GGR112_F2021_Lab 4 Measuring Stream Discharge Lab 4: Measuring Stream Discharge Grading: • Answer to the questions at the end of this document are to be submitted on Quercus Due Date: The assignment...

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GGR112_F2021_Lab 4 Measuring Stream Discharge Lab 4: Measuring Stream Discharge Grading: • Answer to the questions at the end of this document are to be submitted on Quercus Due Date: The assignment due date depends on the day of your practical, as outlined in the syllabus. Figure 1: Sawmill Creek 2 hours after a 30 minute summer storm contributing approximately 10 mm of precipitation Introduction: We measure discharge primarily as a tool for flood forecasting and watershed health. Manual measurement of stream discharge is the first step to establishing an automated stream gauging station. Automated discharge monitoring stations are established at points in the stream where the geometry of the stream-bed profile is well described. Knowing the profile of the stream channel allows us to monitor stream stage which is simple to measure (an automated water level sensor is all that is needed) and from that infer discharge using a relationship between stage and flow that is derived from gauging the stream manually. In the broader context, measuring the response of a stream to precipitation inputs can tell us a lot about the nature of the watershed and its resiliency and health. Knowing the discharge in a particular stream is most valuable when monitored continuously and combined with precipitation data for the same time period. Presented together, discharge and rain-fall data show an approximation of the inputs (the addition of water via precipitation or rain-fall) and outputs (discharge, outflow through a stream channel) of a watershed. Objective: In this lab you will compare two methods of determining the discharge of a stream. The first is the traditional method using a current meter, while the alternative method is tracer dilution which is often used in turbulent and low-flow conditions where flow speed varies across the width of the channel or where it too shallow to practically use a current meter. You will work in a large group and gather data on both methods to compare them to the official dataset collected by the Credit Valley Conservation Authority. Materials and Logistics: During your practical session, the TAs will provide an overview of the Current Meter and the Tracer Dilution methods of stream discharge measurement. You must also watch the tutorial videos for each method (below. Tracer Dilution Method Instructional Video (https://play.library.utoronto.ca/play/cLhQPd0hmaiT) Current Meter Instructional Video (https://play.library.utoronto.ca/play/fOgLQTn8pfqZ) The materials used in the Current Meter and Tracer Dilution methods include: • Hach HQ40d portable water quality meter equipped with a CDC101 Conductivity Probe • Swoffer 2100 Current Meter • 2x 30m measuring tape • 18.9 L bucket • 1 litre graduated cylinder • 2 kg of salt • A stopwatch/timer Stream water data (excel file on Quercus, ‘Stream Lab Data Sheet W2021.xlsx’) Field Data: Field data was previously collected from the Sawmill Creek, just West of campus. This data was collected using the Current Meter and Salt tracer discharge measurement methods – a brief overview of the steps involved in each method are outlined below. Please ensure that you also watch the instructional videos provided above. The data - an excel file - is available on the Quercus webpage. For stream gauging with the salt tracer method: 1. Choose an injection site 2. Set up a conductivity measurement point approximately 15-20 meters downstream from the injection point (the exact distance is not critical but must be recorded) 3. Rinse the bucket well with stream water 4. Measure out 10 litres of stream water into the bucket 5. Test conductivity of stream water in the bucket and record it (note the units! Are they mS/cm or μS/cm?) 6. Add 2 kg of salt and stir to mix 7. Test the conductivity of the tracer (note the units! Are they mS/cm or μS/cm?) 8. Record the conductivity value for the tracer 9. Have a partner bring the conductivity meter downstream to the measuring team 10. Downstream team measures and records the baseline conductivity of the stream 11. Coordinate with your downstream partners, signaling when you inject the tracer 12. Inject the tracer in one big pour (the more sudden the better) and begin timing 13. Meet your group downstream 14. Record conductivity every ten seconds 15. The conductivity should rise as the tracer passes by then return back to its baseline value 16. Once the tracer has passed and the value has returned to ambient levels (+- 15µS) your gauging is complete For stream gauging with the current meter: 1. Choose a gauging site (can be between the tracer injection and conductivity measurement point) 2. String the measuring tape across the stream to obtain width and record the width 3. Assemble the current meter if necessary 4. Gauge the depth and velocity of the stream for each 30 cm width interval record the velocity and depth for each measurement Field Data – collected values: For the current meter data, two variables were recorded for each 30 cm width measurement interval. 