Slug Injection Using Salt in Solution
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Volume 8 Number 2 Spring 2005
Introduction to Salt Dilution Gauging for Streamflow Measurement Part III: cloud will cause EC to increase from
its background value to a peak value,
Slug Injection Using Salt in corresponding to the passage of the
core of the cloud, followed by a
Solution decline to background EC as the
trailing edge of the cloud passes,
resulting in a characteristic salt wave
R.D. (Dan) Moore (Figure 2). Longitudinal dispersion
reduces the peak EC of the salt wave
as it travels downstream. The time
Introduction Conceptual Basis required for the peak of the wave to
P revious Streamline articles
introduced the general principles
of stream gauging by salt dilution
In this approach, a volume of salt
solution, V (m³), is injected as a
move past an observation point will
depend inversely on the mean
near-instantaneous slug or gulp at one velocity of the streamflow, while the
(Moore 2004a) and the procedure for location in the stream. Following duration of the salt wave will depend
constant-rate injection (Moore injection, the salt solution mixes Continued on page 2
2004b). While constant-rate injection rapidly throughout the depth of the
is best suited for use in small streams stream and less rapidly across the Inside this issue:
at low flows (discharges stream width as it travels
less than about 100 L/s downstream with the Introduction to Salt
or 0.1 m³/s), slug Slug injection general flow of water. Dilution Gauging for
injection can be used to works well in Because some portions of a Streamflow Measurement
gauge flows up to 10 stream flow faster than Part III: Slug Injection
steep, highly Using Salt in Solution
m³/s or greater, others (e.g., flow tends to
depending upon
turbulent be faster in the centre than
streams. An Inexpensive,
channel characteristics. near the banks), the cloud
Automatic Gravity-fed
Slug injection works of salty water “stretches”
Water Sampler for
well in steep, highly downstream in a process
Investigating Water
turbulent streams, such as the called longitudinal dispersion. This
bouldery mountain channel dispersion results in the cloud having
Quality in Small Streams
shown in Figure 1. This article a leading edge with relatively low Live Gravel Bar Staking
introduces the conceptual concentrations of salt solution, a Channel Stabilization in
basis and field procedures central zone of high concentrations, the Lower Elk River
for slug injection using followed by a trailing edge of
salt in solution. decreasing concentration. A Qualitative
Hydro-Geomorphic Risk
If the electrical conductivity (EC) is
Analysis for British
recorded at some point downstream,
Columbia’s Interior
where the tracer has been completely
Watersheds: A Discussion
mixed across the stream width, the
passage of the salt
Paper
Re-creating Meandering
Streams in the Central
Oregon Coast Range, USA
Results of Streamline
Reader Survey 2004
UpdateContinued from page 1
Published by:
FORREX–Forest Research
Extension Partnership
Suite 702, 235 1st Avenue
Kamloops, BC V2C 3J4
Project Manager:
Robin Pike
Tel: (250) 387-5887
Distribution/Mailing List:
Janet Jeffery
Tel: (250) 371-3923
Technical review committee:
R. Scherer, R.D. Moore, R. Pike
John Heinonen
Technical reviewers this issue:
S. Babakaiff, L. Barr, R. Doucet, B. Eaton,
T. Giles, B. Grainger, D. Hutchinson, P.
Jordan, M. Miles, R.D. Moore, R. Pike, D.
Figure 1. Place Creek at high flow during summer glacier melt.
Polster, P. Raymond, R. Scherer, M.
