OF
T. S. HARDEE,
CITY SURVEYOR OF NEW ORLEANS,
ON A SPECIAL SURVEY
OF
LAKE PONT6HAPiTRAIN,
IN CONNECTION WITH THE EFFECTS OF THE
BONN ET-T CARRE CREVASSE,
AND THE DANGERS FROM
WITH ACCOM]'ANYING CHART.
PUBLISHED BY AUTHORITY OF THE CITY COUNCIL.
NEW ORLEANS:
A. W. IIYATT, STATIONER AND PRINTER, 38 CAMP STREET,
1876.








LAKE PONTCHARTRAIN.
OFFICIAL, REPORT OF THE CITY SURVEYOR.
The effects of Bonnet Carre Crevasse on the physical condition
of the Lake-Impending dangers of overflow from a concurrence of river floods with easterly gales-An appropriation
from. Congress suggested to close the Crevasse.
SURVEYOR'S OFFICE,    )
New Orleans, September 25, 1876.
To the Honorable Mayor and Administrators of the City of New
Orleans:
GENTLEMEN —In response to the resolution adopted. by your
honorable body on the 2d of May, 1876, the United States Government generously placed the revenue cutter Dix at the service of
the city authorities, for the purpose of assisting in a special survey
of Lake Pontchartrain, and I now have the honor to report, as
your engineer officer in charge, that the work has been conducted
to a satisfactory termination.
The object of the survey was to'determine the effect of the
Bonnet Carre crevasse on the physical condition of the lake, and
to deduce from the facts thereby elicited the prospective dangers
which might threaten the city of New Orleans from that source.
The results are not as full and complete in some details as might
have been desired, owing to the fact that there was no previous
survey made by the Government, and therefore no records in
existence -by which to compare present soundings with those of




the past. In consequence of this deficiency, and the want of
means on the part of the city to undertake the amount of work
involved in an original survey, attention was mainly directed to
discover the existence of sedimentary deposits in the lake, and
then by suitable examinations to determine the location and area
of the same, believing that the information thus obtained, in connection with the knowledge of other well established physical peculiarities, would satisfy all requirements in the premises.
With the view of accomplishing the work projected. a new contrivance of a combination rod was ordered to be constructed,
which answered successfully the double purpose ot' taking soundings and borings at the samne time.
PRELIMINARY EXAMINATIONS,
made in a general way over the lake, at various points, from Pass
Manchac to the Rigolets, developed the fact that the deposits were
being made principally in a compact mass at the western end of
the lake, where the rapid discharge of river overflow is finally
ehecked by contact with the tidal expanse, and to this locality the
labors of the survey were chiefly directed.
The present crevasse, which was caused in the spring of 1874 by
a breach in the levee at Bonnet Carrb bend, about thirty-five
miles above the city, is now 1370 feet in width, in a direct line
across the gap, and as the discharge of water courses towards
Lake Pontchartrain, five miles distant, it widens in a fan-like
shape, so that by the time it reaches the shore of the lake the
flow of water has attained a breadth of more than twenty-two
miles, as will be seen by the section colored blue on the accompanying chart. The difference in elevation between the highest water
in the river at the head of the crevasse, and the ordinary tidal level
of Lake Pontchartrain is about eighteen feet, and the discharge of
water through the crevasse when the river is in the neighborhood
of flood height has been estimated to be about one-tenth of the
volume of the entire river which passes at that point. With these
facts kept clearly in mind, and viewed also in connection with the




immense quantity of sediment known to be carried in the stream,
it can be readily conceived what must be the results necessarily
involved in the very nature of such an outpouring. A very large
proportion of sediment is of course precipitated before reaching
the lake shore, but enough is still held in suspension to produce
marked effects when the flow of water is at last checked by
debouching into the lake.
The accompanying map shows the location and
AREA OF THE DEPOSITS,
as determined by the examinaticns. The figures shown in the
section,colored brown upon the chart, represent the thickness of the
deposits at the various points where the rod was used. In recording these different borings care was taken to show only what was
deemed to bear indisputable evidences of river mud or dlluvion,
embracing principally the results of the three successive crevasses
since that of 1874, and possibly embodying those of other overflows anterior to that date, and it may be of some even before the
levee system was adopted. It will be perceived that the greatest
width and depth of deposit has been made opposite Frenier Station,
on the Jackson Railroad, or where the main body of the crevasse
water pours into the lake. At this point the cross section from the
shore outward shows the deposit to have been made over a distance exceeding five miles, and the greatest thickness in that section to be about six and a half feet. The whole area of deposit
embraces a compact mass, some twenty-two miles in length by an
average of four miles in width, and contains, by calculation, about
two hundred million cubic yards of sedimentary matter. To furnish some practical idea of the immensity of this accumulation, it
may be stated, that if the material were distributed over the
inhabited portion of New Orleans, it would be sufficient to raise
the entire ground of the city more than four feet above its present
low level.
The capacity of' Lake Pontchartrain as a reservoir, its entire
water contents being displaced, will allow the substitution there3




