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tions than those that are still in operation in London. What
has been the practical issue of this prolonged and wide
experience 1 Every medical witness that has appeared before



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90 WATER SUPPLIES

us, whether his general feeling was favourable or unfavour-
able to the water, has told us unhesitatingly that he knows
of no single instance in which the consumption of this water
has caused disease. This is the unanimous testimony of the
medical officers of health, of the water analysts, and of the
bacteriological experts, — of all, in short, whose attention has of
necessity been directed to the subject." The Commissioners
therefore think that the risk of disseminating disease, even
by admittedly polluted river water, is, under conditions
similar to those which obtain in the Lea and the Thames,
and where the water is equally carefully collected and
filtered, so small as to be negligible. The serious outbreaks
of typhoid fever in the Tees valley in 1890-91, which were
investigated by Dr. Barry, a Local Government Board inspector
of great experience, were attributed by him to the pollution
of the river Tees by sewage. The Medical Officer to the
Local Government Board, in his introduction to this Report,
says, " Seldom, if ever, has the fouling of water intended for
human consumption, so gross or so persistently maintained,
come within the cognisance of the medical department, and
seldom, if ever, has the proof of the relation of the use of
water so befouled to wholesale occurrence of enteric fever
been more obvious and patent." These outbreaks were
carefully considered by the Metropolitan Commissioners, and
they concluded that Dr. Barry's evidence connecting them
with the polluted Tees water was not conclusive.

Amidst such a conflict of opinion it is safest to suspend
one's judgment ; but even the most ardent advocate of the
use of river water will admit that it should receive as little
sewage as possible, and that the sewage should be previously
subjected to the most effective system of purification. Storage
reservoirs also should be provided, sufficiently large to allow
of the average daily supply being furnished without taking
in any part of the flood-water, and the filters should be kept
in a thoroughly efficient condition. That the neglect to
maintain these conditions might result in an outbreak of



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RIVER WATER 91

typhoid fever or cholera seems possible if not even probable,
and the fact that a town using polluted water has remained
free from such epidemics for a series of years is no proof
that such immunity will be perpetual. In the section
treating of "Diseases disseminated by Potable Waters"
many examples will be quoted in which polluted river water
has been proved, so far as actual proof is possible, to have
been the cause of serious outbreaks of both typhoid fever and
cholera.

The amount of water which can be taken from a river for
supplying a town varies according to (a) the area of the
watershed, (b) the topography and geological character of
the ground, (c) the average rainfall, and the rainfall during
a consecutive series of dry years, ((f) the distribution of the
rainfall throughout the year, (e) the amount of water which
must be supplied for " compensation " purposes, and (/) the
facilities for obtaining storage.

The available watershed, of course, includes only that
portion of the whole watershed which feeds the river above
the point at which the water will be abstracted. This can
only be ascertained by actual measurement, though approxi-
mate estimates may be made from hydrographical maps on
which the river basins are defined.

The contour of the ground surface also affects the supply,
for upon this depends greatly the rapidity with which the
rainfall, especially when heavy, will flow over the surface into
the stream. The character of the surface and of the subsoil
will also affect the amount which will flow directly into the
river, and the amount which will percolate and pass into the
river at a lower level. All the above also will be factors in
determining the amount of evaporation, or, in other words,
of determining the available rainfall. The surface drainage
area does not always correspond with the true drainage area,
since there may be springs within the surface area fed from a
source without that area ; and, on the other hand, rain which
falls on the surface area may pass by underground channels



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92 WATER SUPPLIES

beyond the limits of the watershed. All these possibilities
have to be borne in mind, and the locality carefully examined
to ascertain whether such conditions exist, and to what extent
they will affect the water supply.

The way in which the rainfall in any particular district
can be ascertained has already been described. The minimum
rainfall for a year, or a series of years, can only be determined
from records continuously taken for many years ; but it is
found that, under ordinary circumstances, the maximum
rainfall exceeds the average by one-third, whilst the minimum
falls short of the average by the same amount. The mean
rainfall during the three driest consecutive years is usually
about one -fifth less than the average. Thus, where the
average rainfall for a series of years is 30 inches per annum,
the maximum will be about 40 inches, the minimum 20 inches,
and the mean for the three driest consecutive years 24 inches.
Where careful daily gaugings of a stream have been made for
a few years, the proportion of the rainfall finding its way into
it can be ascertained, and by calculation the amount which
would pass into the river, with the minimum rainfall, can be
approximately determined. The following table, compiled
from the 22nd Annual Report of the State Board of Health of
Massachwetts, shows the rainfall received and collected during
a series of years on the Sudbury River watershed.

