Roy K. Dokka’s research while affiliated with Louisiana State University and other places

A steady-state subsidence forecast model was developed as a proof of concept to estimate changes in surface elevations of the wetlands and evacuation routes across coastal Louisiana for the years 2015, 2025, 2050, and 2100. Subsidence estimates were derived from an empirical study published by the National Geodetic Survey. Forecasted vertical change was subtracted from current surface elevations. Land and evacuation routes estimated to have surfaces at or below 0 m in elevation, NAVD88, were quantified and classified as vulnerable to inundation hazards. The extent of the coastal zone susceptible to hurricane induced storm surge was also evaluated relative to surge models published by the National Weather Service. The results indicate spatially heterogeneous rates of subsidence that are forecasted to consume nearly 50 % of the existing coastal margin wetlands by 2100. The most significant rate increases are anticipated between 2015 and 2050. Relative to the impact on evacuation routes, subsidence occurring between the 2015 and 2025 forecast years expanded at slower rates when compared to the latter half of the century. Subsidence adjusted storm surge forecasts reveal similar patterns. The methods employed and findings produced demonstrate forecasting capabilities that provide emergency managers and transportation engineers with resources applicable to evacuation modeling, hazard mitigation, environmental sustainability research, coastal restoration efforts, and more.

The vulnerability of the Gulf Coast has received increasing attention in the years since hurricanes Katrina and Rita. A quantitative geophysical basis for measuring, predicting, and understanding subsidence rates, their geographic distribution, and temporal variability, is necessary for long term protection of lives and property in addition to being a challenging scientific problem. Analysis of historical and continuing geodetic measurements identifies a surprising degree of complexity in subsidence, including regions that are subsiding at rates faster than those considered during planning for hurricane protection of New Orleans and other population centers (http://www.mvn.usace.army.mil/pdf/hps_verticalsettlement.pdf), and for coastal restoration planning for coastal Louisiana (http://www.coast2050.gov/2050reports.htm) (Dokka, 2011, J. Geophys. Res., 116, B06403, doi:10.1029/2010JB008008). Meanwhile, traditional geodetic data provide precise information at single points, InSAR observations provide geographically dense constraints on surface deformation. Available radar data sources include C and L band satellite, and NASA/JPL airborne UAVSAR L band data. The Gulf Coast environment is very challenging for InSAR techniques, especially with systems not designed for interferometry. The shorter wavelength C band data decorrelates over short time periods necessitating more elaborate analysis techniques. We have early results from new persistent scatterer methods and masking techniques to eliminate areas affected by water level changes, all applied to C-band satellite radar data. Limited L-Band ALOS/PALSAR satellite data are available for analysis using conventional interferometry, unfortunately this Japanese satellite system recently failed. Most importantly, we now have airborne UAVSAR repeat pass interferometry data sets spanning a total interval of 514 days (http://uavsar.jpl.nasa.gov/). These data can constrain geophysical models of crustal behavior, leading to quantitative predictions of future subsidence. Preliminary model results show good agreement between geodetic measurements and geophysically reasonable parameters including sediment load and ~130 m post glacial sea level rise. We review work to date and present newly acquired UAVSAR data.

Disaster response to incidents such as the 2010 Deepwater Horizon oil spill requires rapid access to comprehensive, consumable, and actionable data. Providing effective situation awareness requires data collection methodologies capable of account for the inherent spatial and temporal characteristics of the incident. However, data collection is often encumbered by complex technologies that require specialized knowledge for use. Consequently, these requirements can impede the effectiveness of disaster response. To compensate for these challenges, an easy-to-use and spatially accurate incident reporting system was designed for responders tasked with identifying the location and extent of oil infiltration within marshes and bays of south Louisiana following the disaster. This workflow was assembled around a Global Positioning System (GPS)-enabled digital camera capable of receiving positioning corrections from GPS reference networks. Images depicting oiled beaches, habitats, and wildlife were automatically georeferenced and displayed using common geographic data visualization applications. Whether uploaded to data servers or printed, the imagery was shared across a wide audience, fostering collaboration among all response agencies.

