Hydrologic Realities and the Limits of Protective Structures

Despite its strategic and economic importance, the New Orleans region always has been vulnerable to flood and hurricane storm surge hazards. The Mississippi River delta is a low-lying region surrounded by waterbodies—namely, the Mississippi River, Lake Pontchartrain, and Lake Borgne—that rise and can overflow during hurricanes and floods. The proximity of New Orleans and southeastern Louisiana to the large, shallow continental shelf of the northern Gulf of Mexico make the area highly vulnerable to Gulf hurricanes and storm surge.

The origins of today’s New Orleans hurricane protection system date back to U.S. Army Corps of Engineers’ planning studies in the mid-1950s and issuance of a 1962 interim survey report for the Lake Pontchartrain and Vicinity Hurricane Protection Project (LP&VHPP; Woolley and Shabman, 2007). The major principle guiding the system’s construction and maintenance, as well as post-Katrina repairs and strengthening, has been to “make the city safe.” In this large region of varying topography and elevation, and as demonstrated by Hurricane Katrina and past storms in New Orleans and elsewhere, this guiding principle—although noble—is flawed.

Modern protective structures and diligent maintenance and repair efforts can help reduce the risks of hurricanes and storm surge. In fact, the ability of these structures to help protect against storm surge was demonstrated in New Orleans during Hurricane Gustav in early September 2008. The drama surrounding the storm surge of Hurricane Gustav, which nearly (but did not) overtopped a protective (T-wall) structure along New Orleans’s Inner Harbor Navigation Canal was broadcast to a national viewing audience. In that instance, that protective structure clearly resulted in a reduction in flood damages in the low lying areas behind that structure.

Protective structures, however, do not provide certain protection against all storm surges. They can be overtopped in large storms and there always is the risk of future—even with well-constructed and maintained structures. Thus, even in areas behind well-built structures, some risk—referred to as “residual risk”—will exist to inhabitants and structures. Structures can reduce some hydrologic risks but all flood and hurricane storm surge risks in this region never can be fully eliminated. It therefore is critical to consider a guiding principle for these protective structures as one that seeks to reduce risks from hurricanes and storm surge—but recognizes that such risks cannot be fully eliminated and, as such, augments flood and hurricane protection by protective structures with complementary measures such as floodproofing of buildings, evacuation plans, and comprehensive land use planning. In fact, the IPET recognizes and supports

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