The Bonneville Dam is not the only dam that has or will at some point be affected by an earthquake, and it is important to realize this when we create situated projects in order to get the top and bottom of the hourglass. Here are three examples dams that were in some way impacted by the occurrence of an earthquake
The Lower San Fernando Dam in Southern California was impacted by the 1971 magnitude 6.7 San Fernando earthquake, however the dam did not fail. The earthquake created a liquefaction failure at a water storage facility known as the Lower San Fernando Dam and Reservoir in the San Fernando community on the northern edge of the greater metropolitan Los Angeles area. Another facility known as the Upper San Fernando Dam also suffered some damage, but it was not as serious as at the Lower Dam. The earthquake shaking initiated a major failure on the upstream side of the dam. The failure led perilously close to a catastrophe. Had the head scarp been slightly lower, the outflow from the reservoir would have quickly eroded the dam and flooded many communities downstream. Considering the extremely precarious situation, some 80,000 people over an 11-square-mile area were evacuated while the reservoir was emptied over a period of three to four days. To study this particular dam it was important to take into account the liquidation and soil properties, and that the dam was constructed primarily by hydraulic fill placement. In 1994, a similar earthquake happened, however due to information gained from the 1971 earthquake, the replacement dam was able to survive with little impact.
Although the Bonneville Dam is much bigger, and therefore presumably stronger than the San Fernando Dam, the San Fernando earthquake was only magnitude 6.7, while the Cascadia megathrust earthquake is expected to be magnitude 9.0.
The May 12, 2008 Wenchuan magnitude 7.9 earthquake in Sichuan Province, China, on the other hand, led to severe landslides in association with the main earthquake and aftershocks. The effects of the landslides triggered by the Wenchuan earthquake include: 1) landslides triggered during the main earthquake and aftershocks, which were responsible for mass destruction at the failure site or within their run out paths, burying inhabited river valleys, and preventing quick access to affected areas, 2) landslide dams that formed as landslide material blocked valley bottoms — which became an immediate flood hazard depending on the size, as well as where, how, when and if they failed, 3) increased erosion and flooding associated with the re- distribution of unconsolidated landslide material as rivers worked to re-establish their courses, as well as sediment fluxes which increased during the rainy season(s) that followed the earthquake, and 4) landslides and debris flows unrelated to earthquake shaking that originated on hillslopes that were destabilized during the removal of vegetation associated with the main earthquake. Analysis used satellite mapping, GIS tools, and google maps to determine how the shift in land during and after the earthquake created the landslides. Since the earthquake, China Geological Survey has identified 4,970 potentially risky sites, 1,701 landslides, 1,844 rock avalanches, 515 debris flows, and 1,093 unstable slopes. Rock avalanches and landslides caused many fatalities directly and disrupted the transportation system, extensively disrupting rescue efforts and thereby causing additional fatalities. Landslide-dammed lakes not only flooded human habitats in upstream areas but also posed threats to potentially inundated downstream areas with large populations. Debris flows become the most remarkable geohazards featured by increasing number, high frequency, and low triggering rainfall. Earthquake-triggered geohazards sequentially induced and transformed to additional hazards.
In this case, these dams were naturally made, and by current and previous landslides, however the landslide impacts on the surrounding ecosystem can be applied to the Bonneville dam because the landslides happened upstream from a populated area, and the techniques used to mitigate the aftermaths can be applied to Portland and surrounding counties.
On March 11, 2011, the Magnitude 9.0 Tohoku Earthquake in Japan triggered an extremely destructive tsunami that killed thousands of people, caused wide spread liquefaction, and resulted in tens of billions of dollars in damage. The dam failure itself caused eight fatalities. Prior to the Tohoku Earthquake, the Fujinuma dam retained a small reservoir known as Fujinuma Ike that was used for irrigation and leisure. The main dam was reported to have begun breaching within 20 minutes of the earthquake and a photo, taken approximately 25 minutes after the earthquake, shows almost the entire length of the dam being overtopped. Despite the failure, this dam was constructed very well and has suitable maintenance after finishing construction in 1949. GIS mapping can help explain the destruction and failure of this dam, however is is still unclear how the earthquake caused the destruction of this dam in such a strong way.
It is important to review other instanced of dam failure when understanding how the Bonneville Dam may fail during the magnitude 9.0 Cascadia earthquake. Observing previous earthquakes, landslides, spillovers, and tsunamis will help us when we model the potential dam failure of the Bonneville Dam.
Works Cited:
Pradel, Daniel, et. al “Failure of Fujinuma Dam During the 2011 Tohoku Earthquake” Conference on Earthquake Engineering, 2012.
Ouimet, William B. “Landslides associated with the May 12, 2008 Wenchuan earthquake: Implications for the erosion and tectonic evolution of the Longmen Shan” Amherst College Dept. of Geology, 2009.
Yin, Yueping, et al “Landslide hazards triggered by the 2008 Wenchuan earthquake, Sichuan, China” Landslides, Volume 6, Issue 2, 2009.
“The Lower San Fernando Dam” GEO-SLOPE International Ltd, 1992.