1. Velocity (m-1·s-1) as measured in each 30cm width interval 2. Depth (m) as measured in each 30cm width interval For the tracer injection, three variables were recorded: 1. Background conductivity of the stream (prior to injection) (μS·cm) 2. Time-series conductivity data measured every 10 seconds (μS·cm) 3. Dry mass of NaCl (salt) added to the stream Data Analysis and Presentation: Discharge Measured Using the Current Meter The table below depicts an example of how to calculate discharge data for the stream. Multiply the velocity (m-1·s-1), width (m) and depth (m), then sum all products to obtain the total stream discharge. In this example the discharge of the 90cm wide stream is 0.047 cubic metres per second. Since a cubic metre is 1000 L this stream discharges only 47 litres per second. Sawmill Creek will have a higher discharge since there will likely be between 6 and 15 30 centimetre intervals across its width. Table 1: Example of discharge calculation for data collected using the current meter Interval # Velocity (m-1·s-1) Width (m) Depth (m) Interval Discharge (m3·s-1) 1 0.48 × 0.3 × 0.15 = 0.0216 2 0.36 × 0.3 × 0.08 = 0.00864 3 0.55 × 0.3 × 0.1 = 0.0165 Add all products à 0.047 m3·s-1 Discharge Measured Using the Tracer Injection Method To compute the discharge based on the tracer injection method, the math is considerably more complicated. We need to determine the change in conductivity that our salt solution caused in the stream. The magnitude and duration of increased conductivity as the salt solution passes the measuring site is used to calculate discharge. This works because the stream has a baseline conductivity which we modify using a known volume of high-conductivity tracer. The larger the volume of water discharged by the stream, the lesser the impact of our tracer (because it is more dilute). We have prepared an Excel spreadsheet for you to calculate discharge based on the provided field data. The spreadsheet includes the following inputs from the measurement: • Distance from injection point to measurement point [Cell B1] • Background conductivity value (measured in stream) [Cell B5] • Stream conductivity values measured at regular intervals (10 sec) [Cell B20 and down] • Mass of NaCl (salt) added [Cell B11] Your field data will look something like Table 2. You can see the peak at approximately 60 seconds following injection, and a gradual return to baseline after 4.5 minutes. You can see this example in the Excel file we have provided on Quercus. Table 2: Example of field tracer injection conductivity data TIMESTAMP Conductivity sec mS cm-1 0 1218 10 1223 20 1225 30 1360 40 1560 50 1758 60 1838 70 1862 80 1770 90 1691 100 1616 110 1550 120 1497 130 1447 140 1395 150 1363 160 1336 170 1320 180 1304 190 1285 200 1280 210 1281 220 1273 230 1268 240 1264 250 1260 260 1258 270 1234 280 1220 Assignment Questions and Assignment Submission: You will submit your answers to the questions below on Quercus. Data from the conservation authority will be distributed on Quercus following your labs. Tracer Dilution Questions 1. What was the mass of NaCl (salt) added to the stream? (0.5 marks) 2. What was the peak concentration of salt (after the background concentration is subtracted) at the monitoring point (0.5 marks) 3. By what factor was the conductivity of the tracer higher than the background conductivity of the stream? ( !"#$%!&'('&)!"#$%" ) (1 mark) !"#$%!&'('&)&#$'(")*+, 4. How long, in seconds, did it take for your tracer (I.e., maximum concentration) to arrive at the measurement site? (0.5 marks) 5. What was the stream velocity (m1 s-1) and discharge (m3 s-1) as calculated by the data entry spreadsheet for the tracer injection method? (0.5 marks) 6. Assuming that the cross-sectional area at the monitoring site is 0.6 m2, what was the stream velocity (ms-1) (1 marks)? 7. Consider carefully what factors might impact the accuracy of the tracer injection method. Name and describe at least two factors that you think would negatively affect the accuracy of this method to gauge the discharge of the stream. Consider small streams like those around campus, as well as larger rivers. (3 marks) Current Meter Questions 8. Complete the following table by calculating the discharge for each interval as well as the discharge for the entire stream. This table can also be found in the provided excel spreadsheet. Show your work by providing the calculation for interval #1 (3 marks). Interval # Velocity(m·s-1) Width (m) Depth (m) Cross Sectional Area (m2) Interval Discharge (m3·s-1) 1 0.33 0.3 0.08 0.024 2 0.36 0.3 0.08 0.024 3 0.25 0.3 0.17 0.051 4 0.62 0.3 0.18 0.054 5 0.12 0.3 0.06 0.018 6 0.43 0.3 0.08 0.024 7 0.12 0.3 0.09 0.027 8 0.25 0.3 0.09 0.027 9 0.29 0.3 0.11 0.033 10 0.32 0.3 0.13 0.039 SUM 0.321 9. How many 30cm width intervals were there at the current meter measurement site? (0.5 marks) 10. Considering each of the reported stream intervals independently, what was the maximum velocity of the stream measured using the current meter? (0.5 marks) 11. Describe what