Schnorbus, K. Swift, P. Teti, R. Winkler
on the amount of longitudinal where T represents the salt wave
Publication and Web Site Support: dispersion, which, in turn, depends on
Jesse Piccin, Satnam Brar, Julie Schooling
duration (s). Equation [2] can be
how variable the stream velocities are rearranged to solve for Q:
Graphic Layout: SigZag Design across the stream. The author has
V
Editing: Ros Penty
found that the time required for the Q= [3]
Cover Illustration: William McAusland
salt wave to pass typically varies from
a couple of minutes (e.g., Figure 2) to
ò RC (t)dt
McAusland Studios, Kamloops, B.C.
over 20 minutes. Under low-flow
T
Streamline is published twice a year by conditions with low velocities, the In practice, RC(t) is determined at the
FORREX. All articles published in duration can be longer than desired downstream measurement point at a
Streamline are reviewed to ensure reliable
and technically sound information is for accurate measurements (e.g., well discrete time interval Dt (e.g., 1 or 5
extended to our readers. Content over 30 minutes). s), and the integral is usually
published in Streamline reflects the approximated as a summation:
opinions and conclusions of the At any time (t) during the salt wave
ò RC (t)dt @ å RC (t)Dt
contributing author(s), not those of passage, the discharge of tracer
FORREX, our editorial staff, or our funding [4]
partners. Please contact Robin Pike, solution q(t) (L/s or m³/s) past the
Streamline Project Manager, for further point will be approximated by: T n
guidelines on article submission or with where n is the number of
your comments and suggestions. q(t ) = Q × RC (t ) [1] measurements during the passage of
This publication is funded in part by the the salt wave. The relative
where Q is the stream discharge (L/s
British Columbia Ministry of Forests concentration can be determined
through the Forest Investment Account, or m³/s) and RC(t) is the relative
from EC:
Forest Science Program, and by the concentration of tracer solution (L/L)
USDA Forest Service. in the flow at time (t). Equation [1] RC (t ) = k[ EC (t ) - ECbg] [5]
assumes that q(t) is much smaller than
ISSN 1705-5989 Q, which should be true in virtually all where EC(t) is the electrical
Printed in Canada cases. If the tracer discharge is conductivity measured at time t, ECbg
© FORREX–Forest Research Extension integrated over the duration of the is the background electrical
Partnership
salt wave, and if the stream discharge conductivity of the stream, and k is a
Printed on recycled paper
is constant over that time, then the calibration constant. The calibration
http://www.forrex.org/streamline following equation should hold for a constant, k, depends primarily on the
conservative tracer (i.e., one that does salt concentration in the injection
not react with other chemicals in the solution and secondarily on the
water, bind to sediment, or otherwise chemical characteristics of the
change as it flows downstream): streamwater. Combining Equations
[3], [4], and [5], the following
V = ò q(t ) dt = Q ò RC (t ) dt [2] practical equation can be derived for
computing discharge:
T T
2 Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 2005V available. In addition, the salt solution is mixed in one container
Q= [6]
kDt å[ EC (t ) - ECbg] concentrations and durations of
exposure normally involved in
then decanted into a second,
pre-calibrated container (e.g., Østrem
n discharge measurement are less than 1964). This procedure ensures that
To apply Equation [6], we need to thresholds associated with deleterious the salt in the injection solution is
know V, the volume of salt solution effects on organisms (Moore 2004a). completely dissolved, and allows
injected; measure the resulting Wood and Dykes (2002) observed accurate measurement of the injection
transient increases in invertebrate drift volume.
changes in EC at intervals of Dt until
during slug injection, but concluded
EC returns to background levels; and Required Volumes of Injection
that salt injection had a relatively
determine the calibration constant, k. Solution
short-term effect and is unlikely to
Field Procedures have any long-term deleterious The accuracy of a measurement
Choice of a Measurement Reach impacts on invertebrate communities depends on how much EC increases
