Environmental Hydrology – Lab 5
Discharge measurements and channel characterization
Introduction
This lab introduces a series of techniques to characterize river channels and quantify river discharge. The techniques include surveying channel cross-sectional geometry, measuring flow velocity across a cross section, and characterizing channel bed roughness using a Wolman pebble count. Our measurements will be conducted on Ranch Brook at the USGS gaging station at Ranch Camp and on the Little River a few miles downstream.
Instructions (to be conducted in small groups at each site)
A. Channel cross-section
1. Use the auto-level to set up a surveying station on the river banks at an elevation that will allow a view of the entire channel cross section, including the floodplain surface.
2. Stretch a measurement tape across the channel and secure it or hold it tautly so that it can be accurately used to measure linear distance across the channel.
3. Use the stadia rod to survey level lines across the channel at roughly 1 meter increments. Record your X (distance), Y (elevation) measurements on the datasheet provided.
B. Discharge measurement
1. Stretch a tape across the channel to establish a cross section. Divide the channel into a series of relatively homogeneous subsections with respect to roughness and hydraulic characteristics. Record the widths of these subsections on the datasheet provided.
2. Use the wading rod (with current meter attached) to measure the depth of each subsection and record the depth on your datasheet. Depths on the wading rod are indicated by a single hatch for each 2cm increment, double hatches for each 10cm increment, and triple hatches for each 50cm increment.
3. Adjust the wading rod so that the current meter records velocity at 0.6 times the water depth (0.6D) from the water surface. This is done by depressing a lever at the top of the wading rod and lining up numbers on the cylindrical rod with those on the hexagonal rod to indicate the channel depth at the measurement site. The channel depth is represented in tens of cm on the cylindrical rod and cm on the hexagonal rod. For example, if the channel is 1.0 meter deep (100 cm), you would line up the markers at the top of the rod to align the “10” on the cylindrical rod with the “0” on the hexagonal rod. This places the current meter 40 cm above the channel bed or 60 cm below the water surface.
4. Read the velocity from the datalogger and record the velocity for each subsection. Note, you should complete steps 2, 3 and 4 on each subsection, before moving on to the next subsection.
C. Wolman pebble count
- Choose an area of the channel in the vicinity of your cross-sectional survey and discharge measurement sites. To minimize bias in your sampling, determine a set of “rules” for sampling (i.e. take two steps between each sample; measure the particle at the toe of your shoe or below your finger as you blindly reach down to the stream).
- Measure the intermediate axis on the particle you select. The intermediate axis is neither the longest nor shortest axis of the particle. Record your measurement on the datasheet.
- Repeat step 2 to collect and record the size of 100 particles from the channel. You may move across the channel and then systematically downstream as you repeatedly cross the channel until your sample is complete.
Each group should complete the three exercises at a single measurement site and collectively enter the data into a spreadsheet posted on the class website. Spreadsheets should be emailed to by 5:00pm on Thursday, September 27 to be made available to the entire class. Note that the data may be submitted in a single excel file (containing all three datasheets) or in three different excel files (one sheet per submission) if members of each group elect to do the data entry for one of each of the three exercises.
Data reduction/analysis
Use the data for the entire class (i.e. both sites) in each of the steps below.
1. Use the cross-sectional data from part A to plot the cross section from each site on a single graph. You will need to reverse the scale of the y-axis on your graph or establish an arbitrary datum and subtract the elevations from that datum in order to plot a profile that characterizes the channel shape. Use Figure 6.23 of Ward & Trimble as a model (note that this figure shows the cross-section of a single channel over multiple periods of time; your graph should show the cross-sections of the two channels at a single period in time).
2. Use the data from part B to estimate the discharge (Q) at each site, using two approaches: (1) multiply subsection width * depth to get subsection area (A); average the velocity (v) across all subsections, and multiply it by the total subsection area to get a discharge, and (2) multiply subsection width * depth * velocity and sum the answer to get discharge using the relation
Q = v * A
Present these calculations in a table (you may do the calculations in excel and embed the excel table into your report) expressed in units of m3/sec and ft3/sec.
3. Use the data from part C to plot a bed material distribution curve using Figure 6.37 of Ward and Trimble as a model (note, I would only like the cumulative % curve) for each site on a single graph. To do this, you will need to use the excel “sort” function to sort the 100 particles at each site from smallest to largest, add a new column called rank ordered from 1 to n, where n = 100 if each particle has a unique size but n will be less than 100 if more than one of your particles is the same size. Use excel to calculate the percent finer than value for each particle size from your sample of 100 particles.
Reporting
Hand in the following items in a report:
o a single graph showing the cross-sectional geometry for the two sites
o a single graph showing the cumulative particle size distribution for the two sites
o a table showing your estimates of discharge for each of the two sites. Show your estimates in m3/s and ft3/s.
o a paragraph in which you compare and describe any differences that you note between the two sites in the following metrics: wetted channel width, bankfull width, average channel depth, average channel velocity, discharge, and median particle size.
o a paragraph commenting on sources of error or uncertainty in your estimates. In this paragraph, compare your discharge estimate at Ranch Brook to the value reported by the U.S.G.S. on their real-time monitoring record for this site (see lectures slides for link to USGS web site) and give the unit-area discharge, based on your field measurements, for this site.
Submit your report in printed form or by email (as an attached MS Word file) by Friday, October 5 as l to . Please place the words “Lab 4 report” into the subject line of your email message if you submit it electronically.
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