New measurements of ongoing subsidence of land proximal to the city of New Orleans, Louisiana, and including areas around the communities of Norco and Lutcher upriver along the Mississippi are reported. The rates of vertical motion are derived from interferometric synthetic aperture radar (InSAR) applied to Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on 16 June 2009 and 2 July 2012. The subsidence trends are similar to those reported for 2002–2004 in parts of New Orleans where observations overlap, in particular in Michoud, the 9th Ward, and Chalmette, but are measured at much higher spatial resolution (6 m). The spatial associations of cumulative surface movements suggest that the most likely drivers of subsidence are groundwater withdrawal and surficial drainage/dewatering activities. High subsidence rates are observed localized around some major industrial facilities and can affect nearby flood control infrastructure. Substantial subsidence is observed to occur rapidly from shallow compaction in highly localized areas, which is why it could be missed in subsidence surveys relying on point measurements at limited locations.

1 Introduction Loss of deltaic lands and the associated increase in flood risk is a major issue facing coastal communities across the globe [Wassmann et al., 2004; Ericson et al., 2006; Morton et al., 2006; Day and Giosan, 2008; Mazzotti et al., 2009; Teatini et al., 2011; Kay et al., 2015; Liu et al., 2015; Tessler et al., 2015]. The extent to which this subsidence is exacerbated by human influence [Syvitski, 2008; Syvitski et al., 2009; Blum and Roberts, 2012; Giosan et al., 2014] is in dire need of improved quantification, especially in the face of the complicating factor of an ongoing, and possibly accelerating, rise of global mean sea level [Church and White, 2011; Hay et al., 2015]. The Mississippi River Delta is losing its natural coastal barriers, including deltaic wetlands and barrier islands [List et al., 1994; Day et al., 2000; Glick et al., 2013], increasing the flood risk across the area. The primary response to an increasing flood threat is investment in infrastructure and restoration activities to protect human populations and areas of high economic value [Tessler et al., 2015]. However, at the most basic level, improving resiliency requires understanding the underlying processes driving subsidence. The processes are diverse and heterogeneous, and as a consequence improvements in our first‐order observations will enable more reliable modeling that can accurately predict future subsidence and guide effective implementation of remediation and subsidence reversal actions [Turner, 1997; Day et al., 2005, 2007; Törnqvist et al., 2008; Yuill et al., 2009; Louisiana Coastal Protection and Restoration Authority, 2012]. Southeast Louisiana is a Holocene landscape built upon a coastal delta created by the Mississippi River during the past ~7000 years, with major land building initiating after post‐Pleistocene sea level rise rates slowed [Coleman and Roberts, 1988; Coleman et al., 1998]. Prior to human intervention, natural subsidence was offset by a combination of sediment deposition during Mississippi River floods and organic soil production from decay, primarily of the wetland vegetation prevalent in the region. Construction of flood control levees to protect the Gulf Coast economy and local population interrupted the sediment supply, with the direct consequence of net increase in land subsidence [Gagliano et al., 1981; Coleman et al., 1998]. In Greater New Orleans, the local geology plays a major role in flooding and subsidence [Fisk, 1960; Saucier, 1963; Snowden et al., 1980; Dixon et al., 2006]. The city lies along the current path of the Mississippi River and is built on a combination of modern and relic natural levees and buried or artificially drained swamps and marshes. It is located in an area that received sediment from multiple historic Mississippi River deltaic lobes [Saucier, 1963], which constructed a complex network of sediment and soil layers from the different depositions along historic distributaries and in former interdistributary troughs, swamps, or marshes, including peat accumulations of 0–4.9 m (0′–16′) depth underlying much of the extended city, sometimes interfingered with inorganic facies [Fisk, 1960]. Modern subsidence in the Mississippi River Delta is the integrated effect of numerous natural and anthropogenic processes that operate at several different spatial and temporal scales, so motion at any point is dependent on a unique set of local and regional conditions. Natural processes include sediment compaction/consolidation, faulting, salt evacuation and kinetics, and load‐induced crustal downwarping. Anthropogenic processes include extraction of fluids, specifically water, oil, and gas; aquifer and reservoir compaction; induced fault motion from human activities, e.g., mining and fluid withdrawal; and organic sediment decomposition and compaction due to drainage projects. Measurement, interpretation, and modeling are all complicated by the fact that each contributing process varies in areas affected, spatial extent, and induced subsidence rate, duration, and reversibility. There is clear evidence that subsidence across the Mississippi River Delta is neither constant nor spatially uniform. It is well documented that subsidence in the wetlands is both higher and displays greater temporal variation than on the coastal plains and along relic and current‐day levees [Morton et al., 2009], but the root cause and relative apportionment of wetland subsidence between geological and anthropogenic factors remain contentious [Gagliano et al., 2003; Morton et al., 2006; Kolker et al., 2011; Olea and Coleman, 2014; Turner, 2014]. Subsidence rates in New Orleans have been measured using data from 1955 to 1995 [Dokka, 2011], 1995 to 2006 [Dokka et al., 2006], and 2002 to 2005 [Dixon et al., 2006]; all observed high subsidence rates in the Michoud area, where ground water withdrawal and a tectonic component of subsidence are documented [Dokka, 2011]. The most spatially comprehensive of the studies reported an average subsidence rate of 8 mm/yr across most of the populated areas of Greater New Orleans on both banks of the river [Dixon et al., 2006]. In general, the numerous studies of subsidence of the Gulf Coast, especially those local to the Mississippi River Delta, would appear to provide an inconsistent picture. Some report minimal subsidence [Törnqvist et al., 2004, 2006]; others report significant recent subsidence regionally [Shinkle and Dokka, 2004; Karegar et al., 2015] or locally [Dixon et al., 2006]. Much of this disagreement comes from the fact that the measurements cover processes acting at different depths or time scales, having different spatial extent, or acting at different locations, so that they do not quantify the same phenomena. Some more recent studies report isolation of a single mechanism, as in the isolation of the lithospheric flexural subsidence from glacial isostatic adjustment [Yu et al., 2012], possibly allowing improved understanding of the relative roles of sediment, ocean loading, and glacial isostatic forebulge migration [Ivins et al., 2007; Ivins and Wolf, 2008]. Rates of subsidence in the deeper subsurface have been estimated from a combination of modeling and geologic constraints to be 2 mm/yr or less [Wolstencroft et al., 2014] in areas outside the bird‐foot delta [Fisk et al., 1954]. Clearly, other mechanisms must be acting in some areas because rates can be locally very high, indicating that there are multiple natural and human‐induced processes