THE
DESIGNING AND CONSTRUCTION
ON
STORAGE RESERVOIRS.
BY
ARTHUR JACOB, B. A.,
LATE EXECUTIVE ENGINEER FOR IRRIGATION H. M.
BOMBAY SERVICE.
NEW YORK:
D. VAN NOSTRAND, PUBLISHER,
23 MURRAY AND 27 WARRN STREET.
1873.








THE DESIGNING AND CONSTRUCTION
OF
STORAGE RESERVOIRS.
Before entering upon such considerations
as affect the selection of reservoir sites and
their construction, a brief allusion to some
of the most ancient works for impounding
water may not be uninteresting. Of these
the most prominent examples are undoubtedly to be found in Hindostan, where the
magnitude and antiquity of the storage
works cannot fail to arrest attention. These
great works, surpassing in their immensity
what are conventionally esteemed to be the
wonders of the world, the production of
other countries and nations, took their origin in the necessities of the people and the
variableness of the climate of India, and




were, in fact, great public works on which
the welfare of the people mainly depended.
The climate of India, although singularly
uniform in some respects from year to year,
is remarkably variable as regards the rainfall; and in order to guard against the disasters of famine and sickness, inevitably
attendant on a scanty monsoon, *the native
princes were wont to make such provisions
as large resources and an almost unlimited
power enabled them, in order to obviate
the difficulty that they had to contend with.
The rain records of India for several
years past show that a scarcity of rain is
indicated by periods of about five years, or
that every fifth or sixth year is marked by
a scanty rainfall over certain districts. The
recurrence of these periods is, of course,
not very clearly marked, but still it is sufficiently so to warrant, with approximate
correctness, the prediction of scarcity and
famine; and such deplorable recurrence is,
as all are aware, now reigning in India,
and visiting with destruction, by sickness
and hunger, some thousands whose sole dependence is upon a fair season of rain, and




5
the successful maturing of their little crop
of grain.
The natural expedient for guarding
against the recurrence of these periodical
calamities was evidently to be found in
husbanding a scanty supply of rain-water
for the purpose of irrigation, and this the
people of India appear to have understood.
They took advantage, in certain districts, of
every nook and ravine, whether large or
small, and converted them into storage reservoirs by throwing across banks of earth,
or bunds, as they are termed, producing, in
certain districts, such an elaborate and
complete system of irrigation as can only be
compared, for cost and completeness, to our
railway system in England. Taking fourteen districts in the Madras Presidency,
where tank irrigation was most generally
relied upon, the records of the Indian Government show that there are no less than
43,000 irrigation reservoirs now in effective
operation, and as many as 10,000 more that
have fallen into disuse, making a total
number of 53,000 storage works. The average length of embankment is found to be




about half a mile, the extreme limit of the
series being a dam of the immense length
of 30 miles. This ancient reservoir, called
the Poniary tank, is no longer in use, the
cost of maintaining such a length of bank
in adequate repair having probably been
found disproportionate to the advantage
derived from the supply. The work, embracing an area of storage of between 60 and
80 sq. miles, remains however as a record
of what the Hindoos are capable of. To
quote a second example, there is the Veranum reservoir, now in actual operation as
a source of supply, and yielding a net revenue of no less than ~11,450 per annum.
The area of the tank is 35 sq. miles, and
the storage is effected by a dam of 12 miles
in length. In order to bring the immensity of this system of storage works within
the reach of statistical minds, it has been
calculated that the embankments contain
as much earth as would serve to encircle
the globe with a belt of 6 ft. in thickness.
To show that these are not singular examples, one other embankment of remarkable
size may be alluded to. This embankment,




of somewhat singular construction, was
built on the island of Ceylon, and bears
testimony that the' Singalese monarchs
were not behind their neighbors in public
spirit or enterprise. The embankment was
composed of huge blocks of stone strongly
cemented together, and covered over with
turf, a solid barrier of 15 miles in length,
100 ft. wide at base, sloping to a top width
of 40 ft., and extending across the lower
end of a spacious valley.
Thus it will appear that the practice of
embanking across valleys, for the purpose
of retaining the surface water, has for ages
been in operation. There is no doubt that
the disposal of some of the most remarkable works in India is not what it might, with
advantage, have been; the fact remains,
however, that the desired end was attained,
and if the earthworks were disproportionately extensive, it was a source of satisfaction at least for the projectors to know
that they cost, as a general rule, little or
nothing, the practice in those days being to
press whatever labor was required, rendering in return nominal wages or none at
all.




8
The two main questions that- it is proposed to submit for consideration are, first,
the selection of a reservoir site; and, secondly, the leading principles to be observed in the designing and construction of
storage works.
The purpose or purposes for which the
work may be required will, of course, affect materially the choice of a position, as
well as the details of the structure itself;
but certain general principles are available
for our guidance in every case, after considering which, it is proposed to dwell upon
such points as apply to the special purposes
for which reservoirs may be constructed.
The first and most essential point for accurate determination by the engineer is
undoubtedly the amount of rainfall, both
maximum and minimum, that may be expected in the district under examination;
and, having arrived at reliable data on this
point, the next consideration will obviously
be, what amount may be made available,
due allowance having been made for evaporation and absorption.  When we know
that the annual depth of rainfall taken all




9
over the world varies, according to the locality, between zero and 338 in. or 28 ft.
deep (which excessive amount was on one
occasion registered in the hill district of
Western India), it will be obvious how
little ground there will be for assumption,
in the examination of any district hitherto
unexplored, with regard to the question of
its rainfall.  In the examination of any
given country, however, there are certain
phenomena connected with the rainfall that
will be found of almost invariable acceptation, and may with advantage be borne in
mind.
The rainfall will, as a general rule, be
greatest in those districts that are situated
towards the point from which the prevailing winds blow. If Great Britain for instance be taken, the western districts will
be found the most rainy. The very reverse,
however, of this phenomenon is noticed in
the neighborhood of mountain ranges. If
the wind prevails from one side rather than
from the other, it is found that the greatest
rainfall is on the leeward side of the range,
and the probable solution of the matter is,




14)
that the air, highly charged with moisture,
is carried up the hills by the wind until it
comes into a cold region of the atmosphere.
Condensation of the watery vapor immediately takes place, and the result is a fall of
rain on the side of the mountain range remote from the prevailing wind.
To this cause may also be attributed the
fact that the rainfall is always greatest in
mountainous districts, while it by no means
follows that elevated plains are more abundantly supplied with rain than land lying
nearer to the sea level. The principles are
remarkably exemplified in the southern
part of the Bombay Presidency, where the
author has had occasion to study the subject of rainfall. The Western Ghauts run
parallel to the coast, rising to a height of
4,500 ft. above the sea, and form the western support of the great table-land of the
Deccan, the mean elevation of which may
be taken at 3,000 ft. In the rainy season
the south-west monsoon, blowing from the
sea, impinges against the ghauts, and while
passing onwards to the Deccan, parts with
its moisture to the average annual amount




11
of 254 in. On a spur of mountain that
runs eastward, the pluviometers are found
to register but 50 in.; and about 40 miles
farther inland the rainfall is not more in
some places than 15 in., which is considerably less than that registered in the lowerlying districts of the Presidency.
In civilized countries like our own much
valuable information is as usually available regarding the rainfall, if not applying actually to the district under examination, then probably to some neighboring districts, enjoying the same physical
characteristics; but when any project of
great importance is in contemplation, it will
not be sufficient to take the returns of adjoining districts as accurate information of
the rainfall at the exact locality fixed upon
for the construction of the works. It will
be necessary to establish rain-gauges at
different points over the catchment basin
of the valley from which it is intended to
obtain the supply; and daily observations
of these gauges must be taken for comparison with a series of simultaneous observations taken and recorded at the nearest sta



12
tion at which the rainfall has been regularly and carefully noted.  It is evident
that a comparison of the several observations taken over the area of water-shed with
those registered at the permanent station,
will convey a just estimate of the amount of
maximum and minimum rainfall that may
be relied upon.
The amount of rain falling upon the
ground is not, however, the point to be
determined, though it will aid considerably
as a guide to the engineer. A considerable
quantity of all the rainfall is either absorbed by the ground or evaporated before it
reaches the point at which it can be made
available for storage. Regarding, then, first
the question of absorption, it must be apparent that no two districts, unless they are
exactly identical in soil, inclination of surface, and under similar circumstances of
cultivation, can give on examination the
same comparative result of rainfall and
evaporation. If one district or unit of area
be similar to the other in all respects but
the surface inclination, that which has the
greatest slope will, as a rule, give the larg



