Fig. 1.3.1 The coast of Oregon and its principal communities.
2. The Oregon Coast
Important to the geology and geomorphology of the Oregon coast is its tectonic setting, being located within the zone of collision of three of Earth's tectonic plates, the oceanic Juan de Fuca and Gorda plates, and the continental North American plate. The crust of the ocean plates is formed at the spreading ridges and is then carried eastward toward the continent, where being denser the ocean crust slides beneath the less-dense continental crust and is subducted. In most locations where plate subduction occurs major earthquakes are generated by the plates scraping together, but there has not been a subduction earthquake off the Oregon coast since the settlement by Euro-Americans. However, evidence has been found that such earthquakes occurred in the prehistoric past, evidence that includes estuarine marsh sediment buried by layers of sand, which had been transported far inland by huge tsunami waves that had accompanied each earthquake (Atwater 1987). Based on the numbers of such layers discovered by geologists along the coast, it has been concluded that catastrophic earthquakes and tsunami have occurred repeatedly in the past, with intervals ranging from 300 to 600 years. Carbon-14 dating of the buried marshes indicated that the most recent event occurred about 300 years ago, with its exact date having been established by Satake et al. (1996) to have been 26 January 1700, based on the arrival of the generated tsunami along the coast of Japan where it destroyed a number of villages. From the size of the tsunami waves that reached Japan, it was concluded that the earthquake must have been about magnitude 9, with the Oregon event having been comparable to the Sumatran subduction earthquake and tsunami in 2005.
As the oceanic plates are being subducted beneath the continent, sediment that had accumulated on the seafloor as the plate moved slowly toward the coast is scraped off and added to the continental mass. This is the origin of the Tertiary mudstones and siltstones that form the sea cliffs along the Oregon coast, while the rocky headlands are composed of volcanic rocks whose origins are also associated with this tectonic setting. Nearly all of western Oregon has been formed by continental accretion of ocean sediment and a series of volcanic seamounts and islands, or by entire blocks of the sea floor.
Fig. 1.3.2 The sea cliff and offshore stacks at Bandon on the southern Oregon coast. The numerous stacks are interpreted as evidence for the rapid erosion of this shore following the 1700 earthquake when this area abruptly subsided, while the near absence of erosion today is attributed to the ongoing tectonic uplift of the land, which exceeds the rate of rise in sea level.
Due to the tectonic setting of the Oregon coast, there are significant land elevation changes that affect the relative sea level, the change in the elevation of the land relative to that of the sea, which is globally rising at a rate between 1 and 2 mm/every year. The studies of the buried marsh deposits in estuaries that documented past occurrences of subduction earthquakes also supported the conclusion that nearly all of the Oregon coast abruptly subsided during those tectonic events, often by 1-2 m (Atwater 1987). In contrast, much of the coast is presently rising, with that south of Florence being uplifted faster than the global rise in sea level. This has been documented through analyses of benchmarks used by surveyors, which the government re-surveys every few years to determine their elevation changes. South of Florence the tectonic rise of the land represents a net rate of land emergence above the sea of the order of 1 mm/every year, while along the north coast the relative sea level rise is on average −1.5 mm/every year (the negative value signifying that the global rise in sea level is faster than the change in land elevations). The consequence of this difference is that during historic times coastal erosion has been substantially less along the southern Oregon coast where the land is emerging from the sea, evident in the vegetated sea cliff at Bandon (Fig. 1.3.2 ), in contrast with the significant erosion that is occurring along the northern Oregon coast (Komar and Shih 1993). The change from abrupt subsidence of the land duringa subduction earthquake, compared with the progressive aseismic uplift now experienced, is interpreted in terms of the present accumulation of subduction strain between earthquake events, causing the slow rise of the land while the plates are locked together; the sudden release of that strain at the time of an earthquake results in the immediate subsidence of the land.
Fig. 1.3.3 Sea cliff erosion at Gleneden Beach south of Lincoln City, the cliff being composed of uplifted Pleistocene terrace sands, with their erosion being the main source of sand to the beach.
Fig. 1.3.4 The condominiums constructed on the remnant of terrace adjacent to the Jump-Off Joe Landslide, with slippage of the bluff having stressed the foundations so the windows were broken, evident in this 1981 photo.
Landslides are also found on the basaltic headlands, or more precisely within the loose debris shed from the headlands that had accumulated along their flanks. Massive landslides associated with headlands have affected a few private homes, but in particular park lands such as Ecola State Park on Tillamook Head north of Cannon Beach. A number of large inactive landslides are found along the coast, believed to have formed at the time of the 1700 subduction earthquake; a few experienced renewed movement when their forest cover was removed by commercial logging.
Fig. 1.3.5 The Lincoln City Littoral Cell, with Cascade Head to its north and Cape Foulweather forming its southern boundary. Siletz Spit is positioned midway along the cell, with the remainder of the beaches backed by sea cliffs, the erosion of which is now the main source of sand to the beach as seen in (Fig. 1.3.4 ); sand carried down the Siletz River is deposited within the Bay, so that it represents a minor contributor to the beach.