operating at different scales. Localized measurements from Global Positioning System (GPS) give some information about the relative contribution of deep and shallow processes. For example, the MARY station in Michoud records 1.3 mm/yr subsidence at a depth of 2000 m (Table 1 and Figure S4 in the supporting information), which would imply that in the same general area there is >1 cm/yr subsidence within the shallower layers based upon measurements from below the base of local Holocene deposits [Dokka, 2011] or at the surface (including on buildings of unknown footing depth) [Dixon et al., 2006]. Table 1. Global Positioning System (GPS) Continuously Operating Reference Station (CORS) Locations, East/North/Up (ENU) Velocities (IGS08 Frame), Site Geology, and Station Mounting Information Station ID Location Latitude Longitude V EAST (mm/yr) V NORTH (mm/yr) V UP (mm/yr) Installation Date Monument Foundation Foundation Depth Surface Geologyb NOLA Loyola University 29.93437 −90.12019 −12 −1.2 −0.2 6/19/2012 Brick 12.2 m Silt/mud DSTR Destrehan High School 29.96456 −90.38223 −12 −1.2 −1 11/18/2005 Concrete Roof Unknown Silt/mud LUCH Lutcher, LA 30.04651 −90.69351 −13.8 +2.6 −2 1/11/2006 N/A Unknown Unknown MARY NASA Michoud Assembly Facility 30.02298 −89.91301 −11.6 −1.5 −1.3 12/22/2010 Wellhead Cap 2012 m Bedrock/clay/conglomerate/gravel/sand ENG5 English Turn 29.87896 −89.94173 −12.5 −1.3 −0.8 9/20/1995 Steel Mast Unknown Bedrock/clay/conglomerate/gravel/sand LWESa Lakewood Elementary School 29.90037 −90.34941 −11.6 −0.1 −4.1 2/7/2006 Concrete Roof Unknown Silt/mud Better knowledge of subsidence rates at the surface and, in particular, at higher spatial resolution can help identify the shallow subsidence processes whose rates can far exceed those of the deeper subsidence. A geographically continuous subsidence map is best provided by interferometric synthetic aperture radar (InSAR), where image swaths are 10–100 km in width. InSAR is a technique that enables mapping of centimeter‐scale surface deformation at ground resolutions of ~10–100 m by forming interferograms of sequential synthetic aperture radar observations [Gabriel et al., 1989]. Here we report subsidence rates between June 2009 and July 2012 in the metropolitan New Orleans area and at two locations upriver from the city, one encompassing Norco, Louisiana, and the other around Lutcher, Louisiana, derived from InSAR applied to data from the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR), an airborne instrument operated from 12.5 km altitude. Both of the upriver scenes are composed of small residential areas adjacent to large industrial facilities. This work uses high‐resolution L‐band InSAR, which maintains interferometric temporal coherence over longer time periods when compared to shorter‐wavelength instruments, such as the C‐band SAR used in the 2002–2005 study [Dixon et al., 2006]. Although the datum documents cumulative movement from all subsidence mechanisms at work, in some areas it suggests a primary mechanism. In those cases, we compare the subsidence rates to local infrastructure, industry, and geology using publically available geographic information system (GIS) layers and maps from the earlier work documenting conditions in the Mississippi River Delta. A discussion of the GIS methodology and a list of GIS sources are included in the supporting information.

2 Data and Methods InSAR is used to determine cumulative ground deformation between successive observations by measuring phase changes in the line‐of‐sight distance to the target, a method sensitive to changes of a small fraction of the radar wavelength. An interferogram formed from two UAVSAR images with 1112 day separation from 16 June 2009 to 2 July 2012 is used for this study and is available as a geocoded, 3 × 12 multilooked (slant range x azimuth), standard UAVSAR product with 6 m pixel spacing (see supporting information). UAVSAR is an L‐band (23.8 cm wavelength) synthetic aperture radar with high signal‐to‐noise ratio [Fore et al., 2015] and repeat flight track accuracy typically < ±5 m [Hensley et al., 2009]. Being an airborne instrument, UAVSAR has both higher signal‐to‐noise ratio than spaceborne SARs and different predominant systematic error sources. For example, at L‐band and given the aircraft flight track repeatability, the topographic errors are negligible, but the aircraft flight track is not as constant or well known as a satellite orbit so errors from aircraft motion are larger. For the lines used in this analysis, the track was maintained to < ±4 m, with the exception of one excursion to 7 m during the second acquisition. The height ambiguity for a 7 m baseline is hundreds of meters at UAVSAR incidence angles and wavelength, far exceeding the topography of the low‐lying study area, so the error from the cross‐track baseline is negligible. Because UAVSAR operates from 12.5 km altitude, the pulses are subject to tropospheric delays. Spatial and temporal variations in pressure, temperature, and relative humidity cause spatially varying tropospheric noise signal in InSAR interferograms [e.g., Hanssen, 2011; Bekaert et al., 2015a]. Even over flat terrain and with a spatially uniform, stratified troposphere, the large UAVSAR look angle variation can introduce a strong tropospheric phase ramp in the range direction. Tropospheric noise is reduced by estimating a correction from the ERA‐Interim global atmospheric model [Doin et al., 2009], provided by the European Centre for Medium‐Range Weather Forecasts [Dee et al., 2011] and computed using the Toolbox for Reducing Atmospheric InSAR Noise (TRAIN) [Bekaert et al., 2015b]. TRAIN was modified to account for an airborne platform, which includes the integration of the refractivity from the surface to the aircraft altitude and the variable UAVSAR acquisition time. The main error source is undetermined aircraft motion, which contributes either a phase ramp in the slant range direction (across track, primarily accounted by atmospheric correction) or phase bands bunched along the azimuth direction (along track) [Jones, 2016]. It was visually confirmed that there were no apparent bands or ramps correlated with azimuth and range image offsets, which would indicate motion‐induced phase artifacts. To cope with spatial long‐wavelength errors, the scene was cropped into smaller areas, each of which contained a Continuously Operating GPS/GNSS Reference station (CORS GPS) that was used as the local phase reference. Figure 1a shows the full extent of the acquired UAVSAR data, the cropped scenes' extents, and the location of GPS stations. The three cropped scenes were selected to encompass the entire metropolitan New Orleans on the east bank of the Mississippi River and a small portion on the west bank (Figure 1b); an area near Norco, Louisiana, extending across the east and west banks of the Mississippi River and including the Bonnet Carré Spillway (Figure 1c); and an area around Lutcher and Gramercy, Louisiana, that includes the Hester‐Vacherie salt dome (Figure 1d). The incidence angles for the three scenes were ϑ inc = 31°–65.2° (Figure 1b, NOLA reference), ϑ inc = 38°–62° (Figure 1c, DSTR reference), and ϑ inc = 50°–65.2° (Figure 1d, LUCH reference), with the incidence angle increasing from south to north, as shown in Figure 2a. Figure 1 Open in figure viewer PowerPoint (a) Overview of the study area in the Mississippi River Delta near New Orleans, Louisiana (USA). The white boxes outline the areas analyzed for our study. Stars indicate the GPS stations in or near the study areas. The UAVSAR aircraft was flying east, and the radar look direction is to the north. (b–d) Subsidence rates calculated from an interferogram formed from two UAVSAR images acquired on 16 June 2009 and 2 July 2012 (1112 day separation) for the Greater New Orleans area (phase reference NOLA, Figure 1 b); Norco, Louisiana (phase reference DSTR, Figure 1 c); and Lutcher, Louisiana (phase reference LUCH, Figure 1 d). Negative vertical velocity indicates subsidence. