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6 Responses of the Natural Shoreline to Sea-Level Rise S ea-level rise affects the natural shoreline in several restored tidal wetlands in providing protection from fu- ways. Higher water levels erode beaches, dunes, ture inundation and the impact of waves. The objective and cliffs; inundate wetlands and other low-lying was to summarize existing knowledge, not to predict areas; and increase the salinity of estuarine systems, dis- specific future shoreline responses or to assess coastal placing existing coastal plant and animal communities. impacts of sea-level rise and storminess (see Box 1.1). These coastal environments provide a protective buffer to areas further inland, as wetlands can reduce flooding COASTAL CLIFFS AND BLUFFS and cliffs, beaches, and dunes protect coastal property from storm waves. Cliffs and bluffs are dominant features along the The distribution and character of coastal habitats west coast of the United States, and they have been and geomorphic environments varies along the Cali- retreating for thousands of years. The rate of coastal fornia, Oregon, and Washington coasts, as does their cliff and bluff retreat is controlled by the properties of response to sea-level rise. The coast of California is the rock materials and the physical forces acting on the dominated by uplifted terraces fronted by low cliffs, cliffs. Important rock properties include the hardness but also includes steep coastal mountains and areas of or degree of consolidation or cementation, the presence coastal lowlands and dunes. Oregon's coast is similar of internal weaknesses (e.g., fractures, joints, faults), and is characterized by rugged volcanic headlands and the degree of weathering. Rates of cliff retreat are separating areas of uplifted marine terraces and river generally well documented along the California coast mouth estuaries, dunes, and beaches. The southern (Dare, 2005; Hapke and Reid, 2007), and range from coast of Washington is dominated by low relief sand a few cm per year in granitic or volcanic rock to tens spits, occasionally backed by bays. The northern coast of cm per year or more in sedimentary rocks or uncon- and Olympic Peninsula are rocky and rugged, whereas solidated materials (Griggs, 1994; Griggs et al., 2005). Puget Sound retains the signature of Ice Age glacia- Moore et al. (1999) found cliff and bluff erosion rates of tion--a crenulated coastline with islands, embayments, 220 cm yr-1 for 19321994 in San Diego County, and and typically sandy bluffs. 614 cm yr-1 for 19531994 in Santa Cruz County. In This chapter summarizes what is known about California, cliffs and bluffs made of sedimentary rocks (1) the responses of coastal habitats and geomorphic typically erode at rates of 1530 cm yr-1 (Griggs and environments--including coastal cliffs and bluffs, Patsch, 2004). beaches, dunes, estuaries, and marshes--to future Fewer bluff retreat rates are available for the Oregon sea-level rise and storminess along the west coast of and Washington coasts. Komar and Shih (1991) and the United States and (2) the role of coastal habitats Komar (1997) described the temporal and spatial vari- (including benthic habitats), natural environments, and ability in cliff and bluff erosion along the Oregon coast, 109
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110 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON noting that cliff erosion is slower where uplift rates are the cliff or bluff more frequently, thereby increasing the highest and the base of the cliff has been raised to an rate of cliff retreat. elevation seldom reached by wave runup. Priest (1999) Cliff and bluff retreat is an episodic process whereby found that cliffs and bluffs in Lincoln County, Oregon, large blocks fail suddenly under conditions of heavy generally retreated at rates less than 19 cm yr-1 for rainfall, large waves at times of elevated sea levels or 19391991. In landslide areas, bluff retreat rates were high tides, or earthquakes, followed by periods of little somewhat higher, ranging from 1150 cm yr-1. or no failure. In steep, mountainous areas, failure is The physical forces driving cliff and bluff erosion often through large landslides or rock falls (Figure 6.2), include marine processes--primarily wave energy and usually driven by excess or prolonged rainfall during the impact, but also tidal range or sea-level variations-- winter months. With very large landslides, such as the and terrestrial processes, such as rainfall and runoff, Portuguese Bend slide on the Palos Verdes Peninsula, groundwater seepage, and mass movements such as the shoreline may actually be extended seaward for a landslides and rockfalls. As discussed in Chapter 4 decade or more before basal wave action removes the ("Short-Term Sea-Level Rise, Storm Surges, and protrusion (Orme, 1991). The episodic nature of cliff Surface Waves"), waves may be getting higher (e.g., retreat, combined with the frequent absence of an Figure 6.1). Increased wave heights mean that more identifiable edge or reference feature, makes it difficult wave energy is available to erode the coastline. Rising to quantify or verify cliff erosion rates in mountainous sea level would exacerbate this effect because waves will areas over short time intervals, such as a few decades, break closer to the coastline and will reach the base of or to project future erosion rates (Priest, 1999). FIGURE 6.1 Boiler Bay, Oregon. Some evidence suggests that waves have been increasing in height off the west coast. SOURCE: Courtesy of Erica Harris, Oregon State University.
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 111 FIGURE 6.2 Large-scale landsliding along the Humboldt County, California, coast at Centerville. SOURCE: Copyright 20022012 Kenneth & Gabrielle Adelman, California Coastal Records Project, . Cliff and bluff erosion is not reversible. The most are usually designed for a particular set of wave and common human response has been to armor the cliff sea-level conditions. If sea level increases substantially base with rock revetments (Figure 6.3) or seawalls and wave heights continue to increase, the original (Figure 6.4). Ten percent of the California coastline freeboard will be gradually exceeded and overtopping has now been armored, including 33 percent of the will become more frequent. coastline of the four most developed southern Cali- fornia counties (Ventura, Los Angeles, Orange, and BEACHES San Diego; Griggs, 1999). Shoreline armoring also has increased over the past several decades in Oregon and Beaches respond quickly to the forces acting on Washington. Approximately one-third of the Puget them as waves and littoral currents easily move the Sound shoreline is now armored (Shipman et al., sand. Along the west coast, beaches change seasonally 2010). Despite this protection, coastal storm damage in response to the different winter and summer wave has increased over the past several decades because of climates. These fluctuations in beach width are predict- intense development and the occurrence of a number of able and temporary, and the losses of sand experienced severe El Niño events, raising questions about the long- each winter are normally recovered the following term efficacy of existing coastal protection structures summer. Longer-term fluctuations in beach widths (Griggs, 2005; Shipman et al., 2010). Moreover, while associated with the El Niño-Southern Oscillation and seawalls and revetments may provide current protection the Pacific Decadal Oscillation (PDO) also have been for oceanfront development and infrastructure, they documented in southern California (Orme et al., 2011).