During the sixteen years, 1875-90 inclusive, the average
rainfall was 45*8 inches. The calculated maximum rainfall
on this area is 61*1 inches, and the minimum 30*5 inches.
The observed maximum and minimum were 57*9 and 32*8
inches respectively. The mean rainfall for the three driest
years (1882-84) was 38*8, whilst the calculated mean is 36*6
inches, so that doubtless the calculated amounts will closely
approximate to the truth when the records for a much longer
period of years are available. The percentage of rainfall col-
lected does not vary directly with the rainfall, and neither
the smallest nor largest proportion collected corresponded
with the lowest and highest rainfalls ; but the results do not



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RIVER WATER



93



vary to such an extent as to render it difficult to determine
approximately the minimum amount available. The cause
of this variation is due in part to the seasonal variation in
the rainfall, and in part to the variation in the amount
evaporated.



Year.
1875


Rainfall.


Raiufall collected.


Per cent collected.


45*49


20*42


44-9


1876


4956


23-91


48-2


1877


4402


25*49


57*9


1878


5793


30-49


52-6


1879


4142


1877


45-3


1880


38-18


12-18


31-9


1881


4417


20-56


46-6


1882


39-39


18-10


45-9


1883


32-78


11-19


341


1884


47*13


23-78


30-5


1885


43*54


18*92


43-4


1886


4606


22*82


49-5


1887


42-70


24-23


56-7


1888


57*46


35-75


62-2


1889


49-95


29-06


58*2


1890


53-00


27-00


50-9


Mean for 16 yrs.


4580


22-67


49-5



A knowledge of the seasonal rainfall and the seasonal
variation in the flow of the stream is also absolutely neces-
sary, since upon these factors depend in a great measure the
amount of storage which will be required to collect the
water during periods of abundance for use during periods of
drought. During the sixteen years' records of the Sudbury
River, the mean daily flow during the month when the river
was lowest was only 60 gallons per acre of the watershed ;
during the driest three months it was 148 gallons ; during
the driest twelve months, 777 gallons ; whilst the mean daily
flow for the whole period was 1686 gallons. From these
records the reporters to the Massachusetts State Board of
Health have calculated a table showing the "amount "of
storage necessary to make available different quantities of
water per day from each square mile of watershed, where the



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94



WATER SUPPLIES



conditions are similar to those which exist at Sudbury River."
To obtain 100,000 gallons per day per square mile, the storage
reservoir must be capable of holding 2,200,000 gallons per
square mile of watershed; to obtain 1,000,000 gallons per
day, the reservoir must hold 540,000,000 gallons. For
intermediate quantities the original table must be consulted. 1
Of course these results can only be used where the conditions
which obtain resemble somewhat those of the watershed
under consideration. The following table for the river
Thames is calculated from data given in the Report of the
Royal Commission on Metropolitan Water Supply : —



Year.


Rainfall.


Rainfall collected.


Per cent collected.


1883


28-4


13-3


46-8


1884


22*9


70


30*8


1885


29-15


8-3


28-5


1886


31-1


11-1


35-7


1887


213


8-2


38-5


1888


28-45


8-9


31-3


1889


25-6


9-1


35-5


1890


22-8


5-7


250


1891


333


9-8


29-3


Average of 9 yrs.


27*0


9-05


335



If this table be compared with the corresponding one for the
Sudbury River, it is evident that a considerably larger pro-
portion of the rainfall is available from the watershed of the
Sudbury than from that of the Thames.

