcement along the side of a fale slab in Poloa, A.S.
Figure 131: Fale in Poloa, A.S., completely destroyed by excessive tsunami flow depth
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Elevated Houses There were a few examples of houses that were elevated on concrete columns. One example is the two story house shown in Figure 132 located in Leone, A.S. The house appeared to be derelict and abandoned prior to the tsunami but, despite this, is a well built house. The ground floor of this house was completely washed out during the tsunami with walls that failed and broke away, but the main structural support columns remained undamaged allowing the top floor to survive the tsunami. The masonry walls at ground level were attached to the slab at the base of the wall but not attached to the columns or second floor slab (Figure 133). This allowed the walls to break away without applying loads to the upper level structure. It demonstrated that the concept of breakaway non‐structural walls, even when they are built of a robust material like masonry, are effective at minimizing structural damage during an extreme tsunami or flood situation where a structure is located in a flood or tsunami inundation zone. If the ground floor were used for low occupancy purposes, then the financial losses could also be minimized.
Figure 133
Figure 132: Two story house with breakaway CMU walls at ground floor in Leone, A.S.
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Figure 133: Masonry wall anchorage at the ground floor of house in Leone, A.S. Another example of a two story house with open lower level was observed east of Pago Pago in Aua, A.S. (Figure 134). This house was not apparently subject to tsunami inundation but is relatively unique in its combination of an enclosed wood framed house on the upper level with an open walled reinforced concrete fale on the lower level. This structure is very robust with reinforced concrete columns, beams and second floor slab. The beams cantilever in both directions to increase the upper floor area. There is also a central line of beams and columns to reduce the beam spans. This is an ideal combination of the enclosed house and open fale to minimize the risk of damage to the structure and possessions while also having an open shaded and breezy gathering place and guesthouse. Given the good performance of open fales, this is considered a good way to rebuild the destroyed houses where moving the population to higher ground is not a feasible or practical option. A typical house would not need to be this large, and if smaller, could have just a single perimeter ring of columns. If the tsunami flow depth is not anticipated to reach the second floor, the reinforced concrete second floor slab could be replaced by a less expensive wood framed second floor with plywood sheathing.
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Figure 134: House elevated above rectangular reinforced concrete fale in Aua, A.S. An elevated residential building constructed very close to the shoreline in Aufaga, I.S. experienced significant damage due to hydrodynamic uplift (Figure 135). This building consisted of an open fale elevated above the beach with a CMU wall residence attached to the landward side of the fale. The fale was supported by reinforced concrete posts, beams and two‐way slab (Figure 136). The posts were founded on rock boulders, or on concrete filled metal drums embedded in the beach sand. Tsunami waves flowing under this elevated floor would have induced significant uplift on the slab and beams, resulting in separation of the slab from the beam (Figure 137) and cracks in both slab and supporting beams (Figure 138). A narrow strip of the floor between the fale elevated slab and the house slab had been washed out by the tsunami flow, possibly acting as an unintended breakaway panel (Figure 139 and Figure 140). This allowed water to flow up the shoreline and through this opening, possibly reducing the uplift loads on the fale elevated slab. The CMU wall separating the house from the fale had collapsed into the void left by the breakaway panel. Damage to the ceiling of the fale indicates the extent of inundation at this location (Figure 140).
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Figure 135: Residence elevated above the beach in Aufaga, I.S.
Figure 136: Fale elevated on reinforced concrete columns, beams and slab
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Figure 137: Separation of concrete slab from supporting beams due to hydrodynamic uplift
Figure 138: Damage to concrete slab and beams due to hydrodynamic uplift
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Figure 140
Figure 139: Narrow slab‐on‐grade section of floor that acted as breakaway panel
Figure 140: View through strip of slab that acted as breakaway panel
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Bridges and Roadways This section examines tsunami induced damage to bridges and roadways in American Samoa. Based on our survey, there are two bridges and two roadways that sustained significant damage. As we shall see, it is interesting that damage to bridge decks were induced by failure of the adjacent abutments rather than structural failure directly due to tsunami fluid forces. Damage to roadways was mainly due to scour of substrates supporting the road surfaces.