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112 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 6.3 Erosion of poorly consolidated sedimentary cliffs at Pacifica, south of San Francisco, is threatening these apartments, and residents have had to move out. Riprap protection has been placed at the toe of the bluff in an attempt to slow the erosion. SOURCE: Hawkeye Photography. FIGURE 6.4 Seawalls and revetments fronting coastal cliffs and bluffs in California and Oregon. (Left) Concrete and timber seawalls protecting cliff top homes in Solana Beach, California. SOURCE: Copyright 20022012 Kenneth & Gabrielle Adelman, California Coastal Records Project, . (Right) Rip rap protecting bluff top housing along the central Oregon coast. SOURCE: Courtesy of Gary Griggs, University of California, Santa Cruz.
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 113 Periodic El Niño events both enhance storm wave dammed for water supply, flood control, hydroelectric activity, leading to severe beach erosion, and increase power, or recreation--provide most beach sand along rainfall and runoff, increasing sand delivery to the the west coast. Willis and Griggs (2003) determined shoreline and thus sometimes leading to wider beaches that more than 500 dams have reduced the average an- in subsequent months. More frequent storms during nual sand and gravel flux to California's coastal water warmer PDO cycles can lead to extended periods when sheds by 25 percent. Sherman et al. (2002) calculated beach widths are narrower than average. Over the long that 28 dams and more than 150 debris basins in the term, rising sea level will cause landward migration water sheds of eight major rivers in southern California or retreat of beaches. The retreat is caused partly by have impounded more than 4 million m3 yr-1 of sand. inundation of the beach by the rising sea and partly Statewide, approximately 152 million m3 of sand that by offshore transport of sand to maintain the beach would have been delivered to the shoreline to nourish profile. Because the berm or back beach is essentially beaches since 1885 has been trapped by coastal dams a horizontal surface, even a small rise in sea level may (Slagel and Griggs, 2008). The long-term effect of lead to a horizontal retreat that is considerably larger declining sand supply works in concert with rising sea than the sea-level rise (Edelman, 1972). level to progressively narrow beaches. Beaches also can undergo erosion or long-term Barrier spits or other sandy peninsulas, which are retreat in response to a reduction of sand supply. common along the northern Oregon and southern Coastal rivers and streams--many of which have been Washington coastlines (Figure 6.5), will tend to erode FIGURE 6.5 Oregon's Cape Lookout State Park on Netarts Spit, which is backed by Netarts Bay. Long sand spits commonly form at the mouths of estuaries along the central and northern Oregon coasts and Washington coast. SOURCE: Courtesy of Erica Harris, Oregon State University.
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114 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON or migrate under elevated sea levels and large storm photographs, and, more recently, lidar (light detection waves. Erosion or landward migration of sand spits or and ranging) were used to determine both long-term barrier bars will occur more frequently with sea-level (1800s to 19982002) and short-term (1950s1970s to rise (Pilkey and Davis, 1987). 19982002) rates of shoreline or beach change. More Back-beach barriers can slow or halt the natural than 16,000 transects revealed that the shoreline eroded inland migration of beaches because of rising sea level. 0.2 ± 0.4 m yr-1 over the short term. The average rate of Where a seawall, revetment, or structure exists, the long-term change was 0.2 ± 0.1 m yr-1, an accretional shoreline cannot advance landward and the beach is trend, although 40 percent of the transects showed net progressively inundated (Figure 6.6). This process, erosion. This net accretional trend was attributed to known as coastal squeeze or passive erosion, has been the large volumes of sediment that were added to the documented in a number of locations along the west system from large rivers and to the impact of coastal coast. Similarly, barrier spits that have been developed engineering and beach nourishment projects (Hapke and then protected with revetments cannot migrate et al., 2006). A similar assessment effort is planned for with sea-level rise (Figure 6.7). Depending on the rate the Oregon and Washington coasts. of sea-level rise, all west coast beaches with hardened or constrained back beach edges will gradually be COASTAL DUNES inundated. Only a few studies have quantified rates of change Cooper (1958, 1967) mapped and described the along the sandy shoreline of the U.S. west coast. coastal dunes of Washington, Oregon, and California, Kaminsky et al. (1999) found widely varying rates and found that extensive coastal sand dunes accumulate of change for the sandy shoreline of Pacific County, when the following conditions are met: (1) a large sup- Washington, ranging from +0.8 to +14.2 m yr-1 for ply of fine-grained sand, (2) a barrier such as a headland 18701926, -13.6 to +8.8 m yr-1 for 19261950, and to trap littoral drift and accumulate sand, (3) a low- -7.0 to +4.2 m yr-1 for 19501995. Sand spits eroded or relief area landward of the beach where sand can accu- accreted, depending on sand supply, wave energy, and mulate, and (4) a dominant or persistent onshore wind. relative sea level. Coastal land change along the sandy Large dune fields are best preserved in areas that have shoreline of California was assessed as part of the U.S. undergone either net subsidence or limited uplift dur- Geological Survey's National Assessment of Shoreline ing the Quaternary (Orme, 1992). Dunes back about Change program (Hapke et al., 2006). Maps, aerial 45 percent of the Oregon coast and 31 percent of the FIGURE 6.6(Left) Passive erosion in front of a revetment, illustrating the loss of beach where the structure restricts the shoreline from migrating landward. The beach continues to migrate inland on either side of the revetment. (Right) Recovery of the beach following removal of the revetment and bluff top structure. SOURCE: Copyright 20022012 Kenneth & Gabrielle Adelman, California Coastal Records Project, .
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 115 FIGURE 6.7 Developed sand spit at Stinson Beach in Marin County, California, where a revetment has been constructed in an effort to protect the homes. This spit cannot migrate with sea-level rise. SOURCE: Copyright 20022012 Kenneth & Gabrielle Adelman, California Coastal Records Project, . Washington coast (Komar, 1997). Many of the dune to the beach, is an ephemeral and unstable feature areas exposed along and inland from the west coast (e.g., McHarg, 1969). Sand dunes typically accrete or shoreline today formed during the lower sea levels of expand under the force of onshore winds and an ample the past. At the end of the last ice age, when sea levels supply of sand, but they can erode quickly under severe were about 120 m lower than today, the entire conti- wave attack at times of high tide or elevated sea level. nental shelf was exposed. Sand from rivers and streams The hazards of building on the frontal dune have been was deposited across this extensive plain, and onshore known for centuries (McHarg, 1969). Nevertheless, winds produced large dune fields, such as those in the many housing developments in California, Oregon, and Coos Bay area of central Oregon, which extend along Washington have been constructed on dunes and are the coast for nearly 240 km and are encroaching into periodically threatened or damaged (Figure 6.9). Dunes, some developed areas (Figure 6.8; Komar, 1997). As sea whether modern or Pleistocene, can be expected to level rose, many of the dunes were cut off from their retreat quickly under rising sea levels and larger waves. vast reservoir of offshore sand. Dunes still form and are active today along the shorelines of all three states, but RETREAT OF CLIFFS AND BEACHES they have a lower supply of sediment and are much less UNDER SEA-LEVEL RISE extensive than those that formed in the past. Decades of observations of coastal dunes around the Coastlines have been retreating globally since sea world have shown that the frontal dune, which is closest level began rising at the end of the last ice age, ap-
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116 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 6.8 Dunes along the central Oregon coast at Florence are encroaching into development. SOURCE: Courtesy of Phoebe Zarnetske, Oregon State University. FIGURE 6.9 Construction of private homes on the frontal dunes. (Left) Homes in central Monterey Bay were threatened by erosion during the high tides, elevated sea levels, and large storm wave of the 1983 El Niño. SOURCE: Courtesy of Gary Griggs, University of California, Santa Cruz. (Right) Placement of riprap during storm conditions to protect development on dunes in Neskowin, Oregon. SOURCE: Courtesy of Armand Thibault.