Mr. Beardmore calculates that during summer, the Thames,
Severn, Loddon, Medway, and Nene, which flow over a variety
of geological strata, only carry off less than one-eighth of the
rainfall, whilst the Mimram and Wandle, which arise in and
flow through chalk districts only, yield nearly half the total
rainfall. Certain rivers are much more constant in their flow
than others, the result depending chiefly upon the conforma-
tion of the watershed and the character of the subsoil. If
the stream be fed chiefly with surface water the variation
1 State Report 1890, page 342.



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RIVER WATER 95

will be very considerable, whilst if fed chiefly from the sub-
soil the flow will be comparatively uniform. All these factors,
therefore, have to be taken into consideration when estimating
the available supply and the amount of storage necessary.

Where there are riparian owners having a right to the use
of the water for any purpose, as for manufacturing, or as a
motive power, further complications are introduced. Sufficient
water must be allowed to pass down the river to satisfy all
their reasonable requirements. Only the amount in excess of
this can be appropriated, and as during seasons of drought
they may require the whole flow of the river, the impounding
reservoirs must be large enough to store water during seasons
of abundance sufficient to tide over these periods when none
can be collected.

The quantity of water which must be stored to equalise
the supply during the longest period of drought which may
possibly occur can only be determined when the average daily
demand is approximately known, and the whole of the con-
ditions above referred to have been carefully investigated.
The number of days* storage required varies in this country
from 120 to 300; the smaller quantity only being required
on the western side, where the rainfall is heavy and the
number of rainy days considerably above the average. In
the eastern counties, where exactly the opposite conditions
obtain, about ten months' storage may be necessary.

The amount of storage required may be calculated from
the rainfall statistics only, or from the stream gaugings, but
both must be considered if the result is to be reliable. The
gaugings may be effected by various methods : (a) by means
of sluices ; (b) by aid of current metres ; (c) by means of
weirs ; (d) by gauging the surface velocity. Where a rough
approximation only is desired, a straight portion of the stream
may be selected which is tolerably uniform in width and
section, and where the water flows smoothly, or where by a
little labour such uniformity may be produced. By plumbing
the depth at different points across the stream and measuring



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96



WATER SUPPLIES



the width, the cross section can easily be calculated. The
length of the selected portion, 20 yards or more, must be
marked off, and the time noted which it takes a chip or float
to traverse this length in mid-stream on a calm day. The
mean velocity of the whole body of the water may be taken
as 75 that of the surface velocity. These data are sufficient
to give the volume required.

For example, the area of a section of a stream is found to
be 45 square feet, and the time taken by a float in traversing
a distance of 60 feet is 80 seconds. Required the flow in
gallons per day.



45 x 60 x 75
80



= 25 "3125= flow in cubic feet per second.



25*3125 x 60 x 60 x 24 x 6*25 = 13,668,750 gallons per 24 hours.

The ratio of the mean to the surface velocity is not a constant,
and its value is variously estimated by engineers from the
results of actual experiments. It varies with the rapidity of
flow, the nature of the channel, depth of water, or form of
cross section, but the first named is probably by far the
most important factor. Mr. Beardmore adopts the formula
U = V + 2*5 - \/5V where U equals the mean, and V the surface
velocity per minute. This formula gives the following values
for U :—



Surface Velocity




Value of IT


in Feet
per Minute.


Mean Velocity.


in terms of V.


5


2-5


•5


10


5 5


•55


20


12-5


•625


50


36*5


•73


100


80*2


•802


200


170*9


•855



Where greater accuracy is required and the stream is large, a
current metre may be employed.

" Having fixed on the station where the cross section of a



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RIVER WATER 97

large river is to be taken and the velocities ascertained, take
a number of soundings across the stream, at 8, 10, or 12
points, according to the breadth. These lines of sounding
divide the section into a number of trapezia, and the area of
each of these is to be calculated. Then, at a point half-way
between each of the two lines of sounding, is to be fixed a
small boat containing the current metre (Fig. 10), by means
of which 5, 6, or 7 velocities are to be determined in the same
vertical line. The arithmetical mean of these is then to be



Fio. 10.

multiplied by the area of the trapezium to which they apply.
The sum of these products is evidently the discharge of the
river — it is equivalent to the total sectional area multiplied
by the mean velocity " (Hughes's " Waterworks," quoted from
D'Aubuisson's Traite d } Hydraulique a Vusage des Ingenieurs).
In artificially constructed channels of uniform cross section,
such as canals, culverts, and pipes (the two latter may be
running full, but must not be under pressure), various formulae
have been devised for estimating the flow from the fall per
mile and the hydraulic mean depth. 1 Beardmore's modification
of Eytelwein's formula is the one usually employed —

1 The hydraulic mean depth is the sectional area of the water divided

H



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9 8 WATER SUPPLIES

U = 55V2HH,

where U equals the mean velocity in feet per minute, R the
hydraulic mean depth, and H the fall in feet per mile.

