EM 1110-2-1100 (Part II)
30 Apr 02
(f)
Monitoring of relative mean sea level behavior is at present inadequate for measuring
the possible global result of future climate warming due to rising greenhouse gases.
(g)
The risk of accelerated mean sea level rise is sufficiently established to warrant
consideration in the planning and design of coastal facilities. Although there is substantial local
variability and statistical uncertainty, average relative sea level over the past century appears to
have risen about 30 cm relative to the east coast of the United States and 11 cm along the west coast,
excluding Alaska, where glacial rebound has resulted in a lowering of relative sea level. Rates of
relative sea level rise along the Gulf Coast are highly variable, ranging from a high of more than 100
cm/century in parts of the Mississippi delta plain to a low of less than 20 cm/century along Florida's
west coast.
(h)
Accelerated sea level rise would clearly contribute toward a tendency for exacerbated
beach erosion. However, in some areas, poor sand management practices or navigational
modification at channel entrances has resulted in augmented erosion rates that are clearly much
greater than would naturally occur. Thus, for some years into the future, sea level rise may play a
secondary role in these areas.
(i)
As noted previously, the two response options to sea level rise are stabilization and
retreat. Retreat is most appropriate in areas with a low degree of development. Given that a
"proper" choice exists for each location, selecting an incorrect response alternative could be unduly
expensive.
(j)
There does not now appear to be reason for emergency action regarding engineering
structures to mitigate the effects of anticipated increases in future eustatic sea level rise. Sea level
change during the design service life should be considered along with other factors, but it does not
present such essentially new problems as to require new techniques of analysis. The effects of sea
level rise can be accommodated during maintenance periods or upon redesign and replacement of
most existing structures and facilities. There are very limited geographic areas where current
subsidence rates may require near-term action as has been the case in Japan and Terminal Island,
California.
(4) The above conclusions represent the state of knowledge on the subject of relative sea level change.
For additional information, the reader is referred to the Committee report. It presents a complete and
comprehensive investigation of the subject based on known facts and engineering and scientific principles.
(5) For the purposes of this report, the primary conclusion is that, with some regional exceptions, sea
level is not rising at a rate to cause undue concern. Results of the report indicate an average sea level rise
over the past century of approximately 30 cm/century on the east coast, and 11 cm/century on the west coast,
and a range along the Gulf of Mexico coast of less than 20 cm/century along the west coast of Florida to more
than 100 cm/century in parts of the Mississippi delta plain. The above summary remarks lead to the
conclusion that normal design criteria should be followed in which the design life of a project should consider
the possible local relative sea level rise rates shown above.
g. Changes in lake level datums. As with long-term change in the relative mean sea level of the
oceans, lake levels also change in time. Although lake levels are subject to the same tectonic types of relative
movement, the primary form of change is due to shorter-term phenomena. For example, lake levels are
subject to seasonal and annual variations in precipitation and freshwater inflow resulting from regulated
reservoirs. These hydrologically related effects produce water surface elevations in the Great Lakes that vary
irregularly from one year to the next. The annual cycle consists of water surfaces that consistently fall in
elevation to their lowest stage during winter. Falling stages are due to the fact that the majority of
Water Levels and Long Waves
II-5-37
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