13
est percentage of water available for storage, because of course there will be less
time for the rain to be absorbed. Again,
the degree of cultivation will materially affect the result when two areas, otherwise
precisely similar in their physical conformation, come to be compared one with the
other, it being evident that an open and
well-drained soil will be more favorable to
the retention of water falling upon it than
compact and impervious land. In every
case the physical features of a district will
each and every one of them force itself on
the attention, as influencing the conclusion
to be arrived at. If any general rule can
be applied, it may be said that the greater
the slope of the valley, the more rapidly
it will throw surface water off; the more
denuded the surface is of soil of any kind,
the less will the escape of rain-water be retarded; and the more compact the rocks
composing the geological structure of a district, the better will the circumstances be
for impounding water. The volcanic rocks
and those of the granite order will be as favorable as any that could be desired;




14
while, on the other hand, porous rocks,
such as the sandstones, chalk, etc., are too
absorbent to offer the desired conditions for
storage. It is not here asserted that all
the water absorbed by porous rocks is necessarily intercepted from passing away to
contribute to storage supply; much of it
may be lost by evaporation and absorption
by vegetables, but a considerable portion
will often be found to contribute in the
form of springs, if the disposition of the
strata be favorable.
As a further source of loss, evaporation
from the ground as well as from the surface of the reservoir, must be taken into
consideration. The circumstances attending
the latter source of loss will be considered
further on, as this does not affect the question of how much of the total rainfall may
be made available.
The question how much water will be
evaporated at any moment from the surface
of land is one involved in considerable difficulty; and so many disturbing elements
enter into the solution of the problem, that
its accurate determination may be-regarded




as hardly possible of attainment. The hygrometric state of the ground's surface, the
aspect of the sky, the amount of wind, and
the temperature, will all, in their degree,
exercise a sensible influence on the amount
of water that the ground will give off from
its surface; so that, in fact, it is doubtful
whether any reliable and philosophically
correct conclusions can be arrived at. The
resultant facts from  such experiments as
have been carefully conducted afford, after
all, the only data for the engineer to arrive
at any general conclusion by; and for forming a rough estimate for the probable available rainfall of a district, the following
proportions of available actual rainfall may
be accepted as furnishing general data; but
they are not meant to obviate the necessity
of a careful and specific examination of the
circumstances likely to affect the design of
any particular work:
Steep surfaces of granite, gneiss, and slate    100
Moorland and hill pzsture................ 60 to 80
Flat cultivated country................. 40 to 50
Chalk................................. 0 to O
In order to arrive at more specific, and
truly reliable results, the engineer will have




16
to make a series of accurate observations
on the discharge of the stream or streams
that carry away the rainfall of a district;
and by doing so, and at the same time
comparing the result with the amount of
rain registered by the gauges-which
should also, of course, be kept with accuracy in the locality under examination-an
approximately true estimate of the availaable rainfall will be arrived at.
If there is time in the preparation of a
project to make the necessary examination
of a district, it is evident that the results
will speak for themselves; and there will
be no necessity to enter into abstract speculations concerning the theory of the influences affecting loss by either evaporation or
absorption.
In proportioning the size of a storage
reservoir to the area of the catchment basin, the engineer will, of course, in the first
instance be guided by the requirements of
the work. The object of the undertaking
may be any one of the following:
To husband a scanty rainfall.
To check the injurious effect upon the
country by floods.




17
To add to the discharge of a stream, by
preventing the escape of the flood waters.
The amount of storage will always be
part of an engineer's data in designing
works. It will either be his object to store
the whole of the water that the drainage
area will afford, which will be the case in
impounding water for irrigation, for example; or a certain fixed demand, governed
by the want of a town or other requirements, will determine the amount of the
rainfall that it will be necessary to retain
for supply. In England the demand for
water supply may be reckoned at from 150
to 180 days, depending on the amount and
the constancy of the rainfall; as a rule, the
six months' supply will be the safest to
adopt. The following Table, extracted
from Mr. Beardmore's work, shows the proportions that have been observed in designing some of the best constructed reservoirs.
(See Table A.)
From this it appears that the proportion
between the amount stored and the total
rainfall varies between one-half and onefourth.




18
TABLE A.
Height
LOCALITY..             Heiht          C
above Sea. 
ft.    ft.    sq. m.
Bann Reservoiis, 1837-38...   400 to 28(0      5.15
Greenock, 1827-28, flat moor   512 to 1000     7 88
Bute, 1826................   200 to 350       7 80
Glencorse, Pentland Hills..   734 to 1600     6.00
Belmont, 1843, moorland...  850 to 1600        2.81
" 1844        "..........................
"     1845,.............................
"    1846, ".............................
Rivington Pike, 1847......    800 to 1545   16.25
Longendale     ".....    500 to 1800 
Swineshaw      "             500 to 1800
Turton and Entwistle, 1836   500 to 1300       3.18
"           "' 1837.................
B;,lton Waterwork........    800 to 1600.80
Ashton    "       1844....   800.59




19
TABLE A -Countinued.;..~
r4                        C
c..                                   r
pemr n.                           mill
600 0   100.0   22.3    3  0        766  3 10
630.4   224.3   50 7    63 4    2.8 
412.8   146 4   33 3    50 0..............
28800   176 7   40.0    5.5    29 6 
416.6   1981.3   41.0    60 2    38.0    2-5
548.2   18129    0412        55. 
411.3   146 3   33.2    49 8
548.2   125.23  32.7..2....]-3
40.7    65.5   15.5    40.0    21.0    4-7




The rule suggested by Professor Rankine " for estimating the available capacity
required in a store reservoir, that founded
upon taking into account the supply as well
as the demand," is probably the best that
can be adopted in designing waterworks for
the supply of a town; "for example, 180
days of the excess of the daily demand
above the least daily supply, as ascertained
by gauging and computation in the manner
above described." In order that a reservoir of the capacity "prescribed by the
preceding rule may be efficient, it is essential that the least available annual rainfall
of the gathering grounds should be sufficient to supply a year's demand for water."
In calculating the capacity of a storage reservoir, the consideration of the surface
evaporation must not be disregarded, especially when the works are designed for
tropical or very dry climates. The amount
of loss will in some cases be very considerable, for whatever depth of water be assumed to pass away into the air, it.must be regarded as extending over the whole surface
of the reservoir; or, in fact, the cubic quan



tity will be equal to the product of the
depth evaporated away and the mean surface area of the reservoir as the water rises
or falls throughout the year. Some have
gone the length of asserting that the amount
of evaporation from the surface of large and
deep bodies of water is probably nothing at
all, or, at any rate, not worthy of consideration; whilst others assume a much larger
amount of loss than appears to be supported by observation. The following extract
from  the article "Physical Geography,"
published by the Society for Promoting
Useful Knowledge, expresses intelligibly
the conditions that tend to promote evaporation:
" Other things being equal, evaporation
is the more abundant the greater the
warmth of the air above that of the evaporating body, and least of all when their
temperature is the same.  Neither does
much take place whenever the atmosphere
is more than 15 deg. colder than the surface upon which it acts. Winds powerfully
promote evaporation, because they bring the




22
air into continual as well as into closer and
more violent contact with the surface acted
upon, and also, in the case of liquids, increase by the agitation which they occasion,
the number of points of contact between the
atmosphere and the liquid.
" In the temperate zone, with a mean
temperature of 5'2 deg., the annual evaporation has been found to be between 86
in. and 37 in. At Cumnana, on the coast of
South America (N. lat. 10.), with a mean
temperature of 81.86 deg., it was ascertained to be more than 100 in. in the course of
the year; at Guadaloupe, in the West Indies,
it has been observed to amount to 97 in.
The degree of evaporation very much depends upon the difference between the
quantity of vapor which the surrounding
air is able to contain when saturated and
the quantity which it actually contains. M.
Humboldt found that in the torrid zone the
quantity of vapor contained in the air is
much nearer to the point of saturation than
in the temperate zone. The evaporation
within the tropics, and in hot weather in
temperate zones, is on this account less




23
than might have been supposed from the
increase of temperature."
Thus it appears that evaporation, under
highly favorable conditions, may take place
to the extent of 9 ft. in depth-an allowance that will demand careful consideration
in designing storage works. In India,
where from the extreme dryness of the atmosphere the evaporation is found to be
considerable, the usual allowance made by
engineers for the evaporation from the surface of storage reservoirs is at the rate of
I in. of depth per diem for eight months in
the year. Regarding the results that have
been arrived at in Bombay, this allowance
would appear to be about double what is
necessary, for the observations extending
over five years give a mean daily evaporation of less than 1 in. In Bombay, however, the atmosphere is much more humid
than that experienced on the great tableland of the Deccan; and in Madras, where
reservoirs are the specialty, it is probable
that the actual loss is not far from being a
mean between the two fractions. In Great
Britain the mean daily evaporation is found
to average less than the tenth of an inch.




