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5 Effects of Sea Level Rise in the Coastal Zone
Pages 40-71

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From page 40...
... It Is possible to divide the coasts into physiographic regions for consideration of their response to relative sea level rise. For instance, the conditions in Louisiana do not apply to the coast of Mane because the Mississippi delta region is very flat, undergoing pronounced compaction and subsidence, while northern New England is characterized by nonerodible cliffs and portions are experiencing neotectonic uplift.
From page 41...
... SEA LEVEL RISE IN THE COASTAL ZONE 41 1971) , resulting in a narrow continental shelf and an essentially nonexistent coastal plain.
From page 42...
... This physiographic region has the only tide-dominated barrier islands along the U.S. Atlantic and Gulf coastal plains.
From page 43...
... However, coral reefs can continue to grow vertically, which is nature's response to sea level rise, as long as anthropogenic pollution does not kill these ecosystems. The Florida Gulf peninsula contains a diverse coast of sandy barrier beaches and swamps.
From page 44...
... The Chancleleur Islands and Isles Dernieres barrier chains are being fragmented by hurricanes, and it appears that these islands will be lost during the next 100 years even under the present conditions of relative sea level rise (PenIand et al., 1985~. The straight barrier coast of Texas has been wed studied (McGowen et al., 1977~.
From page 45...
... Rising relative sea level tends to cause shoreline recession, except where this trend is offset by an influx of sediment. The primary reason that a sea level rise would induce beach erosion ~ that natural beach profiles are concave upward; this geometry results in the wave energy being dissipated in a smaller water volume than without sea level rise, and thus the turbulence generated within the surf zone is greater.
From page 46...
... shoreline, the link between shore retreat and sea level rise is based on more than circumstantial evidence; it can be stated that the relationship is causal in nature. III some areas, it is clear that human actions have caused sum spatial erosional pressures.
From page 47...
... Onset of the present transgressive pulse, attested to by marked beach and dune erosion, has varied geographically depending upon local differences In sand supply ~d wave energy. Information from 73 correspondents in 39 coastal countries showed that less than 10 percent of the length of the worId's sandy shorelines have prograded, more than 60 percent have retrograded, and the balance have been relatively stable or have shown no consistent trend during the past century (Bird, 1976~.
From page 48...
... Army Corps of Engineers (1971~. Historical Records Historical records also indicate the prevalence of shore recession during at least the past century.
From page 49...
... Slope is the controlling variable: steep-sloped areas wig experience little horizontal shoreline displacement with each increment of water level rise, while gently sloping shores will undergo a much broader area of dooding for a given sea level rise. This Is the preferred methodology for immobile substrates, such as
From page 50...
... Several approaches to shoreline recession that have been employed to date are largely based on the erosional potential of sea level rise: (1) extrapolation of historical trends (Leatherman, 1984b)
From page 51...
... The method of projecting shoreline movement due to accelerated sea level rise is as follows (Leatherman, 1984b)
From page 52...
... The easily eroded, unconsolidated sediments and gently sloping, low-lying topography make the projections straightforward, except where modified by coastal engineering structures. The underlying assumption of this analysis is that shorelines will respond in similar ways in the future, since sea level rise is the
From page 53...
... in which R represents shoreline recession, S is sea level rise, W is the width of the "active portion of the profile participating in the adjustment, and h* is the vertical distance over which the adjustment
From page 54...
... found reasonable agreement between the predicted and actual erosion rates along the southeast coast of Florida. In general, it was found that a rapid rise In sea level of S causes a shoreline recession of about lOOS, which translates to about 1 m/yr for his study area.
From page 55...
... Rising water levels establish a potential for erosion, but realization of the potential requires sediment redistribution, that is, work that depends on energy being available. It may be useful to view the allied roles of sea level rise and wave energy by considering sea level changes as setting the stage
From page 56...
... With some qualifications, the Great Lakes research may prove to be a useful analog In considering the response of open-ocean shores to long-term sea level rise. For example, the m~-AtIantic Coast is subject to both extratropical (northeasters)
From page 57...
... If there were no long-term averaged landward transfer of sediment to maintain barrier width (Leatherman, 1979a) , the shore recession accompanying sea level rise would cause the island to erode away literacy and "drown in placed Since overwash and inlet sand are being lost to the beach and nearshore profile, this generalized approach would always predict greater retreat than does the Bruun rule.
From page 58...
... have shown that these linear sand ridges are dynamic, rather than relict features, and are believed to be initiated on the shoreface. With continued sea level rise and concomitant landward barrier retreat, these large sand bodies are essentially left behind, albeit reworked surficially.
From page 59...
... There are major differences in sediment budgets from site to site, and each area must be evaluated individually with respect to the existing sediment budget and the effects of present and future sea level rise. Dynamic Equilibrium Model The dynamic equilibrium model attempts to account for the transient response characteristics of a beach profile due to changes in the forcing function (i.e., changing water level and wave conditions)
From page 60...
... Application of the dynamic equilibrium mode! with the sediment budget overlay indicated that the existing rate of sea level rise accounts for about 20 percent of the historical shoreIme retreat rate for Ocean City, Maryland (1.9 ft/yr, Leatherman, 1985~.
From page 61...
... ˘3ruun rule includes only the impacts of sea level rise. Brulm rule adjusted includes 2.6 ft/yr due to factors other than sea level rise.
From page 62...
... As previously mentioned, cliffs of crystalline rock are essentially stable with response times to sea level rise much longer than those of sandy shorelines. Thus, for parts of the Pacific Coast and ahnost Al of the rocky Maine coast, cliff position is unchanged over historical periods of record.
From page 63...
... Shallow bays surrounded by extensive wetlands will expand rapidly in response to a rise both because of the gentle slope and the deterioration of the marshes in response to water level increases. Barataria Bay, Louisiana has increased its surface area about 1~15 percent over the last century in response to about 1 m of local relative sea level rise in that area.
From page 64...
... The continuing human destruction of wetlands should be kept in mind for the proper perspective when considering sea level rise and its potential effects on wetlands deterioration. Ecological conditions in coastal marshes range from marine to nearly terrestrial.
From page 65...
... , and the same qualitative result would be anticipated as a result of accelerated sea level rise. Estuarine (Brackish)
From page 66...
... , but in a dynamic equilibrium condition onsite production of organic materials and influx of mineral sediments cause vertical accretion, balancing the loch rate of sea level rise. In view of the geographic range of the measurement sites and the local variability within coastal marshes, it is rather remarkable that the measured sedimentation rates all fall within the same order of magnitude.
From page 69...
... The comparative resistance of marshy shorelines to wave attack suggests that with rapidly rising sea levels, most marshes will be Tong since submerged before extensive shoreline erosion occurs. A more probable catastrophic mechanism of marsh loss with a large increase In sea levels will be the formation of extensive interior ponds allied with general tidal creek bank erosion and headward growth as tidal prisms increase.
From page 71...
... With Holocene sea level rise, these salt marshes have been naturally translated landward through tune. With the construction of landward-flank~g bulkheads, which are prevalent along the mainland bay shores of many coastal states, these marshes will literally be squeezed out of existence with a sea level rise (Figure 5-4~.


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