CEE 370 Lab Fall Lab Exercise #1: Measuring Streamflow

Transcription

1 CEE 370 Lab Fall 2015 Lab Exercise #1: Measuring Streamflow Objective: Measure volumetric flow rate (Q) and mean velocity (v) of a stream. Introduction: Streamflow or volumetric flow rate/discharge is defined as the volume rate of flow of water (including any sediment or other solids that may be dissolved or mixed with it) (Buchanan and Somers, 1969). Hundreds of thousands of streamflow measurements are done every year. They can be done on a wide array of water body discharges, from still waters to floods. Since the flow velocity varies at different points in a stream cross section, calculating the average velocity at many points within that cross section is highly recommended. There are numerous methods for measuring volumetric flow rate/discharge and linear flow velocity in a water body. These include (the items in bold will be completed during this lab): 1. Floating markers 2. Tracer dilution 3. Mechanical current-meters 4. Drogues 5. Acoustic current meters 6. Laser Doppler meters Methods: 1. Floating Markers: In a nutshell: Used as a measure of the time it takes for an object to float a specified distance downstream. Commonly used items are oranges and lemons (biodegradable, highly visible, good size), stress balls, and Frisbees. Benefits: fast and convenient method Drawbacks: can only measure the velocity of surface water, need to estimate mean velocity using correction factor, accuracy This method is considered to be one of the easiest methods to measure surface velocity (vv ssssssssssssss ) as the ratio of the travel distance to the travel time of the object (Eq. 1). 1

2 vv ssssssssssssss ( LL tttttttttttt dddddddddddddddd ) = tt tttttttttttt tttttttt Eq. 1 Since surface velocities are typically higher than the mean velocity, a mean velocity (vv mmmmmmmm ) is calculated using a correction factor (k) of 0.8 for rough beds and 0.9 for smooth beds (Eq. 2). A correction factor of 0.85 is commonly used. vv mmmmmmmm ( LL tt ) = kk vv ssssssssssssss Eq. 2 The discharge (volumetric flow rate, Q), is then calculated by multiplying the cross sectional area (A cross ) by the mean velocity (Eqs. 3 and 4). A cross (LL 2 ) = WWWWWWWWh DDDDDDDDh Eq. 3 QQ ( LL3 tt ) = AA cccccccccc vv mmmmmmmm Eq. 4 Procedure: 1. Choose a suitable reach (location) with minimum turbulence (have the length be at least 3 times the width of the channel) and mark the start and end points of your reach whereby travel time exceeds 20 seconds (See diagram below). 2. Drop the object into the stream at the designated start point and simultaneously start the timer. Stop the timer when the object reaches the designated end point. 3. Record the time it took for the object to go from the start point to the end point. 4. Repeat steps 2 and 3 at least three times to be able to get an average later on. 5. Measure stream width and depth across at least one cross section. If possible, measure depth across the stream width at the start and end points and average the two (if measuring one cross section, choose the downstream side). Use a marker rod, a yard or meter stick to measure the depth at regular intervals across the system (at least 10). 6. Average your cross-sectional areas (A) to compute Q. D1 D2 D3 W1 W2 W3 2

3 2. Tracer dilution (instantaneous): In a nutshell: Used as a measure of the downstream concentration of a tracer (known volume and concentration) discharged/injected instantaneously (sudden/slug) upstream over time until the concentration reaches the background level. Criteria for a tracer in a tracer dilution method is that it (Rantz et al., 1982): 1) readily dissolves in water; 2) is either absent or present at very low concentrations in water; 3) is not decomposed, is not retained or absorbed by sediments, plants, etc ; 4) can be detected at low concentrations using simple methods; and 5) is harmless. Common tracers include chloride (measuring conductivity) or organic dyes such as rhodamine (detecting fluorescence). Two methods: -The constant rate method of injection method (not the focus of this lab): For details on this method, visit: -The instantaneous injection method: assesses longitudinal dispersion and general mixing. By instantaneous injection upstream, downstream sampling will produce a curve similar to the one portrayed in Figure 1. Calculating discharge from slug injection involves integration, or calculating the area under the curve of concentration vs. time (Eq. 5). Figure 1. Concentration-time curve at downstream sampling site for a suddeninjection of a tracer (Rantz et al., 1982). 3