24
In estimating the quantity of storage
water that will result from the drainage of
any particular district, it will be essential
to consider carefully the geological disposition of the strata characterizing the locality
in which it is contemplated to establish the
works. This, although a matter that may
influence the effectiveness of an undertaking
to the extent of success or failure, will appear to the purely practical man to imply a
degree of refinement that is uncalled for.
There will be no difficulty, however, in
showing that the geological conformation
of a district may be such as, on the one
hand, to materially contribute to the efficiency of a storage reservoir, or on the other to prove so defective that no engineering
skill or pecuniary outlay could remedy it.
A condition of geological structure perhaps
the most favorable that could be imagined
is that shown in Fig. 1. This diagram
represents a geological section taken at
right angles, or nearly so, to the axis of the
valley that it is proposed to convert to the
purpose of storage. This somewhat peculiar
structure is what is geologically termed syn



FIG. 1.
SECTION ACROSS SYNCLINAL AXIS.




26
clinal, the beds inclining away from the
axis of the valley, and is the result of an
upheaving force having taken place underneath the points of greatest elevation. Subsequent to the upheaval and consequent
displacement of the strata, the process of
denudation has taken place, cutting the upper beds, and leaving the outcrop exposed,
not only inside the basin, but in the adjoining valleys at O and 0. Now, it is evident
that if the highest ridges bounding the valley be taken to mark the line of watershed, and therefore limiting the area of the
catchment basin, it is possible that the estimate of the amount of supply may be found
far short of what the district will yield. A
certain proportion of the rain falling upon
the outcrop at the points 0 0 will be absorbed by such of the strata as are porous,
and the water, percolating through the bedding, till an impervious stratum is met
with, will find its way down the course of
the stratification, till it ultimately reaches
the reservoir in the form of springs, and
contributes more or less to the maintenance
of the supply. The converse of this condi



FIG. 2.
SECTION ACROSS ANTICLINAL AXIS.




28
tion of things will be readily understood by
reference to Fig. 2. It also represents a
section taken directly across the valley of
the proposed reservoir.  Here the strata of
the earth's crust incline against each other
consequent upon some disturbing force
having taken place to elevate them, dnd
are said to be anticlinal to the axis of the
valley. In order to account for the formation of a valley on the summit of the ridge,
that at first was thrown up, it is to be un.derstood that the upper beds suffered fracture in the process of upheaval, and subsequently were exposed to denudation. These
valleys of elevation are evidently not to be
desired as situations for the establishment
of storage reservoirs. The area of the
gathering grounds will be much more limited than the extent of the watershed would
appear to indicate; and cannot safely be
relied upon to give an estimate of the quantity of water that the valley will afford. A
certain amount of water will undoubtedly
pass over the surface in times of heavy and
continued rain, before it can be absorbed;
but there is no doubt that of all the water




29
absorbed by the ground, by far the greater
portion will follow the inclination of the
strata, and come out as springs in the adjoining valleys.
Fig. 3 shows a geological section that
combines in it favorable and unfavorable
conditions for the storage of water. On one
side the outcrops of the strata are found to
extend beyond the highest point of watershed line, whilst on the other side the strata
incline away, producing such a condition
as would favor the escape from the valley
of the water absorbed.
Certain rules are in general use for estimating the quantity of the total rainfall
that will be lost by absorption and evaporation, with a' view to determining the
proper proportion to be observed between
the reservoir and the area of the catchment
basin. Two-thirds of the whole fall is
sometimes taken to represent the loss that
may be expected from the drainage of any
district, in general terms, one-third being
assumed as the amount that may actually be intercepted for utilization. Some authors leave a much smaller margin, and




FIG. 3.
VALLEY OF DENUDATION,




31
state that fully two-thirds of the total rainfall may fairly be taken as available for
storage. This is a large discrepancy when
the application of the rules is taken to be
general; but when the statements are applied to separate districts and different
countries, there is nothing irreconcilable in
them. General rules are undoubtedly of
much value if they be received with qualification, and are not adopted as of absolute,
ly universal application.  They  cannot,
however, with safety be substituted for
specific investigations, when so much depends on starting with accurate data.
RESERVOIR SITES.
The special requirements of each particular case will, as a general rule, go far
towards determining the selection of a site
for the establishment of storage works.
Assuming, however, that there is a considerable extent of country situated advantageously in relative position to the locality
at which it is proposed to utilize the water,
and that there is a choice of ground, the
point to be considered chiefly will be the




32
nat ral lie of the country. To throw an
embankment across a valley at any point
without due regard to the configuration of
the ground would most probably result in
an expensive and ill-designed scheme;
for under such circumstances the cost of the
dam would bear a very large proportion to
the quantity of water stored. It will rarely
happen that, in the examination of the resources of any particular piece of country,
some special features will not present themselves, favorable to the situation of storage
works. The most advantageous disposition
of the ground will be when two spurs of
high land approach each other, forming a
narrow outlet for the stream, and leaving a
wide space above above them in the valley
for storage. Such a configuration is not
uncommonly met with at the junction of
two streams, as shown in Fig 4. This is
merely a sketch from memory, by the
author, of a reservoir that he designed in
India for purposes of irrigation; and it will
be evident that the disposition of the
ground was.singularly favorable in every
respect for the construction of a large




FIG. 4.
IRRIGATION RESERVOIR.




34
storage work. The area of the reservoir,
as designed, was about three square miles,
and the maximum depth 90 ft., the area of
the catchment basin being about 60 square
miles. Such favorable situations for storage
are of somewhat rare occurrence; for when
the contour of the land is what is desirable,
it may be that the area of water-shed is
not adequate, or possibly the geological
condition of the ground may be unfavorable,
or the materials for the construction of a
sound bank are not available. In examining large tracts of country in India, with a
view to the establishment of irrigation reservoirs, the author found that more reliance was to be placed on' a careful examination of the map in the first instance,
than on the common plan of making personal explorations of the country. A good
map will show at a glance, especially if the
hill-shading has been carefully engraved,
the points at which the supply will be found
sufficient to justify the undertaking; and
will probably furnish a pretty true indication of sites at which embankments may be
advantageously constructed.




35
In tropical climates, where the rainfall is
in places very scanty, and where the land is
not of great value, it not unfrequently happens that such situations prove available
for the establishment of large storage
works as would not under any circumstances be made available in England. These
sites are to be found, not at the head of a
valley, but at some considerable distance
down the course of a stream, where, the
general inclination of the country being
slight, a low embankment serves to store a
very large area of water. The apparent
disadvantages of such a site for storage are
the large area of land swamped and lost to
the cultivator and to Government, and the
great surface exposed to evaporation under
a tropical sun and the influence of a dry
wind. In India, the first objection is one
of comparatively little moment, considering
that in those districts where irrigation is
most required the value of land is very
trifling. From Is. to 2s. is about an average
rent per acre, where land is under dry
crops; but when.water is available, the
cultivators can, with profit, afford to pay




36
30s. per acre. It is therefore evident that,
so far as Government is concerned, there is
no sacrifice in the matter, but, on the contrary, an unspeakable benefit is conferred
on those landowners who hold farms below
the reservoir; and an ample supply of water
is stored in the dryest seasons to mature
those crops whose failure almost inevitably
reduces the people to the verge of starvation. The evaporation from these lakes is,
beyond question, a source of very considerable loss, and one that admits of no possible
abatement.  Estimated as above, at about
half an inch vertical for eight months of the
yPar, the loss frequently amounts to onethird of the whole body of water stored.
As a set-off against this and other objections, the facilities for constructing these
reservoirs of great extent, are considerable.
In the first place, the embankments, being
very low, are rapidly and cheaply constructed by native workmen; and when finished,
the head of water even at the deepest point
is not sufficient to try the work to any great
extent. Further, the greater the extent of
the reservoir, the less inconvenience is ex



perienced from silting. The streams, owing
to the suddenness of the rainfall, come down
heavily charged with earth in suspension,
the mass of which is deposited like a miniature delta at the influx of the reservoir, instead of passing on and resting near the
embankment, as invariably occurs in reservoirs of small extent. The immense consumption of water necessary to confer any
appreciable benefit by irrigation is of itself
the strongest argument in favor of these
broad and shallow reservoirs; for it is not
possible to find in the upper part of a valley
such sites as would store the requisite quantitv of water without an embankment of
excessive dimensions; and moreover, the
catchment.area in such situations is not
usually sufficient to serve, with a scanty
rainfall, for the supply of a very large reservoir. It is not, of course, maintained.
that this mode of storing water is by any
means applicable in England, for the cir —
cumstances and requirements in each cases
are wholly dissimilar.