Bridge abutment collapse, Leone, A.S. A two‐span bridge at Leone, located on the southwest coast of A.S., experienced a run‐up estimated at 5 to 7 meters (16 to 23 ft). Each span of the bridge was approximately 7 m (23 ft) long (center to center between the abutment retaining walls and the center pier) and 6 m (20 ft) wide with two lanes of traffic (Figure 141). The bridge spans consist of a poured‐in‐place continuous flat slab deck built integral with the supporting abutments and center pier (Figure 142). The bottom of the bridge deck was about 3 m (10 ft) above sea level. The two spans were supported at the center by a tapered concrete pier (1.25 m thick at the base and 1 m thick at the top). The width of the center pier was identical to the width of the bridge deck. The northwest span collapsed during the tsunami and was subsequently repaired by filling gravel over the collapsed span, and paving over the fill (Figure 143). It is suspected that the collapse of the northwest span was due to scour of the substrate at and near the northwest abutment (Figure 144). As the abutment failed, the northwest span became a cantilever beam spanning from the center pier. The deck failed in shear adjacent to the center pier (Figure 145 and Figure 146). On the other hand, the abutment at the southeast end (right side of bridge in Figure 141) sustained much less damage, possibly benefiting from the protection provided by a large number of palm trees on the seaward side. The substrate at this abutment was relatively intact. With both of its supports intact, the southeast bridge deck was able to withstand the tsunami fluid loading without any noticeable damage. This is attributed in part to the continuity between the bridge deck and supporting abutment and center pier.
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Figure 141: A two‐span bridge at Leone damaged by the tsunami and temporarily repaired
Figure 142: Undamaged southeast span showing continuous deck integral with abutment and center support pier
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Figure 143: The collapsed northwest span of the two‐span bridge at Leone was repaired by filling the void with gravel and paving over the surface
Figure 144: Failure of the northwest abutment of the two‐span bridge at Leone due to scour of substrate at and near the abutment underneath the roadway
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Figure 145: Shear failure of northwest span adjacent to center pier support
Figure 146: Shear failure of the northwest bridge deck at the center support
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Bridge pier scour/settlement, Amanave, A.S. A single‐span bridge at Amanave, located near the western tip of A.S., experienced a run‐up estimated at more than 5 meters (16 ft). The bridge was 4.3 m (14 ft) long between the supporting abutment retaining walls, and 6.4 m (21 ft) wide with two lanes of traffic (Figure 147). The flat slab bridge deck was 0.7 m (28 in) thick and built integral with the abutments. Based on our survey, the bottom of the bridge deck was about 6.7 m (22 ft) above sea level, and 1.7 m (5’‐6”) above the stream bed below the bridge. The bridge was supported on concrete abutment retaining walls. The width of the abutment walls was the same as the bridge deck width. It was observed that the sandy support underneath the southeast abutment liquefied and the abutment wall sank approximately 1 meter (3 ft) on the seaward side (Figure 147) and 1.3 meters (4 ft) at the landward side (Figure 148). A significant amount of the backfill materials at the abutment were also scoured away by the tsunami flow. The settlement of the supporting abutment induced a large bending moment at the opposite (northwest) end of the bridge deck causing it to fail in bending at the deck‐column interface (Figure 147). Similar to the Leone bridge discussed above, the abutment at the opposite end of the bridge sustained practically no damage. It appears that the presence of the tree on the seaward side of this abutment may have provided protection against scour. The substrates at the undamaged abutment appeared intact. It also was noted that the bridge deck withstood the tsunami fluid load and did not sustain noticeable damage (other than the bending failure at the northwest end). This is attributed to the integral nature of the bridge deck and supporting abutment walls. Repair work was performed on the road and bridge decks by filling the resulting void above the bridge deck and adjacent to the southeast abutment with gravel and repaving the bridge deck and roadway surfaces.