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 117 proximately 21,000 years ago. At that time, the western sea-level rise or wave climate. Where data are available, shoreline of North America was located at the edge of projections for future coastal retreat could be made by the continental shelf (Shepard, 1963; Nummedal et al., extrapolating existing erosion trends (e.g., Box 6.1) and 1987), which for Oregon and Washington is typically adding an appropriate safety factor to accommodate 2550 km offshore (Komar, 1997). Off the California expected future sea-level rise and potential increases coast, the shelf width varies, averaging 1530 km, but in storm wave heights. Because projected rates of sea- narrowing to 5 km or less off Big Sur and parts of level rise are moderate in the near term (Chapter 5), southern California, and widening to 40 km off San extrapolation of current erosion rates is likely reason- Francisco (Figure 6.10). The average rate of coastline able to at least 2030. retreat over the post-glacial period of sea-level rise can An alternative approach to projections, developed be estimated by dividing the width of the continental by PWA (2009), relates rates of shoreline change to the shelf at a specific location by 21,000 years. For example, coastal geology, then applies changes in total water level a shelf width of 5 km corresponds to an average retreat at the shoreline in exceedance of the elevation of the rate of 23.8 cm yr-1, and a 40 km wide shelf corresponds base of the bluff or cliff to predict erosion (Figure 6.11). to an average rate of 190 cm yr-1. Of course, the actual Based on this approach, the central and northern Cali- rate at any given time and place may be significantly fornia coast is projected to lose 81 km2 of land by 2100 higher or lower, depending on variations in the rate relative to 2000 for 1 m of sea-level rise and 99 km2 of of sea-level rise over the 21,000-year period as well as land for 1.4 m of sea-level rise (Table 6.1; Heberger et geographic variations in coastal geology, regional wave al., 2009; PWA, 2009). Due to their differing resistance climate, offshore bathymetry, and the degree of coastal to erosion, dunes and cliffs will respond differently to armoring. rising sea levels. Under the scenario of 1.4 m of sea- Few studies have projected future shoreline and sea level rise by 2100, Revell et al. (2011) predicted that cliff retreat rates under rising sea level. For example, cliffs would erode an average distance of 3360 m, a Federal Emergency Management Administration- depending on assumptions about geologic variability, sponsored effort to assess future coastal erosion haz- and that dunes would erode an average distance of ards (Crowell et al., 1999) simply projected historic 170 m in the 11 counties studied. However, projected erosion rates without considering changes in rates of land losses vary significantly within each county and along the coast. In Del Norte County, for example, the average distance cliffs are projected to erode is 85 m by 2100 and the maximum distance is 400 m (Revell et al., 2011). The variability in how far cliffs are expected to erode under sea-level rise is illustrated in Figure 6.12. Such uncertainties in land losses, combined with uncer- tainties in exactly how sandy shorelines with back beach barriers or armor will respond to sea-level rise and with uncertainties in rates of future sea-level rise, make precise projections of future beach retreat or erosion in these areas problematic. Wave Energy and Coastal Erosion Wave-induced cliff and shoreline erosion is a sig- nificant problem along the west coast of the United States, and an increase in wave energy will only in- crease the rates of retreat. The amount of wave energy FIGURE 6.10Sea-level rise has moved the San Francisco expended at any position on the coast is determined by shoreline eastward by about 40 km since the last Ice Age ended. the effects of wave height, tidal elevation or sea level, SOURCE: Griggs (2010).
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118 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON BOX 6.1 Technology, Tools, and Resources for Evaluating Sea-Level Rise and Coastal Change Most historic assessments of coastal change have relied on stereo vertical aerial photographs, which can be used to measure coastal erosion or retreat over time. However, most vertical photographs are in university libraries or must be obtained at considerable cost and time from aerial photographic companies or state or federal agencies. California has an online resource of oblique aerial photographsa as well as a selection of vertical photos. Ken and Gabrielle Adelman began flying and photographing the entire coast of California in 2002 and have rephotographed the coastline in 2004, 2005, 2006, 2008, and 2010. Three additional sets of oblique color slides taken in 1972, 1979, and 1987 by state agencies and some vertical aerial photographs have been scanned and added to the site. More than 90,000 high-resolution color photographs, covering every kilometer of the California coast, are available from the website. Using a time comparison option on the site allows users to immediately access photographs spanning 40 years of coastal change in California. Lidar systems use a laser to precisely measure ground surface elevations or topography. Airborne scanning lidar can be used to estimate eleva- tion every few square meters over tens to hundreds of kilometers of coast, allowing precise assessments of the spatial variability of beach and sea cliff changes (Sallenger et al., 2002). The first lidar topographic survey of the California coast was flown in October 1997 as a large El Niño event was approaching the west coast. A second survey of the same areas was flown in April 1998, after sea levels and storm waves returned to their normal state. The two surveys provided the first accurate comparison of the coastline before and after a severe event, and documented how much erosion or beach scour occurred (Figure).b FIGURE(A) Photograph of the Pacifica region where extensive sea-cliff erosion occurred during the El Niño winter showing threatened houses at the top of the cliff. (B) Three-dimensional view using lidar data acquired prior to the El Niño winter of the area shown in (A). Note that the buildings are clearly shown. Superimposed on the topography is vertical change with warm (red) colors indicating loss over the El Niño winter. SOURCE: Sallenger et al. (2002). a See . b See .
