The record-union., 1899-05-23

representatives. Oh IDS NHOhc CITY VIM
fairly well improved, and it so happens
that their capacities per unit of surface
drained agree fairly well with that
above set forth as being desirable ana
determined by an Independent line OF
reasoning, thus at least Strengthening
the conclusion that run-off OF OUo
cubic feet per second per acre should
be made the basis of calculating re
quired capacities of storm water sewers.
The calculated capacities of the sew
ers now in service are not perfectly
reliable guide as to required capacities
of new sewers, for the reason that 1t IS
difficult to make such calculation
close approximation, and because their
capacities, calculated on the asSump-
ticn of full sewers, are exceeded some
what by the combined capacities OF
their several principal branches. The
sewers may, in fact, have at times been
called upon to serve in excess OF their
capacities as above tabulated, though
such excess has probably never greatly
exceeded 10 per cent.
When, therefore, the rate of .O6 Gu
bic feet per second per acre is recon-
mended as a unit fer determining ca
pacities of the outlying districts of the
city, it should be remembered that this
rate is most likely to prove correct for
the city taken as whole; that the run
off must be expected to exceed this
rate somewhat in districts of small
area, and in those districts which are
closely built upon and we'll paved-that
is, in the business section of the city.
No special reference has been made
in this discussion, Of house drainage
or sewage proper, because the proper
ton it bears to the maximum flow of
surface drainage is small, and the fig
ures noted in the foregoing table will
suffice to show that the whole proposed
system of sewers tributary to the main
sewer should be revised.
To illustrate: The proposed Pinch
sewers from the south on Twenty
fourth and Twenty ninth streets would
have only about one-fourth of the out
fall capacity which the districts that
they are to serve may ultimately re-
quire.
The sewers from the north on Twen
ty-ninth street. from the south on
Thirteenth street, and from the south
on Ninth street under the proposed
project would each have about one
third of the capacity that should ulti-
mately be provided, and those from
the north on 25th street and from the
south on Second street about one-half.
That a deficiency in the capacity of
the S-street sewer near it's outfall end
exists has already been noted; the de-
ficient capacity is still more pronounced
at points further from the river
Your City Engineer has made a rain
fall of one inch in twelve hours
the basis of his calculation of required
drainway capacity, and has assumed
that 80 per cent. of this rainfall will
reach the S-street sewer in eighteen
hours from that part of the city north
of S street, eastward from the river to
Fifteenth street. He has assumed that
from the rest of the city about 10 per
cent. Of the rainfall in twelve hours
will reach the main sewer within TWEEN
ty-iour hours. Sewers planned on this
basis would discharge an amount of
water equivalent to nearly three-quar-
ters of an inch of rain in twenty four
hours from the sparsely built up areas
and over one inch from the more dense
ly populated and well built up portions
of the city-amounts which approx
mate the average requirements, but
which fall considerably short of what
must be expected to be a maximum
run-off rate
1t may be suggested that even the
rate of .O6 cubic feet per second per
acre may at some time in the future
when all streets are paved with i'm
pervious material, be exceeded, be-
cause the area of street surface alone
approximates one third Of the whole
area Of the city, and only one fifth of
the area has been assumed impervious
in determining this rate
Could it be known that the city will
within reasonable period of time, say
thirty years. have all its streets paved,
then such an objection might have some
force; but unfortunately, this is not
likely, and in case it should appear by
actual experience that a greater al
lowance should be made, it will still re-
main possible to add additional con-
duits from centers most requiring the
same.
1t appears, so far as my investigation
has extended that all the water which
the main sewer receives must be dis
posed of by delivery into the river To
let it go by natural flow southwark
along the line of the old drainage canal
means that it will become a burden
upon the reclamation districts below the
city, where it will either augment flood
waters, increasing inundated areas, or
must be handled at some expense.
The river is higher than a large por-
ton of the city during half the year,
and is continuously too high for grav-
ity discharge of sewage or storm water
from the city into it Pumping is
therefore necessary, and pump capac-
ity is to be determined in practically
the same way as sewer capacity, on the
basis of the maximum runoff during
severe storms.
Conditions in the past have been ia
vorable for the control of surface drain
age by pumps of comparatively small
capacity. Not only was the city only
partially severed and the water from a
large part of its surface greatly imped-
ed in its flow to the pumps, but there
has been a large extent of lowland be-
tween r and Y streets subject to in
undation, which has served as a re-
ceiving basin, in which water in ex-
cess of pump capacity could accumu-
late, and in which it would be held un-
til the runoff again fell below the pump
discharge.
without a receiving basin of large
capacity, the pump capacity to be pro-
vided should be at least equal to the
estimated maximum output of all the
sewers.
No matter how carefully a system may
be planned and how carefully the work
may be executed, there may be acci-
dents particularly to mechanical appli-