38
SUPPLY.
The reservoir site being supposed everything that could be desired, as regards the
disposition of the ground, the supply will
next engage attention as a matter of course.
Assuming that the gathering grounds
are sufficiently extensive, it is presumed
that the reservoir will be constructed to
contain sufficient water to meet the maximum demand, whatever that may be calculated at; and in order to determine with
accuracy what capacity the reservoir will
have with different heights of embankment,
it will be necessary to carry out certain levelling operations over the ground. The least
elaborate manner of proceeding will be to
run a series of cross-levels through the
valley, referring all to the same datum, and
by comparing these levels to ascertain what
the average depth will be for a given height
of bank. Having decided the height of the
water-level, the next operation will be to
contour round the basin, and to survey the
boundary-line. In this way may be acquired sufficient knowledge as to the storage
capacity, to justify the procedure with the




work. When the execution of the project
has been determined upon, it will be advisable to make a more accurate survey of the
bed of the valley, and this can best be done
by covering the whole plan with a series of
contour lines at a vertical distance from
each other of about 5 ft. This kind of
survey will be of lasting value to the engineer, for it will enable him to calculate
what quantity of water the reservoir will
contain at each foot of depth; and, consequently, he will know, from a mere inspection of the gauge in the reservoir, how
much water he has at his disposal for
service.
It has been assumed that the gathering
grounds are sufficient to maintain the requisite supply in the reservoir; but it may
be well to pause and inquire what extent of
water-shed will be sufficient to furnish a
given supply, and what method may be
adopted for supplementing an insufficient
drainage area. It has before been remarked that the only reliable information, when
there is any question as to the sufficiency
of the rainfall or the area of the catchment




40
basin, canll be derived from careful gaugings
of the stream or streams that may be depended upon to contribute to the supply. If the
catchment area is very large as compared to
the capacity of the reservoir, a mere inspection of the map and an exploration of the
ground will generally be conclusive as to
the sufficiency of the supply for storage.
Should there not be such conclusive evidence on this point, it must be determined
by measuring the quantity of water that
absolutely flows off the ground, at the same
time gauging the rainfall. This latterprecaution would appear unnecessary, but in
truth it is of great value, for it will furnish,
by comparison with the rainfall registers
that have been kept through the same year,
and a series of previous years, evidence as
to the amount of available rainfall that may
be expected during terms of comparative
drought. If the supply of a town with
water be the desideratum, the rule to be
rigidly observed is that of making a minimum supply meet the maximum  demand,
and therefore it is of the highest importance to determine beyond any doubt, what




41
the minimum yield of a catchment basin
will be.
As a mode of supplementing an insufficiently large drainage area, catchment drains
or feeders have frequently rendered good
service. These are cuts that are carried
outside the water-shed line to arrest the
surface drainage and catch the contributions of small streams, and conduct the
water into the reservoir. The greater the
area enclosed between catchment drains
and the water-shed line, the more valuable
will they be as aids to the supply of the
reservoir. They of course virtually extend
the area of the catchmnent, adding so many
square miles or acres to the rainfall.
DESIGNING OF WORKS.
Knowing the exact requirement of a
given population, or rather having fixed,
after every consideration, the daily consumption of every individual that it is proposed to supply, there will be no difficulty
whatever in proportioning the reservoir to
the demand upon it. It is sometimes necessary, however, to provide reservoirs for




42
the purpose of preventing damage to'the
country by floods, and in' this way the inconvenience and injury  naturally  consequent upon very sudden and excessive falls
of rain may be to a great extent obviated.
The duty of the reservoir will be to arrest
all water in excess of what the stream can
carry within its banks, and to dispose of this
excess water, so to speak, in detail, after the
excessive rainfall has become moderated.
A comparison of a stream's discharge, taken
at highest floods, with the quantity that it
can carry without overflowing its banks will
show the excess that has to be retained by
the reservoir; and these data can only be
arrived at through a carefully kept record
of the extent of the floods and of their duration. The maximum flood in this consideration will not be that which rises to
the greatest height for a short time, but
will be the product of the excess above
what the river can discharge by the length
of time the flood lasts; which will, in fact,
be the necessary capacity of the reservoir.
The Table given on a preceding page will




43
afford an interesting study when compared
with the following Table, extracted in part
from the same work. The first gives a
comparative view of the volume of water
gauged and stored in small hill districts,
the last column indicating the proportion of
the total available rainfall to the amount
actually intercepted for storage. The following Table shows the ordinary summer
discharge of various rivers, streams, and
springs, as unaffected by immediate rain.
(See Table B.)
Where the reservoir is designed to check
the injurious effects of floods, the proportion of the storage to the rainfall will, in
most cases, be much smaller than what
would be necessary to provide for the
better part of a whole year's fall of rain.
for it is not probable that the maximum
known flood can ever exceed the amount
that it would be necessary to store for
economic purposes.
PROPORTIONS OF BAUNK.
The proper proportion to be given to an
embankment for the support of water is a




44
TABLE B.
Height
RIV'RS.                 above Sea.
Valley Hill.
ft.   ft.
Thames at Staines, chalk, greensand,
Oxford clay, oolites, etc.........    40 to 700
Severn at Stonleebench, silurian.....    400 to 2600
Loddon (February, 1850), vreensand.   110 to 700
Nene,  at  Peterborough,  oolites,
Oxford clay and lias............    10 to 600
7limram. at Panshanger. chalk....   200 to 500
Lee, at Lee Bridge, chalk (Rennie
April, 1796).......          30 to 600
Wandle, below Carshalton, chalk..      70 to  350
Mecdway, dryest seasons (Rennie,
1787), clay......................
Ditto, ordinary summer run(Rennie,
1787)...........................................
Verulam, at Bushey Hall, chalk....   150 to 500
Gade, at Hunton Bridge, chalk.....   150 to 500
Plvm, at Sheepstor, granite........    800 to 1500
Wo(udhead Tunnel, millstone, grit..      1000
Glencorse Burn...................   750 to 1600
1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~




45
TABLE B -Continued.
_        H.
sq.      c. ft.    c. ft.      in.    in.
miles.   per min. per min.
3086      40,000    12.98      2.93    24.5
3900      33,111     8.49      1.98
221.8      3,000    13.53      3.01    25.4
620.0     5,000      8.45      1.88    23.1
50.0      1,200     2.4       5.5      26.6
570 0;8,880    15.58      3 b3
41.0      1,800    43.9       9.93    24.0
481.5      2,209     4 59      1.04.....
481.5      2,520     5 23      2.19
120.8      1,800    14.9       3 37
69 5      2,500    36.2       8 19
7.6       500    71.4       15.10    45 0
139. 46 0
6.0       130    21 6        4.9      37.4




46
question that appears to admit of a good
deal of difference of opinion, some designers
taking one view, some another, of the proper theory that is to determine the dimensions of a bank. Some few, with whom
the author cannot agree on this point,
maintain that a bank ought to be designed
with strict reference to its theoretical power
of resisting hydrostatic pressure, or the
effort of the water to displace it. Regarding
the question in its abstract form, it will be
evident that any structure intended to sustain the pressure of water may be supposed
to fail in one of two ways-either, in the
first place, by yielding to the horizontal
pressure of the water and overturning, or
by progressive motion, i. e. sliding on its
base. In considering the first theory, that
of resistance to overturning, the easiest
method of examining the question will be
to take a simple example of a vertical
rectangular wall, and ascertain what power
it exercises to resist the pressure of water.
The pressure of water upon any plane
surface immersed is known to be equal to
the area of that surface, multiplied by the




47
depth of its centre of gravity below the
level of the water and by the weight of a
unit of water. Generally speaking, the
unit adopted in calculations is a foot; and
the unit of water being taken at a cubic
foot, weighing 62.5 lbs., the resulting product from the multiplication of the three
quantities will give the pressure in pounds
on the surface immersed. Let it be supFIG. 5.
D          E
posed, for simplicity, that water to the
depth of 10 ft. has to be sustained by a
vertical rectangular wall, as in Fig. 5. It
is usual to take but 1 ft. length of the




48
wall for the calculation, though it will not
affect the result whether 1 ft. or 100 ft.
be the length assumed. We then have
the surface under pressure = 10 sq. ft., the
depth of the centre of gravity - 5 ft., and
the weight of a cubic foot= 62.5 lbs., the
product of which quantities gives us 3,125
lbs. pressure on 1 ft. length of the wall. But
this pressure is not the whole of the force
that the wall has to resist; the leverage
that it exerts must also be taken into
account. In the example under consideration-viz. that of a vertical plane with one
of its sides coinciding with the surface of
the water, as in Fig. 5-the whole of the
pressure is so distributed as to be equal to
a single force acting at a point one-third of
the depth from  the bottom.  Thus, the
total force to be resisted by the wall is
3125 X 3.33  10,406, which is the moment
tending to overturn the wall.
It is evident that a certain weight of the
wall must be opposed to this overturning
force; and as the height of the wall and
the length are determined quantities, the
thickness alone remains for adjustment.




