New Orleans Republican. [volume] (New Orleans, La) 1867-1878, 1875-05-25
Captain Fad. at the Chamber Of Com.

Captain Eads at the Chamber Of Com
merce.
| The Chamber of Commerce was tillei
| last evening with large assemblage O.
| gentlemen. who came on the announcement,
| that Captain Eads would deliver an address
on the jetty question.
The President, General Bussey called the
meeting to order and explained it's object
tie said there was a mistake in the call
That Captain Eads had not expected tc
! address public meeting, but to explain te
few gentlemen bis maps and plans, and
consequently was unprepared to address &C
many. He then introduced Captain Eads.
Captain Eads regretted that he was com
PEELED to apologize for want of preparation
to meet such an assemblsge. He had only
expected to meet fifteen or twenty gentle
| men, who could gather round a table and
examine the plans. He would always be
pleased to meet any number of gentlemen
and explain his theories and plans. The
substance of what he said was as follows:
To properly comprehend the jetty system
Az 40 in pOItduV W lully uuutrSC6NO tuG
cause OF the continual advance of the river
into the sea. Its mouths are now thrust
out about sixty miles into the Gulf and
their progress goes steadily forward in con
sequence of some of the simplest laws of
which We have knowledge.
The opponents OF the .jetty system main
tain that the chief portion of the sedi-
mentary matters transported by the river
are pushed along the bottom. If those who
believe this error will only discard it for q
few moments, and permit their minds to
picture the sand and earthly matters of the
river carried in suspension, and that the
magnitude OF the burden thus carried is
propOrtioned to the velocity of the stream.
l will promise to make plain to them every
phenomenon presenter at the mouth of the
river and throughout the 1000 leagues of its
alluvial bed.
In proof that it carries eand in suspen-
sion you have only to dip up a cupful of
water from its surface, and you will find
1t not only opposite your city but tar be-
yOnd the crest of its bars out in the Gulf.
So long as it moves onward it holds these
matters in suspension, and just as its cur
rent is slackened will it drop the heavier
particles and sustain the lighter ones on to
the last.
Accidental causes may show that the
water is sometimes charged with less or
greater amount of sediment than is due to
given depth and velocity, but every appar-
ent exception to the law that all alluvial
streams carry their sediment in exact pro-
portion to their velocity modified by the
depth of the stream, can be easily recon
ciled with the law. 1t is this law which gives
the stream the power to regulate the size
OF its channel to suit it's own wants. Where
it is too small for the flood the current is
accelerated. it scOurs up bigger burden
there, and coon ecOurs out the requisite
room for itself. If the channel at another
place is too large the current is there slack
ened and the excess of load is dropped
until the channel is diminished and the cur
rent restored. When the channel is big
enough the scour ceases. When it is sutii.
ciently reduced in size the depOsiting action
ceases. The bend caves in because the
river channel above is too big. There the
slack current has been depositing its excess
load and forming sand bars. In the bend
the channel is too contracted and the cur
rent is then made too rapid for its reduced
load, and it scours up the deficiency out Of
the bottom and side of the bend.
Let us apply this law to the advance Of
the Southwest bar. On tie bar we find the
depth fifteen feet, and throughout the pass
not less than sixty feet. The bottom is
therefore iorty-five feet higher on the bar
than it is at the foot of the pass, where
nature has completed her jettiee, between
which the river flows in q magnificent chan
mel of almost uniform width and great
depth. This hill, forty five feet high, begins
to rise just where the banks of the pass
begin to widen out to the sea. and between
these two points the river is building its
jetties. Evidently the effect of the hill
against which the river is discharging is to
check the velocity of the stream which is
flowing down with the load Of sediment due
to it's speed. Checking the velocity means
dropping, the excess Of load. The current
trail channel, and hence they are gradually
built up to the surface, and the continual
dropping OF sediment on their margins
gradually narrOws them in until they be-
come so steep that no more deposit will rest
upon them. When thus narrowed in they
are completed and are Of the width Of the
channel above.
Now let us note the effect upon the bar of
this building up and narrowing in OF these
incipient banks. The bar is to-day about
seven and ont third miles from where the
mouth of the pass begins to widen. When
Talcot surveyed it in lS38-thirty seven
years ago, the bar was seven and one third
miles from the narrow part Of the pass. but
the pass has by this narrowing in process
been gradually concentrating more rapid
current against the bar, and this increased
power has cut out the hill on the
upper side and transported the material
over its crest, and dropped 1t on the sea-
ward side of it.
In proof of the narrowing in of this un-
finished part Of the pass, will refer to the |
fact that five miles above the bar of Talcot |
the pass is now only about 1000 feet wide, |
it was $500 feet wide when Talcot |
surveyed it. Colonel Longs map of 1857
shows the width to have been three-eighths |
If mile greater at Pilot Town, about six |
niles above the bar, than it is to-day. 1t
8 evident that the river is building at the |
Southwest Pass parallel jetties about 1900 |
eet apart, and that it preserves depth be. |
ween them of at least sixty feet of water. |
q it not evident that the hill or bar is an
Ibsolute necessity in this natural jetty |
work, without which the river could not |
hrow down it's sediment on it's two incipient |
cities. At the rate the river is building at
his pass it would take it l7S years to build
ts ietlies seven and One-third miles, or from
where they are completed out to the crest
# the bar. Ifman were to extend two arti-
bird miles between the present narrow
erminus of the finished ietties and
he crest of the bar, and Of the
anne width, say 1900 feet it is evident
his sudden contraction of the present
Q, 900 foot channel over the bar would raise
he water in the pass momentarily. and
realty increase the current, giving it q
realty increased carrying or securing
over which would rapidly cut down the
ill between the artificial jetties and thus
over the water above, diminish this sud
enly increased current, and finally, when
he iill was cut through, and sixty feet of
epth secured out, we should have at the
nd of the jetties current almost as great
9 that now running seven and q half miles
bove the bar, which current is about thirty
er cent more rapid than that at the bar.