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. Figure 2 Open in figure viewer PowerPoint Total estimated uncertainty in the rates, calculated as the random phase error and systematic uncertainty added in quadrature, in (a) radar line‐of‐sight direction and (b) projected onto the vertical direction assuming there is no horizontal contribution to the line‐of‐sight displacement. The radar incidence angle versus position in the cross‐track direction is shown in Figure 2 a. Names used in the text to refer to different communities and parts of the city are shown in Figure 2 b. (c) Ratio of the measured vertical rate (Figures 1 b– 1 d) to the total estimated uncertainty (Figure 2 b). Service Layer Credits: Esri, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. In support of this study, CORS GPS data were collected and analyzed through Louisiana State University (LSU) to determine subsidence with high accuracy at many locations along the Gulf Coast. The work at LSU used both the National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey's CORS network and stations at sites established by LSU. Table 1 contains information about the six GPS stations located within or near the cropped scenes (Figure 1a) that were in operation during 2009–2012. A discussion of the processing method and plots of the East‐North‐Up (ENU) velocities (Figures S1–S6) are included in the supporting information. The LUCH station is unique in showing northward horizontal movement of statistically significant magnitude (2.6 mm/yr) (International GNSS Service of 2008 (IGS08) reference frame), in contrast to all other stations, which show southward movement at ≤1.5 mm/yr. The results for this location were confirmed through independent analysis of the LUCH station's Rinex data (R. Muellerschoen, GPS analysis results, Jet Propulsion Laboratory, personal communication, 2015). The eastward movement is comparable at all six sites (−11.6 to −13.8 mm/yr). The full UAVSAR scene contains industrial and urban areas separated in some cases by marshland or open water, which for the most part did not maintain phase coherence and in any case are unreliable because the phase change could arise from water level change in areas with standing vegetation. A mask was applied to remove pixels with coherence < 0.60, and the scene was further masked to remove areas of water, swamps, and intermediate, brackish, and salt marsh using vector GIS data from the Louisiana Coastal Marsh‐Vegetative Type Database [Louisiana Department of Wildlife and Fisheries, 2001]. The relatively high coherence threshold ensured that most pixels imaged hard structures, usually either on the structure or including its base, e.g., multibounce scattering from the ground and the structure. Because of low pixel coherence at some GPS station locations, the phase reference was calculated from high coherence ( ≥ 0.85) pixels in a 600 m × 600 m box surrounding the CORS site, following verification that the general subsidence within the box was uniform. The extensive amounts of water or marsh in the scene and the long temporal baseline precluded successful application of a standard phase unwrapping algorithm. Therefore, to permit manual phase unwrapping with high fidelity, each cropped scene was limited in size to (1) contain a GPS site, (2) exclude extended areas showing temporal decorrelation, e.g., water and marsh, (3) not exceed one interferometric fringe (6.28 radians) in large‐scale line‐of‐sight phase change across its extent, and (4) retain a continuous, high coherence path between all unmasked areas in the scene. Average line‐of‐sight velocities were calculated from the line‐of‐sight displacements assuming a constant rate of change over the 1112 day interval. All line‐of‐sight rates were projected to the vertical direction following determination from the GPS measurements that there was no large‐scale differential horizontal velocity component across the scene at a level above the systematic uncertainty. The general horizontal trend measured with GPS in the study area is 12 mm/yr west and 1.3 mm/yr south. The westward component is not observable by the UAVSAR instrument, which was north looking, because it has no component in the radar line‐of‐sight direction, and the southward horizontal movement projected onto the line‐of‐sight direction is <1 mm/yr across the entire swath, below the uncertainty in the InSAR‐derived velocities. InSAR determines movement at all locations in the scene relative to an in‐scene reference of known movement, e.g., the location of GPS geodetic measurement. Because of this, InSAR's absolute accuracy is inherently limited by the accuracy of the measurement at the reference location and generally worsens with distance from the reference. This introduces a systematic uncertainty in the line‐of‐sight displacement that depends upon a pixel's distance from the reference point. Setting a high coherence threshold resulted in a very small random error from phase variance (~1–2 mm/yr in line‐of‐sight displacement), so the uncertainty in the derived rates is dominated by the systematic error. The systematic uncertainty was estimated in two parts, as follows, with the first accounting for the error in the velocity at the reference location and the second for systematic errors in the velocity at all other locations in the scene. The first contribution was estimated to be 2 mm/yr from the difference between the CORS GPS Up velocity component (Table 1) and the InSAR‐derived vertical rate at the reference CORS site. The second component was estimated using the data itself to account for the uncertainty from all unknown sources (e.g., motion artifacts and residual atmospheric noise). A single scene was formed to encompass the individual New Orleans (Figure 1b) and Norco (Figure 1c) scenes and its interferogram normalized to the NOLA CORS site. The line‐of‐sight velocity for the pixel containing the DSTR CORS site was compared to the GPS‐measured rates projected onto the line‐of‐sight direction and used to estimate a distant‐dependent uncertainty that increased with increasing distance from the phase reference location, similar to the estimation used by Dixon et al. [2006]. This procedure yielded an estimate of 0.38 mm/yr/km for the distance‐dependent line‐of‐sight velocity uncertainty. Figure 2a shows the line‐of‐sight velocity uncertainty, calculated as the systematic and random uncertainties added in quadrature, and displaying circular symmetry about the phase reference locations. The vertical velocity uncertainty (Figure 2b) increases to the north because the radar incidence angle increases in that direction. Figure 2c is a binary valued map showing the locations where vertical velocities exceed the uncertainty. In areas where the ratio is less than unity, the uncertainty sets an upper limit on the subsidence rates. Systematic errors apply across broad regions, so relative subsidence measurements between nearby points are much more accurate than the plotted absolute uncertainty. Because the systematic uncertainty dominates the total uncertainty, the relative accuracy between two locations in close proximity can be estimated from Figure 2 as the difference in the uncertainties at the locations being compared.

