The record-union., 1899-05-23

E, 1S93-January Both. LOG inches;
March 11th, LIS; November 24th, l.
1S94-January 10Th, 2.25 inches; Feb
ruary 10Th, 2.16; December Sth, l.lO;
10Th. 1.26; Dist, ITO.
1S95-January 4th, 262 inches; 10th,
1.16.
1S96-January 18th, 11S inches; 18th,
1.2O; Goth, 1.S4; 9th, 1.7S; April 24th,
LOO; November 23d, 1.60.
1S97-January Doth, ] SO inches; 31st,
| 1.1O; February 1st, 1.25; March 2Sth.
| 11S
! 1s9S-February 24th, Ito inches:
May 18th, 1.
1S99-March 14th, 119 inches; 18th,
| Leg; 10th, LA; 22d, lis.
Eighty-two days with more than l
inch and less than inches.
Twelve days with more than inches
and less than inches.
Three days with more than inches
and less than inches.
! No days with more than and less
I than inches
One day with more than 5 inches.
On April 20, 1880, two inches Of rain
fell in twenty-four hours, between noon
and 4 p. M, and the phenomenal rain
! fall of 5.2S inches on that day was
Immediately followed by 800 inches of
rain in the next twenty-four hours.
So far as records of daily rainfall are
at my disposal QS61, 1862 and 1877 to
date, there has been no other storm
4 during which the rainfall in twenty -four
hours exceeded five or even four inches,
and the rainfall of three inches was
barely exceeded on only two other
! Tn, s1.ate 7s,,i,1 ooffi Of
| storm water sewers it will. therefore,
be reasonable to make a fall of three
i inches per day the basis of the calcula-
ton. The maximum hourly rate has
never been determined for Sacramento
1t is of considerable Importance to
/ know this rate, at least approximately,
as basis fer determining the capacity
of sewers, particularly of those which
are to carry off the surface drainage
from closely built up and well paved
sections of the city.
For districts of considerable area, the
rate to be applied may be determined as
followc:
The distribution of the rainfall to each
hour of any twenty four hour period is
probably never uniform. If the rain
fall had been recorded for periods of
3 tNOA' AiOUt IiSlCdQ VI lhCuly -lOul,
there can be no doubt that the rate per
hour would have been somewhat great
er than the average maximum rate as
! determined from the daily fall. If it
/ be assumed. and the assumption is not
unreasonable, that 80 per cent. more
rain falls in one-half of a twenty-four-
hour period of heavy rain than in the
other half, and that likewise, the fall of
+ ra in in two cO1caplIfiva ciy-holIr rari0dc
of rainfall maximum, show the same de-
gree of variation. then the maximum
rate of rainfall for six consecutive
hours will be 0.1S inches per hour in
stead Of q. 10 as would appear if three
inches per twenty-four hours were sup
posed to be evenly distributed to the
! several hours.
! The phenomenal local rainfall of two
inches in four hours on April 20, 1880,
may be disregarded except as an indi-
cation that all drain capacities designed
/ to meet ordinary storm conditions may
at great intervals and for very short
/ time periods be found inadequate to
handle storm waters as fast as pre-
/ sented. 1t may safely be assumed
I that if a rainfall of three inches in
/ twenty-four hours is made the basis
/ of the calculation, that the facilities for
i handling the runoff will meet all ordi-
nary requirements. If the rainfall of
storm with more than three inches
/ be uniformly distributed to a twenty
! four-hour period, the storm water con-
duits calculated on this basis may still
be adequate to handle the water with
out inconvenience, as the capacity
would be ample for the runoff during
a steady rain aggregating 4 81 inches
per day. A long continued excess over
( this rate per day would cause surface
water to accumulate temporarily, and
/ possibly very generally, throughout the
city, until rainfall rate again falls be-
low the assumed maximum rate.
! Three inches per twenty-four hours
were exceeded on only three days since
July, 1877. and then so slightly that
sewer system planned on the 3-inch
basis would have been amply adequate
I to carry off the rain water
| At three inches per twenty-four hours
! the. average hourly rate is one-eigffth
! inch, and the average hourly rate for a
/ period of six hours may. as above
! shown, be taken at 0.1S inches. which is
/ equal to 0.1S cubic feet per second per
acre. 1t is likely, and assumed for the
I purposes of this demonstration. that
within six hours water flowing across
4 the surface of lots or streets into the
/ drainways, can reach the outfall point
of the main sewer from the most re-
mote section Of the city; that, therefore.
| within this time impervious and per-
vious surfaces will not only be thor
oughly wet, but may be assumed to be
contributing runoff water, no matter
where they are located.
Were there no evaporation and were
! there no pervious areas, then sewer
capacity could with a fair degree of ap-
proximation be r,ctly determined from
the rate of rainfall: but the areas under
roof, the paved streets and walks are
the only portion of the city which may
/ be classed as impervious. As the den
! sity of population increases, this Will
gradually approach one-fifth to one
/ fourth Of the entire area of the CITY
The areas not built upon, not rooted
over or covered with some imperyious
4 or practically impervious material. such
as street pavements, owing to the flat
ness of the city and the pervious nature
of the soil, take the rain water almost
as fast as it falls.
The runoff from the areas 20 be
classed as impervious will be about GO
per cent. of the RAINFALL that from the
| rest of the city about % per cent. With
| these percentages and fifth as the
ratio of impervious area to that of the