Tale of Two Cities (and two hurricanes): New Orleans

Tom Juster
University of South Florida, Tampa
Created: April 18, 2012

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Summary

2cities-New Orleans-title slide
In this Spreadsheets Across the Curriculum activity, students explore the origins of subsidence in the city of New Orleans. Students first learn about the history and geology of New Orleans, and the three main reasons it is presently subsiding: (1) compaction of deltaic sediments; (2) downward movement along a normal fault; and (3) compaction and oxidation of organic soils that have been exposed by drainage. They then create a spreadsheet that calculates the cumulative subsidence of the city since 1720 incorporating all three sources of subsidence. This estimate is then adjusted to account for sea level rise. The final task requires them to adjust the subsidence rates within limits to see how the same net subsidence can be produced by various contributions among the three processes.

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Learning Goals

Students will

  • Learn about the history of hurricanes in the Gulf of Mexico and specifically Hurricane Katrina.
  • Discover why New Orleans is located where it is.
  • Learn about the geology of New Orleans, and how it is related to the elevations of the city.
  • Learn about the system of pumps and canals needed to keep water out of New Orleans.
  • Learn about the three main sources of subsidence: (1) compaction of deltaic sediments; (2) tectonic movement along a fault; and (3) oxidation and compaction of organic soils exposed to the atmosphere.
  • Combine estimates of all three processes in a spreadsheet to compute the cumulative subsidence from 1720 to 2010, and then estimate the amount in the future to 2050.
  • Explore the relationship between absolute and relative subsidence by adding in the effect of sea level, which has been rising continuously during this time.
  • Discover how there is no unique solution to the contributions of each factor in producing the total subsidence.
     

In the process the students will

  • Discover that New Orleans was founded to facilitate the portage of boats and materials from the Mississippi River to the Gulf of Mexico via Lake Ponchartrain.
  • Learn about natural levees and the role they played in the development of New Orleans.
  • See how the subsidence of New Orleans is inexorably linked to its geology and development history, especially the draining of low-lying swamps which exposed their organic soils to the atmosphere.
  • Learn that subsidence can be caused by several processes, some of which can be induced or accelerated by human activities.
  • Discover how rising sea level exacerbates the effect of subsidence in coastal regions.
  • Learn why New Orleans is below sea level.
  • Work with real data to study a problem that burst into everyone's consciousness following the devastation by Katrina in 2005.

Context for Use

This module was designed for use in the Hazards of the Earth's Surface service course at USF. It assumes that students are familiar with basic Excel operations, especially the use of relative and absolute cell references and functions. The topic of subsidence would be appropriate in other geology courses as well.

Description and Teaching Materials

The module is a PowerPoint presentation with embedded spreadsheets. Click on the link below to download a copy of the module.

Optimal results are achieved with Microsoft Office 2007 or later; the module will function in earlier versions with slight cosmetic compromises. If the embedded spreadsheets are not visible, save the PowerPoint file to disk and open it from there.

The above PowerPoint presentation is the student version of the module. The embedded spreadsheet consists of a template for students on which students complete their work and answer the end-of-module questions, and then turn in for grading. Since this module is designed as a stand-alone resource, instructions for extracting and saving the embedded spreadsheet are included in the PowerPoint presentation.

This module is offered in two versions: a traditional SSAC version and a new auto-feedback/graded (AFG) version. The AFG version: (a) provides automatic and immediate feedback to incorrect answers, including formulas; (b) requires students to complete tasks sequentially by not allowing them to advance until they've completed a task perfectly; and (c) automatically computes a grade and encrypts it into a code the students submit to verify successful completion. The files needed for this version can be accessed here .

Teaching Notes and Tips

This module is constructed to be a stand-alone resource. It can be used as a homework assignment, lab activity, or as the basis of an interactive classroom activity. It has been used as a mid-course module in Hazards of the Earth's Surface, an online service course at USF designed for non-majors, for the last two years. Although they are introduced in the module, ideally students should be familiar with basic river terms like levees and floodplains, and to be familiar other concepts such as organic soils and faulting. The concept of computing a cumulative subsidence is difficult for some students, and may require assistance from the instructor.

Assessment

There is a slide at the end of the presentation that contains end-of-module questions. The end-of-module questions can be used to examine student understanding and learning gains from the module. The answer key for the end-of-module questions is found at the end of the instructor version of the module.

References and Resources

Burkett, V.R., Zilkoski, D.B., and Hart, D.A., Sea-level rise and subsidence: Implications for flooding in New Orleans, Louisiana, available at: .

Dixon, T.H., Amelung, F., Ferretti, A., Novali, F., Rocca, F., Dokka. R., Sella, G., Kim, S-W., Widowinski, S., and Whitman, D., 2006, Subsidence and flooding in New Orleans. Nature 44, 587-588.

Dokka, R.K., 2006, Modern-day tectonic subsidence in coastal Louisiana, Geology 34 (4) 281-284.

Dokka, R.K., Sella, G.F., and Dixon, T.H., 2006, Tectonic control of subsidence and southward displacement of southeast Louisiana with respect to stable North America, Geophysical Research Letters 33, L23308.

Dunbar, J.B. and Britsch III, L.D., 2008, Geology of the New Orleans area and the Canal Levee failures, Journal of Geotechnical and Geoenvironmental Engineering 134 (5), 566-582.

Grossi, P. and Muir-Wood, R., 2006, Flood risk in New Orleans, Risk Management Solutions Inc., available at: http://www.rms.com/Publications/NO_FloodRisk.pdf.

Hart, D., 2006, Exploration of subsidence and elevation in Orleans Parrish.

Meckel, T.A., ten Brink, U.S., and Williams, S. J., 2006, Current subsidence rates due to compaction of Holocene sediments in southern Louisiana, Geophysical Research Letters 33, L11403.

Rogers, J.D., 2008, Development of the New Orleans flood protection system prior to Hurricane Katrina, Journal of Geotechnical and Geoenvironmental Engineering 134 (5) 602-617.

Snowden, J.O., Drainage-induced land subsidence in metropolitan New Orleans, Louisiana, USA. Available at: http://iahs.info/redbooks/a151/iahs_151_0507.pdf.

Tornqvist, T., Wallace, D.J., Storms, J.E.A., Wallinga, J., Van Dam, R.L., Blaauw, M., Dersken, M.S., Klerks, C.J.W., Meijneken, C., and Snijders, E.M.A., 2008, Mississippi delta subsidence primarily caused by compatction of Holocene strata, Nature Geoscience 11, 173-176.

US Army Corps of Engineers, 2009, Performance Evaluation of the New Orleans and Southeast Lousiana Hurricane Protection System, Volume 1: Executive Summary and Overview. Available at: https://biotech.law.lsu.edu/katrina/ipet/Volume%20I%20FINAL%2023Jun09%20mh.pdf.

Waltham, T., 2005, The flooding of New Orleans, Geology Today 21 (6): 225-231.

Yuill, B., Lavoie, D., and Reed, D.J., 2009, Understanding subsidence processes in coastal Louisiana. Journal of Coastal Research 54, 23-36.