COLLECTION OF INFORMATION CONCERNING STREAMS
1. =Preliminary Remarks.=--The information which is required concerning
streams depends on the character of the stream and on the nature of the
work which is to be done. For the present let it be supposed that the
stream is large and perennial. Other kinds of streams will be dealt
with in _Arts. 6_ and _7_. In dealing with a large perennial stream it
is nearly always necessary to know the approximate highest and lowest
water-levels, and these can generally be ascertained by local inquiry,
combined with observations of water marks; but a higher level than the
highest known and a lower level than the lowest known are always liable
to occur, and must to some extent be allowed for. If navigation exists
or is to be arranged for, the highest and lowest levels consistent with
navigation must be ascertained. The highest such level depends chiefly
on the heights of bridges. A plan to a fairly large scale is also
necessary in most cases.
If an embankment to keep out floods is to be made along a river which
is so large that its flood-level cannot be appreciably affected by the
construction of the work, it may be necessary to obtain information
only as to the actual flood-levels, and as to the extent to which
the stream is liable to erode its bank. If a length of the bank of a
stream has to be protected against scour, it is necessary to know of
what materials the bed and bank are composed, and whether the channel
is liable to changes and to what extent. It is also desirable to know
to what extent the water transports solids, if any. In some kinds of
protective work these solids are utilised.
But in cases where the stream is to be much interfered with, it is
necessary to have full information concerning it, not only as regards
water-levels, changes in the channel, and transport of solids, but as
regards the longitudinal profile and cross-sections, and the discharges
corresponding to different water-levels. The collection of some of this
information, particularly as to the water-levels and discharges at
different times of the year and in floods, may occupy a considerable
time.
Methods of ascertaining the quantity of silt carried in the water of
a stream are described in CHAP. IV., _Art. 4_. Remarks regarding the
other kinds of information required--the stream being still supposed to
be large and perennial--are given in _Arts. 2_ to _5_ of this chapter.
The degree of accuracy required in the information depends, however,
on the importance of the work, and sometimes the procedure can be
simplified. Detailed remarks on gauges and on the instruments used and
methods adopted for observing discharges and surface slopes, are given
in _Hydraulics_, CHAP. VIII. and Appendix H.
2. =Stream Gauges.=--Unless the stream being dealt with is an
artificial one, it is unlikely that the flow in the reach with which
the work is concerned will be uniform. The rise and fall of the water
at one place cannot therefore be correctly inferred from those at
another. It will be desirable to have two gauges, either read daily
or else automatically, recording the water-level, one near each end
of the reach concerned, with intermediate gauges if the reach is very
long. If, in or near the reach, there is already a gauge which has been
regularly read, it may be sufficient to set up only one new gauge,
and to read it only for such a period of time as will give a good
range of water-level, and to compare the readings with those of the
old gauge. The readings of the new gauge for water-levels outside the
range of those observed can then be inferred, but if the stream is very
irregular this may involve some trouble (_Art. 4_).
In the case of a large stream which shifts its course, the reading
of a gauge does not give a proper indication of the water-level. In
other words, the distance of the gauge from the two ends of the reach
is subject to alteration. The case is the same as if the stream was
stable and the gauge was shifted about. In such a stream there ought,
if accuracy is required, to be a group of two or more gauges for each
point where there would be only one if the stream was not a shifting
one. Also, owing to erosion of the bank or the formation of a sandbank,
it may often be necessary to shift the gauge. When possible it should
be kept in a fixed line laid down at right angles to the general
direction of the stream. When shifted, its zero level should be altered
in such a way that the reading at the new site at the time of shifting
is the same as it was before shifting. When the gauge is moved back
to the original site its zero should be placed at its original level,
though this may give rise to a sudden jump in the reading for the
reason given in the first sentence of this paragraph.
3. =Plan and Sections.=--Making a survey and plan, and laying down on
it the lines for longitudinal and cross-sections, and taking levels
for the sections, are ordinary operations of surveying. If any land
is liable to be flooded, its boundaries should be shown on the plan
and on some of the cross-sections. Unless the water is shallow, it is
necessary to obtain the bed levels from the water-level by soundings,
the level of a peg at the water-level having been obtained by
levelling. All the sections should show the water-level as it was at
some particular time, but the water-level will probably have altered
while the survey was in progress, and allowance must be made for this.
The pegs at all the cross-sections and on both banks of the stream--for
the water-levels at opposite banks may not be exactly the same--may,
for instance, be driven down to the water-level when it is steady,
and thereafter any changes in it noted and the soundings corrected
accordingly.
In order to ascertain what changes are occurring in the channel it may
be necessary to repeat the soundings at intervals and, if there is much
erosion of the bank, to make fresh plans.
4. =Discharge Observations.=--For a large stream it is necessary to
observe the discharges by taking cross-sections and measuring the
velocity. If there is a sufficient range of water-levels, it will
be possible to make actual observations of a sufficient number of
discharges. If soundings cannot, owing to the depth or velocity, be
taken at high water, they must be inferred from those previously
taken, but this does not allow for changes in the channel, which are
sometimes considerable and rapid. If there is not a sufficient range of
water-level, the discharges for some water-levels must be calculated
from those at other water-levels. In this case observations of the
surface slope will be required, and the discharge site should be so
selected that no abrupt changes in the channel will come within the
length over which the observations are to extend. This length should
be such that the fall in the water surface will be great enough to
admit of accurate observation. If the cross-section of the stream is
nearly uniform throughout the whole of this length, or if it varies
in a regular manner, being greatest at one end of the length and
least at the other end. the differences in the areas of the two end
sections being not more than 10 or 12 per cent., then the velocity
and cross-section of the stream can be observed in the usual manner
at the centre of the length; but otherwise they should be observed at
intervals over the whole length, or at least in two places, one where
the section is small and one where it is great, and the mean taken. Or
the velocity can be observed at only one cross-section and calculated
for the others by simple proportion and the mean taken. The coefficient
C can then be found from the formula C = V/√(RS). To find the discharge
for a higher or lower water-level, the change in the value of C
corresponding to the change in R can be estimated by looking out the
values of C in tables, and the discharge calculated by using the new
values of C and R and the new sectional area, S remaining unaltered.
