New Orleans Republican. [volume] (New Orleans, La) 1867-1878, 1876-07-02

of tho area described was about seventeen

of the area described was about seventeen
feet; now it is about ten feet.
For the middle third this average deptl
has changed from las feet to sc feet.
For the lower third it has changed from
Il46 feet to 9.5 feet. On comparing tn,
aoundings in the west entrance to Eoutl
Pass, on sheet No. l, herewith, with those
of Mr.. Marindin's survey of June, 1875, nc
very marked change appears; certainly nc
| east entrance.
1t is not always easy to assign the G fl
Gent cause for shOaling in q special portion
of the river. The ehOalirg in the east en-
trance may have arisen from some general
| causes, which would have acted bad nc
attempt been made to improve the river.
But the changes which have been made in
the South Pass might produce shoaling-
l. At the lower end of the jettes at the
mouth Of South Pass the velocity is much
| greater than it was year ago.
This increased velocity requires an in
creased head to produce it, and the sur
| face of the pass between the etties has,
| at least in high river stages, become ele-
waited. Grand Bayou has been partially
| obstructed, raising its water surface.
IL is probable that both these cause raisa
the water level somewhat in the South
Pass at its head, thus diminishing the
velocity. of the water entering it from the
man river. Ii the entering velocity ia
sufficiently diminished, some of the SCI
ment carried must be dropped.
have no precise data for estimating the
rise in the water surface between the Jet
ties, produced by their cons truction. Some
months since there seemed to be differ
ence of eight or ten in shes in water level on
the inside and outside of Kipp dam.
If We suppose the water level Of the pass
at that place 10 have been raised by six
inches above it's orig inal position, hydraulic
formula (which are, however, cOnfessedly
based on imperfect theories, but which are
the best We have), give resulting rise of
about three inches at head of South Pass
when the stage of the river is q medium
one, giving slope of one foot in this ten
and half miles distance, the pass being
supposed of indefinite length.
.. The jetty at the cad Of South Pass,
where not raised to the water surface, is
now crossed at an angle of 200 or 800 by
water running with considerable velocity
into Pase-aIOutre.
1t acts, therefore. to some extent as a
dam, diminishing the velocity OF the water
above it, and perhaps ,using it t. deposit
part of its sediment.
Which of these causes has been most
effective in producing shoaling am unable
to say.
On April Is, 1876, you gave me instruc-
TONS to report 1.-THE material used, the
character and permanepcy OF the jetties
and auxiliary works, and neither they are
SO thoroughly substantial and permanent
a. to maintain the improved channel for all
time after their cOmpletiOr;" also to state
40 what degree they fall short of it, and
further report to what degree the jetties
and auxiliary works are less massive, less
strong, less enduring, and less costly than
the works planned by the board of en-
glneers constituted under the river and
barbnr act of June %, 1874, of which Lieu.
tenant Colonel Wright was president, and
yourself . member, the report and plan of
which board was submitted to Congress
and was before that body when the river
and harbor act of March 3. 1875 was passed.
and the contract with Mr.. Z ADS catered
into"
The fret question is wheth the jetties
and auxiliary works are go Substantial
and permanent as to maintain the improved
channel for all time after their compile |
ton" 1t liz cludes the question whether the |
works are in themselves permanent, and |
whether they will maintain q permanent |
deep channel.
At present the jetties at South Pass are
of superposeu mattresses,
'ne side of the embankment being vertical,
he other side so sloping that while the
width of the jatty at the water surface is
wenty five feet, the width at the bed of
he river is from thirty five 10 fifty feet.
The critical side of the embankroent is in
contact with a row of piles; where the
wave action is strong, piles have also been
lriven along the other edge Of the top mat
Tess to prevent its displacement by the
waves. A part OF the mattresses have
'een sunk by rubbiestone thrown upon
hem, part by allowing mud to accum-
ate in them while at1oat over their proper
'OsitlOns. In their present condition these
cities are not permanent structures. AS
lr. E ads has not furnished me with copies
t his plans, am unable to state definitely |
that further work he proposes to com.
lete them, but understand it ia intended |
0 widen and cover til an with stone.
AS to whether if these works are made
ornament they will, vi moderate occa-
onal extensions, maintain the improved |
hannel for all time after their completion,
would state that the information on which |
ldgment is to be based is, with some ad-
itiOns, the same as that before the board
engineers established under the act OF
congress of June %, 1874, which proposed |
10 improvement of the South Pass, and |
rat gave definite plan for the work.
The additional information on which |
lace reliance is that already referred to in
scour, derived from the CON-
arson OF the coast survey map of June.
SIS, and sheet No. l, transmitted herewith
ne comparison shows an average ad vance
front OF the jetties of the outer face of /,
10 bar, az defined by the twenty foot :
rive, Of about $50 feet.
1t seems probable that part Of the de- :
,sit producing this advance is that scour- :
from between the jetties and rolled :
ong the bottom, or carried in intermitting | :
spension near it, to be dropped as soon as =
comes to deeper and stiller water outside =
the bar and below the level of It's crat |
4 acceleration in the advance Of the bar :
ould naturally be expected while this | ;
our was going on. and will continue SO | I
Gg as the scOur continues; when that scour | !.
asses the advance Of the bar will be indie !,
ndent of this temporary and extraordi- -
ry cause Of advance.
The present rate OF advance, arising in .
1T at least from scour, gives, in my .judg- h
snt, no definite information as to what a.
9 ultimate rate will be, and hence does | 4
t give sufficient data to modify the r
inicn expressed by the board of engi. :.
ers, already referred to, as to the perma- y
nee of the channel when properly jettied, :,
with moderate extensions Of the ietties | ~
IN time to time. -
IN the second place am directed to re- ~ y,
rt Ito what degree the jetties and f.
Hillary works are less massive, less ~,
ong, less enduring, and less costly than :.
} work planned By the board of en-. a;
eers' already referred to. e.
The board proposed for the first Zoo feet N
the east jetty lnatlress-dike ten feet ~, ~
10 on top. with side slopes of twoi birds r. e,
it's present condition the first 5000 feet a
sists of two rows of piles, line of sheet a
ng. and two layers Of mattresses The 90
It 2lOO feet has cross-sectional mattress 81
a somewhere near that proposed by the he
lrd. From this point on, the jetty in it's NC NC
sent state has less cross section than 49, ~
t proposed by the board, the difference Sh
oming very great on reaching deep ha
Ter, where the board proposed top NO NO
th OF fifty feer, with side slopes of one ="
rter, while the construction has q top fs1