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 119 FIGURE 6.11 Example of projected sea-level rise hazard zones, defined as the historic erosion rate times the percent increase in total water level, in map view. SOURCE: PWA (2009). TABLE 6.1 Projected Land Loss for 11 Central and Northern California Counties Under 1.0 m and 1.4 m of Sea-Level Rise Cliff Land Lossa Dune Land Lossb Total Land Loss Year (km2) (km2) (km2) 2025 5 21 27 2050 21 2225 4346 2100 5361 2738 8199 SOURCE: Adapted from PWA (2009). NOTE: Low end of the range is for 1.0 m of sea-level rise, and the high end of the range is for 1.4 m of sea-level rise. a Includes 2 standard deviations of the historic shoreline change rates. b Includes erosion associated with a 100-year storm event. offshore and beach profile/slope, and beach width/ effective in causing basal cliff erosion (Lee et al., 1976; height. Combined, these factors may significantly in- Kuhn and Shepard, 1984; Griggs and Trenhaile, 1994; fluence wave run-up and thus exert a major control on Benumof and Griggs, 1999). Any significant increase the hydraulic forces applied to the cliff, bluff, dune, or in wave heights or the amount of wave energy reaching beach face (Benumof and Griggs, 1999). Conventional the cliff will, therefore, lead to an increase in the erosive wisdom is that waves are the primary agent for seacliff forces and the erosion rate. erosion at the base of the cliff (Sunamura, 1992; Shih A detailed investigation of cliff erosion in San and Komar, 1994). Large storm waves occurring during Diego County, California, found significant variation high tide or times of elevated sea level are particularly in the rate of erosion, as well as in intrinsic proper-
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126 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON FIGURE 6.18Mean annual inflows for U.S. west coast estuaries for 20002010. N.D. indicates that no data were available. SOURCE: Data from the U.S. Geological Survey, . San J oaquin rivers to the San Francisco Bay and noted sea level provides space for soil accumulation to proceed that, on average, 88 percent of the annual suspended- in areas where it is otherwise limited. sediment load was discharged during the wet season Much attention has been paid to the issue of land- and 43 percent was discharged during the wettest ward migration of tidal marshes as a result of sea-level 30-day period. rise. Such migration will occur only if the landward Dams and human actions have a large impact on margin of the marsh is unobstructed (e.g., Kraft et estuaries. One of the best described examples concerns al., 1992). The rate of migration is determined by the the effect of hydraulic mining and dams on sediment slope of the land and the rate of rise. New marshes may delivery from the Sacramento River to the north- develop at the landward margin, depending on the level ern San Francisco Bay (Gilbert, 1917; Wright and of development. The limited availability of suitable land Schoellhamer, 2004; Jaffe et al., 2007). The huge pulse along the California coast is described in Heberger et of sediment released during mining activities gradu- al. (2009). But whether marshes at a particular location ally moved down the Sacramento River and into San survive in the long term will be determined by their Pablo Bay. Recent losses of sediment from San Pablo ability to build elevation. Figure 6.19 illustrates how Bay likely indicate the progressive movement of the marshes can be created by sea-level rise then lost if sediment pulse toward the ocean. they cannot maintain their relative elevation. Although The response of coastal marshes to sea-level rise is the figure shows an idealized uniform slope, many influenced by changes in sediment dynamics, mediated west coast shorelines steepen abruptly landward of the by physical forcing, biotic factors, and plant growth. marsh, which would limit the extent of marshes as they Dating of buried salt marsh peats suggests that salt move inland in response to sea-level rise. marsh surfaces are frequently in equilibrium with local Projecting the sustainability of salt marshes under mean sea level (see Allen, 1990, and references therein), future climate scenarios is complex because it depends as would be expected in areas where salt marshes sur- on the relative importance of organic matter to marsh vive for long periods. It is well established that the vertical development, the factors governing organic surface elevation and, in many cases, the accretion rate matter accumulation during rising sea level, the impor- of marshes can change to keep pace with sea-level rise. tance of subsurface processes in determining surface However, it is unclear whether the elevation change is elevation change, and the role of storm events and stimulated by increased inundation or whether rising hydrologic changes in controlling sediment deposition,
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RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 127 soil conditions, and plant growth. A good example of this complexity and the challenge of isolating the effects of sea-level rise from other climate-related influences is described in Kirwan et al. (2009), who found evidence of an increase in the productivity of a dominant east coast salt marsh grass with increases in temperature. Other studies have found changes in the productivity of some marsh plants with increased atmospheric CO2 levels (e.g., Cherry et al., 2009). This report's assessment of the response of estuaries and marshes to future sea levels is therefore only one part of the climate change story. Fully assessing the fate of west coast marshes under climate change, including sea-level rise, is further hampered by the lack of long-term data on many west coast marshes and the differences in species composition compared to other more-studied systems. Response of Mudflats and Marshes to Future Sea- Level Rise and Storms The regional projections presented in Chapter 5 show substantial differences in the magnitude of sea- level change along the west coast. If space is available for landward migration, the rate of sea-level change over biologically important timescales will determine the fate of tidal marshes. Most models of marsh re- sponse to sea-level rise ignore interannual variability in sea level and assume a consistent monotonic pattern of rise (Kirwan and Temmerman, 2009). The potential consequences of the monotonic rise in the sea levels projected in Chapter 5, as well as the effect of potential interannual variations or other conditions that could FIGURE 6.19 The change of marsh surface elevation is impor- modulate those responses, are discussed below. As tant to successful landward migration under sea-level rise. De- pending on the difference between the rate of sea-level rise and discussed in Chapter 4, however, local vertical land mo- the rate of marsh accretion, a narrow or wide band of wetlands tions (subsidence or uplift) may be significantly larger will be present under any sea-level condition, but the area of than the regional land motions used in the projections, marsh will not expand unless elevation change can keep pace and thus relative sea-level change at any particular with sea-level rise. (Top) Sea level has risen from T1 to T2. New marshes are created at the landward margin of the marsh and place along the coast may differ from the committee's existing marshes lose relative elevation as the rate of sea-level regional projections. rise exceeds the elevation increase. (Middle) Sea level rises to T3. Newly inundated marshes in T2 are now losing elevation and new marshes are created at the landward margin. (Bottom) Central and Southern California Sea level continues to rise to T4. Existing marshes continue to lose elevation. Approximately 9095 cm of sea-level rise is expected between 2000 and 2100 south of Cape Mendocino, but the value could be as high as ~167 cm and as low as 42 cm (Figure 5.9). The projected value of this study falls between two sea-level rise scenarios