4 QQ ( LL3 tt ) = CC tt VV tt 0 (CC CC bb )dddd = CC tt VV tt aaaaaaaa uuuuuuuuuu cccccccccc Eq. 5 where Ct is the concentration of the tracer solution injected into the stream; Vt is the volume of the tracer solution injected into the stream; C is the measured tracer concentration at a given time downstream; Cb is the background concentration of the stream; and t is time. The terms (CC CC bb )dddd is the total area under the 00 curve. Benefits: does not require a large reservoir for tracer, suitable for remote sites, fairly quick, inexpensive. Drawbacks: sampling interval has to be short enough to catch the peak, tail might be long due to eddies, tracer might be lost, incomplete mixing. Procedure: 1. Prepare a salt solution with a known concentration of 1 kg of salt per 6 L of water (done by TA). 2. Pour a known volume (100 ml) of prepared solution into a graduated cylinder to be poured at a chosen reach. 3. Record background electrical conductivity and the water temperature using a conductivity meter upstream (where the tracer will be injected). 4. Set up the conductivity meter probe at the downstream point, record background conductivity and temperature, wait for tracer discharge. 5. Inject the known volume of salt solution instantaneously upstream. 6. Begin recording the time as well as the conductivity downstream at 3-second intervals. Continue until the conductivity returns to background level. 7. Plot the salt wave (conductivity vs. time) and calculate the discharge. 3. Mechanical Current-Meter: In a nutshell: Relates rotational speed of the meter to linear flow velocity ==>rotational (angular) velocity. Benefits: decrease the impact on velocity measurement of turbulence Drawbacks: depend on the precision of equipment Procedure: 1. Decide on a cross section along the width of the stream channel and divide this cross section into numerous vertical subsections (Figure 2). 4

5 Figure 2. Vertical sub-sectioning of a river to determine discharge (obtained from USGS: 2. In each vertical sub-section, measure the width and depth of the subsection to determine the area (Eq. 3). 3. In each vertical subsection, decide the depth to measure the velocity (Figure 3): a. In a deep stream subsection, the average velocity is estimated by the average of velocities measured 20% depth (0.2D) and 80% depth (0.8D). b. In a shallow stream subsection where measurement at two depths is difficult, the average velocity is determined by measuring velocity at 60% depth (0.6D) 0.6D 0.2D 0.8D Figure 3. Schematic subsection of river to determine depth of velocity measurement 4. Place the current meter in the stream in one of the vertical subsections and reset the current meter and your stopwatch to Start your stopwatch for a specified time period and record the number of rotations on the meter. This is your water velocity at that subsection. 6. Repeat steps 3 and 4 at different vertical subsections. 7. The discharge in each subsection (Q1, Q2, Q3 Qn) is computed by multiplying the subsection area (A1, A2, A3 An) by the measured velocity (v1, v2, v3 vn) (Eq. 4). 8. The total discharge is then computed by summing the discharge of each subsection (Eq. 6). 5

