7. STREAMBED TEXTURE ANALYSIS

Transcription

1 Geomorphology 7. Streambed Texture Analysis 7. STREAMBED TEXTURE ANALYSIS 50 Points The purpose of this exercise is to analyze the size characteristics of the sediment samples collected in the field. We will ultimately use these measurements to test some hypotheses relating stream geometric and hydraulic characteristics to the stream bed texture. Save your work in Excel file 07_Bed_Texture_Analysis in your team folder on the department server. YOU SHOULD BE ABLE TO: Use a sieve shaker and balance to determine the particle size distribution of a sediment sample; and, Create cumulative frequency distribution graphs of sediment grain size in Excel and use these graphs to determine descriptive statistics for grain size, such as the median grain size and the degree of sorting. PROCEDURE There are two sets of activities involved in particle size analysis. The first activity is to sieve the sediment and record the weight retained in each size class. The second activity involves graphing the particle size distribution as a cumulative distribution for each sample, and then using that graph to extract particle sizes at specific percentiles (e.g. the 50 th percentile). You can use the particle sizes you extract from the graph to represent the particles found on the bed of the river, or you can use those particle sizes to calculate other descriptive statistics (e.g. the degree of sorting) regarding the bed sediment. Sieving Using the RoTap Sieve Shaker 1. If your sediment was drying in several beakers in one of the ovens, remove the sediment from the ovens and recombine each sample into a single container (i.e. if sample 1 is in three beakers, combine the sediment in those three beakers into a single container). 2. Select a clean container to weigh all your samples in. Weigh this container while it s empty and tare the scale so it reads a weight of zero. You do not want to include the container weight when weighing sediment. 3. Weigh each of your sediment samples. Record the total weight for each sample in Table A, the first worksheet in your Excel workbook (Table A. Percent Calculations) using the computer in the lab room. If your sample weighs too much for the balance, split the sample in two and then add the weights together. 4. If any of the particles in your sample have clumped together while in the oven, you must break up all the clumps. Pour the sample into the shallow rectangular pan and use the rolling pin to break up the clumps. 5. By hand, sieve out the coarse pebbles and larger sized particles (Table 7.1 below lists the various size classes). Weigh and record the sediment weight in this size category in Excel Table A. Note that the coarse pebbles and larger class is at the bottom of the Excel table, not the top. Do not put these larger particles in the sieve shaker. Return this weighed sediment to a clean container, to which you will gradually add the remainder of the sample. K.A. Lemke UWSP 47

2 7. Streambed Texture Analysis Geomorphology 6. If the weight of the remaining sediment is approximately 600 g or less, proceed to sieve the entire sample. If you run samples much larger than approximately 600 g, there is a very good chance that not all the finer particles will have the opportunity to fall through the sieves, giving you inaccurate results. As a result, if the weight of the remaining sediment is much more than 600 g, split the sample, run each portion separately, and then combine the retained weights from the two portions. TABLE 7.1 Description of Particle Sizes Description Sieve Mesh Size (mm) Size (in) Phi Units Wentworth Size Class Gravel Boulder Cobble Pebble Granule Sand Very Coarse Sand Coarse Sand Medium Sand Fine Sand Very Fine Sand Silt Coarse Silt 7. Arrange the sieves on the shaker from coarsest at the top to finest at the bottom. The very bottom container must be a pan, not a sieve. 8. Pour the sample into the coarsest sieve. Cover the top sieve with a pan lid without a ring, and place the top plate with the cork in it on top of the pan lid, cork-side up. Follow the instructions posted in the lab room for running the sieve shaker. Allow the shaker to run for minutes 10 minutes for smaller samples, 15 minutes for samples of approximately 600 g. 9. Weigh the material retained in each sieve. Pour the sediment retained in the first sieve into one of the large flat pans. You may need to shake the sieve or run a brush around the edges and across the bottom of the sieve to get all the sediment out of the sieve. If particles are still stuck in the sieve, try tapping the sieve on a desktop, upside-down, or brushing the top and bottom sides of the sieve with a brush. Be sure all of these particles end up in the large flat pan. If after brushing the sieve, particles still remain in the sieve, do not force them through the sieve; leave them. Pour the sediment into your weighing container, and after weighing the retained sediment, record the weight retained in Excel Table A under the correct size class. Note that in the Excel worksheet the size classes are in the reverse order in which you are unstacking your sieves. As a result, you will enter weights starting at the bottom of the table and work your way up; the first weight you enter will be for the coarse pebbles and larger class; the second weight will be for the pebble class, and so on. After weighing the sediment retained in a particular class, add the weighed sediment to the container containing your coarse pebbles and larger sediment. Repeat this procedure with each of the remaining sieves. When finished with each sample, the total sample should be reassembled in one container. Ultimately, you should one labeled container for each sample. Do not throw your samples away or mix them up. Occasionally teams have had to re-sieve their sediment. 10. Before starting on your next sample, clean the sieves with a toothbrush one more time to remove particles stuck in the sieve. If particles still remain, do not force them through the sieve; leave them. 11. Repeat this procedure for each of your samples. 48 K.A. Lemke UWSP

