Oregon Coastal Landforms
The Pacific coast of Oregon extends for nearly 700 km from the Washington border on the Columbia River in the north, to the border with California in the south (Fig. 1.3.1). The Oregon coast is characterised by considerable geomorphic variability, with long stretches of rocky shore and major headlands formed of resistant volcanic rocks, which isolate sections of sandy beaches ranging in lengths from small pockets to littoral cells that extend for 10 s to over 100 km distances along the coast; the longest is the 250 km beach from Florence to Coos Bay, backed by the Oregon Dunes Recreation Area, the most extensive coastal dune sheet in North America.
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.
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 during a 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.
The long-term net tectonic uplift of the Oregon coast, together with the cycles of sea level rise and fall during the Ice Ages, has given rise to marine terraces that in places form stairways up the flanks of the Coast Range. Along the south coast the highest and oldest terrace in the series reaches elevations up to nearly 500 m. The lowermost terrace, extending along much of the coast, dates back about 80,000 years. The Pleistocene terrace sands (former beaches and dunes), together with the underlying Tertiary mudstones and siltstones, are being eroded by the waves, forming sea cliffs that back sand beaches. Many of Oregon’s coastal communities are situated on this nearly level terrace; cities such as Cannon Beach, Lincoln City and Newport have suffered property losses as the cliffs progressively retreated (Fig. 1.3.3). State Parks have also been impacted, with the loss of picnic grounds and camping facilities. In total, sea cliff erosion and property losses affect hundreds of km of the Oregon coast (Komar 1997).
Landslides are common along the Oregon coast. The sea cliffs cut into the marine terraces are particularly susceptible to mass movement on various scales, with the occasional formation of large landslides (Komar 2004). This susceptibility is due in part to the consistency of the cliffs, landsliding being most active and destructive where the cliffs are composed of Tertiary mudstones and with their layers dipping toward the ocean. Most destructive in terms of losses of private property has been the Jump-Off Joe Landside in Newport (Komar 1997). Its initial movement began during the winter of 1942–43, affecting about 15 acres along with the loss of 15 homes. An attempt was made to re-develop the site in 1980, with plans to build condominiums on the down-drop block of the landslide. This plan was prevented when the State rejected the developer’s request to construct a sea wall along the toe of the slide, to prevent further wave erosion and its continued seaward movement. The developer instead constructed the condominiums on a small remnant of the marine terrace to the immediate north of the Jump-Off Joe Landslide, beyond which was a second older landslide of comparable size. As the condominiums approached completion in 1981, slippage in the remnant terrace undermined the foundation, leading to their destruction (Fig. 1.3.4). With the developer having gone bankrupt, the city had to cover the expenses of tearing down the condominiums.
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.
The differential rates of erosion and long-term retreat of the different rock types found along the Oregon coast have given rise to its irregular shore, with headlands of resistant basalt separating embayments formed by the more rapid retreat of the less resistant mudstones and siltstones. These embayments are the principal sites of beach sand accumulation, each constituting what is termed a littoral cell, basically a stretch of beach that for the most part is isolated by the large headlands that prevent the exchange of sand between adjacent cells: Figure 1.3.5 is a typical example of a littoral cell. The individual cells contain different quantities of sand, apparent in the widths of their beaches, with the quantities depending on the existence of sand sources such as rivers or sea cliff erosion, which differ from cell to cell. A few cells have virtually no modern-day sources, the sand in its beaches instead being relict, having reached the cell thousands of years ago. In a study of the mineral contents of the sands within the cells we found that the metamorphic rocks of the Klamath Mountains in southern-most Oregon and northern California had been a prime source of sand, even to the cells along the central to northern Oregon coast where the numerous headlands now make it impossible for the Klamath sand to reach those beaches (Clemens and Komar 1988). We concluded that the Klamath derived sand had been transported northward during times of lowered sea levels in the Ice Ages, unhindered by the presence of headlands when the shores were seaward on what today is the continental shelf. With the melting of glaciers and the rise in sea level, those shelf sands were pushed landward within the migrating beaches, having become trapped between headlands in the littoral cells roughly five thousand years ago.
