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Estimating Flow Through an Earthen Dam - Lab Exercise

In this lab exercise, students will learn to make field measurements, develop a graphical model, and apply Darcy's Law to estimate seepage in an earthen dam. They will also discuss assumptions, sources of error, and analyze data to estimate hydraulic conductivity and specific yield.

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Estimating Flow Through an Earthen Dam - Lab Exercise

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  1. Estimating flow through an earthen dam lab and homework exercise – Kaye Savage, Wofford College Context: Hydrology and Water Resources course - environmentalstudies majors (both BA and BS) - small liberal arts college - class of 4-12 students - lab session (4 hours) Content and skills goals: - make field measurements relevant to flow calculations (authentic data) - use field notes effectively to record data for subsequent use - develop a graphical model (flownet) from field measurements - apply Darcy’s Law to estimate seepage Higher-order goals: - link an abstract concept to a physical example - consider error in measurements and resulting estimates - make and justify assumptions

  2. Site: The Cleveland Preserve • 5 acre pond • owned by land conservancy • 15 minutes from campus Field Supplies • long tape measures • soil texture by feel (laminated) • staff and hand levels • cameras Field Measurements • dam dimensions • water levels • soil texture

  3. Data Analysis Expectations • draw a credible flow net (on an idealized cross-section of dam) • estimate hydraulic conductivity and specific yield based on field analysis of soil texture • discuss whether dam material is likely to be isotropic or anisotropic • estimate seepage across dam • explain assumptions and discuss sources of error Assessment • field notes with sketch, clear documentation of measurements, and observations • credible flow net • correct use of their own flow net and measurements to estimate seepage • reasonable values for specific yield and hydraulic conductivity based on soil texture • consider grain size and textural maturity for K. Use ternary diagram for Sy. • discussion of error with K identified as biggest uncertainty • discussion of assumptions such as depth to impermeable bedrock, homogeneity

  4. Student work • reasonable K range: 10-4 to 10-6 cm/sec • flow nets generally reasonable • common problems with analysis • failure to account for length of dam • incorrect dimensional analysis • ignore possibility of permeable medium below lake and stream

  5. Helping students succeed • they may be unsure how to begin translating physical observations to conceptual model • they need to make appropriate assumptions: depth to low-K layer, homogeneity, isotropy • they always need more reminders about units and unit consistency… Discussion and reflection in class • compare approaches, assumptions, measurements, error estimates, and outcomes • discuss range of answers; consider importance of skepticism in context of field studies • resource for soil depth estimate – consider using NRCS database re. soil thicknesses in area • (was used in a prerequisite course). What else could you do to find depth? Adapting this exercise • could use other site types. Main factors: simple geometry, size manageable, measurable. • e.g. hill-slope to river or lake • could do 100% in lab period with less detailed calculations (e.g. focus on flow net development) • ?

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