3 Geomorphology 7. Streambed Texture Analysis 12. When you are finished, clean up after yourself. Put the sieves back where you found them, store your sediment samples in the designated space, wash any dirty containers and set them out to dry, and sweep up any material spilled on the floor. Completing Table A 1. In Table A, sum the retained weights for each sample (=sum function). This sum should be within a few grams of the total weight that you started with. If these two weights are not within a few grams of one another, you may need to resieve your sample. 2. Calculate the percent of each sample falling into each size class by dividing the weight in a specific class by the total weight of the sample (what you started with, not what you got when you summed all your weights) multiplied by 100. Use formulas in Excel to do this; do not do it with a calculator. 3. Calculate the cumulative percent for each size class. The first cumulative percent value will equal the percent in the pan (medium silt and finer) class. The cumulative percent for the coarse silt class will equal the percent in the pan class plus the percent in the coarse silt class. Continue to sequentially add the percent in each class to the cumulative percent of the previous classes. By the time you get to the coarse pebbles and larger class, you should be at approximately 100%. Use formulas in Excel to do these calculations; do not do the calculations on a calculator. Creating a Cumulative Frequency Distribution 1. Copy and paste the cumulative percentages for each sample from the first worksheet (Table A. Percent Calculations) to the second worksheet (B. Cumulative Frq Graphs). Since the cumulative percentages are formulas, be sure to use paste special and values because you want to paste the result of the formulas, not the formulas themselves, in the second worksheet. 2. The table is designed such that after entering the percentages for each of your four samples, the last column of the table will automatically calculate the average cumulative percent in each size class. 3. Create two scatter plots, one for the upstream sediment samples and one for the downstream sediment samples. Each scatter plot should have five lines, one line for each of your samples, and a fifth line for the transect average. To create a scatter plot: a. Highlight the block of cells containing the phi-values and the cumulative percentages for your four samples and for the transect average (i.e. select the entire table except for the labels in the first row). b. Go to the Insert tab in Excel and click on Scatter Plot. Select a chart with smooth lines and no dots. c. Format the x-axis. The y-axis will appear in the middle of your chart rather than on the left side because by default, Excel always places the y-axis at zero on the x-axis. To move the y-axis to the left side of your graph, click on the numbers on the x-axis and a box will appear around the x-axis labels. Do a right-mouse click and select Format Axis. The top-section of the dialog box Axis Options, allows you to determine the minimum ( 5.0) and maximum (+5.0) values for your x-axis. If 5.0 and +5.0 don t appear here, click the fixed option and type in these values. If the major unit is not 0.5, click the fixed option and type in 0.5. Because negative phi values correspond to large particle sizes and positive phi values correspond to small particle sizes, check the box for Values in reverse order. Numerically, the larger numbers will now be to the left and the smaller numbers to the right on the x-axis, but if we converted the phi values to millimeters, the smaller values would be to the left and the larger to the right (which is how we normally expect the x-axis to be set up). Near the bottom of the right side of the dialog box is a section headed Vertical axis crosses. Click on Axis value and then type in the maximum phi value (5.0). Now your x-axis should be scaled from +5.0 to 5.0 at intervals of 0.5 and the y-axis should be on the left side of your graph. K.A. Lemke UWSP 49