for of' about ten thousand million cubic yards of earth or
sedimentary matter. With the knowledge, then, that in a given
number of years the capacity of the lake has been reduced to the
extent of what two hundred million cubic yards of sediment would
displace, it is within reach of arithmetical computation to calculate
the length of time it would require to fill up the body of the lake,
provided the crevasse should remain permanently open, with the
same average discharge of water annually, and with the same
general laws governing its action in the future as in the past.
But as the number of years in which this estimated deposit has
been accumulating cannot be ascertained with any degree of
accuracy, it fbollows that no deductions can be made which would
fix, even approximately, the happening of such an event at any
specified date or period in the future. It can safely be predicted,
however, that many generations will come and go before the
citizens of New Orleans would be deprived of the benefits of Lake
Pontchartrain as a navigable water. Agencies more active than
the slow one of shoaling must, therefore, be taken into considera
tTon as liable to produce anything like the immediate dangers
which have been apprehended from the altered physical condition
of the lake.
It has been clearly established' that the
EFFECT OF THE CREVASSE,
up to the present time, has been to raise considerably the ordinary
level of Lake Pontchartrain, which abnormal condition lasts
generaily about four months in the year, or during the flood:period in the Mississippi River. This elevation of the surface of
the lake is almost exclusively due to the immense volume of
foreign water constantly precipitated into its basin from the crevasse, which must necessarily create for itself a new plane of
discharge to the Gulf by the consequent forced raising of the
water.
With this great cause of danger from the large amount of fresh




water, are combined the injurids which breaten from the periodical conditions of Lake Pontchartrain by overflow caused by
accumulations of sea water.
It has been contended that the obstructions caused by the
bridges and earthworks of the Mobile Railroad, between the
Gcntilly ridge and the highlands east of' Pearl River, have contributed largely to the elevation of the water line in the lake, by
damming and backing up the crevasse v ater, thus impeding its
free discharge into the Gulf. A carefill investigation of the line of
road,,when the water was at its highest mark, did not develop any
facts which warrant such an assertion. On the contrary, the construction of the railroad embankments across the lowlands or
marshes between the points mentioned is deemed a positive benefit
to the city of New Orleans as a prevention of overflow. It
unquestionably acts as a barrier to shut out the Gulf water, which
in severe easterly gales rolls towards Lake Pontchartrain in a
solid mass of' thirty miles in width, and of a depth sufficient to
inundate simultaneously the whole intervening marsh or lowland,
upon which it is,pecipitated in a few hours' time. This massive
wave, when it comes, is now arrested by the railroad embankment,
acting as a protection levee, and the supply to the lake basin can
only be made through the limited openings where the bridges are
constructed. Thus the influx of this immense quantity of water
is rendered more gradual, and would occupy days before it would
pass through these narrow inlets, during which the subsidence of
the gale or a fortunate change of wind would arrest its further
accumulation and increase, and tend to discharge back into the
Gulf by the outflow what had already been received. It will thus
be seen that, in the cobnfinement of the
RECEPTION  OF SEA'WATER
to these circumscribed entrances, these inlets serve to a certain
degree the purposes of locks in lessening and delaying the admission of water into the lake, when without the embankments the
whole expanse of thirty miles would afford the opportunity for the
5




rapid introduction of the whole bulk and force of the mammoth
wave in a fbw  hours. While it is plain, therefore, that the
embankments act as barriers, and the openings as embouchures
regulate the admission of water, it is equally evident, that if by a
series of' easterly gales, continuing through a considerable length
of time, the lake should receive the maximum of water, and
thereby overflow the city from the rear; the same reasoning shows
that these narrow outlets would retard the discharge back into
the Gulf, delay relief from the extraneous surplus of foreign water,
and necessarily prolong the inundation. The effect is best illustrated by a comparison of the lake to the body, while the openings
would correspond to the narrow neck of a bottle, the smaller the
latter is, the slower the process is in filling the vessel, and when
once filled, the' emptying of its contents, for the same reason, must
be correspondingly gradual. The latter contingency could arise,
however, only by the occurrence of a continued series of' violent
easterly gales to an extent hitherto unknown, and happening in
rapid succession, before the distiarge could be effected. Such a
state of things could scarcely be anticipated, anel at the worst the
damage would have already been inflited, and would only be
slightly aggravated by a somewhat longer continuance of the
overflow.
In view of this theory, therefore, which is believed to be correct,
the apparent obstruction of' the railroad: is converted into bn
absolute advantage, for the great damages by back overflow have
always been produced by the quick and sudden precipitation of
this (gulf water into the lake basin, and its consequent encroachments upon the contiguous lowlands in the rear.
THE  E(ECORD)S
in this office show a slight annual increase in the height of the
water in the lake for the past three years during the flood period
of the crevasse, and before any examinations were made, it was
presumed that the shoaling, which was believed to be progressing
more extenisively than has been found to be the case, might
{?