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Figure 147: Tsunami induced failure of concrete bridge at Amanave, A.S. (View from seaward side)
Figure 148: Tsunami induced failure of concrete bridge at Amanave, A.S. (View from landward side)
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Coastal road damage, Fagasa, A.S. The coastline of Fagasa Bay on the north coast of American Samoa experienced a run‐up estimated at about 12 meters (39 ft), one of the highest levels on the island. The riprap, which consisted of dark color rocks of various sizes, and the road along the Fagasa Bay coast, especially at the western end, sustained significant damage. Figure 149 shows the view from the western end of the road. The rock armor placed over a light color rock foundation collapsed seaward and a significant amount of the rocks were submerged (Figure 149). This failure mode indicates that the riprap, which was designed to prevent wave‐induced erosion of the natural material underneath the road, performed well in protecting the road from the tsunami run‐up (in the direction of the normal attacking wave). However, the large magnitude of the run‐up lead to a corresponding very large scouring force on the run‐down in the opposite direction of the normal attacking waves. Since the riprap was not designed to resist large horizontal forces in the seaward direction, it collapsed seaward under a combination of large horizontal force, buoyancy force, and slope instability. The direction and magnitude of the movement of rocks are indicated in Figure 149 by the position of the light color rocks at some distance from the shoreline. In Figure 150 and Figure 151 the metal wire‐net used to tie down the layer of riprap closest to the substrate underneath the road had moved some distance seawards from its original location. Debris from the landward side had been deposited in the resulting scoured cavity. Figure 150 and Figure 151 also show the large amount of scour of the armor rocks and the exposed substrate underneath the road. Note that a significant section of the riprap was completely washed away and the corresponding section of the road it was meant to protect has collapsed.
Figure 149: Failure of riprap and partial collapse of roadway at Fagasa, A.S.
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Figure 150: Seaward displacement of the wire‐net encased riprap and scour of the substrate underneath the roadway induced by tsunami run‐down (view toward western end of roadway)
Figure 151: Seaward displacement of the (dark color) rock armor and scour of the substrate underneath the roadway induced by tsunami run‐down (view toward eastern end of roadway)
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Culvert roadway repair, Alega, A.S. A roadway along the coastline at Alega, located on the south shore of Tutuila, experienced a run‐up estimated at about 6 meters. Figure 152 shows a section of the roadway that has been repaired after damage caused by the tsunami. The surface of the roadway is about 25 feet above sea level. The damaged section was directly above a concrete culvert (see Figure 153 and Figure 154). The culvert on the landward side (Figure 154) is connected to an inland stream (Figure 155). According to residents in the area, the roadway above the culvert was washed out after the tsunami. Because it is the only road that permits residents east of Alega to access the major city Pago Pago, the road was repaired immediately after the tsunami. The distribution of the vegetation on the banks and condition of the culvert on both sides of the repaired roadway indicated that there was little to no damage to the banks and culvert. In fact, damage was limited to 2 to 3 feet directly below the road surface. The evidence indicates that the tsunami overtopped the roadway and scoured the weak substrate underneath the roadway during run‐ up and subsequent run‐down, causing significant damage to (probably collapsing) the road surface. The run‐up and run‐down would have been significantly higher at and near the culvert because of the small “valley” that channels fluid through the region, whereas the steep cliff on either side of the valley would have stopped the tsunami flow in the landward direction.
Figure 152: New concrete roadway covering culvert at Alega, A.S.
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Figure 153: Seaward side of culvert underneath the damaged and repaired roadway
Figure 154: Landward side of culvert underneath the damaged and repaired roadway
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Figure 155: Stream running into culvert under the Alega roadway
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Port Facilities
Foundation/uplift failure of new ferry pier, Malaela, I.S. A new ferry pier in Malaela had been constructed to support ferry service between Samoa and American Samoa. The ferry pier, shown in Figure 156, was built on a jetty constructed of fill. On the landward side of the pier was a shallow basin separating the jetty and pier from the shore (Figure 157). The pier faced seaward and would have felt the full impact of the tsunami as it approached the shore.
Figure 156: New ferry pier in Malaela. I.S.