With the beaches being contained within littoral cells bounded by large rocky headlands, they are in effect pocket beaches, even though they may have long shore lengths of 10 to over 100 km. In general, during the summer months the waves arrive predominantly from the west to northwest, and this causes a southward displacement of the sand within the cells. In contrast, the waves of winter storms mainly arrive from the southwest, moving the sand back to the north. As a result, there tends to be a seasonal north-south oscillation of beach sand within the littoral cells, but with the long-term net littoral drift effectively being zero. The existence of this net-zero longshore transport of beach sand was evident when jetties were constructed during the early twentieth century on the inlets to estuaries and bays. In contrast to jetty construction in Southern California and along most of the U.S. East Coast, which did block a net longshore transport of beach sand so that it accumulated to one side of the jetties while erosion occurred in the down-drift direction, the jetties along the Oregon coast generally resulted in sand accumulation both to their north and south sides, locally where the shoreline is partially sheltered from the waves by the jetties (Komar 1997). Thus, jetties constructed on the Oregon coast have not been a problem in terms of having induced erosion and property losses. However one exception was dramatic, where early in the twentieth century the community of Bayocean was lost to erosion, but that occurrence can be attributed to only a single jetty having been constructed, not the usual pair, with the single jetty having caused the beach sand to be swept through the inlet into Tillamook Bay, lost from the sand spit on which the community had been developed (Komar 1997).
Fig. 1.3.6 The erosion of Siletz Spit during the winter of 1972-73, with one home being lost and others on promontories of riprap placed to protect them. The erosion was caused by the coincidence of high storm waves and elevated tides, enhanced by seaward flowing rip currents that locally cut embayments into the beach.
These variable quantities of sand within the littoral cells and the resulting widths of their beaches depend on the existence of modern-day sources as well as the volumes of relict sand. This determines the rates of backshore property erosion, which varies from cell to cell, governed by the capacity of the fronting beach to buffer those properties from wave attack.
The Oregon coast is one of the world's most dynamic environments, with the extremes of its waves and tides accounting for occurrences of erosion like that seen in (Fig. 1.3.6 ). These extremes have a direct connection with the Earth's evolving climate, including the intensification of storms and the waves they generate, which may be due to global warming, and the periodic occurrences of major El Niños. The Oregon coast is noted for the severity of its winter storms, which typically generate waves having deep-water significant wave heights (the average of the highest one-third of the waves) greater than 10 m, and with the significant wave heights during the most severe storms having reached 15 m, at which time the highest individual waves would have been about 25 m, the height of a 10-storey building.
Daily measurements of waves off the Oregon coast have been collected by buoys since the mid-1970s. Of concern, those measurements demonstrate that the wave heights on an average have been progressively increasing, with the significant wave heights of the strongest winter storms back in 1975 having been about 9 m, having increased from 12 to 15 m in recent years (Allan and Komar 2006). While our analyses were based on wave-buoy measurements, data for the storm intensities in terms of wind speeds and atmospheric pressures extend further into the past, and demonstrate that the increases in wave heights likely began at least as early as the mid-twentieth century. Although the exact cause is uncertain, the increases in storm intensities and wave heights may be associated with global warming, though it has also been suggested that particulate pollution in the atmosphere may be important, drifting across the Pacific from China.
The increases in deep-water wave heights and periods measured by the buoys off the Oregon coast have produced parallel increases in the processes active on its beaches, in particular the sizes of the breaking waves and elevations reached by the swash runup of the waves when they reach the shore (Komar and Allan 2002). The runup elevations on beaches are particularly important in that they combine with the high tides to produce erosion of dunes and cliffs backing the beaches. Our analyses have shown that the progressive increase in wave heights has resulted in parallel increases in average swash runup levels at rates that are greater than the global rise in sea level, a factor that undoubtedly has played an important role in the increased property erosion experienced along the Oregon coast.
Oregon's tides are classified as mixed: there usually are two highs and two lows each day, but with the highs reaching different levels. With an average range of about 2 m and a maximum spring tide range of 4 m, they are further classified as mesotidal. These are the predicted astronomical tides based on the forces of attraction of the moon and sun on the ocean's water. The actual measured tides on the Oregon coast can differ significantly from those predictions, with the difference primarily being of interest when the measured tide is substantially higher than predicted, since such occurrences can result in beach and property erosion. One cause of elevated measured tides is the occurrence of a storm surge, created by the onshore-directed winds and low atmospheric pressures of major storms. Measurements of surges on the Oregon coast show that they elevate tides of the order of 1.0-1.5 m, and although they are much smaller than storm surges produced by hurricanes along the U.S. East Coast, due to the low slopes of Oregon's beaches its storm surges shift the mean-water shoreline landward by some 25-40 m, increasing the impact of the storm-wave runup on shore-front properties.
