On the other hand it is possible to have a loss rate which positively
indicates the need for lining where no damage to adjacent lands is
evident. In this case the loss of water is important.

Seepage rates from canals can vary widely throughout the year. On
one ponded canal the seepage varied from 0.58 cfd in the spring to
0.24 cfd inthe fall. A variation in rates such as this is not unusual
especially where silt deposition or other sealing takes place over a
period oftime. The causes for seepage variations can be attributed
to the factors listed at the beginning of this article.

Sources of Error

Accuracy of loss rates established by the ponding method will depend on
the attention given to relatively simple details in preparing for and con-
ducting the tests. Generally speaking, the end of the irrigation season
is considered the most appropriate time for making seepage tests. At
this time, bank storage has been satisfied and the water table in the
adjacent areas has stabilized in elevation so that conditions are more
nearly representative of those during the irrigation season.

Because of the time required to refill a pond and to repeat a test, mak-
ing the test after the irrigation season provides sufficient time for re-
peat tests. Otherwise with limited time the temptation may be to accept
seepage rates established in a single filling. When seepage rates are
other than small, this practice could lead to erroneous conclusions
concerning the true losses.

Dikes and any turnouts in the pond should be inspected periodically to
be sure they are not leaking. If check structures have been used, they
should also be inspected to be sure stoplogs and canvas are in place.
Leakage around the canvas can usually be averted by timely placement
of more fill material at the edges.

Hook or staff gages not provided with stilling wells can yield erroneous
readings of water surface levels even when only a light breeze is blow-
ing. Also, it is easy to misread the gage by a full 0.1 foot; but check-
ing the previous reading or comparing loss increments when upstream
and downstream gages are used will usually prevent the error from
being recorded.

In any pond which shows a rapid drop of the water surface, elevations
must be determined frequently. The longer the time interval between
readings, the less accurate will be the average wetted perimeter and
average water surface widths used in the calculations.

The survey which establishes elevations on all gages is very important.

Closed circuits should be used, and the allowable error of closure should
be in keeping with the quality of results expected. At some time during

16


a. Water surface width according to elevation. - This is the
water surface distance across the canal.

b. Wetted perimeter according to elevation. - This is the dis-
tance along the ground surface under water, or that part ofa
cross section of the canal wet by the water.

Note: It will not be necessary to make a separate measurement
of the length of the pond, since length enters into calculations
for seepage loss through the cross-section stationing, as dis-
cussed on page 14, and shown in the basic equation at the bot-
tom of Table IV, on page 21.

The first step toward calculation of the seepage loss is the compiling
of Tables I and IJ, as shown on pages 18 and 19. This includes plot-
ting the canal cross sections to a scale that will make possible the
measuring of water surface widths and wetted perimeters to within
0.1 foot as shown in Figure 7, on the opposite page. Noting the range
of water elevations to be covered in the tests, water surface widths
and wetted perimeters are scaled on each cross section from an ele-
vation just below the lowest test elevation to an elevation slightly
above the highest test elevation. The increments of elevations used
will depend on the size of the canal, depth in the pond, and range of
test depths. For shallow canals of short bottom width, it is advis-
able to compile tables of water surface widths and wetted perimeters
for each 0.1 or 0.2 foot of depth. In deeper and wider canals, the
increment may be changed to 0.3 foot or more without seriously affect-
ing the accuracy of computations.

Table I and II also show each column under a particular elevation aver-
aged for all stations to obtain the representative characteristics for
the entire pond. Since elevations measured in the course of the tests
will rarely correspond exactly to those in the table, it will be neces-
sary to interpolate between average values for correct water surface
widths and perimeters.

Filling the Pond

Gravity flow or pumping may be used to fill the test pond. The method
will depend on the conditions that prevail at the site and the size of the
canal. The following possible courses of action are suggested:

a. Release water to form a supply reservoir at the upstream
dike, and pump the water over the dike into the pond as shown
in Figure 8 on page 12.

b. Allow water to flow into the pond by gravity (over a dike
covered with plastic).