Geodetic leveling observations from Biloxi, MS, to New Orleans, LA, and water level gauge measurements in the New Orleans-Lake Pontchartrain area were analyzed to infer late 20th century vertical motions. These data were used to test the validity of previous subsidence rate measurements and the models that predict the location and causes of subsidence. Water gauges attached to bridge foundations and benchmarks affixed to deep rods that penetrate Holocene strata subsided as much as 0.8 m locally between 1955 and 1995. The observed deep-seated subsidence far exceeds model predictions and demonstrates that shallow processes such as compaction and consolidation of Holocene sediments are inadequate by themselves to explain late 20th century subsidence. Deep-seated subsidence occurring east and north of the normal faults marking the Gulf of Mexico basin margin can be explained by local groundwater withdrawal, and regional tectonic loading of the lithosphere by the modern Mississippi River delta (MRD). Sharp changes in subsidence coincide with strands of the basin margin normal faults. Displacements are consistent with activity and show motions consonant with fault creep. Deep subsidence of the region to the south, including New Orleans, can be explained by a combination of groundwater withdrawal from shallow upper Pleistocene aquifers, the aforementioned lithospheric loading, and perhaps, nongroundwater-related faulting. Subsidence due to groundwater extraction from aquifers ˜160 to 200 m deep dominated urbanized areas and is likely responsible for helping to lower local flood protection structures and bridges by as much as ˜0.8 m.

The causes of multi-decadal coastal subsidence that exceed the rate of 20th-21st Century mean sea-level rise are numerous. A host of causal relationships between anthropogenic activity and coastal erosion and subsidence are both compelling and well understood. A short list of these processes should include: fluid extraction, reservoir compaction; fluid-induced motion of shallow growth-faults; decomposition of organic sediments (left in the wake of major water diversion projects) and sediment empoundment and/or diversion. Non-anthropogenic post-glacial sediment loading-induced subsidence has also been demonstrated to be a candidate mechanism that could dominate the GPS-mapped subsidence in the Mississippi-Atchafalaya Rive Delta system (Ivins, Dokka and Blum, GRL, 34, L16303, 2007). More recent changes in sediment load can also have a substantial impact on the active subsidence of the crust and mantle. These can be of both anthropogenic and natural origins. Here we examine, in varying degrees of detail, the world's major delta systems wherein the necessary and sufficient conditions are met that allow sediment plus water load to drive gravitationally-layered, hydrostatically pre-stressed viscoelastic Earth model simulations far enough away from gravitational equilibrium that a detectable geodetic signal in tide-gauges and GPS vertical position would be predicted. Sediment-load-induced subsidence occurs over horizontal length scales, lambda, comparable to thickness of the lithosphere: lambda >= 40-60 km and has amplitudes ranging from of 0.5 to 8 mm/yr (Ivins et al. GRL34, 2007; Syvitski Sustain. Sci., 3, 2008). The cases of the pan-Arctic deltas, the Atchafalya -Mississippi River Delta system, the Yangzte River and Yellow River Deltas of China, the Danube River Delta Plain, and the generally complex postglacial water-sediment loading of the Black Sea and Caspian Sea.

Hurricanes Katrina and Rita focused attention on the vulnerability of the U.S. Gulf Coast. Significant improvement in geophysical understanding of subsidence rates, temporal variability, and geographic distribution is not only an interesting scientific challenge, it is necessary for long term protection of lives and property. An integrated geophysical approach using precise and accurate geodetic measurements is the only way to gain physical insight into the myriad of possible processes at work and provide accurate predictions of future subsidence rates. In particular, southeast Louisiana is a Holocene landscape built on a coastal delta created by the Mississippi River during the past ~8,000 years as sea level rise slowed. Prior to human intervention natural subsidence was offset by sediment deposition by the Mississippi River during floods, and in situ organic sediment production in marshes. Currently, several processes have been documented to contribute to subsidence, including wetland loss due to lack of present day sediment flux, land subsidence due to sediment compaction, sediment oxidation, fluid withdrawal, salt evacuation, tectonics, and also crustal loading. One of the least studied subsidence driving phenomena is the effect of crustal loading due to Mississippi River sediments, and the geologically recent ~130 m (427 ft.) rise in sea level. We model subsidence rates expected from these loads using geophysical methods developed for post-glacial rebound. Our model predicted, and geodetically observed, vertical subsidence rates vary between 2 - 8 mm per year over areas of 30,000 to 750 square kilometers, respectively. This viscoelastic flexure is the background crustal deformation field, upon which larger amplitude, but smaller spatial scale, subsidence occurs due to other factors. We are extending subsidence measurements from traditional geodetic techniques (including GPS), to geographically comprehensive measurements derived from synthetic aperture radar interferometry (InSAR) using both satellite and airborne radars. The Gulf Coast is a very challenging environment for InSAR techniques and we are developing new persistent scatterer methods to apply to available C-band satellite radar data. More recent L-Band PALSAR satellite data are suitable for conventional interferometry. We are also making new observations with NASA/JPL's new airborne interferometer system UAVSAR (http://uavsar.jpl.nasa.gov/). The high spatial resolution UAVSAR data has the potential to monitor levees and other critical infrastructure better than satellites. We review work to date and present newly acquired UAVSAR data.