at most locations. above background during the salt
Successful application of the slug
wave passage, relative to the accuracy
injection technique requires a stream The salt concentration in the injection
of the conductivity probe. The change
reach that generates complete lateral solution should be high enough to
in EC during the salt wave passage
mixing in a short distance. Selected increase EC reasonably when using
depends, in turn, on the volume of
reaches should have as little pool volumes of solution that can be easily
handled, but it also needs to remain salt solution and its concentration, as
volume as possible because the slow
less than the solubility. Given the low well as the mixing characteristics of
exchange of tracer between the pool
temperatures often associated with the stream. Those streams with less
volume and the flowing portion of the
field conditions, the maximum longitudinal dispersion will exhibit a
stream will greatly increase the time
concentration that will dissolve readily more peaked salt wave with higher
required for the salt wave to pass. An
is about 20%, or about 1 kg of salt in concentrations, and will require lower
ideal reach begins with an injection
5 L of water (Østrem 1964; Kite injection volumes.
site upstream of a flow constriction
(e.g., where the flow narrows around 1993). We have found that a mixture Kite (1993) suggested that peak EC
a boulder, promoting rapid lateral of 1 kg of salt with 6 L of water should be 50% higher than
(roughly a 17% solution) provides a background, while Hudson and Fraser
30 suitable (2002) suggested that peak EC should
compromise be at least 5 times higher than
25 between strength background. Background EC in B.C.
and ease of streams typically ranges from about
20
dilution. 10 mS/cm for stormflow conditions in
EC (mS/cm)
15 The injection streams draining catchments
solution does not underlain by granitic bedrock, to over
10 need to be mixed 400 mS/cm for low-flow conditions in
from local streams sustained by groundwater
5
streamwater. discharge. The author suggests that
Where access to increasing EC by 100–200% of
0
0 60 120 180 240 the stream does background should be adequate for
Time (s) not involve a long streams with low background EC (less
hike, it is often than about 50 mS/cm), while Kite’s
Figure 2. Example of salt wave in Place Creek.
convenient to (1993) guideline should be reasonable
mixing) and contains no pools or pre-mix the for streams with background EC
backwater areas below the injection solution to allow generous greater than about 100 mS/cm.
constriction. A rough guideline is that time for dissolution and to minimize
Table 1 summarizes the
the mixing length should be at least time spent at the field site.
masses/volumes of injected salt/salt
25 stream widths, but complete Note that the volume of the injection solution used by various authors. The
mixing may require much longer or solution will be greater than the range reflects the diversity of channel
shorter distances, depending on volume of water used to mix it. We morphologies and discharges
stream morphology (Day 1976). have found that when a 1-kg box of encountered in the different studies.
Mixing the Injection Solution salt is mixed with 6 L of water, the The author recommends starting with
We use NaCl (table salt) as a tracer resulting solution has a volume of 1 L of 15–20% solution per m³/s.
because it is inexpensive and readily 6.36 L (±0.01 L). Commonly, the salt Greater volumes of injected salt
Continued on page 4
Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 2005 3Continued from page 3
placed cobbles) so that it will not detailed description of the procedure
solution may be appropriate for wider move during the measurement. To and the calculation of k.
streams that require longer mixing position the probe in a strong current,
Although ideally the calibration is
reaches, while lower volumes may it may be useful to attach the probe
performed in the field, particularly to
work for narrower streams. To avoid to a rod weighted at the end that is
maintain water temperature as close
excessive salt concentrations in the placed in the water. to stream temperature as possible, it
stream, one or more trial injections
In some cases, the background EC can also be conducted in the
should be conducted with low
may vary. One possible cause is an laboratory. To perform the calibration
volumes, working up to larger
overly sensitive conductivity meter. off site, two 1-L samples of
volumes as required.
streamwater should be measured
Table 1. Volumes/masses of injected salt used in different studies accurately into sample bottles using a
Author Mass of salt Equivalent volume (L) volumetric flask. A sample of the
injected per m³/s of 20% salt solution injection solution should also be taken
streamflow (kg) (1 kg salt in 5 L water)
in a small glass (not plastic) bottle to
Østrem (1964) 0.5 2.5 avoid potential problems with salt
Church and Kellerhals (1970) 0.2 1 sorbing onto the walls of a plastic
Day (1976) 0.3 1.5 bottle. The calibration can then be
Elder et al. (1990) 5 25 conducted following the procedure
Hudson and Fraser (2002) 2 10 described by Moore (2004b).
