Responding to Changes in Sea Level Engineering Implications (1987) / Chapter Skim
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6 Alternative Responses
6 Alternative Responses
Pages 72-95
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From page 72... ...
Other countries have coped with relative sea level rise for thousands of years. Alternative responses to sea level rise, derived from worldwide experiences are described in this chapter.
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From page 73... ...
As the sea level rises, the retreat of the dune line may leave the groin susceptible to flanking during high or storm tides, thus permitting sand to bypass the groin, reducing its electiveness. The more readily the structure is flanked during normal weather conditions, the less the groin's sand-trapping and stabilizing capacity.
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From page 74... ...
provide a means, based on the concept of an equilibrium beach profile, to predict the amount of change in the beach profile due to changes in mean water level. Sea level rise can be incorporated into the design of a sea wall in two ways.
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From page 75... ...
, increasing the elevation in the future as dictated by the relative sea level rise actually experienced and/or projected over relatively short (about a decade) time frames.
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From page 76... ...
as the original beach sand will require far larger volumes of sand as the water level rises and the beach will become increasingly unstable. An alternative is to utilize coarser sand in future beach fills.
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From page 77... ...
* The present relative sea level rise rate is 30 cm/century, which appears to include a eustatic component of 12 cm and a neotectonic (subsidence)
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From page 78... ...
Method II This method is much more direct. The annual average shorn line retreat rate R due to natural causes (relative sea level rise)
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From page 79... ...
Beach Nourishment with Groins The use of groins with beach fill increases the time that the beach nourishment remains on the beach and reduces the downdrift erosion since the filled groins will begin to bypass sand immediately after construction. The response to sea level rise is the same as groins and beach fill, mentioned earlier.
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From page 80... ...
Sea level rise will affect a perched beach in the same manner as beach nourishment, with the exception that the sill structure will become less efficient as the sea level rises, resulting in reduced sand retention. The sill should be anchored by shore-perpendicular return walls situated well inland in order to prevent flanking.
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From page 81... ...
As relative sea level rises, the factors of safety of these structures will be reduced. Other Devices There are numerous other devices used for beach erosion control.
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From page 82... ...
in the Netherlands; the Harrison County, Mississippi beach nourishment project; Miami Beach, Florida beach nourishment; and the Tybee Island, Georgia sea walls, groins, and jetties. Galveston, Texas The city of Galveston is located on Galveston Island, a long barrier bounded on the east by the Bolivar Roads Inlet to Galveston Bay.
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From page 83... ...
(1983~. Using the Bruun rule and Leatherman's figures, most of the shoreline loss can be attributed to relative sea level rise.
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From page 84... ...
The Netherlands The foremost international example of a people coping with high relative sea level are the Dutch. Buffeted by catastrophic storm surges every several decades with the loss of thousands of lives and faced with the continuing subsidence of the land as the underlying peats and clays are compacted due to dewatering, the Dutch have spent centuries fighting the encroaching sea.
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From page 85... ...
This massive storm created the pressure for the Delta Project, the worId's largest coastal engineering work, which has resulted in the closing off of three major estuaries In the Rhine-Meuse delta region. This project will no longer permit intensive storm surge flooding.
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From page 86... ...
Additionally, the resort community of Scheveningen, nearly a part of The Hague now, has very wide beaches held in part by groins. In the north, the Frisian Islands beaches along the adjacent West German coast have been maintained in the face of relative sea level rise by migrating, as documented by Nurnmedal and Peniand (1981~.
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From page 87... ...
(1983) the est~rnated relative sea level rise over the period encompassing the beach restoration project (1952-1985)
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From page 88... ...
Erosion control measures have included shore parallel structures (revetments and sea walIs) , groins, and beach nourishment.
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From page 89... ...
/} ski 11 Lo_.` : ( // . : ~ ~ /} ii ' ','3~fe ~ ~ ~ / LEGEND / N 1 867 1875 1900 1918 1931 FIGURE 6-1 Tybee Island mean-high-water shoreline positions for various years.
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From page 90... ...
The sea wall construction program completed in the early 1940s "fixed" the shoreline position against severe storms. Beach nourishment during 1975-1976 contributed to the formation of recreational beach areas, still present after 10 years.
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From page 91... ...
The experiences in the Long Beach/Term~nal Island area resulted in measures to counter an extreme relative mean sea level rise. The changes occurred much more rapidly than those expected with rising relative sea levels elsewhere, and they includes]
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From page 92... ...
In Maine, new buildings must be set back far enough to permit 100 years of erosion. Both states assume that current erosion trends will not accelerate as a result of projected sea level rise.
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From page 93... ...
Recognizing that a retreating shoreline provides a sand source to downdrift shorelines, in situations in which shoreline stabilization is deemed justified, the state of Florida requires annual rrutigation through sand placement in the beach system to offset the material prevented from entering the system through natural erosion processes. Clearly, if some segments of the shoreline remain In place and others are allowed to move back in response to a rising sea
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From page 95... ...
Nevertheless, the time that it would take to replace completely our fossil fuel infrastructure suggests that it wiD be very difficult to limit the global warming expected in the next several decades.
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