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128 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON considered by Stralberg et al. (2011), who examined the of high sediment supply, the opportunity for sediment fate of San Francisco Bay marshes under varying rates deposition increases. Under normal tidal inundation, of organic matter accumulation and sediment supply. times of flooding may not coincide with periods of high Their study showed that some types of marshes (e.g., sediment availability. Further, periodic marsh flooding those lower in the tidal frame, known as low marsh) during storms can allow sediment to be deposited with- are sustainable under even 1.65 m of sea-level rise as out subjecting marsh plants to prolonged inundation long as there are sufficiently high rates of suspended stress. Zedler (2010) found that storms were important sediment supply. This implies that if the high estimates for delivering sediment and increasing the elevation of are realized, marshes will be sustainable by 2100 only marshes in the Tijuana Estuary in southern California, under optimal conditions of sediment supply. Marshes although the lack of subsequent tidal flooding may lead respond more to rates of sea-level change over several to high soil salinities and changes in species composi- years than they do to the absolute change in elevation. tion in high marsh areas. In some bar-built estuaries, Observations suggest that marshes in San Francisco especially those subject to natural closure ( Jacobs et Bay can keep pace with a sea-level rise of 6 mm yr-1 al., 2010), sea-level rise and storms may alter the con- (see Parker et al., 2011 and references therein). The figuration of the estuary and either increase or decrease committee projections for 2030 and 2050 yield rates sediment retention (e.g., Schwarz and Orme, 2005). on this order. For the sea-level changes projected by the com- The supply of suspended sediment to estuarine mittee for central and southern California, a series of marshes in central and southern California is driven by storms combined with some increase in tidal inunda- fluvial inputs. The few long-term studies of sediment tion could allow such marshes to persist to 2100, even delivery to estuaries in this area tend to show a decrease under the highest sea levels projected. If storm events in suspended sediment in San Francisco Bay over time increase both sediment resuspension and marsh flood- (Wright and Schoellhamer, 2004; Schoellhamer, 2011). ing, then rather than causing problems for coastal Much of this decline occurred because a significant marshes, they may be essential to their survival. fraction of sediment that would enter the system natu- rally is now trapped in upstream reservoirs. Northern California, Oregon, and Washington For coastal marsh accretion to occur, some of the suspended sediment carried in from rivers must be de- North of Cape Mendocino, the committee projects posited (Reed, 1989). For example, at Morro Bay, high that sea level will rise 6165 cm by 2100, with lower rates of sediment delivery from the adjacent watershed rates to the north (Figure 5.9). The high end projec- doubled the area of salt marsh between 1980 and tions are ~143 cm by 2100. In the southern part of this 1990.1 Sediment deposition also can be influenced by stretch of shoreline, isolated areas of marsh exist at sea-level rise and storminess. Ruhl and Schoellhamer the mouth of several estuaries, such as Humboldt Bay (2004) noted that wind waves can resuspend erodible and Lake Earl in California and the Rogue River in bed sediment. As sea level rises, wind wave stress on Oregon. Most of these estuaries are relatively narrow bed sediment decreases, reducing the potential for sedi- without extensive intertidal flats for storing sediment, ment resuspension (Ganju and Schoellhamer, 2010). so their ability to survive sea-level rise depends greatly An increase of 1 m in water depth, especially in shallow on fluvial inputs of sediment. The Eel River, enter- subtidal areas, could have substantial effects on sedi- ing the coast south of Humboldt Bay, supplies the ment resuspension. Larger storm events, which produce largest amount of sediment to the California coast larger waves, would be required to mobilize sediments (Sommerfield and Nittrouer, 1999). The Klamath River and make them available for marsh accretion. may have a higher discharge than the Eel River dur- The depth and duration of flooding control the ing a storm event, but it carries a lower sediment load opportunity for sediment deposition on the marsh because there is less erodible material in its drainage surface. If storm events elevate water levels at times basin (Pullen and Allen, 2000). Along this part of the coast, the supply of sediment 1 See
RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 129 river flooding and by management practices. For ex- Role of Mudflats and Marshes in Providing ample, Willis and Griggs (2003) reported that dams on Protection from Future Inundation and Waves the Klamath River control 46 percent of the drainage and have reduced sand transport to the coast by 37 per- Few controlled field studies have examined the cent. Although sand may not be necessary for marsh role of coastal habitats in protecting inland areas from survival, reduction in the supply of sand will modify the inundation and wave damage during sea-level rise, bathymetry of the estuary with potential consequences coastal storms, or tsunamis. Some small-scale studies for tidal exchanges and the resuspension of sediment (e.g., Möller et al., 1999) have detected a relationship for transport to marshes (see discussion above regarding between specific vegetative characteristics and wave the potential importance of wind wave resuspension of attenuation, although bathymetric change appears to sediment availability within estuaries). play a more important role. Several field studies have The committee's projection of sea-level rise by 2100 noted the importance of vegetation morphology or is slightly lower than that used by Glick et al. (2007) architecture in attenuating both tsunami waves (Tanaka to study the effects of sea-level rise on coastal habitats et al., 2007) and wind-waves (Mazda et al., 1997). of the Pacific Northwest (69 cm). Using the SLAMM Field observations, measurements of wave forces, and 5.0 model (Clough and Park, 2007), Glick et al. (2007) modeling of fluid dynamics associated with the 2004 predicted that salt marsh would expand, partly at the south Asian tsunami suggest that tree vegetation may expense of more inland fresh marsh areas. However, shield coastlines from tsunami damage by reducing one of the drawbacks of the SLAMM 5.0 model is wave amplitude and energy (e.g., Danielsen et al., that it uses historic accretion rates to drive inundation 2005). However, it is difficult to separate the effect of and the vertical component of marsh response (Clough the vegetation from other aspects of coastal topography and Park, 2007). Rates of accretion may change with (Dahdouh-Guebas and Koedam, 2006; Feagin, 2008). sea-level rise, and accretion is only one of several The question of whether vegetation structure reduces dynamic factors that determine the response of marsh coastal damage directly through wave attenuation or elevation to sea-level change (see discussion above). For indirectly through alteration of the landscape has not example, if historical measured rates of marsh accretion been settled. are limited by the accommodation space provided by Modeling studies of hurricane storm surge and the highest level of tidal flooding (e.g., Krone, 1987; surge attenuation suggest that decreases in marsh eleva Allen, 1990), then an increase in sea level could increase tion, which increases the water depth, and increases marsh accretion. Glick et al. (2007) set accretion rates at in bottom friction generally reduce storm-surge levels 3.63.75 mm yr-1 for coastal marshes in their study. For (e.g., Loder et al., 2009). Reductions in marsh con- much of the Pacific Northwest, these rates are slightly tinuity increase coastal surges. Wamsley et al. (2009) higher than sea-level rise projected by the committee found that the extent to which wetlands attenuate surge for 2030 and similar to the rise projected for 2050. If depends on the storm and landscape characteristics. accretion rates subsequently increase in response to sea- The effect of vegetation on bottom friction or level rise, the Glick et al. (2007) predictions for 2100 roughness can be approximated from detailed measure- (e.g., salt marsh expands at the expense of other marsh ments of plant morphology and assumptions about types) will not be realized. stem density and flexure (see Feagin et al., 2011, for a For 2030 and 2050, local influences, including detailed review). However, isolating this effect from the changes in tidal hydrology and riverine sediment de- larger coastal configuration with which the storm waves livery, as well as development pressures, can be more or tidal flows are interacting requires numerical experi- of a threat to marsh sustainability than sea-level rise. ments. The depth of flooding and its interaction with If the highest estimates of sea-level rise are realized for plant stems and leaves is yet another nonlinear relation- this part of the coast, only marshes in areas with a high ship as field studies of wave attenuation in seagrass beds local sediment supply (e.g., at the mouth of major river have demonstrated (e.g., Koch et al., 2009). All of these estuaries) will persist in their current form. studies point to the difficulty of generalizing the role