The causes of multi-decadal coastal subsidence are numerous. A host of causal relationships between anthropogenic activity and coastal subsidence and erosion are both compelling and well understood. A short list of these processes should include fluid extraction, reservoir compaction, fluid induced motion of shallow growth faults, decomposition of organic sediments (left in the wake of major water diversion projects) and sediment empoundment and/or diversion. Among natural causes are long-term aseismic vertical tectonics, large earthquakes, volume change of the ocean, changes in wind direction and strength (Fukumori et al. J. Phys. Oceanography, 37, 338-358, 2007) and global (long wavelength) glacial isostatic crustal and geoid change. We have determined that under some special circumstances sediment loading may also have a substantial impact on the active subsidence of the crust and mantle. Those special circumstances apply near river delta systems capable of large changes in volume and/or position of deposition, for these circumstances are precisely what is required to drive the gravitationally-layered, hydrostatically pre-stressed viscoelastic Earth away from gravitational equilibrium. Sediment-load-induced subsidence occurs over horizontal length scales, lambda, comparable to thickness of the lithosphere: lambda >= 40-60 km and has amplitudes ranging from of 0.5 to 8 mm/yr (Ivins et al. Geophys. Res. Lett., 34, 2007; Syvitski Sustainability Science, 3, 23-32, 2008). Here we discuss the theory, the necessary conditions for, and the present-day predicted amplitudes that apply to the cases of the Atchafalya -Mississippi River Delta system, the Yangzte River Delta, the Danube River Delta Plain in the Black Sea and the massive glaciomarine sediments of the Weddell Sea. In the case of the Gulf Coast in the US a variety of subsidence mechanisms operate simultaneously. We attempt to provide a model-based template for delineating the optimum terrestrial and space-based data sets that are best suited to ferreting out competing processes that cause present-day and past subsidence.

New, more efficient methods of measuring road surfaces, including using moving vehicles, are being developed and deployed. Testing the veracity of such data presents its own problems. Busy, heavily traveled highways do not lend themselves to easy occupation for careful measurements by conventional means. The advent of GULFNet by the Louisiana State University (LSU) Center for Geoinformatics (C4G) provides the ability to get highly precise, accurate positions anywhere within the state tied to the National Spatial Reference System (NSRS), which enabled the establishment of benchmark sections of roadway against which the performance of new methods may be tested. The LSU C4G with the cooperation of Louisiana Department of Transportation (LADOTD) personnel accessed and measured the elevations and locations of points, quickly and safely, along test sections of highways in each LADOTD district. The precisions of the measurements, as reported by the Real-Time Kinematic (RTK) engine, averaged better than 3 cm at three standard deviations (3σ). Recommendations are made for using the measurements at the sites for testing the Moving Vehicle Rapid Mapping (MVRM) systems to assess the precisions reasonably to be expected by these systems under a variety of circumstances.

Earth scientists rarely influence public policy or urban planning. In defiance of geologic reality, cities are established on or expanded into floodplains, wetlands, earthquake faults, and active volcanoes. One exception to our lack of influence is that shortly after a major natural disaster, there is a brief window of heightened public awareness that may lead to sensible regulation or relocation of infrastructure. After the 1933 Long Beach earthquake, for example, California building codes were improved to reduce earthquake hazard. After Mississippi River flooding in 1993, several U.S. cities designated parts of their low-lying floodplain as green space. How have we done with New Orleans and southern Louisiana, devastated by hurricanes Katrina and Rita in 2005? Unfortunately, not very well. In the aftermath of those storms, an opportunity existed to educate engineers, policy makers, and the public about long-term hazards associated with land subsidence and sea level rise. This message was not conveyed, and expensive rebuilding has proceeded under the false assumption of relative coastal stability and slow sea level rise.

Sea level rise in the Gulf of Mexico has occurred at a rate of 1.8–2.2 mm/yr during the 20th century, or nearly the same as observed globally due to combined steric and water mass changes. Tide gauges in coastal Louisiana, however, record a substantially larger rate of rise and while a number of causal mechanisms may be responsible, their specific contribution is poorly understood. Using a realistic viscoelastic Earth model, detailed geologic parameters for south Louisiana and new GPS data, we demonstrate that Holocene sedimentary loading in the Gulf and Mississippi River delta is capable of contributing to 1–8 mm/yr of subsidence over areas of 30–0.75 × 103 km2.