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Figure 157: View landward from damaged pier The pier was constructed of reinforced concrete with heavy vertical reinforcing ribs as well as a sloping apron underneath (Figure 158 and Figure 159). The ribs were supported on rock and boulders (Figure 160) In addition, there were tie back beams perpendicular to the pier providing additional support, both for the cantilever action of the pier and to tie it back to the jetty (Figure 161). It should be noted that the entire jetty (Figure 161) was paved with a concrete slab‐on‐ grade prior to the tsunami. The pier was heavily damaged during the tsunami and it will likely have to be replaced. It appears that the primary cause of failure was a loss of the foundation. As the foundation failed, the now cantilevered pier rotated downward, resulting in negative bending failure of the tie back beams (Figure 161). Evidence to support this conclusion includes the failure of the tie back beams and the sinking of the pier (Figure 161), and the loss of foundation of the ribs (Figure 162). It appears that the foundation material was simply washed away. It is quite possible that a contributing factor to the failure was large hydrodynamic pressure that would result from a tsunami bore front entering the space under the pier (Figure 158 and Figure 159). Although there were small holes in the pier (Figure 163, also refer to Figure 159), which were of uncertain purpose, they were likely too small to provide significant pressure relief of the hydrodynamic pressure. There is some evidence that this pressure blew out the back of some of the cells. This flow would have contributed to the loss of the foundation material. It also would have put uplift pressure on the pier, allowing the foundation material to more easily be washed away. In addition, uplift pressure on the concrete apron on the jetty would have contributed to its failure. It should be noted that the apron was either washed completely away, or was so damaged that it was subsequently removed during clean up.
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Figure 158: Underside of pier
Figure 159: Vertical ribs and sloping apron
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Figure 160: Vertical ribs sitting on rock foundation
Figure 161: Damage to tie‐back beams
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Figure 162: Loss of foundation support
Figure 163: 100 mm (4 in) diameter holes through the pier
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Figure 164: Loss of back of pier cells
H‐pile wall collapse, Pago Pago, A.S. The MYD Shipyard is the largest commercial shipyard in the South Pacific. An H‐pile retaining wall forms a channel leading up to the dry dock (Figure 165). The tsunami caused a significant portion of the H‐pile wall to fail, leading to substantial loss of soil on the bank of the channel (Figure 166). The debris from this collapse fell onto the underwater portion of the dry dock tracks, making them inoperable for a month after the tsunami (Figure 167). This delayed the repair of ships damaged during the tsunami.
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Figure 165: Dry dock and channel formed by H‐pile retaining walls
Collapsed H‐pile retaining wall
Intact H‐pile retaining wall
Figure 166: Retaining wall failure and soil erosion
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Figure 167: Debris recovered from dry dock after H‐pile retaining wall collapse
Barge uplift damage to wharf, Pago Pago, A.S. A wharf adjacent to the MYD Shipyard dry dock suffered an interesting type of ‘impact’ damage. As the water receded, the deck of a crane barge dropped below the wharf. As the water level rose again, the bow of the barge was caught under the wharf. The wharf had not been designed for the large upward force from the buoyancy of the barge, resulting in damage to the wharf. Figure 168 shows the wharf damage from the adjacent crane barge, while Figure 169 shows the concrete rubble on the deck of the barge.
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Figure 168: Dry dock damage from barge uplift
Figure 169: Concrete debris from dock on bow of barge
Small boat docks, Pago Pago, A.S. The small boat docks in Pago Pago experienced several incidents during the tsunami. Figure 170 shows the failure of a chain link fence as a result of water flow during drawdown. This failure illustrates the high water velocity on drawdown, and the likely contribution of floating debris damming against the fence, as the fence did not fail during inundation.
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Figure 170: Fence failure from drawdown The tsunami inundation deposited a number of boats onshore. Figure 171 shows a yacht that floated onto the dock at the small boat harbor in Pago Pago. A second boat was lifted onto the dock after the tsunami to undergo repairs. Figure 172 shows another yacht that was washed ashore in Pago Pago, while Figure 173 shows a larger fishing ship that washed ashore near the main road leading South from Pago Pago.
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Figure 171: Boat washed onto dock (left) and boat raised for repair (right), Pago Pago, A.S.
Figure 172: Yacht washed onshore, Pago Pago, A.S.
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Figure 173: Ship washed onshore by tsunami inundation, Pago Pago, A.S.
Ship mooring line failure, Pago Pago, A.S. The dramatic water level change in Pago Pago harbor due to tsunami inundation and drawdown puts unexpected loads on the mooring lines securing boats to their docks. Unless the docks are floating, the mooring lines may not be able to accommodate these water level changes. Many of the boats that washed ashore from the small boat harbor in Pago Pago had broken free from their moorings (Figure 171 to Figure 173). Some were released by their owners in an attempt to ride out the tsunami in the bay rather than risk being buffeted against the dock (Figure 174). Figure 175 shows two large fishing vessels that were captured on video drifting freely in Pago Pago bay. They finally ran aground near Pago Plaza. There were a number of crew members on board, so it is not known whether the vessels broke free from their moorings or were intentionally released.