But if the channel is such that, with the new water-level, a change in
S is likely to have occurred, this change must be allowed for. Any such
change will be due to the changed relative effects of irregularities,
either in the length over which the observations extended or downstream
of that length. The effect of irregularities in the bed is greatest at
low water. The effect of lateral narrowings is greatest at high water.
Since a change of 10 per cent. in S causes a change of only 5 per cent.
in V, it will usually suffice to draw on the longitudinal section the
actual water surface observed and to sketch the probable surface for
the new water-level. If the whole channel is fairly regular for a long
distance downstream of the discharge site, no slope observations need
be made nor need several sections be taken in order to find V. The
changed value of C should, however, be estimated in the manner above
indicated. For this purpose any probable value of S will suffice.
5. =Discharge Curves and Tables.=--Ordinarily it will be possible,
by plotting the observed discharges as ordinates, the gauge readings
being the abscissæ, to draw a discharge curve and from it construct a
discharge table. Unless the channel is of firm material and not liable
to change, there are likely to be discrepancies among the observed
discharges, so that a regular discharge curve will not pass through
all the plotted points. If the discrepancies are not serious, they can
be disregarded and the curve drawn so as to pass as near as possible
to all the points, but otherwise trouble and uncertainty may arise.
The soundings should be compared in order to see whether changes
have occurred in the channel. If such changes do not account for the
discrepancies, the cause must be sought for in some of the recorded
velocities. If no sources of error in these can be found, such as
wind, it is possible that the velocity has been affected by a change
in the surface slope owing to some change in the channel downstream
of the length. Failing this explanation, the discrepancies must be set
down to unknown causes. With an unstable channel and where accuracy
is required, the sectional areas and velocities should be regularly
tabulated or plotted so that changes may be watched and investigated.
To do this it may be necessary to take surface slope observations, or
to set up extra gauges which will show any changes in the slope.
If, downstream of the discharge site, there is any place where
affluents come in and bring varying volumes of water, or where gates
or sluices are manipulated, and if the influence of this extends up to
the discharge site, the water-level there no longer depends only on
the discharge, and a discharge table must be one with several columns
whose headings indicate various conditions at the place where the
disturbances occur.
In order to show how the gauge readings and discharges vary from day to
day throughout the year, a diagram should be prepared showing the gauge
readings and discharges as ordinates, the abscissæ being the times in
days starting from any convenient date as zero. Such a diagram, showing
only gauge readings, is given in fig. 56, CHAP. XII.
6. =Small Streams.=--Small streams will now be considered, those, for
instance, which are too small to be navigable and which occasionally
run dry or nearly dry. If the water of the stream is to be stored for
water supply, power or irrigation purposes, full information as to
discharges and silt carried will be required. If the stream is small
enough the discharges can be ascertained by means of a weir of planks.
The discharge is then known from the gauge readings. Cross-sections
and large scale plans will not be required unless the stream is to be
altered or embanked. If the water, instead of being stored, is to be
got rid of, as in drainage work, the only information required as to
discharges is the maximum discharge. Large scale plans, sections, or
information as to silt or water-levels (except as a means of estimating
the discharge) will not be required unless the stream is to be altered
or embanked.
In all these cases of small streams the information required is
generally, as has been seen, less than in the case of large perennial
streams, but it is generally more difficult to obtain. If the stream
is ill-defined or its flow intermittent, especially if it is also very
small and the place sparsely inhabited, it may be difficult to obtain
any discharge figures except those based on figures of rainfall. The
method of obtaining such figures has been stated in CHAP. II. The
figures required are those of the annual and monthly fall when the
water is to be stored, and those of the greatest fall in a short period
when the water is to be got rid of. Of course a plan of the catchment
area is required.
7. =Intermittent Streams.=--In the case of large streams whose flow is
intermittent, the information required will, as before, depend upon the
circumstances. Such streams occur in many countries. The difficulty
in obtaining information is often very great. To obtain figures of
daily discharge a gauge must be set up in the stream and a register
kept. The chief difficulty in an out-of-the-way place is likely to
be the obtaining correct information as to the maximum discharge.
Information, derived from reports or from supposed flood marks, as to
the highest water-level, may be inaccurate, and information based on
rainfall figures may be extremely doubtful owing to the large size of
the catchment area, the absence of rain gauges, and the difficulty,
especially if the rain is not heavy, in estimating the available fall.
All sources of information must be utilised and, whenever possible,
observations should be made over a long period of time.
8. =Remarks.=--Very much remains to be done in collecting and
publishing information concerning the ratio of the discharges to the
rainfall. By observing a fall of rain and the discharge of a stream
before and after the fall, it is possible to ascertain the figures
for that occasion, but they will not hold good for all occasions.
Continuous observations are required. The chief obstacle is the
expense. Not only have measuring weirs and apparatus for automatically
recording the water-level to be provided, but the weirs would often
cause flooding of land involving payment of compensation. The most
suitable places for making observations are those where reservoirs for
water-works exist or are about to be made.