The most significant climate event in terms of its erosion impacts along the Oregon coast has been the occurrence of a major El Niño, like those during the winters of 1982-83 and 1997-98 (Komar et al. 2000; Allan and Komar 2006). Particularly noteworthy is that an El Niño significantly elevates the measured tides, on average by about 0.30 m but achieving a maximum difference of about 0.60 m between the measured and predicted tides. This is documented by assessments of the monthly averages of the increased tidal elevations, with the maximum occurring during December and January, corresponding to the months that tend to have the greatest numbers and intensities of storms. The increased water elevation itself can be accounted for in part by the thermal expansion of the coastal water, which even in normal years is warmer during the winter due to the presence of cold water in the summer caused by upwelling, the water achieving still higher temperatures and levels in an El Niño winter. Another component results from the northward-flowing coastal currents, with their deflection to the right by the Coriolis force acting to pile water up along the shore; again, which tends to be stronger during an El Niño, resulting in elevated monthly-averaged water levels. Of importance to the resulting erosion of the Oregon coast, this increase in water levels spans the entire winter, in effect representing a sudden increase in mean sea level, even though it later returns to normal when the El Niño ends. This rise in the monthly-averaged water levels during an El Niño elevates the water at all stages of the tides, so for several months there is an appreciably enhanced probability that the runup of storm waves on beaches will impact shore-front properties.
Fig. 1.3.7 The contrasting reversing longshore sand movements within Oregon's littoral cells during a Normal Year versus a major El Niño when strong storms from the southwest result in an enhanced northward transport and the development of 'hot spot' erosion sites.
Fig. 1.3.8 The 1982-83 El Niño erosion at Cape Lookout State Park, with the loss of the high tree-covered dunes and vertical supports of a failed log sea wall seen in the background. The bathroom was lost during the 1997-98 El Niño.
Fig. 1.3.9 The reconstructed dunes with a core of sand-filled geotextile bags, and cobble berm at Cape Lookout State Park, used as an alternative form of shore protection (Allan and Komar 2004).
The use of 'hard' structures - riprap revetments and sea walls - along the Oregon coast has had adverse impacts, especially where they have been used to prevent the erosion of sea cliffs. In some littoral cells the cliff erosion is the primary source of sand to the beach, in locations where the cliffs are composed of uplifted Pleistocene beach and dune sand; for example, the erosion at Gleneden Beach (Fig. 1.3.3 ) within the Lincoln City Littoral Cell (Fig. 1.3.5 ) south of Siletz Spit. There has been a considerable proliferation of structures within that cell, to the extent that the cliff-erosion source of sand to the beach has largely been cut off. Based on the previous rates of cliff retreat within that cell, it was estimated that the quantities of sand added to and building up the beach effectively balanced the relative rise in the sea level at that site (Shih and Komar 1994). With the loss of that sand source due to the installation of shore-protection structures, this equilibrium no longer exists, with the expected prolonged detriment of the beaches within that cell.
Impressive accumulations of dune sands are found on the Oregon coast estimated to be present along about 45% of the coast, either in the form of foredunes backing the beaches or contained within the massive Oregon Dunes Recreation Area that extends from Florence, south to Coos Bay. People have had a major role in altering the vegetationcover of the dunes, which in turn has affected their morphologies, with mixed consequences. When Euro-Americans first settled the Clatsop Plains south of the Columbia River in the nineteenth century, the extensive dune fields were covered by dense grasses. Those native grasses could be eaten by livestock, and overgrazing quickly reactivated the dunes so that by the 1930s some three thousand acres of sand had become mobile. In 1934 this area was planted with European beach grass, which livestock will not generally eat. Its introduction has had unforeseen consequences as it rapidly spread along the coast. On the positive side, it captured sand blowing landward from the beaches, building up substantially higher foredunes than had previously existed with the native grasses, providing a greater degree of protection from erosion and flooding to backshore properties. However, this growth of the foredunes has had negative environmental consequences, particularly to the nesting of Snowy Plover that need areas of open sand; in recent years extensive efforts have been undertaken to locally remove this invasive dune grass, to provide nesting habitat for this endangered shore bird.
The arrival of the European beach grass has also had negative consequences for the Oregon Dunes Recreation Area. A century ago those dunes existed as an unvegetated sand surface extending from the ocean shore to the precipitation ridge of the dunes at their landward edge (Cooper 1958). Sand was free to blow inland from the beach to support the continued growth of the high dunes. However, since the arrival of the European beach grass in the 1930s, large foredunes have grown immediately landward from the beach, where they now capture the sand blowing inland, preventing it from reaching the inland dunes. The impact of that loss was first noted in the area immediately landward from the foredunes, where the ground level was lowered to the water table, permitting the growth of shrubs and other vegetation where the high dunes had previously existed. The aerial extent of the active dunes has substantially decreased, and there is concern regarding their long-term preservation. An attempt was made to remove the beach grass and foredunes along a portion of the Oregon Dunes, but that experiment has not been followed up by a larger-scale implementation of this potential solution.
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