Flooding: A Comprehensive Overview
Flooding is a significant hazard often triggered by earthquakes, and it can have devastating consequences for coastal and inland areas alike. Here's an in-depth look at the various causes and impacts of flooding, with a focus on the Pacific Northwest region.
Earthquakes and Tsunamis:
One of the most well-known triggers of flooding is earthquakes, particularly those originating offshore. When a large earthquake strikes, it can generate tsunamis, which are extremely long waves caused by the sudden displacement of the seafloor. In the Pacific Northwest, both distant and local earthquakes can trigger tsunamis. For instance, a Cascadia Subduction Zone earthquake could produce tsunami waves averaging 40 to 60 feet high along the coast, with some waves reaching an astonishing 100 feet tall. But it's not just major earthquakes that pose a risk; smaller quakes along shallow crustal faults can also cause tsunamis. A hypothetical large Seattle Fault earthquake could trigger a 16-foot-high tsunami, flooding the shoreline within minutes, while an earthquake along the Tacoma Fault could have similar effects on the Puyallup River delta.
Taking Action During an Earthquake:
If you're near the coast and feel shaking or receive an earthquake early warning alert, it's crucial to act swiftly. Drop, cover, and hold on until the shaking stops, then move inland to higher ground immediately. If you witness a sudden rise or fall in the ocean or hear a loud roar, evacuate the coast immediately. Tsunami inundation and evacuation maps are available for Washington and Oregon, providing essential guidance for residents.
Landslides and Seiches:
Flooding can also result from landslides and seiches. Seiches are standing waves in inland water bodies caused by earthquake shaking, and their oscillations can last for hours, causing significant flooding along shorelines. Despite their relatively small height, seiche waves can still flood or wash away houses. Large inland landslides can temporarily fill river valleys, creating unstable dams that flood large areas upstream. Eventually, these dams may fail, leading to even more severe downstream flooding.
Infrastructure Failure:
Damage to flood control and water distribution systems can exacerbate flooding after an earthquake. Most of the infrastructure in the Pacific Northwest, including water pipelines, dams, and other structures, was built before modern seismic design codes. Earthquakes can crack or buckle these systems, potentially releasing large volumes of water and causing localized flooding. However, it's important to note that dams in the region are unlikely to completely collapse during a major earthquake, as evidenced by the rare case of the Shih-Kang Dam in Taiwan.
Spillway Gate Concerns:
A bigger concern for dams in the Pacific Northwest might be the damage to spillway gates. These gates control the water flow from reservoirs into downstream rivers. The Army Corps of Engineers has warned that a Cascadia earthquake could cause spillway gates on the Detroit Dam in Oregon to buckle, leading to devastating flooding in the Santiam Canyon and affecting hundreds of thousands of people downstream, including residents of Salem.
Broken Water Pipelines:
Localized flooding can also occur due to broken water pipelines. Ground displacement and liquefaction can cause buried water lines to snap or pull apart at the joints, creating hundreds of thousands of leaks that can drain entire water systems. The Environmental Protection Agency has developed an earthquake resilience guide for water and wastewater utilities, offering recommendations to address these risks.
ShakeAlert Technology:
One innovative tool to mitigate flooding is the ShakeAlert Earthquake Early Warning System. This technology can quickly detect significant earthquakes, allowing alerts to be sent to affected areas before the shaking begins. Managed by the U.S. Geological Survey, the Pacific Northwest Seismic Network operates seismometers in Oregon and Washington. By integrating earthquake early warning technology into control systems, communities can trigger pre-programmed actions, such as closing output valves for water reservoirs, to minimize flooding and conserve water for essential human consumption.
