1. Estimating flow through an earthen dam lab and homework exercise – Kaye Savage, Wofford College Context: Hydrology and Water Resources course - environmental studies 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 S y. - 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) - ?