6 QQ tttttttttt = (AA 1 vv 1 ) + (AA 2 vv 2 ) + (AA 3 vv 3 ) + + (AA nn vv nn ) = QQ 1 + QQ 2 + QQ QQ nn Eq. 6 Special Topics to include in the write-up In-Situ a) Comment on how velocity changes with position (depth, proximity to bank) in the stream. b) Which of your methods do you think is more accurate and why. c) What are some of the real-world limitations of each method? Can you think of examples where one method may be more appropriate than the other? Ex-Situ 1. Go to the National Water Information System website: ( 2. Select the blue bubble entitled Surface Water. This will bring you to a page that contains all the surface water data measurements USGS has for the nation (some of these data are real time, but a majority of these data are measured by gages that are not updated in real time). 3. Look for the Fort River gage to do the analysis. Select the blue bubble entitled Daily Data (this will bring up a page with check boxes). 4. Check the box State/Territory under the Site Location Field and click submit. (This will help us narrow down our search for the Fort River gage). 5. Select Massachusetts from the State/Territory box and scroll down the page to look over all the parameters the USGS measures (at some point in your career, class work, or research, you may need to find similar data). 6. Look of the parameter Streamflow, ft3///s and check this box (it is under the Water Level/Flow Parameters about two thirds down the page) then scroll all the way down to the bottom of the page and click submit. 7. A long list of stations should now appear. These are all the gaging stations within Massachusetts that the USGS makes measurements at. Scroll down until you find the Fort River gage and select it (site number ). 8. You are now where the data for the Fort River resides. Under the blue box that says: Available data for this site, select Site Map from the drop down menu. Record the drainage area. 9. Now go back to Time-series: Daily Data from the previous drop down menu. First generate a graph and then get some data to put in. Make sure the Graph button is selected and then put in any two years (be mindful of the period of record!). Include a copy of your handmade graph in the lab write-up. 10. To get data, select Tab-separated from the output format box and pick the dates 1/1/1976 1/1/1996. This will give you 20 years of daily streamflow data to play with. Click submit to get a large text file of our data. In your browser, use the select all command to copy the data. Copy this data and open Microsoft Excel and paste the data there. 6

7 11. To fix the format, first delete the header information from the data (This is about the first 27 rows of data in Excel). Now that is done, highlight Column A (or whichever column you pasted the data into). Under the data tab, select Text-to-Columns to separate the data into separate columns for us so we can do some analysis. Choose delimited and then select Next. Check the Space Box and make sure that it looks like it will parse the data correctly. Now click finish. You should have five columns of data (we really only care about the date field and the field that has the streamflow measurements in it immediately to the right of the date field; you can delete the others). You should have 7306 rows of streamflow data. 12. Let s calculate mean annual flow for each year and plot it. Start by creating a column of numbers that begin with 1976 and incrementing by 1 up to 1995 (I used column H). 13. Now that you have the years created, you will need to create a column of values that you can look up. Use the year command in excel to create a column that has only the year values from the date column. Drag this down to make sure each of your 7306 rows has a cell with the year in it. 14. Use the averageif command to find mean annual flow for each year. In the cell next to the column that has the values 1976 through 1995 (mine was Column H from above), I typed =AVERAGEIF($F$1:$F$7306,H2,$D$1:$D$7306), because my YEAR column was in Column F (that is the range in the averageif function), Column H contained my Criteria (i.e., the year over which I wanted to average; with H2 being the cell containing 1976 ), and Column D contained the range of streamflows I wanted to Average Over (Average Range). a. A side note: The dollar signs are used to lock references, a dollar sign in front of a letter means that the column is fixed, no matter how I drag my cell with the equation in it, a dollar sign in front of the number means that the row is fixed. You can play around with this to make sure you get them correct. 15. Your first value (1976) should be cfs and your last value (1995) should be cfs. 16. Create a plot of all the values against time and submit this with your lab report. a. Is there a lot of variability annually? b. What year had the highest mean annual streamflow, which year had the lowest? c. What is the ratio between these two years? d. What is the mean annual streamflow over the period? (Hint: Should be close to 1986 s value) 17. Now we will leave figuring out how to calculate mean monthly streamflow up to you. I would recommend using the =Month() command to create a columns of months next to your year column and following what we did above. 18. Create a plot of average monthly streamflow for our period. It should have months 1-12 on the x-axis and streamflow on the y-axis. a. What month has the highest average flow? Why do you think this particular month has the highest flow? 7

8 b. What month has the lowest flow? Why do you think this month has the lowest flow? (Hint: Precipitation is equally distributed over the year for Western MA, so rain falls about equally in each month. 19. Lastly, we want to find the minimum and maximum streamflow. Use = Min() and =Max() to help with this. What day did these occur on? (Use Find to help you find the value). Hint: Low flow should be in September, High flow in May. 8