Reporting on Hurricane Rita in September, my local TV meteorologist placed
the highest probability of landfall near the mouth of the Sabine River. He added
that Rita-generated flooding broke through levees, and New Orleans was again
flooding. The conjunction of a probability-framed prediction with news about
years of inconsistent and capricious human efforts to challenge absolute natural
forces led me to reflect on an old TV ad: “Don’t try to fool Mother
Nature.” The motto could be amended: Especially don’t offend Mother
Nature by maintaining a big city at the mouth of a big river where a big delta
is building.
New Orleans shouldn’t have been blindsided by Katrina — nowhere else
have delta processes been more studied and measured. Unfortunately, the knowledge
gained has been poorly applied.
Protecting and maintaining a city on a delta is confronting the dynamics of
sediment and water responding to gravity, a basic force in the universe. Gravity-driven
phenomena dominate the delta environment and are major guns in Mother Nature’s
arsenal.
Two processes that cannot be ignored are water’s penchant to always seek
the shortest or steepest gradient to base level, and the fact that particle
size and sediment volume depend upon water-flow velocity. Water with a steep
gradient flows swiftly and carries more and larger particles. Low-gradient flow
is slower and carries a finer load. In a delta setting at sea level, flow slows
and becomes zero. Sediment is deposited — coarse particles first, near
the coast, and suspended clay last, farther from a river or distributary mouth.
These processes, along with continuing regional subsidence (land sinking), are
basic elements of the natural forces that we have been “fooling with”
in New Orleans. They create and destroy land, and abandon and laterally shift
the sites of sediment deposition and concentration. They combine to make deltas
risky places for a major city.
Although early settlers had a good start by founding New Orleans on high ground,
subsequent growth was not well-planned. Many lessons abound in the area, however,
that illustrate the potential effects of a delta’s processes on poor planning.
Several real-world laboratories, such as at Cubits Gap and the Old River Lock,
show that Mother Nature usually prevails, and that the answers to the problems
of the Gulf Coast may be more complex than simply building higher levees.
The Cubits Gap crevasse and splay, located just northeast of the present Birdfoot
Delta of the Mississippi River, began prior to 1838 with a break in a natural
levee. The “gap” and resulting deposits grew, and by 1953 had created
more than 100 square kilometers of land surface. The crevasse or opening recurred
and is now mostly gone — with the sediment subsided, submerged and redistributed.
Since the 1800s, “Ol’ Man River” has been trying to placate Mother
Nature by diverting its flow into the Atchafalaya River at the Old River Lock
site. Currently the U.S. Army Corps of Engineers’ target Atchafalaya diversion
is about one-third of the Mississippi discharge. The capture site is more than
300 kilometers upstream from the present Birdfoot Delta, a long way to sea level
— not a good situation.
In 1880, the Atchafalaya River was diverting 2 percent of the Mississippi’s
flow. By 1960, more than 33 percent was flowing through the Atchafalaya basin.
The new course reaches sea level in about half the present distance. Effects
of the Atchafalaya diversion are obvious. Anyone flying over the central Gulf
Coast can notice that the clay plume from the Atchafalaya River rivals in size
the plume from the Birdfoot Delta. A series of floods will eventually take out
the Old River Lock control installations, and, unless vast sums of money are
spent, New Orleans will become a “sleepy” city on a bayou.
The 200 miles of recent sediment deposits from the Mississippi Delta, stretching
from western Louisiana to Florida, illustrate the dramatic lateral shifts that
occur. The delta is composed of four major and 16 minor deltaic lobes. If land
stability is a desirable characteristic, a delta is not a great place for a
city.
If New Orleans is to be rebuilt, it would be helpful if the media, academia
and government would honestly recognize and appraise the situation. From recent
media accounts, it would appear that there has been much heat and little light
shed on the New Orleans situation. There should be no political correctness
or environmental hysteria, whether for or against “city building on a delta.”
Important issues, such as the collision in New Orleans between human activity
and the effect of gravity on running water, should be in the arena of scientists
and engineers. Earth scientists should look at ways their science can be directly
applied to better management and planning principles.
The processes acting in the Gulf Coast are widespread. If there is any doubt
over well-defined and tested universal “truths,” just look to recent
work at the Texas Bureau of Economic Geology. Geologists there analyzed a delta
on Mars that displays morphology similar to the Mississippi Delta. It shows
evidence of abandonment and lateral shifting processes; in fact, the scientists
recognized six separate depositional lobes. Along with other evidence of fluvial
processes, the Mars delta indicates that laws of nature, if not universal, at
least operate on two planets. Maybe we really should cooperate with Mother Nature.
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