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Figure 174: Yachts released from small boat harbor and washed into shallow water
Figure 175: Large fishing vessels washed into shallow end of Pago Pago bay
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Foundation Scour Tsunami inundation can result in significant amounts of sediment transport and scour around building foundations. Prior studies have shown that much of the scour occurs during drawdown due to increased pore pressures in the soil which result in fluidization (Young, 2009). A few locations were noted where scour had undermined building foundations. Figure 176 and Figure 177 show the results of scour under the foundation of a coastal fale in Fagasa, A.S. The soil was a mixture of sand, pebbles and larger stones. The maximum erosion depth was approximately 1 meter (3 feet) adjacent to the fale foundation slab. The tsunami inundation had also caused erosion of the coastal roadway adjacent to this fale (Figure 178). It appeared that sandbags and rock rip‐rap had been installed along this coastline, possible as a result of prior erosion during storms. This armor had failed as explained earlier in this report. A residential building in Malaela, I.S., experienced scour around the corners of the foundation slab (Figure 179 and Figure 180). The maximum scour depth was approximately 1.2 meters (4 feet) in predominantly sandy soil. A new steel bridge at Safani, I.S., had experienced scour under one corner of the East abutment (Figure 181 and Figure 182). The tsunami flow had also dislodged some of the boulders acting as protection for the river banks around the abutments. Scour and fluidization are also suspected of causing the bridge abutment failure in Amanave, A.S. (Figure 147 and Figure 148).
Figure 176: Scour of approximately 1.0m (3 feet) below fale foundation slab, Fagasa, A.S.
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Figure 177: Coastal erosion, Fagasa, A.S.
Figure 178: Coastal road erosion, Fagasa, A.S.
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Figure 179: Scour of 1.2m (4 feet) below residential foundation, Malaela, I.S.
Figure 180: Scour at other corner of residential foundation, Malaela, I.S.
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Figure 182
Figure 181: New steel bridge at Safani, I.S.
Figure 182: Scour under bridge abutment
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Tsunami Effect on Coral Reef When the tsunami hit American Samoa, it damaged a number of shoreline villages located on the island. Two of the hardest hit locations were Poloa and Leone on the western end of Tutuila. During a reconnaissance survey of the coral reefs on November 4th and 5th of 2009, the coral reefs off the shore of these two locations had everything from broken coral to sheet metal roofs and window frames depicting the effect the tsunami waves had on the sea floor. In addition, along the shoreline at Poloa, rows of chairs and desks were found floating in the waterfront where the local elementary school was completely destroyed by the tsunami. When the tsunami hit the island, the waves crashed onto shore destroying buildings and other manmade objects which then flow back toward the sea with all the sediments and debris entrained during the process. Recent experimental and numerical investigation of tsunami wave impact on coastal fine sand slopes can be found in Young et al. (2009) and Xiao et al. (2009). The sediments and debris carried by the backwash into the sea can be very harmful to the delicate marine ecosystem by introducing bacteria and toxic chemicals, which can harm or kill the inhabitants of the ecosystem. In addition, the hydrodynamic forces and debris can further erode the sea floor and damage coral. To survey the tsunami damage to the coral reefs off the coast of Tutuila, American Samoa, we used a remote operated vehicle (ROV), Seabotix LBV150S2, owned by the Marine Hydrodynamics Laboratory of the University of Michigan. The ROV is a small surface operated robot attached to 152 meters (500 ft) of cable through which power and video are fed (Figure 183). The ROV is approximately 75 cm long by 35 cm wide by 25 cm tall (29.5 by 13.75 by 10 inches). A custom made sand collection unit in the form of a 6 cm diameter by 30 cm long (2.4 inch diameter by 11.8 inch long) cylinder with an end cap was attached to the bottom of the ROV to take sand samples from the sea floor.
