New-York tribune., 1878-10-01
IETTIES Under WATER.

PROJECT FOR THE IMPRnvEAigNT OF THE SNIP
nor or CHARLEsrON s. C., BY MEANS OF LOW
JETTIEs. senate EI Doc No H. XLyth CongIess sec;
Engineers, Breyet Msjor.GeneTai U. S. A.
This document gives detailed description,
illustrated by sheets OF drawings OF the method pro
posed by General Gilmore for permanently deepening
the shipehanne across the Charleston bar 1t Is known
that this bar changed little in either location or
magnitude within hundred years Measured along
its crest Hne or DEPTH the bar is about ten
miles long average width between the inner and outer
eighteen-fOot being about one mile and three
fourhs There appear never TO have been less han four
nor more than six channels the BAR the greatest
dat NJ water-rarcly exceeding thirteen and Onehalf
feet at low tdebeiDg sometimes found In one channel
and sometimes In another Between the channels the
depth OF water along the crest does not exceed three
10 four in many places The mean rise ana ia OF
tides is five and One-tenth feet, and the area OF
the tidal basin formed by the harbor and its
hronches ts about fifteen square miles. The pro
ec nienplates the construction OF two etes OF rip
ran stone resting on of timber and brusl
snringing from sulvan's Island and the other from
Morris Island, located upon cOnyerging curved lines
with the convexty turned toward each other, In such
manner that their ends the slope Of the bar
will be parallel 70 each other, and distant apart from
one-balf TO hve-eghths OF mile. The length Of the
north lefty will be about 5000 feet, and that OF the
south lefty little over 10000 The distinguishing fea-
ture Of the project appears be that the half Of each
iety next the shore ~ kept very lOW The north Jety
will have its crest twelve feet below the level Of
low water where it the sullivans Island Chan
neil while the crest OF the south jety will be fifteen feet
below the same level where the main channel
abreast Of Morris Island From these lowest points the
ietos rise gradually they approach the bar and the
sea ends for length 1900 feet carried up to the
level Of two aud half feet above low water consid
cradle volume Of water will therefore ebb aud flow over
the tops Of the etes and propOrtlOnatey less volume
will pass out and In between them the height OF the Jet
ties and their distance apart being mutually dependent
on each other. The memoir up the chief conditions
Of the plan as foowsi
The complete success OF the works is believed to de-
pend three important conditions, which they
kept In view In preparing the design, VIZ.
l. They should not impede the inflow to such degree
prevent the tidal basin being filled as now at every
influx OF the tidal wave.
10 this end the inner half OF each etty, espec-
tally its central DOrtiOn located In water across the
thread OF the cGsren Is kept several feet below the
water The outer half being nearly parallel TO the di
rection CT the h0w Is built higher and the end for
enough 10 be visible at all stages OF the tide.
q They should control the outflow to such degree and
IN such manner that channel Of the required depth
will be maintained through the bar.
TO this eud, although large portion Of the surface
flow will spread out the tops OF the jetties and
thence the BAR the central How, throughout the
depth along the axia line Of the gorge between Sui
livans and Morris Islands Is aided Ia Its natural ten
dency TO reach the sea along the prolongaon Of that
line by the opening left for IL between tho jetties The
bOttOmOw through the gorge OF the harbor DETECTED
cOnvergnR lines by the jettes. and is therefore
forced In measure 70 concentrate itself In, and flow
through, the gap between them. The outer half
each ety and the adjacent portion Of the shore end
as a trainingwall for this a0W.
g. They should not 70 any considerable extent, cause
movement seaward Of the main body Uf the BARI that
the general position OF the bar should be independent
OF the effects produced between and beyond the heads
IL believed that this condition will be secured by