10


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20


MEASURING SEEPAGE IN IRRIGATION CANALS
BY THE PONDING METHOD

Selecting the Pond Site

Several factors may enter into the selection of a pond site. The pur-
pose for the seepage measurement may automatically dictate the site.
For example, if the water table under croplands adjacent to a cer-
tain length of canal has risen to a level which is causing crop damage,
then the test reach should probably parallel affected properties. If
the purpose is to learn the effectiveness of a lining method or mate-
rial, or to determine a loss rate for a particular soil type, more
latitude may be allowed in placement of a pond.

As a general rule, it is desirable to avoid selecting a reach ina
canal where it is difficult to determine the wetted cross section, and
where turnout leakage cannot be stopped. If possible, the pond should
be free from reaches in which the subgrade materials vary consider-
ably in composition. This is particularly true if the measured seep-
age rates are to be considered as representative of a certain soil
type or condition.

Determining the Length of Pond

The length of the pond is an important consideration. In general, the
length should be great enough to make the sum of the pond end areas a
very small percentage (not more than 3 percent) of the total wetted
area. Since the seepage per unit area through or under temporary
dikes can be greater than through a unit area of the bottom or sides of
the canal, the effect of the end areas on rates of loss can be minimized
by observing this precaution.

As a guide to setting the length, in one canal with a bottom width of

16 feet a test was made in a pond 1, 400 feet long and in another with

a bottom width of 26 feet, the test pond was 800 feet long. The pond
end areas in the first canal were about 0.4 percent of the wetted area,
and the corresponding value in the second canal was about 1.5 percent.
To keep the percentage of end area low the pond should have a mini-
mum length of about 60 times the depth of water.

Constructing the Ponded Section

Many methods or combinations thereof can be used to create the ponded
section. How the dikes are made will depend primarily on the size of
the canal and materials at hand.

Dams may be built of canvas or plastic held in place by a timber at the
top and dirt thrown along the edge on the cross section. Such dams are
usually restricted to small channels in which the water depth is less
than 2 or 3 feet. A heavy-weight canvas treated with water-proofing


the conduct of any test there will probably be a complete absence of
a breeze, and the pond water surface will become obsolutely still.
A check reading on all gages, and particularly on the hook gages
will reveal how well the gages have been set.

When computing the results from field data, many chances for calcu-

lator errors will occur. The existence of large errors can be de-
tected by carefully analyzing trends in tabular values.

17




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19


high loss rate, readings every 1 to 4 hours may be required; on the
other hand, for a pond showing a very slow rate of drop, less fre-
quent readings will be satisfactory. Staff gage readings and the time
of day should be written on the recorder chart each time the gages
are read.

In one canal test with a seepage loss of approximately 1.3 cfd, 1/
gage readings were made about every 4 hours, day and night. On
another canal test where the loss rate was less than 0.05 cfd, read-
ings were taken in early morning, midafternoon, and late evening,
with no readings between 10 p.m. and 8 a.m. Thus personnel must
decide from test circumstances the schedule for taking of data.

A graph of water surface elevation versus elapsed time can be made

to show a changing or steady rate of loss. The graph can be used to
decide whether additional fillings of the pond will be needed or if an
error in water surface level has been made. Ifa water stage recorder
is used, a plotting of the elevation-time curves is not necessary, be-
cause the chart will automatically make the graph.

Computing Loss Rates

Although seepage loss data can be set up for computation by computers,
the manual method using tables is presented here and is dependent
upon knowing the water surface width and the wetted perimeter of

canal ponds for elevations of water surface during the test. These
figures can be obtained from Tables I and II previously discussed.

Table II also previously discussed is used to record information
obtained during the ponding test.