3 Results 3.1 Subsidence Trends in New Orleans and Upriver Communities Figure 1 shows the average vertical velocity between 16 June 2009 and 2 July 2012 for the three scenes in our study area, encompassing most of the metropolitan New Orleans areas and two smaller sites along the Mississippi River to the west. Because the random uncertainty is small (<2 mm/yr line of sight), we have high confidence in relative changes in a localized area. The results show that this 1112 day interferogram is not sensitive to subsidence rates with magnitude below 3 mm/yr anywhere in the scenes but does identify the areas showing anomalously large subsidence, in particular either movement at localized features, like individual structures, or at a broader scale, i.e., the scale of neighborhoods, facilities, or communities. Very high subsidence rates are measured in some locations, up to 70 mm/yr, but these are either on buildings with roofs that clearly need repair or isolated pixels with unreliable phase unwrapping, which can also give very high uplift rates. In New Orleans (Figure 1b), Michoud shows the most subsidence (typically 15–30 mm/yr), with the next highest subsidence occurring in the vicinity of the Upper 9th Ward and eastward in the area between the Mississippi River and Bayou Bienvenue (typically 10–20 mm/yr). The locations of named communities, cities, and parishes are shown in Figures 2 and 3. Subsidence of smaller magnitude, but statistically significant, occurs in eastern Metairie (5–15 mm/yr) and at some locations along the Mississippi River, particularly in Jefferson Parish. A subsiding area often shows up on both banks of the river; e.g., the area on the west bank opposite Chalmette shows higher subsidence than nearby areas on that side of the Mississippi River. Figure 3 Open in figure viewer PowerPoint Subsidence in Jefferson parish near the east bank of the Mississippi River, showing that the predominant scatterers are structures and that the measured displacements are a combination of movement of the ground, apparent as a general trend in the area, and movement of the structures, as in the very high deformation rates seen on individual buildings. The inset drawing at lower right shows the parish locations. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS user community. Subsidence upriver from New Orleans generally decreases, except for high subsidence around major industry in Norco (Figure 1c). In that area subsidence occurs near industry on both banks, with the higher subsidence (≤ ~ 40 mm/yr) in a bowl around a refinery on the south side (west bank) of the river. The subsidence decreases upriver from the industrial locations. Near Lutcher (Figure 1d), subsidence is significantly less, with some indication of higher subsidence along the Mississippi River and in the west part of the scene, although the temporal decorrelation in this particular area is too large to determine the center of the subsidence bowl if there is one. Measured vertical rates in Lutcher indicate much less subsidence than at the downriver locations. The horizontal lines in this map (Figure 1d) are likely to be power lines along east‐west oriented roads. The L‐band InSAR results reported here come from radar backscattering from structures and represent a combination of cumulative subsidence at depths below the foundation of the structure, movement of the structure itself, and from double‐bounce scattering from the ground plus structure, which introduces a component sensitive to surface level subsidence. In previous studies of this area, measurements were either taken at a single point using leveling [Dokka, 2011] or GPS [Dokka, 2006] or across an extended area using InSAR [Dixon et al., 2006]. For the previous InSAR study, no spatial resolution was specified, but the reported results are at the resolution of city blocks or greater, not pixelated at the single structure level. For our results, the spatial resolution is sufficient to resolve displacement at the single pixel (6 m resolution) scale, so we are able to differentiate structural movement from the general subsidence across the area, as in Figure 3, which shows an area in Jefferson Parish where sagging roofs of single buildings stand out from the general movement of the surrounding area. Averaging in this area would inaccurately reflect ground movement. The high resolution of the UAVSAR data is also useful for isolating movement on levees and other flood protection infrastructure and is discussed in section 4.3 below. 3.2 Comparison to Previous Studies Previous studies have reported regional subsidence trends or more localized measurements in the New Orleans area. No subsidence measurements specific to the Norco or Lutcher study areas have been reported. The most comparable previous study is that of Dixon et al. [2006], in which line‐of‐sight displacement rates are derived from persistent scatterer InSAR analysis of RADARSAT‐1 C‐band (5.6 cm wavelength) data acquired in 33 images of both east bank and west bank areas of metropolitan New Orleans between April 2002 and July 2005, a 3.25 year period that ended just before Hurricane Katrina struck the area. There is significant overlap between their study area and our New Orleans scene, particularly on the east bank of the river. In general, there is good correspondence between areas showing the most subsidence during 2002–2005 and those showing the most subsidence in the 2009–2012 time period, particularly at the scale of communities, but we also observe many features at smaller scales, often near major industrial sites or in a single neighborhood. The former includes areas in New Orleans proper along the Mississippi River, Chalmette, and East New Orleans. In areas with lower subsidence rates, the uncertainty for the UAVSAR results is too large to allow reliable comparison, so we here concentrate on the areas showing highest subsidence rates and on general trends across the greater New Orleans area, including only areas with values greater than the uncertainty estimate (ratio > 1, shown in Figure 2c). In 2002–2005, measured subsidence rates were highest, 13–29 mm/yr, in Michoud and an area near the Louis Armstrong New Orleans International Airport. Shinkle and Dokka [2004] had also reported the highest subsidence rates in Michoud, with values of 20–30 mm/yr over the interval 1920 to 1995. High subsidence rates in Michoud near the New Orleans power plant were also measured previously at benchmarks set below the Holocene layer using geodetic leveling and water level gauge measurements [Dokka, 2011], with 70 mm of deep‐seated vertical displacement occurring in 1991–1995 [Dokka, 2011]. The measured rates from L‐band InSAR during 2009–2012 were also highest in the Michoud area, reaching 50 mm/yr at some structures on the west side of the scene. The subsidence rate at the Entergy power plant was 25–30 mm/yr. Throughout this area, the subsidence measured with UAVSAR exceeded the uncertainty by a factor of >1.4 (Figure 2). In contrast to the 2002–2005 results, we find that there was no statistically significant subsidence at the New Orleans airport in Kenner or the nearby area, although the airport shows higher subsidence than the immediately surrounding area, and isolated structures did in some cases show subsidence rates of up to 25 mm/yr. Our results are consistent with the lower range of the previous measurements but make it unlikely that the subsidence rates are currently near the high end of the range in 2002–2005. Dixon et al. [2006] found that in 2002–2005 locations experiencing intermediate subsidence rates, >7 mm/yr, were in several areas bordering Lake Pontchartrain, particularly in Lakeshore and at the eastern half of the East New Orleans district, and in the vicinity of the Upper 9th Ward, Chalmette, and areas to the west along the Mississippi River. We also find high subsidence rates around the Upper 9th Ward and westward, with rates of 5–25 mm/yr subsidence in the area, compared to <18 mm/yr in 2002–2005. Given the high correspondence in the pattern of subsidence and the larger uncertainty in the more recent results, there is generally good agreement. In 2009–2012, stable elevation to relative uplift is seen in East New Orleans bordering Lake Pontchartrain, in contrast to subsidence seen at the east end of this area in 2002–2005.

