Geoscience Reference
In-Depth Information
highest points are on the order of 6 meters
(20 feet).
So what can the city do to prepare for the
rising sea? Miami Beach, the barrier island
fronting the city, will need to have seawalls
on all sides. The beach will be long gone,
no longer maintainable regardless of how
much sand is pumped on it. The city proper
will also need walls or dikes on all sides, in-
cluding on the border with the Everglades
on the western margin of the city.
But the geology underlying the city, as
well as much of coastal southeast Florida,
holds a surprise for the unwary engineer.
Miami sits atop the Miami Limestone, a
layer of sediment laid down during high sea
levels over the last million years. This rock
layer is up to fifteen meters (fifty feet) thick
and is highly porous and permeable, mean-
ing that water flows readily through cavi-
ties in the rock. According to the geologist
Hal Wanless, evidence of the ease of flow
of water through the limestone is found in
the tidal fluctuations of the ocean that can
be observed (on a very small scale) in ponds
within the city.
The engineering ramification of this ease
of flow is that mere walls or levees will not
hold back the sea level rise. The level of the
sea will simply be the same on both sides of
the wall. Thus instead of walls, which are
normally designed to protect the city from
storms, dams will be needed, extending
underground to depths that will depend
on the thickness of the permeable rocks or
sand underlying the city.
This is a problem that has only recently
been recognized, and it is one in need of de-
tailed engineering evaluation. It is a prob-
lem which probably exists along all 5,600
kilometers (3,500 miles) of the barrier is-
land coast of the United States in the Gulf
of Mexico, as well as along much of the At-
lantic Coast. The major ramification of this
is that holding the shoreline in place will
be hugely more expensive than most cur-
rent estimates of seawall costs. A massive
wall, extending four and a half to six me-
ters (fifteen to twenty feet) above the high
tide line, will cost on the order of $33,000
to $82,000 per meter ($10,000 to $25,000
per foot), and the cost of a dam would be
much more.
Miami has no high ground to which to
move. The choices are to hold the line at
great cost or to abandon the city at greater
cost.
shishmaref and
the warming north
One of the most immediate human im-
pacts of the increased temperatures in the
far north is the potential destruction of na-
tive seaside villages along all Arctic shore-
lines, including those in Siberia, Scandina-
via, and northern Canada. In Alaska there
are twelve such villages along or near the
shoreline, in dire need of moving to higher,
safer ground. Shishmaref, Alaska, is a typi-
cal village with typical problems related to
the warming atmosphere. A small Inupiat
Eskimo subsistence village just south of
the Arctic Circle along the shores of the