The information contained in Tables I, II, and III, is sufficient for com-
putation of loss rates. To facilitate these computations, the use of
Table IV on page 21, is also suggested. In using Table IV: -

a. Data from Table II are entered in Columns 1 (date), 2 (time),
and 4 (water surface elevation). Values in Columns 3 (elapsed
time) and 5 (drop in water surface) are computed from Columns 2
and 4.

b. From Table I, an interpolated value for water surface width,
Column 6 can be found.

ce. Column 7 is calculated from Column 6.
T/Seepage losses from canals are generally expressed in Bureau of

Reclamation Studies in cubic feet per square foot of wetted area per
day and abbreviated to "cfd."

14


The gages can be installed on vertical uprights (2 by 4 or 4 by 4 tim-
bers) that have been firmly positioned near the edge of the pond. If
the canal is small, the upright can be placed so that an access plat-
form is not needed to read the gages.

In larger canals, a walkway will probably be needed to reach the gages.
Under no circumstances should the timber for gages be an integral part
of the walkway framing, since repeated trips over the walkway to read
the gages would force the upright into the subgrade and change the gage
setting. Figure 4 shows how the Hook and Staff Gages can be mounted
separate from access platform.

Figure 4, Photo P596-D-18417

The staff gage reading can serve as a check of the hook gage and re-
corder readings; and, should the hook gage be accidentally disturbed,
the staff gage can be read until a resurvey establishes a new elevation
on the hook. A check on the hook and staff gage elevations may be
made when the pond water surface is absolutely still on a calm day.


When earth dikes are to be installed in a concrete-lined canal, it is
impracticable to excavate a cutofftrench. The base width of the
dike must then be great enough to withstand the hydrostatic pres-
sure in the pond, and percolation paths made sufficiently long so
that leakage is reduced to a negligible amount. Piping along the
line of contact must not occur. Generally speaking the base width
of the dike should be greater than that used in an earth canal, and
plastic covering of dikes is almost always advisable.

When water must be passed through a dike to fill a pond or passed
to another pond downstream, it will be necessary to install gated
culvert pipe. The culvert should be provided with one or more cut-
off collars, and the surrounding materials must be carefully com-
pacted to prevent piping along the corrugations.

In some instances it will be convenient to store water in a pond above
the upstream dike for use in refilling the test section. When this is
done it may be desirable to build a second dike to isolate the test
reach, placing this dike so that the toes of facing slopes are separated
a few feet as illustrated in Figure 8, on page 12. Thus it will be pos-
sible to observe the watertightness of the pond dikes. If only one dike
is built it is recommended that it be covered on both sides with plastic.

It is excellent practice to use existing check structures or check drops
to pond water. Canvas or plastic is usually placed over the upstream
side to cover open joints and to prevent leakage around the ends of the
planks. Canvas can be held in place along the edges and bottom by
packing soil against it. Where a pond is formed above a check, unless
another structure is available upstream, a dike should be used at the
upstream end. This will avoid excessive length and near zero depth at
the upstream end with design depth at the downstream end. If the
check is gated, it will probably be necessary to seal each gate against
leakage, either by using canvas as above stated, or by packing the
gate with a watertight material such as oakum.

Test Equipment Required

The equipment needed to conduct ponding tests is simple to operate,
but precautions should be observed when installing it. The equipment
which may be used includes one or two hook gages; one or two staff
gages; a water stage recorder; stilling wells for the hook gages and
recorder; and, in some cases, an evaporation pan. Figure 1, on the
following page, shows a recorder equipped with an 8-day clock, a
spined float pulley, graduated float tape and index bracket to provide
direct indication of gage heights. Figure 2, page 5, shows botha
Hook Gage and a Staff Gage.


will usually function with a minimum of leakage. Heavy plastic mate-
rial may also be used.

Another method very often used for constructing the ponded section,

is to build earth dikes at each end of the pond. For an average size
canal, the material is usually pushed into place with a dozer. If the
canal is dry during dike construction, the dozer can compact the soil
by repeated trips back and forth across shallow lifts. Restricting
each lift to 6 or 8 inches will make it possible to secure adequate com-
paction inthis manner. The base of the dike is usually about as wide
as the total width of the canal. Construction of dikes while there is
water in the canal is not recommended.