produce such effects. But the variation being only a few inches
between the seasons (,f 1874 and 1876, it is concluded that other
causes, such as the differences in the actual volumes of water
passing through, together with the action of the winds, etc., might
have induced these very changes and irregularities.
It is considered, therefore, that the question of increased danger
to New  Orleans depends solely, upon the presence of so large a
quantity of crevasse water in the lake, and the consequent diminution of its normal capacity as a reservoir during high water. thus
limiting its ability to receive and hold the immense volume of additiqnal water which might be blo-wx into it from the Gulf during the
prevalence of easterly gales. But when the two conditions concur
it is evident that the danger becomes intensified, because the
incoming storm wave from the Gulf would completely hold in
check the  outgoing flood of crevasse water, and with the
happening of such a coincidence in an aggravated degree, it can
be easily conceived how New Orleans might be subjected to the
severest and most damaging overflow she has ever experienced.
Such a calamity was barely escaped last spring, when the water
in the lake and in the several car.n] basins was higher than ever
before known. It only required at that time a continuance of the
wind storm for a few hours longer to have so raised the water that
it would have passed bodily over the tops of the low protection
levees in the rear, and even been forced over the higher natural
barrier of Metairie Ridge, thus inundating the entire city from the
river to the lake.
Such being the
CONDITION  OF AFFAIRS,
it was deemed pertinent to the purposes of the examination to
consider and digest the best plan to be adopted which might mitigate or prevent sonic of the evil consequences of the situation,
even if it did not entirely establish perfect security from the
dangers threatened. The best meais practicable, which suggested
themselves, was the closing of the crevasse, in order to shut out




the river water, as one of the greatest disturbing causes of the
normal condition of the lake. Indeed, the immense expense which
would accompany any attempt to preclude the sea water from
encroachment, debarred any effort in that direction, and necessitated the turning of our attention to the control of the river
water, in order to establish comparative safety, relying on the
completion of the back protection levee in the future as the only
measure of complete and permanent security to the city. In view,
therefore, of what has been adduced in this discussion, it is clear
that the interests of New Orleans are deeply involved in the
closing of the Bonnet Carre crevasse, for with this accomplished
a great factor in the problem of danger is at once eliminated. It
is all important also, that the plans for the accomplishment of the
work should be matured without delay, in order that it may be
undertaken in time to control the Mississippi before the recurrence
of another flood.
It is estimated that $200,0Q0 would be required to re-establish
the security of the levee front in the vicinity of the crevasse, and
incidental to it, an additional $50,000 would be needed for the purpose of repairing the levees,all.the way below, which have been
neglected for several years past, on account of the flood-line in the
river being decreased, as an effect of the crevasse above, and the
fears of those interested in planting proportionately removed. A
total expenditure, therefore, of $250,000 is deemed to bet sufficient
to meet both demands of the situation, and to satisfy all interests
concerned.
In' the present
EMBARRASSED FINANCIAL CONDITION
of the city and State, there is scarcely a reasonable hope that the,required amount of money can be obtained from either of those
sources, and reliance, therefore, must be placed upon the strong
arm  of the General Government to afford the relief so much
8




needed. It is suggested, also, that a community of interest exists
on this subject between the seacoast of Mississippi and the city of
New Orleans, and that concert of action between te two, in preparing a memorial to Congress for aid in this particular work, is the plan
which should be adopted, and which promises a measure of success.'The whole coast of Mississippi, from Pearl River to Pascagoula,
embracing millions of dollars of improved property, is offended
with the presence of fresh water from the crevasse at a season of
the year when it is a resort for health and pleasure, and thus the
material interests of that section of the State are very seriously
damaged. Mississippi certainly has some claims upon the Government for the improvement of her seacoast, as she is, perhaps, the
only State bordering on the ocean waters which does not annually
receive appropriations for that object, and although the expenditure in closing the Bonnet Carre crevasse would not be applied
immediately to the coast front, y't the advantages to flow from it
for her benefit would be no less dijrect and positive. Her representatives in Congress would no doubt join in exertions with
those from Louisiana, and use every means in their power to
obtain the necessary appropriation  at the earliest possible
moment. If a bill could be matured and presented among the
first days of the ensuing session in December, it is possible to have
it acted upon in time to enjoy the benefits of an appropriation for
the closing of the crevasse before the coming of the next flood,
otherwise the dangers and damages from the situation must be
endured for another year.
It affords me pleasure, on behalf of the city, to acknowledge the
favors extended by the officers of the United States Customs
Department here, in their uniform courtesy and for the facilities
afforded me in the prosecution of the work; also, for the hospitali9




ties extended to the surveying party by the officers of the revenue
cutter Dix, as well as for the valuable assistance rendered so
willingly by them and their crew.
My thanks are also eminently due to Messrs. Harrod  and
Reynolds, my assistants, and the detailed party of employees, who
so efficiently labored and co-operated to bring the work to a
satisfactory conclusion.
Very respectfully,
THOMAS S. HARDEE,
City Survoyor10