4 Discussion The InSAR technique measures total surface elevation changes resulting from all sources, anthropogenic and natural, deep‐seated or shallow. The InSAR data must be carefully interpreted to disentangle these phenomena, which can operate at different temporal and spatial scales. Understanding the contribution of each driving mechanism relies upon long‐term measurements of surface elevation and establishing a solid geophysical basis for each from the data. Accordingly, here we demonstrate InSAR‐based observations of subsidence, and although we concentrate on reporting measurements, we include some discussion of possible causes for the subsidence trends that we measure. From a public safety standpoint it is important to make these results available so that flood modeling and response strategies to reduce future flood risks are informed by accurate land elevations, and the discussion is intended to motivate modeling and further measurements, not to definitively identify the mechanisms at play. We draw comparisons between observed subsidence and the probable localized subsidence processes that we can relate to public records of wells, past inundations, building construction dates, and geologic and soil records, available as GIS data sets. By overlaying spatially extensive subsidence maps with GIS data sets, the proximate cause for locally anomalous subsidence can be identified more effectively, to help differentiate man‐induced from natural processes. 4.1 Anthropogenic Drivers of Localized Subsidence 4.1.1 Aquifers of the New Orleans Area In the study area, there are four major confined aquifers underlying parts of the area, namely, the Gramercy, Norco, Gonzales‐New Orleans, and 1200 Foot Sand aquifers [Dial and Sumner, 1989], in addition to a number of small, disconnected, unconfined aquifers, referred to summarily as “the shallow aquifers of the New Orleans area” [Griffith, 2003], for which no map was found. The Gramercy aquifer is the shallowest, and in our study area extends no farther east than the west part of New Orleans adjacent to Metairie, and the Norco aquifer the next shallowest, and extends no farther east than the western edge of the Upper 9th Ward and East New Orleans [Dial and Sumner, 1989]. The sediment and soil layers above these four aquifers are referred to as the “New Orleans surficial confining unit,” when describing the layer to which dewatering and surface water level monitoring wells are connected. Depth to the aquifers varies greatly in the study area, and the Gramercy and Norco are closest to the surface in the locations listed above as their maximum eastern extent [Dial and Sumner, 1989]. The Gramercy aquifer lies at ~40 m depth or deeper in the New Orleans area (scene covered by Figure 1b) and is below the footing of the structures whose subsidence is mapped. Approximately 100 km east of New Orleans, and outside the study area, the four aquifers connect to the Mississippi River Alluvial Aquifer, into which they typically discharge, and there is no local connection to the waters of the Mississippi River [Dial and Sumner, 1989]. Recharge occurs in the upland terraces north of Lake Pontchartrain. The only aquifer reliably providing freshwater to New Orleans is the Gonzales‐New Orleans [Dial and Sumner, 1989], so most withdrawal wells connect to it. The saltwater intrusion into the other aquifers decreases to the east as the distance from the Gulf of Mexico increases; e.g., more wells connect to the Norco aquifer in the Norco area than in New Orleans. In this manuscript, groundwater withdrawal refers to withdrawal from the four named aquifers and dewatering (drainage) refers to pumping to lower the surface water table within the surficial confining unit or from the unnamed shallow aquifers. 4.1.2 Groundwater Withdrawal Groundwater withdrawal can be a primary driver of subsidence in urban and industrial areas and has been determined to be a causative agent in the Michoud area of New Orleans previously [Dokka, 2011]. In the areas where we see subsidence associated with groundwater extraction, the available information indicates that withdrawal from the main aquifers occurs at depths below the foundation of structures [Dial and Sumner, 1989]. The association between subsidence and groundwater pumping is evident in our subsidence results in two areas, namely, around the oil refineries and chemical plants in the vicinity of Norco on both sides of the Mississippi River and around major industry located in Michoud. A smaller signal is seen around industry along the Mississippi River in Chalmette. In Michoud (Figure 4), subsidence is 25–30 mm/yr at the power plant, Entergy New Orleans. The higher subsidence around the power plant is consistent with the previously documented influence of groundwater pumping on localized subsidence and increases near the Mississippi River where there are more water withdrawal wells (Figures 4b and 4c). Subsidence initially decreases to the east between the power plant and the NASA Michoud Facility then increases farther east, reaching up to 50 mm/yr on the other side of Michoud Canal. The MARY GPS station, located at the NASA Michoud facility, measures 1.3 mm/yr subsidence at a depth of 2000 m. This means that essentially all subsidence we measure comes from movement in the layers above 2000 m. There is a large potentiometric cone located near Michoud that is directly related to industrial water withdrawals from the Gonzales‐New Orleans aquifer [Prakken, 2009]. There are no oil or gas fields in this area nor active oil or gas wells, and all injection wells we found documented are associated with dry hole oil wells (http://www.sonris.com/). Figure 4 Open in figure viewer PowerPoint Dokka [ 2011 Subsidence in East New Orleans and Michoud. (a) Overview of the general area, including Michoud, East New Orleans, and districts of the city proper bordering Lake Pontchartrain. The location of the Michoud Fault, identified by], is indicated. No distinct change in subsidence is seen along the fault. (b) The outlined area is shown: Subsidence in Michoud near the Gulf Intercoastal Waterway, from the New Orleans power plant (lower left) to the industrial complex immediately east of Michoud Canal. The MARY GPS site is indicated. (c) Active water wells as reported in 2012 ( http://www.sonris.com/ ), and the locations of the Entergy New Orleans power plant, NASA Michoud Assembly Facility, and other local industry. Wells are indicated by type and by aquifer for the withdrawal wells connected to the Gonzales‐New Orleans Aquifer. Subsidence increases near the cluster of withdrawal wells at the power plant and increases again at industrial sites to the east, particularly near Michoud Canal. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. Although there is increasing subsidence to the east, without ground measurements or InSAR coverage beyond the end of our scene, we cannot determine whether the cause is localized or continues into the marshlands to the east. The UAVSAR data in this area did not maintain coherence and could not be used in this study. The trend suggests that subsidence on the east part of the scene arises from a combination of multiple effects, one being groundwater withdrawal. We note that the depth of the peat layer underlying this area increases to the east [Fisk, 1960], so compaction is likely to increase to the east of the Entergy New Orleans plant, where the soil types indicate progressively higher organic content and more clays [United States Department of Agriculture, 2015]. Because most of the lands to the east of the study area are undeveloped and not dewatered, oxidation is not a factor contributing to subsidence. Figure 5 shows a close‐up of subsidence rates around Norco; the location of industrial facilities, which are primarily chemical plants refining crude oil to produce gasoline and other fuel products; and the location of water withdrawal wells, with the aquifer to which they connect indicated. Industry in this area uses cooling water withdrawn from the aquifers, and the subsidence is higher near the wells withdrawing water. The bullseye pattern of subsidence on the west bank across from the Bonnet Carré Spillway indicates that subsidence is related to pumping. As in Michoud, the soil types in this area are complex but generally have more silt and loam and less clay along the Mississippi River bank, extending inland past the industrial sites showing subsidence [United States Department of Agriculture, 2015]. The lower soil organic content also argues for water withdrawal from aquifers being a primary driver of subsidence in this area. A smaller signal of localized subsidence around industry is also apparent in Chalmette, shown in Figure 6c. Figure 5 Open in figure viewer PowerPoint (a) Subsidence rates around Norco, Louisiana, and the location of flood protection levees (white). The DSTR GPS station (black triangle) is used as the phase reference for this scene. (b) Map showing location of water wells active in 2012 ( http://www.sonris.com/ ), local industry, and the Bonnet Carré Spillway. The wells are specified by type and by aquifer for withdrawal wells connected to the major aquifers underlying the area. The highest subsidence forms a bowl within the refinery site to the south of the river, and there is also high subsidence north of the river bordering the levees and the spillway. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. Figure 6 Open in figure viewer PowerPoint Federal Emergency Management Agency, 2006 Fisk, 1960 Snowden et al., 1980 Snowden et al., 1980 Comparison of (a) 2009–2012 subsidence rates and locations of levee breaches during Hurricane Katrina with (b) NOAA Hurricane Katrina flood extent and depth map on 3 September 2005 []. (c, d) Close‐up views of Chalmette, Louisiana, showing the trend toward higher subsidence nearer Bayou Bienvenue, where the peat layers are thicker [.,]. Exceptions to this trend are subsidence bowls around some of the industrial sites (triangles) located on the natural levees bordering the Mississippi River where there is little to no underlying peat [.,]. The levees bordering Bayou Bienvenue, where coherent, are observed to be predominantly subsiding at the rate of the adjacent neighborhoods. Service Layer Credits: Esri, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. 4.1.3 Dewatering/Drainage Although Hurricane Katrina caused widespread flooding across New Orleans in August–September 2005, nearly 4 years before our study began, there is some correspondence between the subsidence rates we measure and the flood depth and extent of Hurricane Katrina flooding. Figures 6a and 6b show flood extent and depth as of 3 September 2005 across the New Orleans area and the 2009–2012 subsidence rates with Katrina levee break locations (Federal Emergency Management Agency, publication FEMA 549, available at http://www.fema.gov/media‐library/assets/documents/4069). Figures 6c and 6d show a close‐up image of the Chalmette area with the location of local industry denoted. Subsidence trends generally follow flood depth across the eastern parts of the city, aside from localized subsidence around industry that is apparent in Michoud and along the Mississippi River in Chalmette. Subsidence between East New Orleans and the Lower 9th Ward occurs in areas near the levee breaks of Hurricane Katrina (Figure 6a). The levees seen in the zoomed image of Figures 6c and 6d, where coherent, subside at the same rate as the adjacent structures. This suggests that subsidence is occurring at depths below the levee footing. More study using time series InSAR, preferably at L‐band to maintain good coherence on the levees themselves, is needed. This relationship between subsidence and land elevation suggests that dewatering, or lowering of the phreatic water level in the shallow aquifers, leads to increased subsidence due to soft soil processes associated with decreased buoyancy or shrinkage of the drained soil, which is consistent with compaction and, near the surface, increased aerobic oxidation of organic‐rich soils. Both compaction and oxidation are strongly dependent upon type and vertical distribution of soils, thickness and depth of different soil layers, and the level of the water table. Compaction can occur from the surface to the top of the named aquifers, within the myriad soil, clay, and peat layers, and can cause subsidence at depths greater than the foundations of the structures measured. The fact that subsidence is high in the vicinity of the current‐day levees and relic natural levees bordering deeply flooded areas, as in the 9th Ward, which includes high ground (Figure 7b), suggests that dewatering causes soil compaction. The levees do not have high organic soils at the surface, so aerobic oxidation will cause negligible subsidence, but they can cover soft soils from former deltaic formations. A relationship between subsidence and soft soil processes is suggested by the fact that high subsidence in the Upper 9th Ward continues eastward along the levees in that area, as the peat deposit thicknesses increase dramatically in Bayou Bienvenue, going from ~1.2 m (4′) thickness in New Orleans to the west of the Upper 9th Ward to >4.3 m (14′) thickness north of the Lower 9th Ward [Fisk, 1960]. Figure 7 Open in figure viewer PowerPoint (a) Overview of subsidence with (d) a close‐up of an area in Metairie showing subsidence. The subsidence feature in Metairie is a white outlined circle on all maps and images. (b) Elevations of the full and (e) close‐up area in Figures 7 a and 7 d. The subsidence does not follow elevation strictly but does include a low area (locally the low elevation point) nearly encircled by higher‐elevation land. (c) Optical image showing flooding from Hurricane Katrina on 30 August 2005 near the east side of Metairie, including flooding of the Metairie and Greenwood cemeteries (white outlined box). The areas at the northeast were not flooded but to the west and south were inundated to varying depths (Hurricane Katrina flood depth shown in Figure 6 b). (Image credit: Google Earth) Service Layer Credits: Esri, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. A subsidence bowl is also evident in Metairie (Figure 7). In the Greenwood and Metairie Cemeteries immediately to the east of the bowl, subsidence generally follows the flood depth during Hurricane Katrina (optical image, Figure 7c; subsidence, Figure 7d; and elevation, Figure 7e). The area to the west (circled in Figure 7) looks very different, with subsidence covering a broad area that does not follow the elevation (Figures 7b and 7d) or correlate with the peat deposit thickness, which is only 0–1.2 m (0–4′) in this area [Fisk, 1960]. Three withdrawal wells connect to the Gonzales‐New Orleans aquifer in this area (Figure 7d), with the two in red being used for watering and the one in black for industry. The subsidence bowl centers on none of these, which indicates that it arises from dewatering or from some other soft soil process. Among the latter, we consider two possible mechanisms for apparent subsidence in this area between June 2009 and July 2012, and note that if related to either of them, this feature is likely to be transitory, as opposed to showing a long‐term trend. Both possible explanations relate to the Mississippi River stage at the times of acquisition of the UAVSAR data. The U.S. Army Corp of Engineers (USACE) Mississippi River gage at Carrollton (29.9347°, −90.1361°) recorded 3.8 m (12.6′) on 16 June 2009 and 0.8 m (2.5′) on 2 July 2012 (http://rivergages.mvr.usace.army.mil/WaterControl/stationinfo2.cfm?sid=01300&fid=RCKI2&dt=S&pcode=HG). The interferogram used for this study was formed between 2012, a very low flow year for the Mississippi River, and 2009, a high water year. Relative subsidence between 2009 and 2012 is consistent with there being less soil saturation in this area in 2012, with less buoyancy and more compaction. We consider two ways in which this could happen at this particular area. The first is through the Gramercy aquifer, which is near its shallowest in this area (46 m (150′)) [Dial and Sumner, 1989]. Although normally discharging into the Mississippi River Alluvial Aquifer far upriver from New Orleans, high water in the Mississippi River can reverse this, causing recharge from the Mississippi River [Dial and Sumner, 1989], in which case discharge would be into the soils above the aquifer and most apparent at the surface in the Metairie area. The second possibility is that water entered this area through the Pine Island sands, a remnant barrier island chain underlying parts of New Orleans that forms a layer of sand and shell at shallow depth, generally lying within 1.5–9.2 m (5′ to 30′) of the surface [Saucier, 1963]. It has been suggested previously that the Pine Island layer could form a pathway through which water from the river or canals moves inland [Nelson, 2013]. Delta wide, an average of about 1000 m3/s of water is estimate to flow underneath Mississippi River levees through relict and buried channels and sandy islands, a value that could be several times greater when river stage is high [Kolker et al., 2013]. A time series analysis is needed to establish whether the relationship between surface water table and the river stage does indeed exist over multiple years and river stages. We note that if the relationship is established, this would be a hydrogeological driver of transitory elevation change, potentially exacerbated by dewatering. 