As the size of the dikes required becomes larger, greater care in con-
struction is necessary. In an unlined or earth-lined canal, a cutoff
trench at least a foot deep should be excavated along the cross section
through sand and silt deposits and into solid material. This allows
placement and compaction of selected soil and destroys any thin layer
of materials which because of gradation, organic content, and mois-
ture condition, may provide relatively low resistance to horizontal
shear forces between the dike and subgrade. It also acts to key the
compacted soil above into the subgrade, and lengthens the percolation
path under the dike. Care should be taken to give sufficient base width
to the dike, and the slopes of the fill should not be steeper than 1-1/2:1,
to prevent slippage of soil into the pond when it becomes saturated.
The space required for pumping equipment in tests in which the pond is
filled from a reserve water supply in the canal may dictate the top as
well as bottom width of a dike.

Leakage from the ponds in even large canals can be eliminated by cover-
ing the pond side of the dikes with sheet plastic from 4 to 8 mils thick.
The plastic must be placed before filling the pond. The edges can be

held in place in shallow trenches about 12 inches deep, with soil shoveled
carefully over the plastic in order not to puncture the plastic sheet. This
treatment is of particular value where the only readily available fill soil
is sandy. A dike so constructed may require less yardage for stability
than one without a plastic cover. It is always good practice to use plastic
on all dikes to insure water tightness and stability, where this is possible.

In small or medium size canals, where water depth is less than 5 or 6 feet,
it is possible to pass water over an earth dike protected with sheet plastic.
Thickness of 4 mils or greater will be satisfactory. The entire dike must
be covered from upstream to downstream inverts, and the plastic should
extend along the canal sides far enough to allow it to be held firmly in
place with fill dirt. The joints should be at right angles to the direction

of flow, and a lap of not less than 1 foot is necessary. If plastic cement

is available, overlapping sheets may be joined. Flow over the dike should
be limited to prevent excessive erosion by the velocity on the downstream
side of the dike.


d. The value in Column 8 is the product of Columns 5 and 7.

e. The wetted perimeter in Column 9 is computed from the data

in Table II. Since the wetted perimeter decreases as the water
surface drops, it is necessary to use a value that is the average

of wetter perimeters for beginning and end of the test time interval.

f. Column 10 is the average wetted perimeter, calculated from
Column 9.

g. Column 11 is calculated from Columns 3 and 10.

h. The desired seepage rate in Column 12 is obtained by dividing
Column 8 x 24 by Column 11.

To facilitate computations, data in Columns 1, 2, 4, 6, and 9 should
be entered on odd-numbered lines; and data in Columns 3, 5, 7, 8,
10, 11, and 12 should be written on even-numbered lines.

Approximate Method of Estimating Seepage

On ordinary earth canals, an approximation of the seepage rate can
quickly be made by using the following equation:

Drop in w.s. x 0.85 x 24
Hours of run

(cfd) =

Note: The factor 0.85 used above is approximately equal to the water
surface width divided by the wetted perimeter, and should be verified
by a sample measurement or calculation.

Interpreting Test Data

The purpose of the ponding test is to measure the seepage rate accur-
ately. Ina general way, a reach of canal might be said to be "tight,"
"borderline," or "leaks like a sieve." A tight canal might have a seep-
age rate of 0.03 to 0.10 cfd. A poorly lined canal or seriously leak-
ing unlined canal might have a seepage rate of 0.50 cfd or higher.
Leakage as high as 3.0 cfd has been recorded on some unlined canals.
Such losses, of course, cannot be tolerated for long periods - on most
canals.

It is possible for a ponding test to indicate a loss rate which alone does
not always provide the information needed for deciding whether to line
acanal reach. For example, land with insufficient natural drainage
adjacent to a canal may have a high-water table during the irrigation
season or water may be standing in shallow depressions. The loss
rate of the canal may be low 0.25 cfd, but it is obvious that the sec-
tion should be lined to prevent extension of land damage.

15


Evaporation pans and rain
gages are not usually neces-
sary; however, if evapora-
tion is significant, an evap-
oration pan should be in-
stalled. Figure 3, below,
shows an Evaporation Pan
set on the dike of a seepage
pond.

