Discharge and Sediment Transport in the Field

Learning Goals

Discharge is a fundamental quantity in geomorphology. Not only is it important in determining when sediment will move, and therefore when erosion and deposition will occur, but on a broader scale it (along with sediment supply) governs the size, shape, planform geometry, and longitudinal profile of alluvial rivers and ultimately shapes entire landscapes. In a more practical sense discharge data is essential in flood frequency analysis which in turn is used for insurance purposes (e.g. FEMA’s delineation of flood prone areas) and engineering considerations such as bridge, culvert and reservoir sizing.

• Develop basic field techniques in measuring discharge and sediment sampling
• Derive empirical coefficients describing channel roughness based on field data
• Use field data, theory, and empirical relations to make predictions about bed mobility

Context for Use

This field activity is done about half way through a comprehensive unit on rivers in an upper level (300) geology class – Physics of the Earth: Surficial Processes. Prior to taking this class students will have taken introductory geology and should have taken one term of physics and applied calculus, although no calculus is required for this particular activity.

Prior to this exercise:

1. Resistance equations (Chezy and Manning) and sediment transport relations will have been derived.
2. Students will have had the opportunity to work through rudimentary examples of these equations.
3. Students will be familiar with surveying.

Teaching Notes and Tips

• Materials: basic survey equipment such as a level, rod, and tape measure and a velocity meter. A float could be used, but you would lose the ability to show how velocity (and therefore shear stress) varies with depth and across the channel. Hip or chest waders are recommended for all but the warmest of weather. The activity should take about 2 hours in the field if the students are skilled at surveying and an additional 2-4 hours outside of class.
• Location: The activity is best conducted on a gravel bed stream that can be waded at modest discharges. Minimum flow depth should be at least 20 cm and minimun width should be about 10m. Across-stream variation in flow depth will allow the students to internalize which areas will have higher flow velocity and why. The Wolman particle count will not work on sand or finer bed streams, but you could sieve finer sediment to determine the sediment size distribution. Sand bed streams are also problematic because of the resistance due to bedforms can dominate over that due to the grains themselves.
• Group Management: Because this class often has fewer than 12 students, I only have to manage 2-3 groups. I am constantly rotating between them to answer questions and keep them on task. By this time in the course I know the group dynamics, strengths, and weaknesses of the individual students. Tyipcally I allow students to work with whomever they chose, but on occasion I will separate particular students or break up 'strong' groups. I have also randomly assigned groups.
• Tips:
1. I encourage the students to develop a plan for how to proceed in the field and ask them to explain it to me before they get too far along.
2. You need to make sure that the field data is good before leaving.
3. I frequently touch base with the different groups posing different questions as they arise. (For example where was flow the fastest/slowest and why?) Many of these questions are in the lab itself.
4. Some students get confused as to how all of the data will be utilized at the end. I tell them the following as appropriate
• We are measuring velocity in order to get an estimate of roughness
• With our resistance equation we can estimate discharge for any given flow depth
• Given a flow depth (and slope) we can estimate the shear stress
• We can then compare this shear stress with critical shear stress (which is a function of the sediment size that they measured)
• Follow up activities: These build upon the skills developed here include sieve and hydrometer analysis of sediment size, extending sediment transport to volumetric relations, and ultimately to include flood frequency data in the context of determining "effective discharge". Students could also be challenged to compare the theoretical derivation of the vertical velocity profile to their field data.

Teaching Materials

• Field Activity (Microsoft Word 43kB Jun29 05)
• Sediment Size Distribution Calculator (Excel 35kB Jun8 05)

Assessment

• Group work – I try to reinforce good field note taking and presentation skills by evaluating the completeness of site sketches and cross sectional plots. The key element is to determine if they have calculated the areas of the subdivisions properly, which is why I require a separate plot of this. If the area is correct then the discharge calculation is straightforward. Individual work is based on the group data, but students are not penalize in the individual portion for group mistakes.
• Individual work – Students are encouraged to discuss and work with classmates, but the end result should be their own. In evaluating this section I make sure that students correctly manipulate the equations and that the understand some of their nuances and limitations. The “right” answer is not as important as the correct application. To determine the right answer it is necessary to carry out the calculations on a case by case basis.

References and Resources

Barnes, H.H., 1967, Roughness characteristics of natural channels: USGS Water-Supply Paper 1849, 213 p.

Ritter, D.F., Kochel, R.C., and Miller, J.R., 2002, Process Geomorphology (4th ed.): WCB/McGraw-Hill, 560 p.

Wolman, M.G., 1954, A method of sampling coarse river-bed material: Transactions, American Geophysical Union, v. 35, n. 6, p. 951-956.

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• Student Resources
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