4.2 Geologic Drivers of Subsidence 4.2.1 Michoud Fault Previous studies have reported subsidence in East New Orleans that shows recent retrograde vertical movement along the Michoud Fault [Dixon et al., 2006; Dokka, 2011], with higher subsidence rates to the east of the fault. Figure 4a shows our results relative to the general location of the Michoud Fault. We do not see the differential subsidence across the fault apparent in the RADARSAT‐1 data of 2002–2005. The subsidence in this area appears to be dominated by groundwater withdrawal to the southeast and possibly dewatering. 4.2.2 Holocene Layer Compaction One of the most widely debated issues regarding subsidence in the Mississippi River Delta is the relative contributions to subsidence from shallow Holocene sediment compaction compared to lithospheric loading from sea level rise and deposits of Pleistocene sediment transported following the melting of the glaciers. There are a few locations where our data provide some information about Holocene layer compaction, although the specific observation is due to man‐made loading in areas of new residential home development from fill added to raise the ground elevation prior to building and from the structures themselves. In this case, the subsidence measured generally extends from the surface downward or from ~4 m downward, as described below, depending upon the foundation used. On the east bank of the river in New Orleans, building codes require that commercial and residential structures be footed on piers seated below the Holocene layer [Dokka, 2011]. However, on the west bank in Jefferson Parish there are areas where pier/pillar foundations are not required for residential structures, or the foundations can be waived if soil is added and compacted prior to construction (Building and Building Regulations, Jefferson Parish Code of Ordinances, https://www.municode.com/library/la/jefferson_parish/codes/code_of_ordinances?nodeId=PTIICOOR_CH8BUBURE). In the event that pillars are used, they are inserted to a depth of 3.7–4.6 m (12′–15′) (B. Smith, Geoscience Engineering & Testing, Inc., Houston, Texas, oral discussion, 2015). Figure 8 shows two neighborhoods that were developed over the same time period, one in Westwego and the other in Waggaman, with different sections constructed in 2009, 2004, and prior to 1998. In Waggaman, pier/pillar foundations are not required, and in Westwego, they are required if soil is not compacted; visual imagery shows these to be raised homes seated on pillars. Soil types at the surface from the Web Soil Survey [United States Department of Agriculture, 2015] are also shown. Although we do not have information about the soil profiles at these locations, the thickness of the surface confining unit above the aquifers in this area is ~40 m [Dial and Sumner, 1989]. In Westwego (Figures 8a and 8b), the newest development is built on clay soil and lies directly adjacent to houses built earlier on silt loam. Although the soil map shows distinct zones, it is likely that there is a transition region along the boundary of mixed soil type. In Westwego, the houses built in 2009 show substantial subsidence during the 2009–2012 period, particularly those farther from the boundary between clay and silt loam soil types. The houses on clay built in 2004 do not show differential settlement relative to houses built before 1998. This indicates that the onset of compaction of shallow soils from loading occurs quickly and continues for a relatively short time, <5 years, following loading. With only a pair of images forming the interferogram, we cannot determine whether the subsidence was still ongoing in 2012 or had abated sometime between the baseline and second image acquisitions. In the nearby community of Waggaman (Figures 8c and 8d), comparable houses built in 2009 on silty clay loam soil subside approximately a factor of 3 less. Although rapid shallow layer compaction is only shown here for an example on Harahan clay and Schriever silty clay loam, the same phenomena is seen in our data elsewhere on the west bank, indicating that it is likely general to the study area. The rapid development and cessation and the spatial localization of shallow compaction makes it more likely to be missed. This shallow compaction could be a substantial component of subsidence that is not detected with benchmarks or the CORS GPS stations located on commercial buildings or on foundations footed in the Pleistocene layer. Figure 8 Open in figure viewer PowerPoint Subsidence of two neighborhoods on the west bank of the Mississippi River in Jefferson parish, (a) Westwego, and (c) Waggaman, with (b, d) the soil types and dates of housing development indicated. Houses built in 2009 or later show distinctly greater subsidence during 2009–2012 than houses built earlier, and houses built on clay show more subsidence than houses built on silty clay loam. The foundation depths of the structures are discussed in the text to relate this subsidence to rapidly occurring compaction of the upper soil layers. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS user community. 4.2.3 Tectonic Conditions Near the Hester‐Vacherie Salt Dome The area around Lutcher/Gramercy (Figure 1d) was included in the study even though it contains relatively few high coherence pixels because the LUCH GPS data are unique in showing northward motion of >2 mm/yr, compared to all the other locations, which show <0.2 mm/yr southward movement (Table 1 and supporting information Figures S1–S6). This area, shown in Figure 9, is unique also in being located at the intersection of two major geofractures, the West Maurepas Alignment to the west and the East Valley Wall Alignment to the south [Fisk, 1944], and contains the Hester‐Vacherie Salt Dome, which extends to within 2070 m (6780′) of the surface [Stipe and Spillers, 1962] and lies adjacent to the West Maurepas Alignment. The study area is on the downdropped block of the West Maurepas Alignment and on the upside block (relatively stable) of the East Valley Wall [Fisk, 1944]; it contains the Vacherie Fault, which ruptured in April 1943 [Fisk, 1944], and is in line with the Frenier regional growth fault to the east [Voelker, 1965]. Although the top of the salt dome is deep, the Vacherie Fault rupture caused up to 20 cm (8″) of surface displacement and extended on the surface over 1.7 km (1 mi) [Fisk, 1944]. Figure 9 Open in figure viewer PowerPoint (a) Line‐of‐sight cumulative displacement in the vicinity of the LUCH GPS site in Lutcher, Louisiana, with the Hester‐Vacherie salt dome, Vacherie Fault, West Maurepas Alignment, and East Valley Wall growth fault zone indicated. The line‐of‐sight direction is to the north, and the incidence angles are given in Figure 2 . (b) Overview of the area showing the fault zones, up and down sides of growth faults, and the salt dome location. The LUCH GPS station indicates northward movement at 2.6 mm/yr, in contrast to all other GPS stations in and around our study area, which show southward movement. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community. Because of the complicated tectonics, the assumption that measured displacement is entirely vertical is not necessarily valid, so the cumulative line‐of‐sight displacement is shown in Figure 9. 