Installing Test Equipment

Each staff or hook gage should
be referenced to canal eleva-
tions so that depths of water in
the pond can be compared with
design or operating depth. An
assumed elevation can be used
for setting the equipment provided
it is referenced to the canal sur-
vey. Zero elevation for staff
gage, hook gage and recorder
should be the same.

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Figure 2, Photo PX-D-25228

Figure 3, Photo PX-D-25230

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C United States Department of the Interior, Bureau of Reclamation

IRRIGATION OPERATION AND MAINTENANCE

4 BULLETIN NO. 65
—} July, August, September 1968
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In This Issue:

Measuring Seepage in Irrigation

Canals by the Ponding Method


Figure 6, Photo P448-D-39233

Accordingly, good judgment must be exercised by the observer in decid-
ing on the necessity for rain or evaporation measurements. Prior to
starting tests, the expected evaporation rate should be related toa
rough estimate of the seepage rate.

For example, if the drop in water surface on a pond is 1.5 feet per day,
0. 04 foot of evaporation would not be important. On the other hand, in
a canal with a low loss rate, the evaporation correction may be signi-
ficant even during periods of cool weather. For instance, if the drop in
a pond from seepage is 0.10 foot per day and evaporation is 0. 02 foot
per day, then the evaporation should be measured.

If evaporation and precipitation are to be considered, evaporation rates
may be obtained from a nearby weather station, and can be used if they
are representative of those of the test site, or a temporary installation
can be made utilizing an evaporation pan adjacent to the pond, as pre-
viously shown in Figure 3. The one advantage of the latter is that the
effect of precipitation at the site can also be determined.


Motion of the water surface makes accurate vertical positioning of the
hook difficult. A stilling well around the hook should be provided to
minimize the movement. While elaborate shop-made stilling wells
have been used, one or more lengths of 6-inch-diameter stovepipe will
serve the purpose quite well, as shownin Figure 4. The stovepipe is
usually nailed to the gage timber at top and bottom. To restrict flow
of water into and out of the pipe, the lower end should be crimped
nearly shut. A damping effect is thus achieved which greatly reduces
the rise and fall of the water in the pipe compared to the fluctuations
in the pond.

A convenient method of installing the water stage recorder is to use a
50-gallon oil drum for a stilling well with appropriate cutouts in the
top for the recorder cables, etc. Other temporary setups can be made
as illustrated in Figure 5, it shows a barrel used as recorder stilling
well and staff gage, and Figure 6 shows turnout gate structure used
for installation of water stage recorder.

Figure 5, Photo P153-D-27829
Relating Effects of Evaporation and Rain

Under ordinary circumstances, as explained previously, evaporation
pans and rain gages are not usually necessary. However, there will
be evaporation from the pond surface; also, of course rain can occur.


If the pond is long or sub-
ject to unusual wind con-
ditions, hook and staff
gages are paired for use

at upstream and down-
stream ends of the pond.
The recorder is usually
installed at the lower or
midpoint of the pond. Dur-
ing windstorms, the water
surface or the pond may not
be horizontal throughout the
length of the pond; but, by
having gages at each end,
average water surface ele-
vations can be determined.
If the pond is reasonably
short (less than a quarter
of a mile), one staff gage, a
hook gage and a recorder
is all that will be required.

It is possible to run a pond-
ing test with less equipment
(such as a staff gage anda
recorder) to determine the
water surface elevation;
however, the precision used
to obtain field data deter-
Figure 1, Photo PX-D-53977 mines, to considerable ex-
tent, the accuracy of the
results obtained in the tests. In all cases, a staff gage is essential.

The staff gage is usually a standard enameled gage frequently used in
irrigation projects to indicate water depths and is scaled to 0.01 feet.

The water stage recorder usually has 1:1 staff gage versus chart read-
ing ratio and a time scale of about 9.6 inches per day.

The hook gage used is usually a laboratory type which can be read to
0.001 foot with the aid of a vernier.