130 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON of coastal habitats in ameliorating the effects of future (The Bay Institute, 1998). In the south bay, more than storms or tsunamis on the west coast. 90 percent of the historic tidal marsh area has been The morphodynamic interactions among topogra- converted to salt ponds, agricultural areas, and urban phy and bathymetry, vegetation, sediment deposition, developments (Foxgrover et al., 2004; Figure 6.20). and turbulent flows are difficult to predict, increasing Many of these areas are protected by an aging collec- uncertainties about the extent to which coastal habitats tion of levees. will mitigate the effects of future sea-level rise and The extensive loss of tidal marsh habitat has storms. A means for reliably determining wave damp- prompted calls for marsh restoration in the San ing by vegetation for engineering studies has not been Francisco Bay Delta (e.g., Goals Project, 1999; developed (Augustin et al., 2009). Models that reliably CALFED, 2000; Steere and Schaefer, 2001). Given predict coastal morphology (independent of the role the large investment required to restore thousands of of vegetation) over decades and under episodic storm acres of tidal marsh, it is important to understand the forcing are not widely available. For these reasons, sig- likely role of restored marshes in attenuating storms nificant tolerance for future coastal habitats, vegetation, and waves and whether they will persist under future and coastal morphology configurations will have to be sea-level rise. built into coastal protection systems. Potential for Marsh Restoration OPPORTUNITIES FOR MARSH RESTORATION AND THE EFFECT OF One of the first steps in marsh restoration is to re- MARSHES ON STORM WAVE ATTENUATION turn the land surface to elevations that can be colonized by marsh vegetation. Many land surfaces within the As shown above, the response of marshes to future delta are currently on the order of 35 m below water sea-level rise and storminess along the west coast of levels. Data from interferometric synthetic aperture the United States depends on local conditions. Marsh radar show that the delta-interior regions are subsid- restoration is also site specific. Consequently, the com- ing 35 mm yr-1 and that local regions in the delta mittee chose two areas where data on prior restoration are subsiding up to 2 cm yr-1 (Brooks et al., 2012). In are available--the California Bay Delta and the Puget areas where subsidence exceeds sediment accumulation, Sound--to explore the potential for marsh restoration it may be necessary to fill low-lying areas to enable given future sea-level rise and the effect of marshes on colonization. Sedimentation rates are low in much storm and wave attenuation. of the delta because fine sediments are slow to settle and waves keep them in suspension (Simenstad et al., Case Study on the California Bay-Delta 2000). In some shallow areas with nearly 100 years of sedimentation (e.g., Sherman Lake and Big Break), California's Bay Delta estuary is one of the largest sediment accumulation has not yet been sufficient to estuaries in the United States. The estuary consists of allow vegetation to become reestablished. a series of interconnected bays and channels connect- Where sediment accumulation exceeds subsidence, ing San Francisco Bay to the Sacramento-San Joaquin vegetation colonization may proceed naturally. For River Delta. Salinity increases from the delta to the example, high vertical accumulation rates of 3 cm yr-1 Golden Gate at the mouth of San Francisco Bay. At for 19551963 and 4.2 cm yr-1 for 19631983 were times of high river flood, fresh conditions can penetrate inferred from 137Cs measurements of marsh cores at into the bay. Alviso in the south bay (Patrick and DeLaune, 1990). The estuary has been modified extensively by Orr et al. (2003) found accretion rates for restored anthropogenic activities over the past 150 years (The marshes in San Pablo Bay of 1870 mm yr-1 for low Bay Institute, 1998; Goals Project, 1999; Brown, 2003). marsh and 910 mm yr-1 for high marsh. At these Approximately 80 percent of the tidal wetlands in San rates, marsh restoration could progress under all except Francisco Bay and 95 percent of the tidal wetlands the committee's high projections of 2100 sea-level in the Sacramento-San Joaquin Delta have been lost rise. However, high rates of past accretion may not
RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 131 FIGURE 6.20 Extent of tidal wetlands in San Francisco Bay in the mid 19th century (left) compared with the extent c. 1997 (right). SOURCE: Courtesy of The Bay Institute. continue in the future. Schoellhamer (2011) found a described above illustrate that with adequate migration 36 percent decrease in suspended solids concentration space and sediment supply, marshes in some areas may in San Francisco Bay from water years 19911998 to be able to survive future sea-level rise. However, if the 19992007. He attributed this decrease to the deple- highest projections for the Bay Delta are realized (1.6 m tion of a large erodible sediment pool ( Jaffe et al., by 2100), marsh restoration will be realistic only in 1998; Foxgrover et al., 2004) within the estuary. The areas with exceptionally high and sustained sediment availability of an erodible sediment pool prior to the supply. late 1990s may have enabled higher accretion rates in restored marshes in the past than would be possible Effect of Restored Marshes on Wave and Storm in the future. Transport of sediment from adjacent Attenuation intertidal and subtidal flats into relatively quiescent restored areas where it cannot be readily suspended The Golden Gate carries storm surges from the would promote accretion in restored marshes at the open coastal Pacific into San Francisco Bay and expense of the erodible sediment pool. The elimination the delta (Bromirski and Flick, 2008), and the border- of the sediment pool would lead to less sediment being ing low-lying lands are vulnerable to the increased available for development and maintenance of restored water levels (Knowles, 2010). Most measurements of marshes around San Francisco Bay. the effect of marsh vegetation on wave attenuation The committee's projected sea-level rise for the in the bay delta have focused on small waves, such as San Francisco Bay Delta is 93 cm by 2100. The studies boat wakes (e.g., Bauer et al., 2002). For example, Ellis