49
But as a rectangular wall in upsetting is
considered +o turn upon a single point, F,
in *the.Fi-r e-viz. the outer line of the
wall-there will be a certain amount of
leverage to assist the wall in resisting
the pressure of the water. This leverage
is the horizontal distance of the centre of
gravity of the wall from the turning point,
F, and when the structure is rectangular
and vertical, it is equal to half the thickness. The amount of the wall's resistance
will then be equal to the number of cubic
feet in one foot of its length multiplied by
the weight of a single cube foot of masonry
and by half the thickness of the wall.
Taking w = weight of a cubic foot of water
62.5 lbs., wl —=weight of a cubic foot
of brickwork, say 112 lbs., x =thickness
of the wall, and h=the height, the conditions of simple stability will be fulfilled
when
wlXhXX      =WXhX-.....    (1)
w'hx2  wh3
2     6
and solving for x, we get




50
= w'2. (2)
the thickness of the wall - 4 ft. 4 in.
A simple example has been selected for
illustration; but of course a rectangular
section of wall would not be found generally applicable in practice, nor would it be
convenient to limit the dimensions of a
retaining wall of whatever kind to the
minimum  that would sustain the pressure.
If this principle of calculation be applied to
ascertain the stability of a bank of earth
with long slopes of 2- or 3 to 1, it can
easily be shown that in every case the
resistance of the bank to overturning is
greatly in excess of the horizontal leverage
exercised by the water sustained.
The only theory, then, in any degree
tenable, is that assuming a bank in yielding to the pressure of water to slide on its
base. In order to conceive how this can
apply, it is necessary to assume the embankment to be a rigid body resting, for a
given length of its section, on a horizontal
plane; and without any adhesion, or a very
small fraction, existing between the sur



faces pressed. The amount of the friction,
however, is just the point upon which the
whole matter hinges, and until it has been
ascertained that the surfaces of earth that
are carefully incorporated with one another
have any such thing as a co-efficient of
friction, it is idle to to pursue the investigation by a mathematical mode of reasoning.
The conditions of stability will be satisfied
when the horizontal component of the
water's pressure against the bank will
equal the weight of the bank, plus the
vertical pressure exercised by the water to
hold it down and multiplied by the coefficient of friction; but nothing is known
of this co-efficient, and consequently the
equation remains incapable of solution.
As a matter of fact, embankments do not
slide bodily forward on their base when
they' fail, but give way from other causes
than mathematical reasoning can supply.
Landslips, it is true, to some extent support
the principle that maintains the sliding of
embankments; but, here, the circumstances
are widely different. Landslips either take
place when a mass of earth rests upon an




52
inclined surface of rock, with an ample
supply of water to lubricate the surfaces in
contact, or else they are the result of cutting
or embanking earth to a higher slope than
the material will stand at; the infiltration
of water also in this case is the chief agent
in producing the effect, acting as a lubricant,
and causing the earth to assume its natural
slope. In each case the surface of separation is an inclined plane, an element that
does not enter into the question of the
stability of embankments, by either of the
modes of reasoning above referred to. The
principles that direct the design of embankments to retain water are not those that
apply to the calculation of the forces to be
resisted or the means to overcome them,
any more than breakwaters and harbor
walls can be designed on mathematical
principles. The whole question naturally
turns on what slope the material composing
the bank will stand at. If earth could be
got to remrain at a slope of 1 to 1, even
though the embankment had no thickness
whatever at top, it would be amply sufficient
in weight to uphold the water in a reser



53
voir. This, however, cannot be accomlplishel
without the assistance of retaining walls,
which would be found in most cases much
more expensive than the additional earth
required to increase the slope to the angle
of stability; and therefore the section is
so disposed that the earth shall stand both
inside and outside the reservoir at such a
slope as will be under all circumstances
permanent. These slopes have been determined by long practice and by success and
failure in pre-existing instances-that is to
say, the limits have been laid down, for it
is not to be assumed that all descriptions of
earth will fall to exactly the same slope
when exposed to the constant action of water
or weather. Earth when subjected to the
contact of water almost invariably loses a
certain amount of its stability, and therefore it is usual to give the inner side of an
embankment a longer slope than the outside. In most of the best existing examples
the inside slope of the bank is either 3 to 1
or 21 to 1, and it is rare to meet any departure from this rule. The outside slope
may be designed at from 2 to 1 to 3 to 1,




54
depending upon the character of the material,
its power of withstanding the erosive action
of the air, and the means used to protect
the surface from being washed off or from
crumbling away. In designing embankments, the impermeability of the earth is a
matter that cannot be relied upon. There
are, it is true, innumerable embankments
now standing that have never allowed the
escape of a drop of water from the
reservoir, although no special precaution
was taken to make them water-tight. Of
these India abounds with examples, the
introduction of a puddle wall being in the
older embankments of very exceptional
occurrence. The earth was merely dug out
close at hand, and carried by the workpeople in baskets on their heads to where
it was deposited, without any regard to the
mode of disposing the material.  The
author has had occasion to construct a
considerable length of levee, or embankment, on this simple plan for the protection
of the country from the flooding of a river;
and although, so far as he is aware, no
flood has yet taken place to test the work,




55
he has, from the study of existing examples,
entire confidence in the result. The earth,
so far as practicable, was disposed in layers,
and before each was completed it was
thoroughly consolidated by the tread of
the workmen. It is not suggested that the
puddle wall should be dispensed with in
designing embankments, for the additional
degree of safety, in most instances, will
more than compensate for the extra expense
it entails; but, in low embankments made
of good retentive clay, the precaution of
puddling is by no means a necessity.
In most of the best examples of embankments in England, the practice adopted has
been to carry up the earthwork in layers of
2 or 3 ft. in thickness, disposed in the manner shown in Figs. 6 and 7, and at the
same time to construct in the centre of the
bank a wall of well-puddled clay, the foundation of which is carried down for whatever depth may be necessary in order to
reach an impermeable bed of earth or rock.
It is not in all situations possible to procure earth exactly suitable and in sufficient
quantity for the construction of an embank



~  \'' I,, 




No. 2.
~0oI                         tI.
-   AI  I I   T
/t        /                      _     _




58
ment, and consequently it is usual and advisable to dispose the best part of the material-that is, the most retentive of water
-in juxtaposition to the puddle wall, as
indicated in Fig. 6. In this example, the
selected material is disposed equally at either side of the puddle; but, as its function is to withstand the admission of water,
it would probably be more consistent, though
less in accordance with practice, to place all
the selected material on the inner side.
The practice of excavating the earth for an
embankment from the inside of the reservoir is one that should not be followede
without caution.  Removing so large a
mass of material would, no doubt, give a
considerable increase of storage room; but
sometimes the bed of a reservoir is covered
by a layer of impervious clay that is of immense value, and if this be cut through
or removed, it is quite possible that a bed
of porous material may be met with sufficient to allow the escape of water when it
comes to be admitted. In specifying for
the dimensions of the puddle wall, a sound
rule for adoption is, that it shall have a




FIGI. 7.
-— i   -_-.
I~~I ~     ~         L                                   __ —---—;~~,,, -Xr A..-!;   -  -r__                --                              __
SECTION 0OF MBANKMENT-RIDDEFORD WATER-WORKS.




6(}
thickness of 10 ft. at the top water-line and
increase in thickness to the surface of the
ground at the rate of 1 in. on each side for
every foot of height. Before any excavation is commenced, it will be essential to
make a sufficient number of borings to ascertain the nature of the soil beneath the
surface.
It may here be mentioned that professional men are not apparently agreed as to
the principles to be kept in view in constructing reservoir embankments; and this
want of concurrence never was more apparent than in the discussion that followed
the destruction of the Dale Dyke reservoir,
near Sheffield. Fig. 8 shows a plan of the
embankment site after the catastrophe.
The bank was 95 ft. high, with slopes of 22
to 1, and a top width of 12 ft. The puddle
wall was 16 ft. in width at the ground-line,
and tapered to 4 ft. at the top of the bank.
This embankment, with the exception of
the puddle wall, was composed of rubble
stone and shale; an additional price having been given by the engineers to insure
the use of the former material; which




FIG. 8.
SHEFFIELD RESERVOIR.