Figure 2 illustrates a salt wave for Another cause of varying background Summary of Field
Place Creek, where the author has EC is incomplete mixing of Procedures
found the salt waves to be highly streamwater and groundwater (which Table 2 lists the equipment required.
reproducible. Injecting 6.35 L of a typically has higher EC than the Suggested steps for conducting field
roughly 17% solution into a flow of streamwater) within and immediately measurements are as follows:
2.66 m³/s produced a peak EC about downstream of groundwater
100% higher than background. discharge zones. Similar problems 1. Mix injection solution (either at
with incomplete mixing can occur office or on site).
Recording Electrical Conductivity
downstream of tributaries. In these 2. Select measurement reach.
Ideally, a data logger should be used latter cases, find an observation point
to record the passage of the salt wave. where background EC is uniform 3. Use a pipette to extract a known
Some conductivity meters have volume of injection solution (e.g., 10
across the channel and constant in
built-in data logging, while others can time. mL) and add to the secondary
output a signal that can be recorded solution bottle. Cap the bottle and
using a separate data logger. If you do Determining k by Calibration store upright.
not have data logging capacity, To determine k, a known volume of 4. Record background EC and water
record EC manually at 5-s intervals. injection solution (typically 5 or 10 temperature at the downstream end
Although this approach may not be as mL) is added to a known volume of of the measurement reach, and
accurate as using a data logger and a streamwater (typically 1 L) to produce upstream of the injection point.
1-s recording interval, it can produce a secondary solution. Known
satisfactory results. In most cases, two increments of this secondary solution 5. Set up the conductivity probe at
people are required to conduct a salt the downstream end of the mixing
are then added to a second known
dilution measurement with manual reach. Record the background EC and
volume of streamwater (typically 1 L), water temperature. If you have a data
recording, while the use of a data to generate a set of EC values
logger allows a single person to make corresponding to different values of logger, start recording EC.
the measurement. relative concentration. The slope of 6. Inject a known volume of salt
The conductivity probe should be the relation between relative solution at the upstream end of the
placed within the main part of the concentration and EC provides the mixing reach.
flow, not in a backwater. Avoid required value for k. This two-step 7. Record the passage of the salt
locations with substantial aeration, as procedure dilutes the injection wave, continuing until EC returns to
air bubbles passing through the probe solution to the relative concentrations background. If EC does not return to
cause spurious drops in conductivity. observed during the salt wave without background, measure EC upstream of
The probe should be firmly emplaced using large volumes of streamwater. the injection point again to determine
(e.g., by wedging it between carefully See Moore (2004b) for a more whether the background changed.
4 Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 20058. Measure a volume V0 (e.g., 1 L) of
streamwater using the volumetric flask Worked Example
and pour into the secondary solution Figure 2 shows a salt wave recorded during a slug-injection measurement
bottle, which already contains the at Place Creek, located about 30 km northeast of Pemberton, B.C. Place
sample of injection solution. Cap the Creek, a steep, bouldery mountain stream, would be impossible to gauge
bottle and shake vigorously to mix the accurately using a current meter (Figure 1). The volume of injection
streamwater and injection solution. solution was 6.35 L. This volume resulted from mixing 1 kg of salt with 6 L
This mixture is the secondary solution. of water (to produce 6.36 L of solution), followed by extracting 10 mL
9. Measure a volume Vc (e.g., 1 L) of (0.01 L) of injection solution for use in the calibration procedure. The
streamwater using the volumetric flask stream EC data were logged at 1-s intervals, and the calibration constant
and pour into the calibration tank. was 2.99·10–6 cm/mS.
