Friday, June 02, 2006

Waves and soil erosion

You mean it wasn't Bush's fault? How can this be?

IPET determined that, after 35 years structures at the Inner Harbor Navigation Canal were more than two feet lower than their design specifications. Other discrepancies, such as outfall canals built one to two feet below their specified elevations, were the result of designs being based on water level datum while construction was based on geodetic vertical datum—the two are not equivalent, spatial and temporal variations between 0.2 and three feet were found between the two.

Government or contractor negligence was not discovered. The practices and design criteria did vary however since 1965. The piecemeal construction of levees, floodwalls and gates over the decades led to “inconsistent levels of protection.” Protection erected around the 17th Street Canal for example was not as strong as those at the Orleans Canal, which incorporated more conservative designs and practices. Materials also ranged in strength and fortitude.

IPET also determined Katrina’s surge levels were as much as six feet higher than design levels in the eastern and southern portions. And the waves were “long period ocean storm waves” allowing them to run over the levees. Some waves generated velocities of 10 to 15 feet-per-second over levees.

All but four breaches were due to overtopping and erosion. A key element leading to failures at the 17th Street Canal and London Avenue was the formation of gaps behind I-walls. The morning of the hurricane, water had already risen 1.7 feet above the tops of the levees and floodwalls to an elevation of 14.2 feet. As the water passed over the levees, it eroded the soil supporting the walls degrading their stability and resulting in catastrophic flooding.

Yet another reason I try and wait for actual analysis before drawing conclusions. Sometimes when you get out in front of the facts, you turn out to be wrong.

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