62
proves, at any rate, that this mode of construction was adopted on principle and not
through ignorance or mistake. From the
evidence given by the engineers of the company, it appears that it was, in their opinion, desirable that the inner part of the
embankment should be permeable to water,
because earth was much more likely to subside and slip than an open and less yielding
material like stone. This mode of construction implies that the puddle shall be fully
sufficient of itself to resist the passage of
water, and that there is no necessity to relieve it of any part of the pressure against
it. Of course, if a bank be composed of
open work, every point in the face of the
puddle is exposed to the full and direct hydrostatic pressure; and if at any point there
is the smallest fissure or imperfection, the
water has full power against it, and will, to
a certainty, take advantage of such point to
breach the dam. The assumption, then, of
the constructors of this and the Agden reservoir evidently was that a puddle wall of
some 25,000 sq. ft. of area was to be constructed without an imperfection of any kind,




63
or a single weak point in the whole surface.
The obvious reason for employing puddle at all, in embankments, is to thoroughly close up any imperfection that may occur
in the earthwork; it is in fact merely an
accessory, and cannot be relied upon of itself to secure the embankment against destruction. If an embankment be constructed of good sound earthwork, properly executed, it is highly probable that the water
may never penetrate half way through to
the puddle wall, and probably, in the majority of examples, has not done so. Earthwork, however, is not always executed
without imperfection; some decomposable
material may be introduced, which, in
course of time, dissolves, leaving a fissure;
one part may be at first less consolidated
than another, and, subsiding, lead to imperfection; or an embankment, be it ever so
well constructed, may be burrowed through
by moles, rats, and other vermin. It is to
meet the first two of these sources of imperfection that puddle is used; and if, by such
fissures as may occur in ordinary earth



64
work, water is admitted as far as the puddle wall, it can only exercise pressure
against it at a few points, the puddle and
earth being, in good work, so bonded and
incorporated with each other that there is
no space left for the water to occupy and
press against the surface. Most who have
read the account of the disaster that occurred in March, 1864, at Sheffield, will recollect how singularly conflicting the professional evidence on that occasion was. Some
of our first engineers were ranged against
each other in order to satisfy the public as
to whether the failure of the embankment
was attributable to bad engineering or to a
landslip; and although the impression
finally remained on the public mind that
" there was not that engineering skill and
attention to the construction of the works
that their magnitude and importance demanded," the engineers were fairly divided
in opinion as to the cause of the disaster.
One section pronounced, without qualification, that the embankment gave way in
consequence of a landslip, and entirely
ignored the fact of the embankment being.




65
defectively constructed; whist the other
gentlemen gave their verdict dead against
the company, and their mode of constructing water-tight banks. The two diagrams,
Nos. 6 and 7, may be taken as indicating
the system of constructing embankments
most generally approved of. The puddle,
as will be observed, is carried up to the
natural surface of the ground without any
batter, and from that point slopes on each
side to the top of the bank; on either side
of the puddle is disposed, in concave layers,
the most sound aud retentive part of the
material, and outside of all comes the ordinary earthwork.
As a security against the eroding action
of the water, and also against the inroads
of vermin, the most desirable, as well as the
most usual practice, is to pitch the whole of
the inner face of an embankment with
stone, carefully laid by hand. Neglect of
this precaution has led to the destruction of
many embankments in other respects securely constructed, and even when ample
height of bank above the surface of highest
water was provided. In all ordinarily in



66
clement weather the disturbance of the
surface of a reservoir amounts to no more
than a mere ripple; but when the surface
is of large extent, and a severe storm blowing, the waves produced are such as to
cause reasonable apprehension, and, in fact,
have, before now, overtopped the bank and
cut it down, till the water flowed over and
caused the destruction of the work. In
most cases, it will be necessary to leave
about 5 ft. between the level of the highest
water and the top of the embankment, and
never less than 3 ft.
A. mode of construction not very generally used, but apparently consistent with
reason, is that shown in Fig. 7, the embankment for the Bideford WYaterworks.
It consists in covering the whole of the
inner face with a layer of puddle, with sometimes a layer of peat outside it. On some
occasions it has been thought desirable to
mix with the puddle a quantity of small
stones or furnace cinders, by way of ob-.struction to vermin-a precaution that is by
no means unnecessary. As an instance in
point, the author is reminded of a masonry




67
dam in India that had to be pointed every
year regularly, because the fresh-water
crabs in the reservoir found it convenient
and promotive of their development of shell
to appropriate the mortar to their personal
use. The joints were cleaned out as effectually at the end of each monsoon as if the
work had been done to order.
The preparation of the foundation for an
embankment is a matter requiring some
care. The soil, consisting of grass, roots,
etc., and other matters of a decomposable
nature, should be carefully removed over
the whole surface to be covered by the
bank, and if any porous material, such as
sand or gravel, be present, it must be removed, until a compact and water-tight bed
is arrived at. The bank must, in fact, be
in contact with some sound and reliable
material that will not admit the passage of
water.
APPENDAGES OF RESERTOIRS.
Under this heading may be considered:
The whole apparatus for allowing the
water to escape, including the pipes, the
valve tower, and the culvert.




68
The waste sluices.
The waste weir or by-wash.
The most economical mode of discharging
water from a reservoir is through a single
pipe passing either through the embankment or immediately under it; but this
plan cannot, under any circumstances, be
recommended, though it is some times found
in existing examples. It is open to several
grave objections, the principal of which,
perhaps, is that the failure of a joint under
the embankment from unequal pressure, or
from whatever cause, will probably produce
the destruction of the embankment, or aEt
any rate, entail a serious interruption to the
supply, by the reservoir having to be
emptied in order to repair the pipe. Buried
in or under an embankment, a pipe is completely out of reach and out of view, and
may be in a very defective state without its
being possible to detect the imperfection.
In order to secure the satisfactory working of a reservoir as a source of constant
supply, it is essential that the outlet pipes,
valves, and all other appendages for controlling and regulating the escape of the




69
water, should be accessible for inspection
and repair.  The usual mode of accomplishing this is to carry the pipes out
through a culvert of brick or masonry of
sufficient dimensions to admit a man. This
culvert communicates with the valve tower,
as shown in Figs. 6 and 7, so that there is
a complete communication between the outside of the reservoir and the inside. When
unavoidable, the culvert is carried straight
under the embankment in the natural
ground; but the safest and most generally
approved mode of construction is to bring
the culvert round the end of the embankment, where it will be out of reach of injury from unequal settlement; a source of
no small apprehension when either culvert
or pipes alone are carried under the bank.
Where possible, it is an excellent plan to
run a heading through the solid ground,
lining it with brickwork and puddling it,
forming a tunnel entirely independent of
the embankment. The principal objection
to carrying either the culvert or pipes
through or under the bank is their liability
to fracture from the unequal settlement of




70
the earthwork. It would appear that their
liability to damage cannot with certainty be
insured by any reasonable depth of excavation, and is, therefore, generally disapproved
of by the best authorities.
In the best constructions the culvert is
situated half way or two-thirds up the embankment, and in such case the outlet pipes
for drawing off the - water in the reservoir
act as syphons when the water surface has
fallen below the culvert. Fig. 6 shows a
plan, as well as a cross section, of a reservoir dam designed for general application
by Mr. Rawlinson. Here the bottom of the
culvert is about 25 ft. above where the inner slope of tile embankment intersects the
ground at the lowest point. The syphon
pipe is also shown passing through the
culvert; the horizontal culvert is connected
with a shaft inside the embankment, in
which are placed the valves for leading off
the supply from the reservoir. The valves
are made to be closed on the inside by
valve spindles and screws, and the inlet
pipes are closed on the outside by plugs
which can be applied from the top of the




valve-tower. Thus the engineer has full
command of the whole of the outlet works;
all the pipes and valves are easily accessible
and under perfect control, so that the supply
can at any time be arrested for the repair
of any derangement that may occur, even
to the removal and replacement of all the
pipes. The inlet pipes are shown in this
example, as well as in Fig. 7, fixed at
different heights in the valve-tower, the
object of which is to draw the supply from
the reservoir from points near the surface.
The outlet pipe, passing through or under
the embankment, may be connected on the
inside of the reservoir by a flexible joint
with another pipe of the same diameter, to
the upper end of which is attached a float.
This pipe is movable in a vertical plane,
being controlled from lateral motion by the
guide-posts. Such an arrangement admits
of the water being drawn off from the surface, where it is least liable to be contaminated with impurities. Whatever arrangement be selected for drawing the supply
off from a reservoir, the system of carrying
the pipes, either with or without a culvert,




72
through or under the embankment, cannot
be sufficiently deprecated; they are, in
such a position, beyond the reach of inspection, and, moreover, are very likely to induce
leakage from the reservoir. It is usual to
puddle carefully the culvert or pipes when
carried under or through the bank, but, even
with such a precaution, the water has under
a considerable head a tendency to creep
along the pipe, and, by soaking into the
earthwork, may cause any one of the many
evils that imperil and destroy embankments.
When embankments are not of great
height, an exceedingly cheap and simple
mode might be adopted for drawing off the
water. This would be by laying a syphon
over the embankment, as was done in the
case of the middle-level drainage in Cambridgeshire, which syphon would at the
inner side have a flexible connection with
another tube having a float attached, as
above described.  Such an arrangement
would apply in principle to heights not exceeding 30 ft., as the pressure of the atmosphere would maintain no greater height. In




73
practice, however, the syphons cannot be
worked with success at much above 20 ft.,
for it is found that after a short time, the
flow becomes arrested by the collection of
air in the upper part of the syphon, and it
becomes necessary to pump the air out constantly, to prevent it from interfering with
the flow, as it would do if not removed. It
would appear a simple matter, where it is
desirable to adopt a syphon, to utilize the
power of the water flowing out for the purpose of getting rid of the air; it might
easily be applied, through a small wheel
and suitable gearing, to work an air-pump
fixed at the highest point of the syphon,
making the whole arrangement self-acting..
The arrangement could be successfully applied to irrigation tanks in India, where the
embankments are frequently less than 30
ft. Each leg of the syphon should be provided with a valve to retain the water, and;
when the supply was intermittent it would.
be essential to have an opening at the:
highest point of the syphon, and some ap-.
pliance, perhaps an air-pump, for. filling i&
with water in case of leakage.




