Posted on September 1, 2020
People don’t generally think about how important sand and mud are in the context of our coastal communities and recreation areas, but in essence, sediments are what shape our coast. Sediment transport is a tricky subject. Even Einstein said it is too difficult a subject to study, but sediment transport in actuality is a force equals mass multiplied by acceleration problem: with bigger forces, like a tsunami, more sediment will move. Before considering tsunami, let’s start by thinking about how sediment transport affects us in the day-to-day.
First, our beaches. The sand spit in Bayshore on the Oregon Coast goes through periods of erosion and accretion. Twenty years ago in Bayshore, homeowners were afraid of their houses getting lost to the ocean. Today the beach is accreting (building) so much that if you visit the community after a really windy few days, the houses there might actually be covered to their roofs by sand. The homeowners need to hire contractors to dig their homes out. The opposite is true of Beverly Beach, between Newport and Depoe Bay, where Highway 101 cuts very close to the shore. As you drive along that stretch you can see the beautiful Pacific Ocean and its breaking waves. Those powerful breakers concern the Oregon Department of Transportation, which is keenly aware that the highway is too close to the eroding beach, and any big storm or nasty winter season could threaten its integrity.
River inlets are important to the economy of Oregon., which has eleven coastal river inlets along a 363-mile-long coast. Many of these rivers lead to ports that feed the Oregon and national economies. Coastal ports are used for fishing, trade, and recreation. Yet, river inlets are naturally dynamic; the sediment in the river channels changes shape with waves, river currents, and tidal currents.
The Portland office of the US Army Corps of Engineers keeps Oregon’s waterways safe and navigable through dredging sandbars and deepening shipping channels to ensure the safe transport for more than $24 billion in waterborne commerce. In order for ships to pass in the Columbia River, dredges must move 35,000 cubic yards of material each day. That is about 3,500 standard dump trucks full of sand every day. If you have not heard of the Columbia River Bar Pilots, look up their story, they help all this traffic navigate the treacherous and dynamic sand bars.
Finally, coastal estuaries are some of the most productive regions in nature. Studies have shown that estuaries are valued on average at $50 million per square mile per year in 2007 dollars for all of the ecosystem services that they provide to habitat and humanity. They are important for food, habitat, recreation, carbon sequestration, and much more. Although waves are not as large in estuaries, wave attacks at fringe marshes combined with sea level rise does cause erosion of marsh muddy sediment. With erosion of sediments, water clarity decreases and nutrient loads (nitrogen and phosphorous) increase and degradation of submerged flora and fauna ensues.
In the day-to-day, sediment transport in beaches, river inlets, and estuaries, is important to Oregon’s ocean economy. But this disaster preparedness series is , about disasters, so what about the projected Cascadia subduction zone earthquake event? The Cascadia event is estimated to happen once every 300 years and predicted to potentially bring a magnitude 9 earthquake that causes a tsunami. As a tsunami approaches the coast its speed will slow because the water gets shallower and to conserve mass the height of the wave must grow, potentially up to 10 times its offshore height or more. People on the beach, in low-lying areas of the coast, and near estuary mouths or tidal flats will be in the greatest danger.
A tsunami can cause bed shear stresses (force imparted from the water to the sediment) that are potentially more than 10 times greater than coastal storm waves. This will cause a lot more sediment to move. Sandy beaches and dunes could be flattened during the inundation caused by the tsunami, especially if it occurs during a high tide. Oregon river mouths will be reshaped by high water levels and flow speeds, which will threaten their navigability. In estuaries, alongside inundation, the biggest threat will be land subsidence from the earthquake shaking. Research from the 1700 earthquake shows that land levels dropped up to three feet in some places; the productive estuarine ecosystems will drown and take hundreds of years to grow back. The Cascadia event would be catastrophic and re-shape our coast and the way we interact with it for years to come.
Source: democratherald