Figure 183: The ROV with 152 meters (500 ft) of power cable attached 131 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
The ROV has significant advantages over underwater investigation by SCUBA diver. The ROV can stay below the surface for long periods and collect hours of video footage without having to resurface. The ROV also has the ability to dive to 152 meters for long periods taking samples and filming the sea floor. Hence, the team was able to collect numerous samples, and capture hours of footage during the two‐day investigation. The approach adopted was to dive the ROV to a particular water depth and drive it towards land with a specific heading using the ROV’s onboard compass (Figure 184). By comparing observations of damage to the coral reef at different depths, the team could then determine the location and approximate extent of the damage region.
Compass Heading Water Depth Date (Top), Time (Bottom)
Figure 184: Operator view through the ROV onboard camera The areas surveyed using the ROV on November 4th and 5th of 2009 are shown in Figure 185. A brief description of the observations is listed in Table 1. The underwater survey focused on the coral reef off the shores of Poloa and Leone, where the maximum wave runup was found to be 10.7 meters and 7 meters [Gelfenbaum et al., 2009], respectively. As noted in Table 1, Fagasa Bay also was surveyed, but no quality results could be reported for that region because of the poor visibility caused by the fine silty sediment suspended in the bay water. Figure 186 shows a bathymetric map of Tutuila, while two‐dimensional reproductions of the bathymetry near Poloa and Leone, where the surveys were conducted, are shown in Figure 187. The video output to the monitor onboard the ship (Figure 188) allowed the team to efficiently locate the damage and debris areas. Detailed analysis of the video capture was then conducted to compare the degree of damage with depth at the various dive locations.
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Fagasa Bay Dives Poloa Dives
Leone Dives
Figure 185: Survey locations of the coral reefs of Tutuila, American Samoa
Figure 186: Bathymetry map of American Samoa (Courtesy NOAA)
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Table 1: Survey Locations with Observations Dive Date Location Observation Coral was healthy with no real damage to the reef. There was no foreign debris found. Water Depth 1 Nov. 4th 2009 Leone Range: 2‐7 meters. Coral looked to be intact with very little damage. A few broken and dead pieces of coral can be found on the sea floor. No foreign debris was found. Water 2 Nov. 4th 2009 Leone Depth Range: 12‐20 meters.
Coral looked to be intact with very little damage. A few broken and dead pieces of coral can be found on the sea floor. No foreign debris was found. Water 3 Nov. 4th 2009 Leone Depth Range: 9‐20 meters (Figure 192). Healthy coral is attached to the reef while the sea floor has an abundance of dead and broken coral. Little foreign debris was found, only a plastic bag stuck on a head of coral. Water Depth Range: 3‐17 4 Nov. 4th 2009 Poloa meters (Figure 194).
Healthy coral found on the reef while there is an abundance of broken and dead coral found on the sea floor. Some of the foreign debris found includes sheet metal roofing, tire, timber, and tree branches. 5 Nov. 4th 2009 Poloa Water Depth Range: 9‐13 meters (Figure 195). Coral seems to be in good condition with a few broken coral scattered on the sea floor. There is little foreign debris found only a car tire was found. Water 6 Nov. 4th 2009 Poloa Depth Range: 3‐8 meters. Very fine silt produced poor visibility (approximately 10 cm (4 in)). The coral found seemed to be intact and healthy while there was an abundance of foreign debris; sheet metal roofing, timber, tree branches. 7 Nov. 4th 2009 Fagasa Bay Water Depth Range: 13‐15 meters. Very fine silt produced poor visibility (approximately 10 cm (4 in)). The coral found seemed to be intact and healthy while there was an abundance of foreign debris; sheet metal roofing, timber, clothing. Water 8 Nov. 4th 2009 Fagasa Bay Depth Range: 9‐15 meters. Very fine sand and poor visibility meant there was no useful visual data captured. Water Depth Range: 12‐ 9 Nov. 4th 2009 Fagasa Bay 15 meters
