Improving Flood Protection - Understanding How Levees Are Different From Dams

Following the catastrophic destruction and loss of life induced by the 2005 flooding of New Orleans by Hurricane Katrina, there are now renewed efforts to evaluate and upgrade many aging levee systems across the country. This devastating flood event demonstrated yet again the fragility of levees and put a face on the countless personal tragedies that result when levees fail and cause deep flooding of an urban area. Many levee owners and public agencies are now evaluating their levee systems for potential defects, re-evaluating their maintenance practices, and looking to revise levee design criteria or standards. This includes the United States Army Corps of Engineers (Corps) which is now in the process of revising its levee design manual and issuing new vegetation and encroachment management standards. These efforts are certainly appropriate given our recent understandings that many urban levees protect more people and have higher risks than do many dams. Some engineering professionals and policy makers are also recommending that we apply engineering criteria and standards that have long been used for the evaluation and design of embankment dams when we are evaluating or improving levees. Such recommendations need to be explored, but new criteria should be developed with the full understandings of where levees are similar to dams, and where they are different. To begin with, dams generally are situated across rivers within a limited footprint, whereas levees run along river systems for extended lengths, sometimes for hundreds of miles. Dams typically hold back reservoir water every day of every year, whereas levees generally are only part-time workers and only hold back water during extreme flood events. There are many differences in how these two types of structures were designed and constructed, with levees often being built without the benefit of any modern engineering methods. There are also differences in how the performance of the structures is monitored and historical performance standards. Further, there are huge differences in how such structures are regulated, governed, and funded. Public impacts in many cases are also quite different. This paper will discuss many of these issues and provide some recommendations for an approach to upgrade levee criteria and performance with an understanding of the differences and legacies of these two different types of structures. 29 pp., 19 figures, 14 references.