It is advisable to use hook gages when seepage rates are low and the
time interval is short between readings; however, the decision whether
or not to use hook gages rests with the test observer.


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18




INTRODUCTION

Seepage losses from canals have been a problem to irrigation dis-
tricts and irrigation engineers for a long time. As the need for
conservation of land and water resources grows, the importance of
accurate knowledge concerning this type of water loss is increasing.
The technique which offers the most accurate method now known for
determining rates of loss is the ponding method, the use and impor-
tance of which is growing rapidly.

Studies using the ponding method can answer such questions as:

(a) How much does a given canal leak? (b) Where are the major
leakage areas? (c) Should a canal be lined? (d) Is an existing
canal lining effective ?

Many factors affect the rate of seepage loss from a canal. Some of
the more obvious are listed here:

Permeability of soil traversed by canal
Depth of water

Wetted area

Location of water table relative to canal invert
Slope of subgrade soil structure

Flow velocity

Soil and water temperatures

Entrained air in soil

Ground-water inflow

10. Atmospheric pressure

11. Soil and water chemistry

12. Capillary attraction

13. Surface seal in canal by silt, etc.

ODIMR OO hwWWH

The relative importance of each of the above factors has not been de-
finitely determined, though it is known that one may offset another,
and some may even alternately increase and decrease seepage rate.
Moreover, with so many potential factors affecting seepage, it is
impossible to write simple equations expressing their relationships.
However, it is the purpose of this bulletin to outline as briefly as
possible the procedures that should be followed and equipment needed
to provide satisfactory results in making a ponding test.

A more detailed treatment of this subject and the source of material
for this article is ''Hydraulic Laboratory Report, HYD-459." It can
be obtained on loan from the Bureau of Reclamation Library, Denver
Federal Center, Denver, Colorado 80225.

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c. When using check structures, release water until it flows over
the stoplogs at the checks, then stop the inflow.

d. Ina series of ponds, fill the reach of canal by gravity to the
upstream dike of the upstream pond, and allow water to flow
through gated culverts in the dikes to downstream sites.

In the ponding test, it is wise to plan on filling the pond at least twice;
and if the test reach is known to leak appreciably provision should be
made for a third filling. Only by repeatedly filling the pond can one
be reasonably sure that bank storage has been satisfied and that paths
of percolation are as active as in an operating canal.

The starting depth in the pond should be slightly higher than the design
or operating depth to secure data above and below that level. This pro-
cedure should be followed even though the operating level is below design
depth on a project not yet fully developed.

When more than a single filling is required, the refilling may be more
difficult than the initial one. Except when gated pipes through the dikes
have been provided, water must frequently be pumped into ponds utiliz-
ing upstream and downstream dikes. Deciding when to refill a pond
will depend on the rate of water surface drop, the depth of the pond, the
length of the pond, and the availability of water to accomplish refilling.
When the water surface drops rapidly, several tenths of a foot per day,
refilling may be necessary by the third day, particularly if the pond
depth is 5 feet or less. However, when the rate of drop is less than
0.1 foot per day, a pond of 4-foot depth or more can be allowed to seep
up to 2 weeks and may not require refilling.

Taking Readings

Measurement of the rate of drop in the water surface begins after the
pond is filled, all gages are set, and the recorder is operating. A sug-
gested form for recording staff and hook gage data during the test is
shown in Table III, page 20. Notations to indicate upstream and down-
stream gages should be used. The reading of each hook gage is recorded
to the nearest 0.001 foot and the staff gage to 0.01 foot. The extra col-
mns may be used to record the wind and wave conditions prevailing at
the time of reading and other information worth noting. The time of
each reading should be recorded to the nearest 5 minutes. This prac-
tice will usually enable the observer to read the gages at both ends of

a pond within a single 5-minute interval. In small ponds where the
seepage rate is high, it may be advisable to record the reading to the
nearest 1 minute.

