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OCR for page 31
3
Relative Sea Level Rise
Effects and Responses
As sea level rises, the shoreline may respond by flooding or
erosion. The magnitude of these effects wiD depend on a number
of factors, including whether the shoreline is on the open ocean
or in a protected bay, the effect of any stabilizing structures, and
population density and activities.
The related shoreline changes will have economic and envi-
ronment~ impacts that will require decisions to define a proper
response. For example, for sandy shorelines, the possible reme-
dies to erosion range from shoreline stabilization (through beach
restoration or protective structures) to retreat from the shoreline.
Each is technically feasible, but the appropriate response will usu-
ally be dictated by ~ combination of economic and environmental
concerns.
~ the case of retreat, planning and timing are important ele-
ments of the process. Given that a definite proper choice exists on
a site-by-site basis, it follows that an inappropriate choice could
be very expensive. For example, continued shoreline stabilization
of an area of high erosional tendency and low economic base could
be too costly. On the other hand, if the natural system is only
slightly out of balance and a large tax base exists, a decision to
retreat could be inappropriate and unduly costly. This latter con-
dition would certainly hold for port cities, which have considerable
investments in infrastructure.
3
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32
RESPONDING TO CHANGES IN SEA LEVEL
Thus, there is a need for development and application of
methodologies for estimating the expected erosion impact and
attendant storm damage. With such a framework available and
the conveyance to the public of plans for responding to a major
problem (rebuilding or retreating), a much more rational response
could be developed for implementation. This would clearly be of
value to those responsible for allocating future financial resources
and directing responses in times of high emotions, losses of per-
sonal property, and possible losses of life.
The decision to retreat or rebuild following a major storm
would be a matter of the gravest concern. Many situations can be
envisioned in which the temptation would be to make the wrong
choice. For example, suppose that a storm of 50~year-return
frequency were to strike an area, causing widespread damage and
major change. Without the knowledge that this was indeed a very
rare storm, the enormity of the damage and emotional impact
could cause decision makers to be swayed toward retreat. On
the other hand, if a relatively frequent storm (say, with a return
period of 40 years) causes major damage due to the relatively high
vulnerability of an area, retreat may be the appropriate response.
The development of a rational decision-making framework re-
quires an understandin g of both complex physical coastal processes
and the economics of the area. Chapters 4 - of this report review
some of the necessary knowledge of coastal processes and the in-
fluence of sea level rise.
Historically, U.S. efforts to cope with relative sea level rise
have been limited in scope and modest compared to those in some
European countries. Until 1929, when the U.S. Army Chief of
Engineers appointed the Board on Sand Movement and Beach
Erosion to carry out field studies of shore processes, there was
no organized effort to study the engineering problems of ocean
shores. The board was appointed to study the processes at work,
not to solve particular erosion problems. The U.S. Army Corps
of Engineers had responsibility for improving and maintaining
navigation works of all kinds, including the tidal entrances to
coastal harbors. In 1930, the U.S. Congress authorized a successor
and permanent organization, the Beach Erosion Board, now the
Coastal Engineering Research Center.
From 1929 to the beginning of World War Il. the Beach Ero-
sion Board and a few universities worked on coastal processes and
engineering problems. Support was limited and progress was slow.
Beginning about 1940, and continuing after the war, amphibious
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RELATIVE SEA LEVEL RISE EFFECTS AND RESPONSES
33
operations, offshore petroleum production, coastal siting of electric
power stations, the growth of coastal communities, environmental
and ecological interest, and other developments caused a tremen-
dous growth in the support for coastal studies and projects and
university prograrrm. The number of professional coastal experts
has increased proportionately with the growth of these programs.
The engineering of works on the ocean coasts deals prunarily
with the characteristics and effects of wind-generated, ocean sur-
face waves. Expertise in this area is the essential qualification of
the coastal engineer. Because of this, categorization of engineering
problems should be made on the basis of "waves" or "no waves,
and this differentiation should extend to the effects of a sea level
rise; those involving wave action differ fundamentally from those
that do not.
Before discussing the effect of rising sea level on specific types
of coastal projects, there are some principles that qualified coastal
engineers generally accept as valid:
1. Structures are expensive and the ocean is a relentless ad-
versary. A development ~set-back" line may be the only action
justified for undeveloped or lightly developed shores.
2. Sandy coastal shores are made of natural units, such as the
length between inlets or a beach terminated by headlands, and
must be treated as such. The effect of any structure anywhere
in one of these units on the remainder of the shoreline must be
analyzed before construction, and the plan should provide for
mitigation of adverse effects, if any.
3. Sand tends to collect in sheltered areas around coastal
structures such as groins or jetties. Plans should provide for pre-
dict~ng the location and capacity of such areas and for filling them
initially with sand from other than the active shoreline, so as not
to deplete other portions of the sand system.
4. The direction and magnitude of littoral transport is the
most uncertain feature in the plans for a coastal project. Site-
specific data are difficult and expensive to obtain, but it is ex-
tremely risky to proceed without them.
5. The choice of coastal structure for erosion mitigation will
depend on site-specific factors. Structures that work satisfactorily
in one location may prove to be totally inadequate (and perhaps
detrimental) in another application.
Representative terms from entire chapter:
level rise
