Estimating flow through an earthen dam

Kaye Savage, Wofford College

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Students make measurements of dam dimensions and water levels above and below a small, accessible earthen dam. They assess the soil texture in the field. They follow up by developing a simple flow net and estimating seepage rate as part of a problem set. They discuss their assumptions and the likely sources and magnitude of error.

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Hydrology and Water Resources class - fulfills an elective for Environmental Studies Majors.

Skills and concepts that students must have mastered

Hydraulic conductivity, Darcy's law, flow net principles, field notes.

How the activity is situated in the course

This exercise occurs a couple weeks into the groundwater section of the course.


Content/concepts goals for this activity

Graphical models of flow (flow nets) and seepage calculations
Use of ternary texture diagram in relation to specific yield
Relationship of sediment texture to hydraulic conductivity

Higher order thinking skills goals for this activity

Developing a graphical model from field measurements
Linking an abstract concept to a physical example
Using authentic data to develop flow estimates
Assessing how measurement error affects results
Making and justifying assumptions

Other skills goals for this activity

Measurements in the field
Assessing data quality
Method of soil texture by feel
Using field notes

Description and Teaching Materials

The site we use for this exercise is The Cleveland Preserve, a tract located in Spartanburg, SC that is protected by a land trust. An earthen dam creates a small (5.5 acre) pond that is used for watering a few cattle and supporting a small restaurant garden. Comparable sites are fairly common but some research will be necessary to find an accessible site in other areas.

Materials taken to the site include long tape measures, cameras, a staff, hand levels, and a "Soil Texture by Feel" decision diagram (Thien, 1979). Students also refer to a ternary diagram relating particle size distribution to specific yield (Johnson, U.S. Geological Survey Water Supply Paper 1662-D, 1967, reprinted in Fetter, Applied Hydrogeology, Prentice Hall).

The assignment is given before going to the field site. Students may work together or individually at the site (their preference; most work with at least one other student). They make measurements of the dam dimensions and water levels. After students have turned in the assignment, we compare their approaches to the problem and their outcomes.

Instructions to students for data analysis:
Use the data you collected at the Cleveland Preserve to assess the seepage through the earthen dam there. Note you may share data collected on-site with one another, but you are on your own to do the tasks requested below. Please type your responses to questions and discussion items on a separate sheet. Turn in a photocopy of your field notes.

a. Estimate a realistic hydraulic conductivity value for the dam, based on the texture of the soil. Describe the method you chose for your estimate. Would you consider the dam material to be isotropic or anisotropic? Why?

b. Estimate the specific yield of the material in the dam, using the diagram presented in Fetter.

c. Draw an idealized cross-section of the dam. Discuss and justify the simplifications you made.

d. Draw a realistic flow net on your diagram.

e. Estimate the discharge via seepage across the dam, using your flow net.

f. Describe the factors that might cause error in your analysis. Which is most important? Why?

Teaching Notes and Tips

Most students are unsure where to begin and need to consider how to visualize the site to develop a flow net. They also need to make several assumptions such as the depth to a low-permeability layer below the dam, homogeneity, isotropy. More resourceful students might think to use NRCS data to find out typical soil thicknesses in this region (they have used this resource in a prerequisite course); we discuss this in class after students turn in the assignment. Caution is needed as students traverse the dam, which is about 2 m wide at the top.


Field notes need to include a sketch, all of their measurements, and additional observations.

Students need to establish a credible flownet on a sketch that captures the major features of the dam. They need to use class notes and/or textbook to recognize how to calculate the seepage based on their flow net, and perform the calculation correctly based on their own flow net. Specific yield should match the information from their soil texture by feel test. Hydraulic conductivity should be within a range appropriate to the soil with discussion of textural maturity and grain size as factors considered.

Discussion of error should include measurements and identify hydraulic conductivity as the largest probable source of error.

References and Resources