Example. — In a circular channel of 2*5 feet diameter,
having a fall of 5 feet per mile, and running exactly half full
of water, what is the flow in cubic feet per minute ?

R=|. H = 5.U = 55V2x5xg = 137'5.

2*5 2 x *785
The area of a section of the water is " - « ~ = 2*453 feet.

This, multiplied by the velocity, 137*5, gives a yield of
337*3 cubic feet per minute.

Streams of any magnitude are usually gauged by engineers
by the aid of artificially constructed weirs. Theoretically
the velocity with which the water passes over the weir is that
which a body would acquire in falling through a distance
equal to the difference between the surface level of the water
above the weir and the surface of the weir itself. A body
falling from rest acquires at the end of one second a velocity,
g, which is approximately 32 feet per second. The mean
velocity at the end of any number of seconds, t, will be

^ = -^, the space traversed, *, in that time will be -X

and the velocity at the end of that period tg. Eliminating t,
we find that v 2 = 2sg = 2 x 32 x s, therefore

v=$\/s.

Theoretically, therefore, the velocity with which water passes
over the actual surface of the weir is eight times the square
root of the difference in level above referred to. But this is
the lowermost stratum of the water only, the strata above
having a less velocity, decreasing upwards as the square root

by the wetted perimeter. In circular pipes running full, S'lid equals the
wetted perimeter, and <f J, 785 the cross section of the water ; R therefore
equals Jrf.



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RIVER WATER 99

of the depth from the surface level The mean velocity of all
the strata will be that of the particles at § the depth of the
lowermost, therefore

Unfortunately friction has to l>e taken into account, and as
this varies with the shape of the weir, its width, etc., the
above formula has little more than theoretical interest.
Numberless experiments have been recorded and many
formulae deduced therefrom for weirs of different kinds.
Here, however, it is only necessary to refer to the one most
frequently employed, that derived from Mr. BlackwelPs
experiment made on the Kennet and Avon Canal on
the flow of water over 2 -inch planks. Let Q equal the
quantity of water flowing over the weir in cubic feet per
minute, then

Q = cw V* 3 .

Where w = the width in feet, * the depth of water in inches,
and c = a constant multiplier, found by experiment and
given in the following table (quoted from Slaggs* Water
Engineering) : —



Depth


$=1 inch


Value of


c = 3'50


>>


= 2 inches


>>


= 4 25





= 3 „


>>


= 4-44


,,


= 4 „


>>


= 4-44





= 5 „


»>


= 4-62





= 6 „


>>


= 4-57


>>


= 7 „


,,


= 4*61


,,


= 8 „


»>


= 4*48


> >


= 9 ,,


>>


= 4*44



For depths of 3 inches and upwards c may evidently be
taken as 4 5. As an example, it is required to calculate the
flow over a weir of 5 feet in width, the level of which is 6
inches below the even surface of the water.

Since s=6, c=4*5 and w=5
Q=4*5x5x \Zg3
Q = 333 cubic feet per minute.



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ioo WATER SUPPLIES

Under certain circumstances, as where a lock gate and
sluice are available, the flow may be determined from the area
of the sluice and the vertical distance between the centre of
the sluice and the level of the water in the stream. Theoretic-
ally the velocity of the water passing through the sluice would
be 8 Js, but from friction and other causes it is always less



Fig. 11.

than this. With very small sluices of from 1 to 16 square
inches area, Poncelet and Lesbros' factor, '62 may be taken
as approximately correct. If therefore the area of the sluice
A be known, the flow per second will be —

Q = A x *62 x S\/s = approximately 5 A\fs.

If A and s be expressed in feet, Q will be the flow in cubic
feet per second.