Immerse the calibration tank in a V 6.35 ×10 -3 m 3
shallow pool at the stream’s edge. Q= = -6
= 2.66 m 3 /s
k Dt S[ EC(t) - EC bg ] ( 2.99 ×10 cm/mS)(1 s)(797mS/cm)
Keep the temperature in the tank as
close to stream temperature as
possible (Moore 2004b). To
where the salt plasticware into the field as a backup
help hold the calibration tank in
Under suitable wave is recorded. in case of breakage.
place, position a “corral” of
In addition,
cobbles around it. conditions, discharge should
If it is raining during the
streamflow measurement, ensure that the
10. Perform the calibration and not change
measurements calibration tank is sheltered.
determine k using the appreciably
Otherwise, rain falling into the tank
procedure described by Moore made by slug during the
may dilute the concentrations below
(2004b), then compute the injection can be injection trial.
the calculated values, producing
discharge using Equation [6].
precise to within Errors may arise biased calibrations.
Errors and Limitations about ±5%. through
The slug injection method may not be
inaccuracies in
Under suitable conditions, appropriate when the channel
measuring the
streamflow measurements contains ice and (or) snow. In such
volumes of
made by slug injection can be precise streamwater, injection solution, and cases, low velocities may result in
to within about ±5% (Day 1976). poor lateral mixing and excessively
secondary solution. These errors can
Accurate measurements require that long salt wave durations, particularly if
be effectively minimized if a
(1) the salt in the injection solution be volumetric flask is used to measure salt solution flows into slush zones
completely dissolved, and (2) the within the measurement reach.
streamwater and glass pipettes used
injection solution be fully mixed to measure the injection and The method will be subject to
across the channel at the location secondary solutions. However, take substantial errors if the measurement
Continued on page 6
Table 2. Equipment list for field measurement of streamflow using slug injection of salt
Item Purpose
1-L volumetric flask Measuring streamwater
1-L plastic graduated cylinder Backup in case volumetric flask breaks
Plastic measuring cup with handle Pouring streamwater into volumetric flask
Squirt bottle Topping up streamwater in volumetric flask
1,2
5- and 10-mL pipettes Measuring injection solution to mix secondary solution
Pipette filler (rubber squeeze bulb) Drawing water into pipettes
1- or 2-L wide-mouth Nalgene water bottle Mixing the secondary solution
1- or 2-L Nalgene beaker or pail Calibration tank
2-, 5-, and 10-mL pipettes1,2 Measuring secondary solution
Plexiglas rod or tubing, 30 cm long Stir stick for calibration tank
Conductivity probe and meter Measuring EC during salt wave passage and for calibration
Data logger (desirable but optional) Recording EC during salt wave passage
1
Separate sets of pipettes need to be used for measuring the injection and secondary solutions.
2
Spare pipettes should be carried in case of breakage in the field. In addition, 10-mL plastic graduated cylinders or graduated pipettes could be carried as backups
Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 2005 5Continued from page 5
reach is not sufficiently long to ensure higher flows (e.g., >5–10 m³/s), it is For further information, contact:
complete lateral mixing. Unlike easier to inject dry salt than to mix
Dan Moore, Ph.D., P.Geo.
constant-rate injection, where lateral and inject adequate volumes of
Associate Professor
mixing can be verified once solution. However, a disadvantage of
Departments of Geography and Forest
steady-state conditions have been the dry salt method is that an
Resources Management
achieved, assessing mixing is more accurate scale to measure the mass of
difficult with slug injection. If two salt or an adequate supply of 1984 West Mall
University of British Columbia
probes are available, then the salt pre-weighed salt in a range of
Vancouver, BC V6T 1Z2
wave can be recorded at two quantities is needed. Where an
downstream distances or on either Tel: (604) 822-3538
accurate scale and pre-weighed
side of the stream. If mixing is E-mail: rdmoore@geog.ubc.ca
quantities of salt are unavailable (e.g.,
complete, discharge calculated from at a remote site over an extended field References
both probes should be in reasonable season), the slug injection method
agreement. If this is not the case, a using salt solution would still be Church, M. and R. Kellerhals. 1970. Stream
longer mixing reach is required. possible because the precise mass of gauging techniques for remote areas
Alternatively, if only one probe is using portable equipment. Department
salt in the injection solution does not of Energy, Mines and Resources Inland
available, successive measurements need to be known, just the volume of Waters Branch, Ottawa, Canada.