132 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON et al. (2002) measured the effect of brush bundles in ment, and 4,020 kilometers of shoreline. About 4 mil- attenuating waves from boat wakes and found up to a lion people live in the Puget Sound watershed, and the 60 percent reduction in wave energy impacting a delta population is expected to reach 5 million by 2020 and levee when the bundles were in place, depending on 8 million by 2040 (Puget Sound Regional Council, the tides. In a study of small waves in a shallow lake, 2004). Commercial fish and shellfish harvesting in Lövstedt and Larson (2010) found an average decrease Puget Sound is an important industry for the state. in wave height of 45 percent per meter within the first Tidal marshes and eelgrass beds are among the 514 m of beds of Phragmites australis. If these results most important coastal habitats in Puget Sound. Ex- are applicable to tules (Schaenoplectus spp.), which are tensive tidal marshes occur at the mouths of rivers that similar in height, extensive tule restoration could result empty into Puget Sound. Eelgrass is found from the in substantial attenuation of waves (produced by wind intertidal zone to the shallow subtidal zone in central or vessels) within the delta. and north Puget Sound. Loss of these habitats has The transition from tules to Salicornia virginica been dramatic. Nearly three-quarters of the original dominated marshes in the bay is accompanied by a salt marshes and essentially all river delta marshes major change in plant morphology. Salicornia virginica in urbanized areas of the sound have been destroyed resembles Atriplex portulacoides above ground, which (Gelfenbaum et al., 2006). Eelgrass habitat is almost has a lower stem density, height, and diameter than the completely gone in Westcott Bay and several other two Spartina spp. (Feagin et al., 2011). This suggests small embayments (Mumford et al., 2003; Wyllie- that Salicornia virginica marshes in the bay may play Echeverria et al., 2003). less of a role on attenuating storm set-up and waves The nearshore environments of Puget Sound than the reed-like architecture of tule marshes in the are maintained by a complex interplay of biological, delta. geological, and hydrological processes that interact Modeling of the propagation of long waves into across the terrestrial-marine interface. Many of these the south bay (Letter and Sturm, 2010) suggests that processes have been significantly affected by human small areas of marsh can ameliorate the effects of activities (Bortelson et al., 1980). For example, dikes storm events on water level. Letter and Sturm (2010) have altered nearshore sedimentation patterns and predicted changes in water level at specific locations eliminated the tidal influence that forms salt-marshes, during simulated storm events, based on the roughness and dams have reduced the magnitude and frequency and extent of vegetation cover and other parameters. of floods, limiting the sediment supply to river deltas. They found that water levels are lower at the edge of More than 33 percent of shoreline in the Puget Sound salt ponds fronted by some marsh than they are at the region has been modified (Puget Sound Action Team, edge of mudflats. For storm tides during the January 2002). 1983 El Niño event, which set records for high sea level The dramatic nature of these changes and the (see "Changes in Ocean Circulation" in Chapter 4), need to accommodate future population growth water elevations on levees not fronted by a small area of without further environmental degradation has led to marsh were higher than those with marsh between the concerted efforts to improve coastal management and levee and the intertidal flats. The extent of marshes in restore ecosystems (e.g., Puget Sound Partnership; the south bay is limited, so whether reductions in water Puget Sound Nearshore Ecosystem Restoration Pro- levels in small areas can be extrapolated to larger land- gram). Such efforts must factor in the effects of future scapes will require more detailed modeling of potential sea-level rise, which is complicated by the strong gra- future landscape configurations. dients in vertical land motion in the area (Figure 6.21). Whether vertical land movements enhance or counter- Case Study on the Puget Sound act the effects of regional sea-level rise has important implications for existing coastal habitats, the viability Puget Sound includes more than 8,000 square of future restoration, and the potential of these habitats kilometers of marine waters and nearshore environ- to help mitigate the effects of future storms.
RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 133 rapidly restoring tidal flow processes. This action could be complemented by modifying channels and making minor topographic changes such as filling ditches and removing road fill. In some areas, the tidal floodplain has been extensively filled and restoration may require resculpting of the land surface to ensure appropriate flooding and drainage of river and tidal waters. Areas where tidal action was recently restored through these measures include portions of the Nisqually Delta and the Skokomish River. In October 2009, after a century of isolation from tidal flow, a dike was removed to inundate 308 ha of the Nisqually National Wildlife Refuge (e.g., Figure 1.13). The Nisqually Indian Tribe restored an additional 57 ha of wetlands, making the Nisqually Delta the largest tidal marsh restoration project in the Pacific Northwest. Studies show more than 3 cm of sedimentation in the first year of resto- ration.2 A smaller scale restoration was carried out on the Skokomish River in September 2007, when tides were reintroduced to a 108-acre site for the first time in 75 years. For such tidal reintroduction projects to be successful, sedimentation (both mineral and organic ac- FIGURE 6.21Vertical land movements in the Puget Sound cumulation) must both raise elevations to a level where area based on interferometric synthetic aperture radar from 2002 to 2006. Surface movements in the radar line of sight marsh flora and fauna can flourish and maintain those range from -4 mm yr-1 (subsidence, blue) to + 4 mm yr-1 (uplift, elevations over time as sea-level rise increases relative red). Black lines are fault locations, and dashed lines are geo- water levels. Within Puget Sound, variations in vertical physical anomalies. SOURCE: Finnegan et al. (2008). land motion (Figure 6.21) either increase or decrease the amount of elevation change required. The supply of river sediment also is important Opportunities for Restoration for maintaining elevation of existing marsh. Dams or road crossings within a delta's watershed may indicate Efforts to restore tidal marshes have focused on the that river systems may not provide enough sediment deltas of the major rivers draining into the sound, where to sustain the elevation of restored habitats. Rates of many of the marshes have been diked for agriculture. sediment delivery from the Puget Sound watershed A recent assessment of restoration needs in the sound vary over time and place, depending on runoff patterns (Schlenger et al., 2011) noted that delta shorelines and land use changes. For example, the Skagit River have been so altered in the Duwamish, Puyallup, and carries more than 2 million tons of sediment per year, Deschutes areas that they are now classified as artificial and streams draining the Olympic Peninsula (exclud- shoreforms. Restoring the tidal hydrology and river- ing the Skokomish) carry generally less than 15,000 ine freshwater and sediment input are key elements tons per year (Figure 6.22). The spatial patterns of of a delta restoration strategy. Tidal hydrology and sediment delivery, combined with general trends in sediment input affect many delta processes, including vertical land motion, can be used to identify areas where distributary channel migration, tidal channel formation restored coastal marshes would most likely survive and maintenance, sediment retention, and exchange of future sea-level rise. In general, areas with high fluvial aquatic organisms. sediment supply and low subsidence or marginal uplift Clancey et al. (2009) identified berm or dike re- moval or modification as the most efficient method of 2 See
