SUBSIDENCE IN NEW ORLEANS MEASURED WITH PERMANENT SCATTERER INTERFEROMETRY

It has long been recognized that New Orleans is subsiding and is therefore susceptible to catastrophic flooding. Here we present a new subsidence map for the city, generated from space-based synthetic aperture radar measurements, which reveals that parts of New Orleans underwent rapid subsidence in the three years before Hurricane Katrina struck in August 2005. One such area was next to the Mississippi River–Gulf Outlet (MRGO) canal: levees failed here during the peak storm surge and the map indicates that this could be explained by subsidence of a meter or more since construction. To make the subsidence map, we used 33 scenes recorded from Canada's RADARSAT satellite. The technique involves interferometric phase comparison of 33 synthetic-aperture radar images taken at different times along the same nominal orbit, and it exploits points on the ground that strongly reflect radar, termed ‘permanent scatterers'. Changes in the LOS distances for the permanent scatterers over the 2002-2005 period indicate that subsidence was widespread throughout New Orleans. If the measured LOS change was due to purely vertical motion, the mean and maximum subsidence rates were 6.4 and 33 mm/yr, respectively. Since global sea level is rising at an average rate of about 2 mm/yr, most of New Orleans is therefore subsiding relative to mean sea level at an average rate of about 8 mm/yr. Although levee failure during the hurricane may have resulted from water overtopping the levees, the high subsidence rates we observe might also reflect active faulting or a weak, easily compacted substrate, promoting failure at or near the levee base.