EM 1110-2-1100 (Part II)
30 Apr 02
Engineering Implications of Changes in Relative Mean Sea Level (CCMSL) was formed to examine existing
knowledge concerning sea level change, to document existing relative rise rates, and to provide recommenda-
tions concerning their conclusions.
(2) Relative mean sea level change can be defined as the difference between local changes in land
elevation and global sea level changes. These changes result from a variety of processes, several of which
can occur simultaneously. The following six processes can contribute to long-term relative mean sea level
change; however, all processes do not necessarily apply to all geographic locations:
(a) Eustatic rise. Refers to a global change in the oceanic water level. Examples of eustatic rise include
melting of land-based glaciers and the expansion of near-surface ocean water due to global ocean warming.
(b) Crustal subsidence or uplift from tectonic uplifting or downwarping of the earth's crust. These
changes can result from uplifting or cooling of coastal belts, sediment loading and consolidation, or
subsidence due to volcanic eruption loading.
(c) Seismic subsidence. Caused by sudden and irregular incidence of earthquakes.
(d) Auto-subsidence. Due to compaction or consolidation of soft underlying sediments such as mud
or peat.
(e) Climatic fluctuations. May also create changes in sea level; for example, surface changes produced
by El Nio due to changes in the size and location of high pressure cells.
(3) The above processes have been evaluated with respect to their historical and potential contribution
to sea level change on U.S. coasts. The Committee report assesses changes in sea level as well as the affected
hydrodynamic processes and the effect on the coastal zone. The report also investigates feasible response
strategies that could be used to mitigate the effects of sea level change. Although it is beyond the scope of
this chapter to reproduce the contents of the report, conclusions relevant to this chapter are reproduced below.
(a)
Relative mean sea level, on statistical average, is rising at the majority of tide gauge
stations situated on continental coasts around the world. Relative mean sea level is generally falling
near geological plate boundaries and in formerly glaciated areas such as Alaska, Canada,
Scandinavia, and Scotland. Relative mean sea level is not rising in limited areas of the continental
United States, including portions of the Pacific Coast.
(b)
The contrasting signals concerning relative mean sea level behavior in different parts
of the United States (and the world in general) are interpreted as due to differing rates of vertical
motion of the land surfaces. Subsidence and glacial rebound are significant contributors to vertical
land displacements.
(c)
Large, short-term (2- to 7-year) fluctuations worldwide are related to meteorological
phenomena, notably shifts in the mean jet-stream path and the El Nio-Southern Oscillation
mechanisms, which lead to atmospheric pressure anomalies and temperature changes that may cause
rise or fall of mean sea level by 15-30 cm over a few years.
(d)
Studies of a very small number of tide gauge records dating more than 100 years (the
oldest being Amsterdam, started in 1682) show that after removal of the subsidence factor where
known, mean sea level has been fluctuating through a range of not more than 40-150 cm (in long-
term fluctuations) for at least 300 years.
(e)
The geological record over the last 6,000 years or so indicates that there has been a
general, long-term rise (with short-term fluctuations) probably not exceeding 200 cm during the last
1,500 years.
II-5-36
Water Levels and Long Waves
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