134 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON 124° 123° 122° 121° Str ait of Ge or r ve gia River Ri aser Fr k ac EXPLANATION British Columbia CANADA ks 0 49° oo ,40 Drainage-basin boundary N UNITED STATES 1 Washington Subbasin boundary 3.3 Annual sediment load, Bellingham Mt. Baker in thousands of tons Samish River ? Published load ? estimates could not be Skagit River found or do not exist Mt V Str 2,800 ait Salish Sea of Juan de F uca Glacier Peak 18 Elwha Dungeness Snohomish River 48° River ? ? River 490 Pu ge Ever Cascade Range t Olympic Mountains So un Big Quilcene River 6 d Dosewallips River 30 3.3 Lake Washington Duckabush River 11 Ship Canal Bremerton Seattle Hamma Hamma River 12 D uw Puget Lowland am 2211 Skokomish River ish 00 410 Ri ve Pacific Tacoma r Ocean Pu ta er s rie ya 80 bu th Olympia tri ll o llu p A 9 ? Ri 47° D ve es r ch 35 ut 12 Rive Mt. Rainier es 0 r 0 25 50 75 MILES 0 25 50 75 KILOMETERS FIGURE 6.22 Annual sediment load of major rivers draining into Puget Sound measured at or near the river mouth. The size of the arrow is scaled to the annual sediment load. SOURCE: Czuba et al. (2011). (e.g., north and western regions of the sound) are the Efforts to restore eelgrass in some areas of the most promising locations for sustainable coastal marsh sound have had only limited success (Thom, 1990; Car- restoration, at least under the committee's projected lisle, 2004; Mumford, 2007). Stamey (2004) found an sea-level rise for 2030 and 2050. Under the highest overall success rate of 1380 percent, concluding that sea-level projections for 2100, a high sediment supply eelgrass transplantation cannot yet be used reliably for and uplift may not be enough for restoration to succeed, mitigation in Puget Sound. Eelgrass restoration costs and additional steps will have to be taken (e.g., filling are high, between $100,000 and $1 million per acre previously subsided areas). (Fonseca et al., 1998). However, if appropriate substrate Linking restoration plans in these areas with land and water quality conditions can be established and use and watershed management plans would improve maintained, the effects of sea-level rise on eelgrass is the sustainability of coastal habitats. Land use plans likely minimal. could include, for example, conservation easements or limits on construction to accommodate the lateral Potential for Wave Attenuation migration of coastal marshes as sea level rises. Water- shed management plans could include changes in dam Eelgrass beds play an important role in nearshore operations to increase the amount of sediment that ecosystems. The plant blades slow water currents and reaches Puget Sound deltas. dampen waves, thereby trapping sediments, detritus,
RESPONSES OF THE NATURAL SHORELINE TO SEA-LEVEL RISE 135 and larvae. Lacy and Wyllie-Echeverria (2011) studied structures, they will eventually be inundated by future the influence of eelgrass (Zostera marina) on near- sea-level rise. bed currents, turbulence, and drag in the San Juan Rising sea levels and increasing wave heights will archipelago of Puget Sound. Zostera marina grows exacerbate coastal erosion and shoreline retreat in at water depths less than 5 m relative to mean lower all geomorphic environments along the west coast. low water along 43 percent of Puget Sound's shoreline Projections of future cliff and bluff retreat are limited (Berry et al., 2003). Lacy and Wyllie-Echeverria (2011) by sparse data in Oregon and Washington and by a measured velocity profiles up to 1.5 m above the sea high degree of geomorphic variability along the coast. floor over a spring-neap tidal cycle, including measure- Projections using only historic rates of cliff erosion ments above and within the canopy. They found that predict 1030 meters or more of retreat along the west eelgrass attenuated currents by a minimum of 40 per- coast by 2100. An increase in the rate of sea-level rise cent, and by more than 70 percent at the most densely combined with larger waves could significantly increase vegetated site, with attenuation decreasing with in- these rates. Future retreat of beaches will depend on the creasing current speed. Even sparse canopies influenced rate of sea-level rise and, to a lesser extent, the amount near-bed flow and significantly attenuated currents. of sediment input and loss. Most Puget Sound shorelines are sheltered, and Some of the coastal damage expected from sea-level waves are generated by local winds with little or no rise and storminess may be mitigated in some areas by energy component from ocean swell. The topographic coastal mudflats and marshes. Mudflats and marshes confines of Puget Sound limit the height of waves protect inland areas from inundation and wave dam- (Finlayson, 2006). Large waves (greater than 0.4 m age, but the specific effect depends on local conditions. significant wave height) occur only during infrequent Some studies have found that certain plants, such as wind storms. Consequently, the effect of eelgrass beds, eelgrass, slow water currents. Other studies have found and to some extent coastal marsh vegetation, on wave that marsh vegetation with high roughness, stem attenuation can be substantial. height, and density--along with coastal topography and bathymetry--reduces wave height and energy. However, CONCLUSIONS this relationship has not been specifically demonstrated for many of the species populating west coast marshes. Sea-level rise and storms along the west coast West coast tidal marshes can survive sea-level rise of the United States have caused significant coastal by building elevation to keep pace with rising water retreat. Cliff and bluff retreat, caused mainly by wave levels, which requires an adequate supply of sediment erosion and terrestrial processes (e.g., landslides, and/or organic matter accumulation. They may migrate slumps, rockfalls, runoff ), ranges from a few centi- inland if the area is unobstructed, but unless they main- meters to tens of centimeters or more annually, with tain elevation under sea-level rise, the area of marsh weaker rocks and areas of lower topography retreating will be limited by the slope of the land surface and the more than resistant bedrock cliffs and headlands. Cliff tidal range. Storms are an important agent for deliver- retreat is not reversible. Although coastal armoring can ing sediment and increasing the elevation of marshes. buy time, today's seawalls and revetments will eventu- For the sea-level changes projected by the committee ally be overwhelmed by sea-level rise and increasing for 2030 and 2050 in central and southern California, wave heights. frequent storms that increase tidal inundation and Sand dunes and beaches, which consist primar- promote sediment deposition could allow marshes to ily of unconsolidated sand, provide little resistance to survive. In northern California and southern Oregon, severe wave attack, especially at times of elevated sea fluvial inputs of sediment, which depend on storms and level. Consequently, beaches and barrier spits may grow water management practices, also are important for and shrink several meters or more per year. Because sediment deposition. Entrapment of sediment behind beaches are nearly flat, a small rise in sea level can cause dams makes marshes less able to survive sea-level rise a large retreat of a beach. Where beaches and barrier in this area. Coastal areas in Oregon and Washington spits are prevented from migrating by coastal armor or are projected to have lower rates of sea-level rise, in
136 SEA-LEVEL RISE FOR THE COASTS OF CALIFORNIA, OREGON, AND WASHINGTON part because the land is rising. In some areas, the ris- supply), hydrological (e.g., floods, storms, dams), and ing land surface will help coastal marshes maintain biological (e.g., accumulation of organic matter) factors their elevation as sea level rises, making sea-level rise that govern marsh survival, all of which combine to a less important threat in this area than other parts of cause significant spatial variability along the coast. In the coast. Should the highest sea-level projections for general, most marshes with natural sediment delivery 2100 be realized, marsh survival will be possible only and unimpaired hydrology will survive the sea levels in areas with high local sediment supply. projected by the committee for 2030 and 2050. For A detailed assessment of the response of west coast 2100, marshes will need room to migrate, a high sedi- marshes to sea-level rise is hampered by the lack of ment supply, and uplift or low subsidence to survive long-term and/or comparable data and by the variety projected sea-level rise. of geological (e.g., vertical land motion, sediment
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