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 northsouth 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).
While the beaches along the Oregon coast are predominantly backed by sea cliffs, a number of sand spits are found separating the ocean from the estuaries and bays, as seen in (Fig. 1.3.5) for Siletz Spit. These spits individually point either north or south, and some in close proximity and within the same littoral cell point in opposite directions; therefore, the direction of spit extension does not provide evidence for the existence or direction of a net littoral drift. While in total the progressive loss of property along the Oregon coast resulting from sea cliff erosion has probably been greater, more dramatic has been the erosion of developments on the sand spits, beginning with Bayocean and more recently with the major erosion that has occurred on Siletz Spit (Fig. 1.3.6), Alsea Spit, and Netarts Spit, with the latter having impacted a State Park.
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 wavebuoy 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.
The occurrence of a major El Niño also tends to, on average result in a degree of increase in the heights of winter storm waves that reach the Oregon coast, but far more important is that the waves arrive more from the southsouthwest than during normal years. This is because the tracks of the El Niño storms are shifted more to the south as they cross the North Pacific, passing over the shores of California rather than Oregon and Washington as they do during normal winters. The result is that unusually large quantities of the beach sand within the littoral cells are transported northward during an El Niño winter, resulting in ‘hot spot’ erosion at the south ends of the cells, to the north of the headlands, as depicted in Fig. 1.3.7. Furthermore, the enhanced northward longshore sediment transport also tends to deflect the inlets to bays and estuaries, forming ‘hot spot’ erosion sites to the north of those inlets (or where present, north of jetties that act as mini-headlands). Following the El Niño winter the sand slowly returns to the south within the littoral cells, eventually reestablishing the long-term equilibrium sand volumes, although this return may take several years during which the erosion within the hot-spot zones persists.
Erosion during the major El Niños of 1982–83 and 1997–98 extended along the entire U.S. West Coast, with that in northern California, Oregon and Washington having primarily occurred in hot-spot zones as depicted in Fig. 1.3.7. Furthermore, along the Oregon coast the 1997– 98 El Niño erosion was significantly expanded during the following winter of 1998–1999 when several storms generated waves that exceeded what then had been projected to be the 100-year extreme event (later to be reanalysed and increased to a 16-m significant wave height). The combined impacts of those two winters were extensive, to both private properties and State parks, in what we characterised as having been a ‘one-two punch.’ Figure 1.3.8 shows the erosion at Cape Lookout State Park on Netarts Spit, an example of hot-spot erosion in that the Park is located at the south end of the littoral cell, to the north of Cape Lookout. The erosion began during the 1982–83 El Niño (Komar 1997), at which time the high tree-covered dunes were eroded away, so the public bathrooms within the campground landward of the dunes were threatened. Although they were temporarily protected by the placement of riprap, it proved insufficient during the 1997–98 El Niño and major storms that swept across the campground during the following winter, damaging the bathrooms to the extent they had to be torn down.
The erosion of sea cliffs and foredunes along the Oregon coast has led to the proliferation of riprap revetments, with an example being those on Siletz Spit seen in Fig. 1.3.6. The use of a ‘hard’ structure was undesirable in Cape Lookout State Park following the El Niño erosion, so it was decided to follow a ‘design with nature’ approach by constructing an artificial dune containing a core of sand-filled geotextile bags and planted with native dune grass, fronted by a constructed cobble berm (‘dynamic revetment’), in essence a nourished cobble beach as found naturally along the Oregon coast, thus acting to dissipate the swash of the waves before they impact the rebuilt dunes. The resulting ‘structures’ (Fig. 1.3.9) have the appearance of their natural counterparts along the coast, and thus far have provided protection to the State Park (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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