Within a few hours after the initial readings of the gages and recorder,
test personnel will have reasonable indication of the rate of fall of the
water surface. From this knowledge, the required frequency of suc-
ceeding readings can be determined. If the pond seems to have a

13


st ane BO NOT REE

UNITED STATES
DEPARTMENT OF THE INTERIOR
BUREAU OF RECLAMATION
DIVISION OF IRRIGATION OPERATIONS
Denver, Colorado 80225

IRRIGATION O&M BULLETIN NO, 65
July, August, September 1968

MEASURING SEEPAGE IN IRRIGATION CANALS
BY THE
PONDING METHOD

CONTENTS

Determining the Length of Pond .......... 066 ccc ee ce ec aes
Constructing the Ponded Section ..............0..0 eee uee
Test Equipment Required
Installing Test Equipment .
Relating Effects of Evaporation and Rain

Observation Wells
Surveying the Pond
Calculations Using Survey Notes 2.0%. secs eee he eens vee
Filling the Pond, ws saree ¢ esis aes 5 we 40
Taking Readings....
Computing Loss Rates. n eiieey mene
Interpreting Test Date, « sures « seis convw w crac somvepe a ome acmmave s nines 15
Sources OL Ber Gr aes a wise save veers wi ome & ewes sans ¥ ataES © tame wow 16





The Irrigation Operation and Maintenance Bulletin is published quar-
terly, for the benefit of irrigation project people. Its principal pur-
pose is to serve as a medium of exchanging operation and mainte-
nance information. It is hoped that the material in this issue, which
summarizes procedures for performing ponding tests to measure
seepage losses from canals, will result in improved system effi-
ciency and reduced maintenance costs.

OK OK OK OK

Division of Irrigation Operations
Office of Chief Engineer
Denver, Colorado

COVER PHOTOGRAPH: .
This is a view of Recorder
Station upstream from Sta-
tion 581+40, on the Eden ‘
Project, Upper Colorado

Basin, Wyoming.

Photo P153-D-27834

*



Observation Wells

The use of observation wells to supplement ponding studies is occa-
sionally worth the added expense. They may be used to log the sub-
surface soils, to locate the water table, to observe the slope in hy-
draulic gradient from uphill to downhill sides of the pond and to indi-
cate the effectiveness of linings in place. However, under ordinary
circumstances observation wells are not needed and so will not be
discussed here.

Surveying the Pond

A survey to establish the shape of the pond is usually required. It

will be an advantage to survey before filling the pond to enable simul-
taneous testing and computing of necessary data from the survey notes,
so that, at the conclusion of the ponding tests, computation of seepage
rates may proceed without delay.

A comprehensive survey is not necessary for alltests. For instance,

in the case of concrete-lined canals, very few cross sections will prob-
ably be required to establish the as-built shape. Ifa recently built canal
is earth or earth lined with little erosion or deposition of materials,

the cross section may be quite uniform. In this case, the cross sec-
tion need be checked only at a few stations.

Where detailed survey is required in older earth canals, cross sections
should be taken about every 50 feet and elevations measured to within
0.1 foot. The survey should establish the shape of the canal to an ele-
vation about 1 foot above the anticipated water test level.

The condition of the canal may be used as a criterion in deciding at
what intervals to cross section. In a canal that is irregular in cross
section, the intervals required to obtain a satisfactory record of shape
may be short. Test personnel must use their best judgment to secure
measurements of appropriate accuracy.

If the pond is short, 1,000 feet or less, consideration should be given
to taking cross sections about every 50 feet to increase accuracy.
When the pond is very long, such as one which has been created up-
stream of a check structure in a canal with a very gradual slope, cross
sections every 100 feet may be reasonable.

As mentioned previously in connection with the installing of test equip-
ment, exact elevation is not necessary to establish temporary bench
mark elevations on nearby structures. These benches can be used to
survey the pond, set the hook gage point and the staff gages, and deter-
mine operating water surface elevation.

Calculations Using Survey Notes

From the survey of the pond, the following are calculated:


Figure 8, Photo PX-D-25225

Double Dike Installation Showing
Four Pumps For Filling Pond