Estimating flow through an earthen damlab 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

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

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

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

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)- ?