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Table 1 Continued: Survey Locations with Observations Dive Date Location Observation Sandy bottom with no coral or debris. Water Depth 1 Nov. 5th 2009 Poloa Range: 29‐40 meters. Very rocky bottom with no coral, or debris present. 2 Nov. 5th 2009 Poloa Water Depth Range: 7 ‐ 30 meters. Sandy bottom with scattered rocks and some coral. Coral is intact and healthy and some foreign debris was found; sheet metal roofing. Water Depth Range: 3 Nov. 5th 2009 Poloa 25 ‐ 35 meters. Very rocky bottom with ravines. Coral on the rocks looks to be healthy while the coral in the bottom of the ravines has been crushed and killed. A blue tarp was found stuck to a head of coral. Water Depth 4 Nov. 5th 2009 Poloa Range: 1‐7 meters. Large rock mounds with healthy coral attached. A large ravine was found and the entire bottom was covered with broken and dead coral which the survey team nicknamed "Death Valley". As for the foreign debris there was; clothes, tires, sheet metal roofing with rafters attached, window frame, and tree branches. Water Depth Range: 3‐17 meters 5 Nov. 5th 2009 Poloa (Figure 189). Rocky bottom with healthy coral and a good amount of dead and broken coral found on the sea floor. Foreign debris found includes tree branches, clothing, sheet metal roofing, and a window frame. 6 Nov. 5th 2009 Poloa Water Depth Range: 4‐24 meters. Large amount of broken coral found on the sea floor. Healthy coral can be found attached to the reef. Foreign debris found includes clothing, tree branches, and sheet metal roofing. Water Depth 7 Nov. 5th 2009 Leone Range: 2‐11 meters (Figure 190). Coral looks to be healthy with a few broken coral heads scattered on the sea floor. No foreign debris was found. Water Depth Range: 2‐22 meters (Figure 8 Nov. 5th 2009 Leone 191). Majority of Coral was healthy with a small amount of coral damaged or killed. There was little to no foreign debris found on the sea floor. 9 Nov. 5th 2009 Leone Water Depth Range: 2‐8 meters (Figure 193).
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Fig. 193
Fig. 195 Fig. 189
Fig. 191
Fig. 192 Fig. 190 Fig. 194
Figure 187: 2‐D bathymetry at Poloa and Leone with location of video images included in this report, water depth range where coral was damaged and distance from shoreline where debris was found
Figure 188: ROV control box with video monitor and joystick control 136 University of Hawaii at Manoa Civil and Environmental Engineering Dept. Samoa Tsunami Reconnaissance January 20th, 2010
The ROV video images allowed the survey team to see the various types of healthy coral physically affected by the tsunami (Figure 189, Figure 190 and Figure 192). Our observations indicate that offshore from Leone, debris could be found in depths up to 20 meters (65 ft). In Poloa debris was found in depths up to 30 meters (98 ft), believed to be due to the larger waves and stronger drawdown in Poloa. The majority of the coral damage in Leone was found in water depths up to 12 meters (39 ft) and in water depths up to 17 meters (56 ft) in Poloa. In general, more debris and greater damage to the coral reef was observed as the water depth decreased closer to shore. A majority of the debris found in both Poloa and Leone consisted of sheet metal roofing used for the houses onshore, although other debris such as clothing, wooden window frames, and tires were also observed (Figure 189). Damage to the coral reefs consisted of everything from broken pieces of coral to entire heads of coral broken off at the base and flipped upside down on the ocean floor. The most severe damage was observed at 7‐13 meters (23‐43 ft) water depth, and approximately 200 to 500 meters (220 to 550 yards) from the shoreline of Poloa, where broken corals of all different species were observed throughout (Figure 192, Figure 194 and Figure 195). This indicated that the damage is most likely caused by the tsunami rather than by natural causes. The sand samples collected during the survey were taken to the Geotechnical Lab at the University of Michigan, where grain size analysis was conducted following guidelines listed in ASTM D 422‐63 (2007). By comparing the grain size distribution of the sand collected onshore and offshore, we hope to be able to tell if the sand was washed down from the shore during the tsunami drawdown.
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Figure 189: Location: Poloa, Nov 5th 2009, Dive #5, 14.2 m depth, heading 001⁰. ROV image showing foreign debris (tire, clothing), along with healthy coral attached to the reef
Figure 190: Location: Leone, Nov 5th 2009, Dive #7, 13.7 m depth, heading 315o. ROV image showing healthy coral offshore from Leone, A.S.
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Figure 191: Location: Leone, Nov 5th 2009, Dive #8, 6.9 m depth, heading 015⁰. ROV image showing example of healthy coral offshore from Leone, A.S.