4.3 Effects of Subsidence on Levees and the Bonnet Carré Spillway The city of New Orleans is at flood risk from the Gulf of Mexico to the east and the Mississippi River running through the south side of the metropolitan area. The greater risk is from storm surges accompanying hurricanes in the Gulf of Mexico, and the city is surrounded by levees to prevent inundation from the east through Lake Borgne, from the north through Lake Pontchartrain and from the south through the wetlands connecting to Barataria Bay. Following the catastrophic levee failures induced by Hurricane Katrina in August 2005, the levees system was augmented by the addition of flood gates and pumps at the canal outlets to Lake Pontchartrain, and with an immense flood wall, the Inner Harbor Navigation Canal Surge Barrier, located to the east of Michoud Canal and protecting the Gulf Intercoastal Waterway and the east side of New Orleans from storm surge through Lake Borgne. Levees along the Mississippi River protect the urban areas from high water in the river. The Bonnet Carré Spillway, located east of Norco, upriver from the city, stands as the last line of protection against springtime riverine floods that would overtop the levees, enabling water to be shunted to Lake Pontchartrain before reaching the city. Figure 10 shows localized subsidence on and near some of the flood protection infrastructure that show the most subsidence. Figure 10a shows the Bonnet Carré Spillway, which is surrounded by subsiding land, most likely from groundwater withdrawal (see section 4.1.2). In this case, there is very high coherence in the pixels north of the spillway, which correspond to backscatter from concrete blocks set into the ground to prevent scour when the spillway is open. We see high subsidence of the ground behind the structure, up to 40 mm/yr. Investigation of possible subsidence impacting the spillway directly is needed. Figure 10b shows the stretch along the Mississippi River‐Gulf Outlet Canal (MRGO) reported to have high subsidence in 2002–2005 [Dixon et al., 2006]. It is obvious that the high coherent scatterers in our data are rocks at the bottom of the waterside slope, and we are unable to quantify subsidence of the levee crown from our 3 year temporal baseline interferogram. Figure 10c shows the Paris Avenue Bridge and the Entergy New Orleans site. Here there are coherent pixels on the levee crown near the power plant, and they follow the subsidence trends of the ground around the plant. To the east of the Michoud Canal (Figure 10d) we see the maximum levee subsidence of any location, up to 50 mm/yr. These levees lie near one end of the Inner Harbor Navigation Canal Surge Barrier. Figure 10 Open in figure viewer PowerPoint Subsidence of flood control infrastructure in relation to subsidence of the adjacent areas. (top) Overview of area showing levees (white) and outlined areas shown in detail. Details of subsidence (a) along the Bonnet Carré Spillway, (b) along the Mississippi River‐Gulf Outlet Canal (MRGO), (c) in Michoud near the power plant, and (d) in Michoud in the vicinity of the Michoud Canal. Service Layer Credits: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, HERE, DeLorme, MapmyIndia, ©OpenStreetMap contributors, and the GIS user community.

5 Conclusions In order to fully understand, measure, and predict future subsidence accurately, it is necessary to understand the partitioning between subsidence due to fluid withdrawal, compaction, growth faulting, lithospheric loading from sediment deposition, glacial isostatic adjustment, and other processes. Both a spatially comprehensive map of subsidence and, at the same time, improved spatial resolution are essential to differentiating the contributing processes. Here we report spatially extensive measurements in and around New Orleans for 2009–2012 using airborne L‐band InSAR that can be used to constrain development of more comprehensive subsidence models for the Mississippi River Delta. In addition to providing InSAR measurements in previously unmonitored areas, the primary contributions of this work compared to earlier spaceborne InSAR studies are the improved resolution and reduction in temporal decorrelation using L‐band data. The longer‐wavelength sensor (L‐band) is particularly useful for quantifying large movement in rapidly subsiding areas, where phase ambiguities might contribute to shorter‐wavelength (X‐band and C‐band) measurements. As an airborne instrument, it is not subject to ionospheric effects as are L‐band satellite‐borne SARs, and in this study we account for atmospheric noise using independent data from a weather model. The major limitation of this specific work is the use of only two scenes to derive linear rates, which means that seasonal and environmental variations contribute to the estimated rates. This highlights one of the limitations of airborne SARs, namely, the high cost and coordination needed for campaigns not local to the home base. Subsidence rates in 2009–2012 support the conclusion that groundwater withdrawal is the primary subsidence driver in areas with major industry around the New Orleans, particularly in Norco and Michoud. This is becoming an understood and observed phenomena in other deltas and coastal areas of the world, e.g., the Yangtse Delta [Zhang et al., 2008], Po Delta, Italy [Teatini et al., 2011], and Mekong Delta [Erban et al., 2014]. Similarly, dewatering is observed to cause subsidence in other areas, e.g., the Sacramento Delta [Deverel and Leighton, 2010] and the Everglades [Hooijer et al., 2012]. In metropolitan New Orleans, dewatering and drainage appears to affect surface elevation; while this is not unexpected, the results from InSAR give an indication of the spatial extent of this phenomenon, which can be mitigated by policy change. The spatial correlation between elevation and subsidence rates shows that subsidence continues in locations that have subsided already and that in aerial extent dewatering is likely to be the most important driver of subsidence in the urban areas. Using the high resolution of the UAVSAR instrument, we are able to identify compaction of shallow sediments and deduce from the dates of residential house construction that this type of subsidence halts relatively rapidly following surface loading, in <5 years, with significant declines in rates after 0.5–3 years. Although this phenomenon is observed as an anthropogenic driver, it has implications for subsidence of either geologic or anthropogenic origin and shows that InSAR time series with frequent acquisitions are needed to capture and quantify the rates of shallow compaction. This requirement of frequent observations holds also for the anthropogenic drivers, which are controlled largely by local water management and environmental factors like precipitation and water levels in the Mississippi River. With the improved resolution of this new InSAR data set, we observe that the subsidence rates in New Orleans and nearby communities can be large, yet spatially localized, and that subsidence centered around facilities can extend to flood control infrastructure several kilometers distant. This type of subsidence is linked to groundwater pumping, so in principle the elevation loss can be recovered when the aquifer recharges. However, this is a real, albeit time varying, loss in height of the flood protection infrastructure and should be included in design considerations. This important observation shows that the spatially comprehensive subsidence mapping provided by InSAR can inform effective planning for long‐term coastal resiliency and the sustainability of New Orleans, informing public safety and water management decisions.

Acknowledgments The authors thank Alexander Kolker, Louisiana Universities Marine Consortium, and Robert Smith, Geoscience Engineering and Testing, Inc., Houston, Texas, for their useful discussions. We are grateful to Ronald Muellerschoen of the Jet Propulsion Laboratory for providing an independent analysis of the LUCH CORS GPS data. The UAVSAR data are courtesy of NASA/JPL‐Caltech (www.uavsar.jpl.nasa.gov) and can be accessed through the Alaska Satellite Facility website (https://vertex.daac.asf.alaska.edu/). This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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