can be made during periods of steady the solution (Equation [6]). Technical Bulletin No. 25.
flow using different distances. Day, T.J. 1976. On the precision of salt
dilution gauging. Journal of Hydrology
Problems can occur if the conductivity Summary 31:293–306.
does not return to background. If the Streamflow measurement by slug Elder, K., R. Kattelmann, and R. Ferguson.
measurements taken upstream show injection of salt solution has been 1990. Refinements in dilution gauging
for mountain streams. In Hydrology in
that the background has truly successfully applied in many locations Mountainous Regions. I - Hydrological
changed, then an average of the around the world. It is particularly Measurements; the Water Cycle.
original and final background values suitable for steep, bouldery mountain International Association for
may be used in Equation [6]. It is streams, which are unsuitable for Hydrological Science (Proceedings of
more problematic if EC has not two Lausanne symposia, August 1990).
gauging by conventional current
IAHS Publication No. 193, pp.
returned to background due to a slow metering techniques. This article has 247–254.
release of stored salt solution within described procedures that the author Hongve, D. 1987. A revised procedure for
the mixing reach, as can occur in has found useful at sites throughout discharge measurements by means of
reaches with pools, particularly at British Columbia. However, there is the salt dilution method. Hydrological
lower flows. In such cases, one Processes 1:267–270.
great scope to vary the details to suit
Hudson, R. and J. Fraser. 2002. Alternative
solution would be to extend the tail of individual circumstances and users are methods of flow rating in small coastal
the salt wave by fitting an exponential encouraged to experiment with the streams. B.C. Ministry of Forests,
decline to the values, although the outlined procedure. Vancouver Forest Region. Extension
actual form of the decline will still be Note EN-014 Hydrology. 11 p.
uncertain (Elder et al. 1990). Ideally, Kite, G. 1993. Computerized streamflow
Acknowledgements measurement using slug injection.
one should find a reach with minimal Hydrological Processes 7:227–233.
George Richards helped me
storage. Moore, R.D. 2004a. Introduction to salt
experiment with variations on the slug
dilution gauging for streamflow
injection method while working at
Injection of Salt in Solution Place Creek. Tim Giles, Dave
measurement: Part 1. Streamline
Watershed Management Bulletin
Versus Injection of Dry Salt: Hutchinson, Scott Babakaiff, and John 7(4):20–23.
A Comparison Heinonen helped refine the methods Moore, R.D. 2004b. Introduction to salt
A number of authors have advocated by asking valuable questions and dilution gauging for streamflow
measurement Part II: Constant-rate
the use of dry salt injection as an bringing relevant articles to my injection. Streamline Watershed
alternative to injection of salt in attention. Comments on earlier Management Bulletin 8(1):11–15.
solution (Hongve 1987; Elder et al. versions of this article by John Østrem, G. 1964. A method of measuring
1990; Kite 1993; Hudson and Fraser Heinonen, Russell White, Michael water discharge in turbulent streams.
Church, Robin Pike, and four Geographical Bulletin 21:21–43.
2002). A future Streamline article will
Wood, P.J. and A.P. Dykes. 2002. The use
focus on streamflow measurement by anonymous reviewers helped improve
of salt dilution gauging techniques:
dry salt injection. The key advantage its clarity. However, any errors remain ecological considerations and insights.
of the method is that, for gauging my sole responsibility. Water Research 36:3054–3062.
6 Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 2005You can also read