Figure 192: Location: Leone, Nov 4th 2009, Dive #3, 11 m depth, heading 014⁰. ROV image showing two pieces of coral heads broken off and lying on top of one another with more broken coral around them
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Figure 193: Location: Leone, Nov 5th 2009, Dive #9, 2.6 m depth, heading 053⁰. ROV image showing healthy coral attached to the reef while numerous broken pieces of coral lie on the sea floor
Figure 194: Location: Poloa, Nov 4th 2009, Dive #4, 17.3 m depth, heading 106⁰. ROV image showing a smaller piece of coral snapped off at its base among other fragments of dead coral
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Figure 195: Location: Poloa, Nov 4th 2009, Dive #5, 13 m depth, heading 095⁰. ROV image showing sea floor covered with broken and dead coral, including two large coral heads broken off by the tsunami
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References Central Intelligence Agency, 2009. The World Factbook. https://www.cia.gov/library/publications/the‐world‐factbook/ Chock, G., 2004. Wind Vulnerability Assessment of Typical Residences in American Samoa. Martin & Chock, Inc. Report, January 31, Honolulu, Hawaii. Gelfenbaum, G., Jaffe, B., Watt, S., Apotsos, A., Richmond, B., Buckley, M. and Peck, B., 2009. “The Samoa Tsunami of September 29, 2009: Preliminary Field Data on Tsunami Inundation in American Samoa.” United States Geological Services (USGS), American Samoa Community College, October 22, 2009. Johnson, J., 2009. Correspondence with Jeanne Johnson of FEMA mitigation team OCHA, 2009. Samoa/Tonga Tsunami, Situation Report #6, 6 October 2009, www.pacificdisaster.net:8080/Plone/samoa‐tsunami/situation‐reports PMEL, 2009. Website ‐ http://nctr.pmel.noaa.gov/samoa20090929‐modeldata.html accessed in November, 2009. Robertson, I. N., Riggs, H. R., Yim, S. C. S., and Young, Y. L., 2006. “Lessons from Katrina”, Civil Engineering, American Society of Civil Engineers, April, pp. 56‐63. Robertson, I. N., Riggs, H. R., Yim, S. C. S., and Young, Y. L., 2007. “Lessons From Hurricane Katrina Storm Surge on Bridges and Buildings”, Journal of Waterway, Port, Coastal and Ocean Engineering, Vol.133, No.6, American Society of Civil Engineers, Nov/Dec. United State Census Bureau, 2003. American Samoa: 2000 – Social, Economic, and Housing Characteristics. U.S. Department of Commerce, Economics and Statistics Administration, June, Washington, D.C. Xiao, H., Young, Y.L. and Prevost, J.H., 2009, “Runup and drawdown of breaking solitary waves over a fine sand beach. Part II: numerical modeling,” Marine Geology, DOI10.1016/ j.margeo.2009.12.008. Young, Y.L., Xiao, H. and Maddux, T.B., 2009, “Runup and drawdown of breaking solitary waves over a fine sand beach. Part I: experimental modeling,” Marine Geology, DOI 10.1016/ j.margeo.2009.12.009.
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Appendix A – Survey Team The survey reported here was carried out by the following team members: Oregon State University, Corvallis Solomon Yim, PhD, P.E. Professor of fluid‐structure interaction University of Michigan, Ann Arbor Yin Lu Young, PhD. Associate Professor of Naval Architecture & Marine Engineering Devin Lee Witt Undergraduate research assistant of Naval Architecture & Marine Engineering University of Hawaii at Manoa Ian Robertson, PhD, S.E. Professor of structural engineering H. Ronald Riggs, PhD, P.E. Professor of structural engineering Krystian Paczkowski Graduate research assistant Martin & Chock, Inc., Honolulu, Hawaii Lyle Carden, PhD, S.E. Structural engineer
Figure 196: Survey team and guides: From left, Sai Mauia, Karl Raynar (local civil engineer), Ron Riggs, Solomon Yim, Devin Witt, Ian Robertson, Krystian Paczkowski, Lyle Carden, Jeanne Johnston (FEMA) and Julie Young 145 University of Hawaii at Manoa Civil and Environmental Engineering Dept.