New Orleans Republican. [volume] (New Orleans, La) 1867-1878, 1875-01-31

i thirty-six years (ab0ut LOO feet per annnn),
that 100 years ago the outer crest of the
South Pass bar was where now there 10 q
deep channel. As the bars move eut to sea,
the river ia, then, all the time eroding a
channel of the characteristic deep water
cross section behind and through them.
The object of jetties is to aid and hasten
this erosion. IT, starting from point in a
pass above it's bar, where there are now
thirty feet of water, We build ietties which
80 confne the pass that it shall have the
width all the way to deep water it now has
at the starting point, we shall be helping
the pass to assume the deep water cross
section it would ultimately take, and by
aiding it, if necessary, by dredging, should
be able to reduces at pleasure the time
required for the process.
This plan is then adopted for the improve
ment of a natural outlet, namely, to keep
parallel dykes at the banks of pass where
there are now thirty feet of water in the
middle, and carry them over the bar to
thirty feet water outside (unless the depth
is Obtained before the dykes have reached
the thirty feet curve) allowing the river to
erode the bottom between the dykes till the
water way between them everywhere has
the same cross section as at their beginning,
aiding the erosion by dredging or stirring
if it is not rapid enough without.
The board considered the question of
limiting the waterway to the cross section
Of thirty feet maximum depth. by converg-
| ing jetties on the bar, and by spu: dykes in
| the pass above, instead of by parallel dykes.
In view of the lack of experience in such
work IN this country, and of the danger of
excessive scour around the ends of spur
| dykes, it was deemed advisable to adopt
parallel dykes as offering fewer contingent
cies and less difficulty of construction. The
| depth of thirty feet has been chosen, in
order that some time may elapse before the
bar, which will form at the sea end of the
jetties, can have less than twenty-five feet
at mean low water upon it, that being the
minimum depth which it is desired to
| maintain.
Having adopted general plan for tie
improvement of one of the natural outlets,
| it remains to fix On that one. As the i'm.
provement Of any will be costly, but one
| should be improved, and that should be
made adequate for all purposes.
The passes which have been most care
fully considered are the South and South
west
In comparing these passes it is seen that
while the average width of tho body OF the
South Pass is 700 feet, that of South west
Pass is about 1400. The greater width is
more favorable to navigation; but in the
opinion Of the board, the South Pass, when
improved, will be adequate to the present
and prospective wants of commerce. The
and maintenance of the works for the inn
provement of the South Pass is 81 942, ITO,
and for Southwest Pass is $l6,O53. 1914. It
is assumed that the Southwest Pass bar
advances about three times as fast as the
South Pass bar.
The South Pass being entirely adequate,
the much greater cost of improving the
larger pass would not, in the opinion of the
board, be warranted either by the some.
what greater ease of entering it in storms,
or Of navigating it when once inside. The
former is twelve and nine-tenths miles long.
the latter being eighteen miles, and is
heightened only about one. third as fast as
the latter.
The cost of improving Pass-a.lOutre
Pass, without advantages sufficient to JUS
tify the increased cost. The South Pass
has the advantage that the works for its
improvement, which would require at lenst
two or three years for their execution,
would in no way interfere with commerce.
The board is, therefore, of opinion that if
any natural outlet is improved It should be-
the South Pass.
The South Pass of the Mississippi is twelve
and nine-tenths miles long, has an average
width of $50 feet, and minimum interior |
channel depth of twenty.nine feet. 1t is |
ii 800 feet from the thirty foot curve inside |
the pass across the bar to thirty feet out |
side. The minimum depth on the bar is |
seven feet. 1t discharges at its mouth about |
81 900 cubic feet of water per second, and |
about 22.OOO, OOO cubic yards of sediment in /
suspension per annum. 1t has shoal at it's |
head, with minimum depth on it in chan
mel seventeen feet.
For the improvement of the South Pass
the board recommends parallel dykes or |
Jetties, constructed of brush, fascihes and
stone, IN tho same general way as those
used by Mr.. Caland at the mouth of the
Mas.
These dykes should begin at the two
banks OF the pass, about 1850 feet below the
South pass lighthouse, where the river has
a width OF 900 feet and a maximum depth |
of thirty feet. They ehould run in straight
lines, parallel to each other, in the direc-
ton OF the pass, to where the water is thirty
feet deep outside of the bar; provided it
should be necessary to carry them 80 far to
secure thirty feet depth. The dykes for
the first 7lOO feet should be ten feet wide
on top; should then widen gradually to
twenty feet in a further distance of 2O5O
feet, should then gradually widen to fifty
fee which is to be the width in thirty feet
Of ter. At present this last length would
be about 275O feet. and the total length of
each dyke II 900 feet. The first 200 feet of
The dykes to have side slopes of two-thirds,
two vertical to three horizontally, the rest |
% have side slopes of one.quarter down to |
fifteen feet below water, and beneath that
lepth slopes of one-half. The top of the
iykes to be rounded and paved, the crown | I
Using to high water Of spring tides. Ii
The question of the average annual ex-
'ense of prolOnging the ietties is a very
erious one. 1t depends on the annual ad.
'ance of the twenty -fve-fo0t curve. that
epth being required. At present, the
muddy water issuing from the South Pass
preats out in somewhat of a fan.shape.
nf no 70 hal Il
he pass, and the ribs several miles in
5ngth.
If the proposed jetties were instantly
ompleted, and the new channel secured t
ut, essentially the same amount of sedi- Il
tent would be spread out in fan shape, but, b
'om the greater velocity of the issuing n
ater, the ribs Of the fan would be longer, il
lille the handle would be narrower. More a
the sediment would at first be deposited | d
# out in the gulf than before. b
But with the present rate of advance, the | Il
venty-five foot curve 120 years ago was cl
bout 1900 feet above its present position; d
ID II the volume Of water carried by the Il
1888 is kept the same, neglecting the slight ie
ference in slope of the gulf bottom out 81
10 the present bar, in about 100 years 81
Nv end for the pass will probably be b,
rmed of the same general shape as the
wer $4,000 feet of the present pass. 1t
axes little difference in the whole time re.
Tired to accomplish the work, whether
same volume of water flows out at no
arting over the present shallow bar or de-
'm between two dykes which force the M
tier to take depth of thirty feet. In an liz
erage OF many years, the rate of progress az
lst be about the same aa now, namely. in
} feet per annum, the volume of water VI;
ing kept as at present; and it Is on this s.
Sis that the average annual cost of ex- ~
lsion, namely, $4,000, has been com fo,
ted. l ~,