74
To insure a constant discharge from a
reservoir with a constantly varying head,
several methods have been adopted; of these,
one of the most ingenious is that used at
the Gorbals Waterworks, near Glasgow.
Fig. 9 represents a transverse section
through the regulator-house, showing the
arrangement by which the discharge is
equalized. To the orifice of the outlet pipe, 0,
is fitted a square-hinged flap valve of wood,
against which presses, by a friction roller,
a lever, B, the arms of which are bent. To
the upper arm is attached a chain that passes over a pulley, and is connected with a
cast-iron cylinder or float, D, that stands in
the reservoir, E, of slightly larger diameter.
At the side of the entrance-door of the
building is placed another cistern, G, of
cast-iron, closed at top, and communicating
by a pipe, R R, with the vertical pipe, H,
which is in connection with the outlet pipe,
and passes up the slope of the embankment, to carry away any air that may accumulate in the main. The cistern, G, is
connected with the reservoir, E, by a pipe,
K, which supplies water to float the cyl



75
~I Ii
LI K?/;;
/~ r.T~




76
inder, D. Now, it is evident that the discharge from the reservoir will be regulated
by the position of the lever, B, and this
again will be controlled by the height of
the float, D. To regulate this height the
supply from the cistern, G, must be selfadjusting, or be regulated by the amount
of water flowing away. The float, N, has
attached to it a spindle, on which are fixed
two double-beat valves that work in the
vertical part of the pipe, K, one of which
admits water from the cistern, G, into the
cylinder, E, and the other allows the water
to escape from the reservoir, E. Now, if
the surface of the water upon which the
float, N, rests should rise above the proper
level, the float forces up the spindle, closing
the supply valve from  the cistern, and at
the same time opening the lower valve.
Thus the supply is cut off and the escape
opened, enabling the float, D, to fall. The
subsidence of the float closes more or less
the flap valve, and checks the discharge, in
consequence of which the surface of the
water falls, and with it the float, N, which
consequently opens the supply valve, and




77
again admits water into the cistern, E.
Thus an almost perfect equality between
the consumption and the supply of water is
preserved. It would appear that the same
effect could be produced by connecting the
lever directly with a float on the surface of
the water, but such an arrangement would
only apply when the pressure against the
flap is trifling.
It is essential that every reservoir should
be provided with some means of gettingrid
of the excess of water that flows into it, and
whether this provision be made by a waste
weir, sluices, or waste pit, it is one that
should not be omitted. The most advantageous position for a waste weir will
generally be' at some point remote from,
and entirely unconnected with, the embankment, and occasionally a natural depression in the ground, as shown in Fig. 4,
will afford remarkable facilities for the
construction of an escape. The level of the
crest of the waste weir with reference to
the top of the dam will require to be carefully adjusted, the minimum difference of
level being 3 ft., and the maximum about




78
10 ft., depending on varying circumstances.
The height of the waste weir will, of course,
regulate the top water level in the reservoir;
and this must be fixed with regard to the
probability of the embankment being overtopped by waves. The circumstances influencing the height of the waves in a
reservoir are the extent of the water surface,
the depth, and the amount of exposure to
-or shelter from wind, all of which will vary
with each particualar case. Under ordinary
circumstances, the height of the top of the
embankment above the crest of the waste
weir should be for
an embankment 25 ft. deep, 4 ft.
50 ft.  "  5 ft.
75 ft.  "  6 ft.
and for greater height of embankment the
difference of level may be proportionately
increased.
When the configuration of the ground
does not afford any facilities for the construction of a waste weir after the manner
described, sufficient provision for the escape
of the overflow is made through a waste
pit. This waste pit, or tower, is generally




79
a circular structure built over the outlet
culvert inside the reservoir, and serves
equally for access to the valves and for the
escape of the flood water. With regard to
the capacity of the waste weir or waste pit,
whichever be adopted, it will be necessary
to make ample provision for the discharge
of the sudden accessions of flood water that
reservoirs are subject to, and which so
seriously imperil their safety. To provide
for this there is an empirical rule amongst
engineers that is supposed to suffice for the
most urgent contingencies. It states that
there shall not be less than 3 ft. of length
of overfall for every hundred acres of
gathering ground, but it is obvious that to
proportion the length of the waste weir to a
given area of country in all cases would be
unreasonable.
The discharge over the weir will not
depend only upon the quantity of rain falling on a certain area of ground, but also on
the extent of the reservoir as compared to
the gathering ground, and on the flat or
precipitous character of the basin. The only
safe mode, then, of proportioning the length




80
of the escape will be to ascertain with
exactness what the discharge of the stream
or streams flowing out of the reservoir was
during the greatest known flood, and then
fixing upon an arbitrary depth for the
water to flow over the weir, say 2 ft. or 3
ft., to calculate what length of overfall will
suffice for the discharge of the excess
water. In India, where large waste-weir
accomodation is essentially necessary, while
it is equally a necessity to save every
gallon of water that is possible, it is a
common practice to form a temporary dam,
of earth and sods, on the top of the waste
weir; this serves to pond up some 3 ft. or,
4 ft. of water over the whole surface of the
reservoir, and does not imperil the security
of the works. In times of heavy floods the
water rises and overtops the temporary dam,
and no sooner does so, than the whole is
carried away, and the water in the reservoir
quickly subsides.
In works designed for the supply of
towns, it is sometimes necessary to make
provision to arrest the entrance of floodwater into the reservoir, as the streams may




81
come down charged with large quantities of
matter in suspension that would injure the
purity of the water for domestic consumption. These streams may be diverted and
carried round the margin of the tank past
the dam, and can be admitted into the
channel of the stream, or be utilized for
mill power. On the Manchester Waterworks are constructed across the mountain
streams weirs of an ingenious design, for
the purpose of separating the flood-waters
from the ordinary flow. The dimensions
are adjusted from observations of each particular stream, so that the discharge up to
a certain amount will take place into the
channel for the supply of the town; but
when the discharge increases, and the water
becomes turbid, it has sufficient velocity to
carry it over the opening, as shown in the
diagram, and flows down to the compensation reservoir for the supply of mill
power.
In determining the dimensions of a weir
of this kind it is first to be ascertained
what the mean velocity of the water flowing over will be for a given depth of water,




82
h, above the crest. The mean velocity, v,
will be
v=     x s 024 Vh-= 5.35 V/h.
If the vertical height of the crest of the
weir above the point to be overleaped by
the cascade be called x, the distance across
will be
2vV/-   4
Y =     = =  a3/ xb
Before concluding, it will be well to give
a brief consideration to the causes tending
to the failure of emabankments.  The foregoing remarks will, in suggesting the best
mode of construction, have anticipated much.
that might be said on the subject of failures;
but there are a few points, the recapitulation of which the importance of the subject
demands.
There are unfortunately on record, accidents, if they can be so called, from the.
bursting of embankments, that if estimated
by the loss of life attending them, are as
appalling as anything within the memory of
man. Thousands of human lives have been
sacrificed to ignorance and false economy,.




83
as well as in some instances to natural defects that it would have been difficult to
foresee.
The existence of springs on the site of an
embankment is an undoubted cause for apprehension, and considerable care should be
taken to carry all water from this source
away, that it may not, as it certainly will if
not checked, force its way between the surface of the ground and the seat of the embankment. In doing so there is every probability that the earth of the embankment
will be washed out by constant trickling till
a fissure is formed of sufficient dimensions
to render the destruction of the bank a cer.
tainty, if the water from the reservoir
should ever penetrate so far. As a provision against this source of injury, all springs
found on the site of an embankment should
be taken up and carried away in proper
drains sufficiently and securely puddled.
Thus the water is confined to a single channel, and has no tendency to soak into the
earthwork and blow it up in endeavoring to
escape. In embankments of all kinds the
presence of water is a most serious evil, and




84
one by which may be accounted for, some of
the most extensive land slips that are on
record.  It is erroneous to assume that
when water is the active element in producing disruption in an embankment or
mass of earth of any kind, that it only acts
as a lubricant between the surfaces in contact. The truth is, the bulk of earth is sen —
sibly affected by the amount of moisture in
it, as is seen in the subsidence of newlyformed railway banks when exposed to rain.
If, then, a sufficient quantity of water find its
way into the centre of a bank that has been
put together in a comparatively dry state, it
will rise and soak into the earth until at
length what was a solid mass becomes
semi-fluid, settles into a smaller space than
it before occupied, and, as a consequence,
will leave a vacuity above it. The inevitable result is the subsidence of the superincumbent earth; but instead of resting, as
at first, on a resisting material, it floats, so
to speak, on the semi-fluid mass underneath,.
and having little or no friction to overcome,
slips away to a lower angle than it before
stood at. Natural springs, therefore, when