4 7. Streambed Texture Analysis Geomorphology d. Format the y-axis. Follow the same procedure to format the y-axis so that the values range from 0 to 100 with a major unit of 5.0. You should end up with y-axis labels of 0, 5.0, 10.0, 15.0, etc. e. Format the grid lines. If your chart does not have horizontal and vertical grid lines, you want to add them so you can easily determine what the particle size is at various percentiles. To do this, make sure you have clicked on the graph (selected it) and on the very top menu bar you should see Chart Tools highlighted with three associated tabs, Design, Layout, and Format. Click on the Layout tab. Under the Axes/Gridlines option select display both major and minor horizontal and vertical gridlines. f. Add axis labels. Click on the chart (select it) and on the very top menu bar you should see Chart Tools highlighted with three associated tabs, Design, Layout, and Format. Click on the Layout tab. Select Axis Titles and give the x- and y-axes labels. g. Give your chart a title. Click on the chart (select it) and on the very top menu bar you should see Chart Tools highlighted with three associated tabs, Design, Layout, and Format. Click on the Layout tab. Select Chart Title and give your chart a title. h. Repeat these steps with your second set of samples. You should now have two scatter plots each with five lines, one line for each sample and one line for the transect average. The charts should have a legend where Series 1, 2, 3, and 4 represent samples 1, 2, 3, and 4, and Series 5 represents the transect average. Calculating Summary Statistics You need to calculate summary statistics for your sediment samples in the third worksheet in your Excel workbook (C. Summary Statistics). We will use the summary statistics to test various hypotheses relating stream geometric and hydraulic characteristics to stream bed texture. In order to do this, however, you need to determine a single value that is representative of all the particles sizes found on the stream bed. From a statistical perspective, the most common values used to represent an entire sample are the mean (the average) and the median (the 50 th percentile, or the middle value of a set of ranked values). The fluvial literature also recommends using the 84 th percentile (calculated as percent finer than). The mean, median, and 84 th percentile are single numbers intended to represent the most typical grain size found on the river bed. In addition to determining the most typical grain size on the river bed, we can also determine how well sorted the samples are. Measurements of sorting determine how the particle sizes are spread above and below the mean. We will calculate two different measures of sorting. The measures of center (or what s most typical) can be calculated using either phi units or millimeters to represent grain size. We will do both. Of our two measures of sorting, one requires grain sizes measured in phi units and the other requires grain sizes measured in millimeters. 1. Use your cumulative frequency graph to determine the particle size at the 5 th, 16 th, 50 th, 84 th, and 95 th percentiles (φ 5, φ 16, φ 50, φ 84, and φ 95) and enter those phi values in Excel Table C. Summary Data. You may want to stretch out your cumulative frequency graph so the values are easier to read. 2. Use equation (1) to calculate the mean grain size. mmmmmmmm = ( ) 3 (1) Use an Excel formula to do this; do not use a calculator. 50 K.A. Lemke UWSP

5 Geomorphology 7. Streambed Texture Analysis 3. Convert the phi values for the different percentiles to diameters in millimeters (d 5, d 16, d 50, d 84, d 95). The formula to convert phi units to millimeters is: dd = 2 (2) where d is the grain size in mm. Use formulas in Excel to do these calculations, not a calculator. In Excel, the carat symbol (^) raises numbers to some power (in this case φ). Put the φ in parentheses. 4. In Excel, calculate the degree of sorting in two ways, using the formulas below. Note that the first formula requires grain size measured in phi units, while the second formula requires grain size measured in millimeters. ( φ ) ( ) 16 φ84 φ5 φ95 sorting measure 1 = sorting measure 2 = * + (3) d d (4) *The numerator for sorting measure 1 usually appears as (φ 84 φ 16) and (φ 95 φ 5). Here, the percentiles are reversed because we calculated the percentiles as percent finer-than, rather than percent coarser-than. Table 7.2 explains how to interpret the value for Sorting Measure 1. With sorting measure 2, the degree of sorting increases as the ratio decreases; the smaller the ratio, the well-sorted the sediment. TABLE 7.2 Interpretation of Sorting Measure 1 very well sorted <0.35 well sorted moderately well sorted moderately sorted poorly sorted very poorly sorted extremely poorly sorted > Record the distance along the tape measure where the sample was collected. You will need to go back to your field data sheets, where you should have recorded this information. 6. Record the water depth at the point where the sample was collected. Again, refer to your field data sheets. 7. Record the average flow velocity at the point where the sample was collected. Once again, refer to your field data sheets. Save all your Excel work in your Team folder. K.A. Lemke UWSP 51

6 7. Streambed Texture Analysis Geomorphology APPENDIX 7.1 USGS Wentworth Chart. USGS Open File Report K.A. Lemke UWSP

7 Geomorphology 7. Streambed Texture Analysis GRADING RUBRIC Upstream Samples Point Value g/class [40 values; 0.4 ea] % weight/class [40 values; 0.15 ea] 6.00 cumulative % [40 values; 0.15 ea] 6.00 cumulative freq graph (5 lines; labels) phi values [25 values; 0.2 ea] d values [25 values; 0.15 ea] 6.25 mean phi [5 values; 0.15 ea] measures of sorting [10 values; 0.15 ea] locations (ft) [4 values; 0.1 ea] depths (ft) [5 values; 0.1 ea] velocities (ft/s) [5 values; 0.1 ea] 0.50 Upstream Points Downstream Samples Point Value g/class [40 values; 0.4 ea] % weight/class [40 values; 0.15 ea] 6.00 cumulative % [40 values; 0.15 ea] 6.00 cumulative freq graph (5 lines; labels) phi values [25 values; 0.2 ea] d values [25 values; 0.15 ea] 6.25 mean phi [5 values; 0.15 ea] measures of sorting [10 values; 0.15 ea] locations (ft) [4 values; 0.1 ea] depths (ft) [5 values; 0.1 ea] velocities (ft/s) [5 values; 0.1 ea] 0.50 Upstream Points Total Points (average of up & down) Points Earned Points Earned K.A. Lemke UWSP 53

8 7. Streambed Texture Analysis Geomorphology 54 K.A. Lemke UWSP