George M. Wells, George R. Goethals, son of the General, Colonel E. J.
Dent, U.S. district engineer at New Orleans, and other engineers who
have studied the problem, say that the dredging of a channel from the
Industrial Canal to the gulf through Lake Pontchartrain, or the
marshes, is feasible, comparatively cheap, and maintenance would be
simple. This would shorten the distance from New Orleans to the sea by
about 50 miles, and would be a vast saving for ships. It is one of the
objects towards which the Hudson Dock Board is working.
It is Uncle Sam's recognized duty to develop and maintain harbors and
channels to the sea. Distance is obviously an important factor;
furthermore, the proposed new outlet would be an important link in the
Intracoastal Canal, connecting with the Warrior River section of
Alabama, which the government is developing between the Atlantic and
Gulf Coasts. A bill was introduced in the Senate in 1920 by Senator
Ransdell of Louisiana, providing for the development of the proposed
channel; it was not pressed because the canal was far from completed.
However, every effort will be made by the Dock Board from now on to
have Uncle Sam take hold.
Colonel Dent has for a number of months been studying the feasible
routes. He, by the way, is thoroughly convinced of the value of the
Industrial Canal to the development of New Orleans, and the commerce of
the nation, and has so expressed himself in public.
The Pontchartrain route has been laid off, by engineers, beginning at
the Canal, paralleling the south shore of the Lake Pontchartrain to the
south draw of the Southern Railway bridge, thence to the Rigolets to
Cat Island Pass, from there to Cat Island Channel and so to the deep
water of the Gulf, a total distance of 75 miles.
Soundings and surface probings have been taken at frequent intervals
over the entire route. These have shown the engineers the following:
Three-quarters of a mile from the south shore of the lake, and as far
as the railroad drawbridge, a hard bottom is found. The material is
principally packed sand, rather fine, with a small amount of clay, and
occasionally some broken shells. Beyond this distance from the shore,
the bottom is softer, consisting of mud mixed with sand. From the
bridge over the remainder of the route, the bottom, with the exception
of a few sand pockets, is soft--a blue mud with a large percentage of
sand. This soft material has so much tenacity, however, that current
and wave wash, which tend to fill up artificially dredged channels,
would affect only the surface.
The government is conducting large dredging operations in Mobile Bay,
Gulfport Channel, Atchafalaya Bay and the Houston Ship Channel. An
outline of the results there will show how feasible the dredging of the
Pontchartrain Channel would be, and how much cheaper in comparison.
The channel connecting Mobile Bay with the Gulf of Mexico has a bottom
very soft for the most part, and with a small percentage of sand.
Towards the outer end, the material is black mud, about equal in
consistency to the softest material found in the Pontchartrain route. A
sounding pole with a 4-inch disc on the end can be easily pushed three
or four feet into the mud and pulled out again. Wave and current action
cause the channel to shoal at the rate of 78,000 to 132,000 cubic yards
per mile per year, depending on the softness of the bottom and the
depth. Where the highest rate obtains, the surrounding material
consists of soft mud, without a trace of sand. Experience shows that
where there is a fair percentage of sand in the material adjacent to
the channel bed, the shoaling is lessened. In general, the material
along the Pontchartrain route contains a greater percentage of sand and
is far more tenacious than that along the Mobile Bay Channel.
Furthermore, the Pontchartrain route is not exposed to such strong
cross currents.
The Gulfport Channel is dredged through very soft material, a
grayish-blue mud of oozy consistency, into which the sounding pole
penetrates six feet with very little exertion. On top, a small amount
of sand is found, but practically none in the lower stratum. The
material is considerably softer than any encountered on the
Pontchartrain route, except for one small stretch. Yet the shoaling is
not great. Where the shoaling is heaviest, between the end of the pier
and Beacon 10, only about 700,000 cubic yards a mile has to be dredged
out every year to maintain the channel. From Beacon 10 out, the average
annual maintenance is less than 200,000 cubic yards a mile. Except for
the four-mile stretch west of the inner entrance to the Cat Island
Channel, the bottom, on the Pontchartrain route, is harder than that of
the Gulfport Channel. Therefore, it is reasonable to conclude that the
maintenance of the Pontchartrain Channel would not average as high as
the outer portion of the Gulfport Channel.
The Atchafalaya Bay Ship Channel, extending from the mouth of the
Atchafalaya River across the shoal waters of Atchafalaya Bay, to about
the 20-foot contour of the Gulf, a distance of fifteen miles, is
through a material of slushy mud, with occasional thin pockets of sand.
The shoaling runs from 540,000 to 1,680,000 cubic yards a mile a year.
The highest rate is obtained in shallow water. Except in the stretch
mentioned, the material on the Pontchartrain route is not as soft as on
the Atchafalaya, nor are the depths as shoal, nor is there the exposure
to cross currents.
In the Houston Ship Channel, the material is composed of soft mud with
a small amount of sand. A two-mile stretch through Red Fish Reef is
practically self-maintaining. For the remainder of the channel, during
the six years from 1915 to 1920, a total excavation of 13,574,000 cubic
yards was necessary to maintain the depth. This is equivalent to
100,000 cubic yards a mile a year.
In summary, then:
1. The Lake Pontchartrain route is practically unexposed to cross
currents, as is the case with the Mobile Bay, Gulfport, Atchafalaya,
and, to a certain extent, the outer portion of the Houston Ship
Channels.
2. The material along and on the sides of the Pontchartrain route is,
with the exception of a small stretch, more tenacious, and contains, in
general, a greater proportion of sand than in the case of the
neighboring channels mentioned.
The channel could therefore be more easily maintained.
Engineers estimate that a channel with a 300-foot bottom would be
needed. On the south shore of the lake, the side slopes should be on
the 1 to 3 ratio, with provision for a 1 to 5 ratio at the end of five
years. Dumped on shore, the material would reclaim considerable
frontage, and eliminate the re-deposit of this material in the channel.
Through the remainder of the route, the original excavation should be
made with side slopes on the 1 to 5 ratio, with provision made for a 1
to 10 ratio in five years.
The dredging of the 75 miles of the Pontchartrain Channel would amount
to 97,200,000 cubic yards, it is estimated by engineers. The cost would
be around $10,000,000. The annual maintenance, during the first five
years, would amount to 8,880,000 cubic yards--an estimate based on a
comparison with the other channels into the Gulf, and the character of
the material to be excavated. This estimate is considered large--but
even at that, it is only 118,400 cubic yards a mile a year, and the
cost would be about $750,000, according to Colonel Dent. After five
years, it would be less.
Another proposed route, investigated by Colonel Dent, is through Lake
Borgne. A canal some miles in length, through the marsh, would connect
the lake with the Industrial Canal. This route has considerable
maintenance advantages over the Pontchartrain route. The character of
the bottom in Borgne is more or less the same as in Pontchartrain.
Sooner or later, one of these channels will be built by the government.
That it has not already been begun is due to the fact that the Canal
has not yet been completed, and the expected development has not taken
place. But there is no doubt that it will.
[Illustration: TYPICAL BRIDGE ON CANAL]
[Illustration: EMERGENCY DAM CRANE]