85
ever they occur, must be dealt with carefully  and  completely.  Exactly  similar
effects to those produced by natural springs
may result from the defective practice of
carrying outlet pipes through or immediately under embankments.  Be the pipes
ever so well puddled, there will be a tendency to trickling along the line of their
direction, and assuredly if this trickle makes
its way to the centre of the bank it will
carry mischief with it. It is true that
springs are occasionally found issuing from
the foot of an embankment, without after
several years causing any appearances
to justify apprehension.  The Doe-park
reservoir is an example in point, and though
at one time fears for its safety were entertained, the embankment is still standing,
and, so far as the author is aware, the
spring is still trickling away. An engineer
of eminence was called upon to report upon
the state of the wqrks, and gave his opinion
that, as the spring came away without any
earth in suspension, there was no mischief
taking place, and that the work was in a
safe condition. There is no doubt that em



86
bankments in this condition require to be
narrowly watched, although the presumption may be that, having lasted for several
years, they will continue in safety.
The empirical and unscientific mode of
proportioning the length of waste weirs has
proved before now a source of danger and
destruction to embankments, from the space
afforded not being sufficient to discharge
the excess water without the surface rising
to such a height as to top the embankment.
To avoid risk, the stream must be gauged
with great care, and the discharge calculated for the greatest known flood; and if
with a given head the length of the weir be
adjusted to discharge this amount, or a
little in excess, there will be no risk to the
embankment.
Regarding finally the whole subject, the
danger that may result from careless -or
unscientific construction, the large outlay
entailed in the establishment of storage
works, and the benefit that may accrue
from them whatever their purpose may be,
the subject cannot be undertaken on merely
rational grounds. Its successful applica



87
tion will rest alone on the study of the
question in its scientific details, and an
ample practical experience.
DISCUSSION.
MIr. H. P. Stephenson said he entirely
agreed with the author as to the impropriety of carrying a pipe through the embankment of a reservoir. He would extend
his objection to the passing of a culvert
through the embankment. If the culvert
were laid on the natural ground, they
would avoid the risks pointed out by the
author, either of the settlement from the
joints of the pipe, or of the water creeping
along between the material and the pipe.
He believed that the true principle of construction for reservoirs was the placing of
a good puddle dam in the centre, and on
each side of this dam layers of earth well
punned in. One reason why he should
prefer the puddle wall in the centre was
that there was less tendency in the puddle
to slip in such a position than when laid on
the slope.
Mr. Albert Latham agreed with Mr.




88
Stephenson in his remarks as to the pipes
and culverts; but he thought it was an
open question whether the puddle wall
should be in the centre of the dam. He
had a strong opinion that it should be on
the face of the dam.
Mr. Cargill said that he believed that the
reason the puddle wall was not required in
Indian embankments, referred to by the
author of the paper, was that the earth
seemed to have been thoroughly consolidated by the continual trample of people upon
it. That thorough consolidation was the
great point in all puddling, and it was on
that account that specifications were generally so stringent as to the thickness of the
layers of the puddle. As to the position of
the puddle wall, he could not see the particular value of having it in the middle of the
dam, and he thought that a far better place
for it would be the face, because the object
of the puddle wall was to prevent the infiltration or the escape of the water. This
could be effected by puddling the whole
slope right down to the permanent strata.
The puddle wall was not required to




promote the stability of the dam. The
question of putting pipes or culverts under
the dam required more consideration.
It was alleged that the putting of a naked
pipe through the dam of the Bradfield reservoir was one of the causes of its bursting.
In some very large waterworks now being
constructed in Dublin there were two distinct sets of main pipes, and they were laid
in two large culverts at the bottom of the
dam. The culverts were large enough for
a man to walk upright in them. If the
foundation were well looked after, there
would be no fear of the arch or dome of the
culvert giving way in consequence of any
inequality of pressure above it, as, if properly constructed, an arch would stand any
amount of pressure short of what would
crush the material.
Mir. Baldwin Latham said he could not
agree with Mr. Jacob that a dam could not
be constructed from theoretical deductions;
for unless regard was paid to theoretical
considerations there might result either a
deficiency of strength or a waste of material
and labor. In the dam shown in the draw



90
ings, and designed by himself, the pipe did
not run through, but on the outside of the
dam, on the solid ground. It was a well
received opinion among engineers that if
you had a pipe or culvert running through
an embankment, that pipe or culvert would
be unsafe.  He believed that well made
and properly tested pipes were quite as safe
as culverts when in the solid ground. A pipe
was simply a small culvert made of iron
instead of brickwork. In cases in which
there was a tendency for the water to creep
along the outside of the pipe, that might be
stopped by having projecting flanges on the
pipe. The same creeping of water might
take place along a culvert as along a pipe.
With regard to the slope of a dam, the inside slope should be greater than the outside slope, because the greater would be
the stability of the dam, and the water
would have less destructive effect on the
dam; he had effectually prevented leakage
by the use of socket-pipes. The square
projection of the sockets was alway presented to the reservoir, and the pipes were laid
in the virgin ground. It was very bad




91
practice to lay the pipes in made ground,
and especially through a dam. Pipes laid
under a dam should be tested under pressure
after being laid and before being covered
up, so that any defective joint might be
discovered. In cases in which he had laid
pipes through dams, they had been so
tested, which resulted in good and effective
work; but he was bound to say that, if the
pipes had not been tested in silu the result
would not have been satisfactory.
Mr. Schiinheyder said that Mr. Jacob had.said that wherever springs occurred they
should be well carried away. Hie (Mr.
Schonheyder) wished lo know how a spring
was to be prevented from diffusing through
the earth.
Mr. Hendry said that he had seen pipes
which were laid through embankments,
but had never seen one that was perfectly
tight. It was almost impracticable to make
it so, owing to the continuity of the puddle
being disturbed at the point where the pipe
passes through.
The Chairman asked what was the
largest diameter of pipe Mlr. iendry had
seen used.




92
Mr. Hendry replied that the largest was
18 in. He had heard of several methods
being tried, but he did not think it was
possible to prevent leaking, more or less,
from the reservoir along the outside of the
pipe. He should like to be informed how
it was possible to connect the puddle with
the pipe; if the pipes be laid in the natural
ground below the foundation of the embankment, then there is no fear of leakage, provided the pipes are properly
laid.
Mr. Jacob, in replying to the discussion,
said, that in the opinions that had been
expressed there were but few points of
disagreement with those that he himself
held. He could not agree with Mr. Latham
in his belief that embankments could be
calculated on mathematical principles. In
order to deal with embankments theoretically, they must be regarded as rigid
masses, and be assumed to rest upon a
horizontal plane. It could be shown mathematically that a rigid body of the same
specific gravity as ordinary earth need not
present the same section as is usually given




93
to embankments, in order adequately to
resist the pressure of water. A right angle
prism with the hypothenuse resting upon
the plane would be quite sufficient to resist the
pressure of water, even supposing the surface of the water to coincide with the upper
edge of the prism. The reason of giving
long slopes to an embankment is discoverable from the fact that banks, when exposed
to the action of water, are found to waste and
slip away to such an angle as will withstand
the action of the water. The chief reason of
the failure of embankments is the infiltration
or soaking of the water from the inner side,
which renders the material semi-fluid and
causes it to subside into a smaller space than
it originally occupied. The superincumbent
mass then sinks and allows the water to
overtop the embankment. The earth used
for making embankments in the Deccan
and in parts of the Madras Presidency in
India is of a most suitable quality for the
purpose. It is what is called " black soil,"
being very dark in color, and of a highly
argillaceous character. The color is, no
doubt, due to the presence of carbon. The




94
clay makes most excellent puddle; but, no
doubt, the consolidation produced by the
tread of the work-people is the real secret of
the earth resisting the pressure of water so
successfully as it does. In North America,
the levees for protecting the country from
flooding by the Mississippi are sometimes
constructed simply of sand; and are found,
for the most part, sufficient for their purpose. As regards carrying away springs
from the seat of an embankment, there is
no difficulty in ascertaining where they
exist when the ground is laid bare, as they
are generally well defined streams. Before
the earthwork is commenced it is necessary
to construct drains of masonry, or brickwork,
or to lay iron piping to carry away the
water clear of the work.
The Chairman said that the paper of Mr.
Jacob was a very interesting one, and the
subject was one which, during the last
year or two, or, he might say, within the
last week or two, had commanded the attention of the whole body of engineers.
Last session a special Act of Parliament
was passed that all reservoirs and embank



95
ments should be constructed to the approval
of the Board of Trade. The subject of
irrigation in India, which was alluded to
in the paper, was one of vital importance.
There was no question that the only means
we had of irrigating that country in an
efficient manner was by the construction of
reservoirs.