Where the river is of considerable dimensions, and it is



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RIVER WATER 101

desired to record the variations in the flow automatically, a
tide-gauge may be used (Fig. 11).

By aid of such an instrument the rise and fall of the float is
recorded on a revolving cylinder, so that not only the extent
of the variations, but the exact time at which they occurred
is registered.

Where the amount of water to be abstracted from a river
is very small compared with its volume, of course all these
elaborate investigations are unnecessary. In such cases also,
storage will only be required to supply the town during periods
when the river is in flood and the water turbid.

In exceptional cases only can river water be abstracted
at a point sufficiently high to supply a town by gravitation.
Usually the water is pumped into storage reservoirs, from
which it flows on to the filter beds, and it may again require
to be pumped after filtration into service reservoirs at such an
elevation as to permit of the water supplying the town by
gravitation. Service pipes *may be attached to the rising
main if houses have to be supplied en route. When pumping
is going on the flow will be from the pumping station to the
houses, but when the pumping ceases the flow will be in the
contrary direction, from the service reservoir. The water
taken from the Thames and Lea for the supply of the
metropolis is all pumped into service reservoirs in order to
obtain the necessary pressure, the height to which it is lifted
being on an average about 200 feet.

Limited supplies of water can be obtained from streams
having a good fall, by aid of rams, turbines, or water-wheels,
when the place to be supplied is at too great an elevation to
be supplied directly by gravitation. These automatic pumping
machines will be described in a later section.

A large number of towns in England derive their water
supplies from rivers. In the Tees valley, Darlington, Stock-
ton, Middlesborough, and several smaller towns are supplied
from the Tees ; Durham is supplied from the Wear, Carlisle
from the Eden, Ripon from the Ure, York from the Ouse,



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102 WATER SUPPLIES

Knaresborough from the Nidd, Leeds from the Wharfe and
Washburn, Doncaster from the Don, Wakefield in part from
the Calder, Ely from the Ouse, Newark from the Trent, 1
Leamington from the Leam, Shrewsbury, Worcester, and
Tewkesbury from the Severn (Gloucester also occasionally),
Plymouth from the Mew, Sandown (Isle of Wight) from the
Yare, etc. On account of the prevalence of typhoid fever in
certain of these towns (Stockton, Darlington, Middlesborough,
York, and Newark, for example) the possibility of obtaining
water supplies from other sources is being discussed. On the
other hand, certain towns are contemplating improving their
present supplies by resorting to rivers. Cheltenham, for
example, is completing works for augmenting its present
supply by drawing from the Severn at Tewkesbury. It is
now supplied in part by private wells, of which there are
over 2000, in part by spring and surface water collected in
reservoirs belonging to the town (this water when stored
has a tendency at certain seasons of the year to acquire a
disagreeable odour from the growth of vegetable matter,
chiefly Cham), and in part by the head waters of the Chelt,
which is also impounded in a reservoir. This reservoir will
hold 100,000,000 gallons, and is usually full to overflowing
about the end of March; it then loses water pretty continuously
until November, when again the feeders exceed the draught.
100,000 gallons a day have to be turned down the Chelt as
compensation water. The closing of surface wells, and the
increasing demand for water for water-closets and for flushing
sewers, and other municipal purposes, has on several occasions
run the reservoirs so low as to cause considerable anxiety.
There is within five or six miles of the town a perennial supply
of pure water from springs, which form the head waters of
the Thames, but Parliament has refused to allow them to be
diverted for the use of the town. In 1881 powers were
obtained for bringing water from the Severn at Tewkesbury,
and for supplying that town and the villages en route. The
1 Vide Chapter IX.



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RIVER WATER 103

severe drought of last year (1893) caused these works to be
proceeded with. The Medical Officer of Health says that
the water is wonderfully good, and the volume magnificent.
That it receives the sewage of several towns along its course
is acknowledged, but that there is any evidence of this pollu-
tion at Tewkesbury is denied. Worcester has taken its supply
from the Severn for forty years, and although the filtration
is said to be far from perfect, it has suffered nothing. This
town, however, pours its sewage into the river at a point
seventeen miles above the Cheltenham intake, and a mandamus



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