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 decline to background EC as the Solution 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 revious Streamline articles In this approach, a volume of salt move past an observation point will Pintroduced the general principles solution, V (m³), is injected as a depend inversely on the mean of stream gauging by salt dilution 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 Part III: Slug Injection gauge flows up to 10 steep, highly stream flow faster than m³/s or greater, others (e.g., flow tends to Using Salt in Solution 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 Paper passage of the salt Re-creating Meandering Streams in the Central Oregon Coast Range, USA Results of Streamline Reader Survey 2004 Update Continued 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 Technical reviewers this issue: S. Babakaiff, L. Barr, R. Doucet, B. Eaton, T. Giles, B. Grainger, D. Hutchinson, P. John Heinonen 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 found that the time required for the = Editing: Ros Penty Q [3] salt wave to pass typically varies from ò RC() t dt Cover Illustration: William McAusland a couple of minutes (e.g., Figure 2) to T McAusland Studios, Kamloops, B.C. over 20 minutes. Under low-flow 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 D and technically sound information is for accurate measurements (e.g., well discrete time interval t (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 contributing author(s), not those of passage, the discharge of tracer ()@ ()D FORREX, our editorial staff, or our funding ò RC t dtå RC t t [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: Tn guidelines on article submission or with where n is the number of your comments and suggestions. qt()=× Q RCt () [1] measurements during the passage of the salt wave. The relative This publication is funded in part by the where Q is the stream discharge (L/s British Columbia Ministry of Forests concentration can be determined or m³/s) and RC(t) is the relative through the Forest Investment Account, 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 conductivity of the stream, and k is a Printed on recycled paper salt wave, and if the stream discharge 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 Vò qtdt Qò RCtdt[2] practical equation can be derived for TT computing discharge: 2 Streamline Watershed Management Bulletin Vol. 8/No. 2 Spring 2005 = V available. In addition, the salt solution is mixed in one container Q [6] concentrations and durations of then decanted into a second, ktD å[() ECt- ECbg ] exposure normally involved in 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 changes in EC at intervals of Dt until volume. during slug injection, but concluded EC returns to background levels; and that salt injection had a relatively Required Volumes of Injection determine the calibration constant, k. short-term effect and is unlikely to Solution 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 salt solution and its concentration, as volume as possible because the slow handled, but it also needs to remain well as the mixing characteristics of exchange of tracer between the pool less than the solubility. Given the low the stream. Those streams with less volume and the flowing portion of the temperatures often associated with longitudinal dispersion will exhibit a stream will greatly increase the time field conditions, the maximum more peaked salt wave with higher required for the salt wave to pass. An concentration that will dissolve readily concentrations, and will require lower ideal reach begins with an injection is about 20%, or about 1 kg of salt in injection volumes. site upstream of a flow constriction 5 L of water (Østrem 1964; Kite (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 S/cm) 15 The injection streams draining catchments m solution does not underlain by granitic bedrock, to over EC ( 10 need to be mixed 400 mS/cm for low-flow conditions in from local streams sustained by groundwater 5 streamwater.