New-York tribune., 1873-05-10

OF the dam over which the water must then rush and
| tear through 1t q wide channel Tor its escape
i THE BoARp's COMPUTATION or DRAINAGE.
now wish to call your attention 10 the IDEAS OF tho
Board ON hydraulics and hydrosatcs as illustrated in
tho report OF their Engineer Mulett submitted 10 tho
Uouae Committee of Investigation In February, 1877
naaes IA and 1l5, In speaking OF that part OF the
canal extending westward from seVenin 10
st. for which they proposed and since nave substituted
| the B-st. wer they say:
Tho area draining into the above portion OF the canal
and north Of t ia about sss the area south OF It,
and draining into 1t, about l9' acres
This s equal total OF SS3 acres, WHOM ior my pros
ent purpose may be supposed be correct But In ther
computation OF the quantity OF water oue W S
of surface they have failed TO include tnas "o '"o 100
acres south OF the canal This omission, on tner own
basis OF one inch an honr amounts 10 7o CUBIC iee'
per hour. for the outflow of which tbey nave N1o Nv
provision whatever And in their computation y, "., ~
would come from the 6ss acres north Of tho canal tney
made an arithmetical error by when tney u'uo ~" "
mated the quantity to the extent Of 324o cuoo o' M
| hour This comes from a Board, having an engineer
among them who pompously stated before the ComniV
tee OF Investigation page 800: 81 do not guess
that kind. do mako approximate estinaVeS' iuo
not do my engineering by guess work; 1t Is ha'oio VI
calculation" As there is record OF ralna oAoceu
ing two inches an hour they adopt that quanhy AS
basis OF calculation determining the size OF te SEVER
TO be substituted for this section OF the canal Oo ~,
IT with the absurd remark ''that only 0O per i ,ts
this amount reaches the sewers the balance nnIi1g M
way by percoatiOn, into the ground ana also pA' '
It ia difficult to deal calmly with such obvious non,
But as the Board have published II as ther o
siyely acted upon 1t IN this city, the taacy must pe ne
and exposed For this purpose 9eR9n m S Ss work
Sewage. 81 about so per cent s supposed 10 DO
raTfSl of about half an inch in "a hours this quite
different thing from two inches IN one houIi oou ~
succession That the proportion thus absorbed would
be much less In the last than In the FIRST Is 700 o
rainfall of inches. inch Of water an hour
win wholly disappear by evaporation ana on ro
duee great irsshe II 49, IN & ae 81 5%a rain we
have had within two years past. II would be far more
consistent to assume that soyenegths hat 49, ~ cIg nc @
| OF a twO-nch rain an hour would hnd us way U 1no
| sewers. II may be shown as fOn0wS' c,tp.
eso acres at one fourth 55o9 !S
s3 acres at one inch %
Error eq1, the difference 2'oJso7
| BLUSDERs IN SEWER.
But for the purpose Of testing the capacity OF the Bt
sewer actually constructed lot be done on their own
basis Of inch hOurequa 10 32O5,290 cubic ieet
aud add Us they do for ordinary drainage AG66 cubic
feet This makes total Of 3246956 cubic feet. TAis Is
to pass through largest part OF the sewern one hour
Dividing this sum by the sectional area OF the sewer
925 square feet lind the velocity OF How required to
be equal 10 feet, 65 miles per hour which Is
equal TO 975 feet per second Now, such velocity OF
| flow through that il not possible. for these
reasons Its length Is nearly 5000 feet Is bottom IS
level in fact slightly higher at lts outlet tdn AT
its opposite end from one to Two rect DelO" tue @ VI
lowest tides and from IL 70 Is feet below tue Level 10
smallest parts of the sewer and flow freely backward
and forward through Its entre length ana oos Heepy
seem that the position of the sewer is that
| of dead or tide level varying only as
Its form before stated is that of anabottomed arch
ing sectional area or opening o go square Ieet IN
lts largest part; this equal TO the OF circle OF
TO- feet diameter through which the water must now,
shown above, with the velocity OF 6Go miles an
fod "A ID
as computed the erroneous basis Of the Board on
true basis OF Il inches hour, 1t will require velocity
of IL CA miles an hour. Now level sewer has not within
itself power impart any current 10 IT'S water hence,
| whatever movement the water within 1t may acquire
must come wholly FROM pressure OF the water now
ing in from tho higher elevations OF It's branches the
mean hight OF which the north side, as given by
The Board about FROM this they give the how
as 1915 per second equal to li42 miles per hour AL
miles Der hour than the mpTacucabe nOw named
above Tor the passage Of only inch an hour Although
the TOW the branches VIII npart motion 10 the wa
within the level sewer ana wOuld undteB pi0luo9
through the required velocity provided us strength ana
that OF the brancnes were a1equate 80 Dear @ nt &
required to produce such YelOcty tne Lact 10 tyg nay
not that strength. The pressure required could only
bJapcbes Tar enough to form hydrostatic tall
about Is eet which would be sure TO burst the nam
in either case result In damaging nundations But
suppose there would be no bursting OF either stl the
water would certainly forced In torrents out OF the
manhOles and other vents and thus lnundae
the valley Just surely, If not quite as speedily as
i'm the other case And above tho valley, all cellars
situated lower than the level 20 which the water might
the branches would be flooded through the house
as I poTt demo5srated by the freshet OF September
sewer and ia projectors most disastrous failures an
foregoing for the Dow Of water are made
diIced for the Information of engneers:
w. d.. L. A. 6Or. In this the diameter by feet is rep
resented by d. the head by h. the length BY and the
| velocity In feet per second by O.
| THE DANGER OF ISUSDATIoNs.
The Board have also fallen into most egregiou
by attempting to determine the proper dimensions of
the Bs from the size Of the old part OF the Tber
| sewer extending through tho Botanical Garden from
Thrdst to the north side of Pennsylvaniaave, which,
they say has proved ample for the drainage OF 3139
acres AT page l4S their report says The sectional
of the at that point Is 225 square feetan
of 10 feet II Is fair 10 assume the conditions OF
the case beng similar that the sectional areas OF Re &
areas drained" But the conditions not similar.
while the Tiber has fall Of about % feet IN length
corresponding with that OF tho Bst sewer this fail
more than nine miles an hour Hence the
conditions pertaining to the two sewers
| that flowing with the great velocity OF
than nine miles an hour such statements are
but element In calculations OF this kind BY
1t quantity Of water ascertaned but the sizes OF
required for carrying OFF the ascertaned guam
y governed by the gradients or falls OF the ducts
ts t%o % ar ~ h.
drained through two separate sewers one having such
and the other feet. Now 1t perfectly plain that
under such circumstances aside from friction, the size
great as that the latter in order make practca
BILE for equal quantities through eacn sewer IN
given This illustrates the astounding character Of
the into which the Board have fallen In estimating
the capacity OF the Bst but IL falls TO express
the entire magnitude of the blunder; for in that
one Of the sewers has no fall whatever, and Is
for this quito inert; while the other has
| so much fall "a to constitute inherent
impellng force of sufficient power to flow OF
than nine miles hour, before stated Another
erroneous statement page 1i5, b THAT the section OF
that part OF the Tber Sewer OF sufficient capacity"
ON the contrary IL Is well known that for want OF su
capacity the has been submerged and floods
passed 10 and the surface Of Pennsyva
MS nd hat % has
been retarded by unwise extension OF
| IN Mary on "A Laval and at