Presentation on theme: "Dr. Roy K. Dokka Department of Civil and Environmental Engineering Louisiana State University Baton Rouge, LA 1 Deformation Monitoring Monitoring Deformation."— Presentation transcript:
Dr. Roy K. Dokka Department of Civil and Environmental Engineering Louisiana State University Baton Rouge, LA 1 Deformation Monitoring Monitoring Deformation Monitoring
What is Deformation and why do we need to monitor it? de ⋅ for ⋅ ma ⋅ tion –noun 1. the act of deforming; distortion; disfigurement. 2. the result of deforming; change of form, esp. for the worse. 3. an altered form. 4. Geology, Mechanics. a change in the shape or dimensions of a body, due to stress; strain. It is change, and not all change is good!
Hazards Associated with Natural Systems Faults and seismicity Volcanism Subsidence and resulting coastal and flooding. Management of the Built Environment Bridges Dams Highways Flood protection systems and flood maps Geodetic networks Integrity validation and monitoring. Flavors of Deformation Monitoring
But how do we measure change? First, a preamble.
“How we measure the world defines how well we can understand it” Quotes to Remember “If you can’t measure it properly, you will never understand it” An example...
. Both methods can measure time. But why then do we use a stopwatch to clock the race?
In fact, the stopwatch is 8,640,000 times more precise than the calendar. 1 day = 24hours x 3600 sec./hr x 100 hundredths of a second/sec. 1 day = 24 x 3600 x 100 = 8.64 x 10 6 The measurement resolution we need to determine the winner Take Away: Your measuring tool needs to match the process causing the movement
Basic Ideas Behind Deformation Monitoring Case 1--Well understood relationship between deformation and underlying process. Motions can signal the time a place of a “tipping point”. Here, you may be able to avoid disaster. Example: Rising water behind a dam. Case 2-- Relationship between deformation and underlying process is not well understood. Motions can be premonitory but do not give clear signal of what will happen. Here, only warnings and heighten preparation is possible. Example: Strain preceding an earthquake.
The measurement of any physical quantity requires: a tool capable of measuring change at the desired spatial and temporal resolution. --subsidence over the design-life of the project. --causative processes operate at the mm/yr level. --subsidence is a 4-D effect. an understanding of the nature of the entity being measured. --You can’t measure it if you don’t under- stand it. You need sufficient domain knowledge so you can develop an effective sampling strategy.
Example: The Deformation and its Effects of Subsidence in South Louisiana Subsidence: the sinking of the land with respect to a datum. 1900 sq. mi. of the Louisiana coast has disappeared since 1930. This affects everything, communities, levees, wetlands, pipelines, navigation on river,.. And the Geodetic Reference network! This affects areas outside Louisiana in terms oil and gas, river commerce, disasters.
13 Dying Coast 2008 River (+dams) levees block sediment and freshwater from reaching coast. Natural subsidence unrelenting. Stupid stuff people do. Inundation 2075 Loss of south Louisiana wetlands Island communities Disrupted Mississippi River
Coastal Louisiana Clearly Has Issues!! It is a dynamic landscape, but how do we approach monitoring deformation? What outcomes do we want? What do we need to know and what technology do we need?
Why Monitor Subsidence? Everybody needs access to a 3-D datum. We need to understand how subsidence is changing that accessibility and figure out ways to overcome it. Engineers, scientists, and planners need measurements that will help provide sufficient scientific insight into present and future subsidence to meet needs, i.e., hurricane protection, restoration, asset monitoring, etc.
Domain knowledge The operative processes establish the requirements of measurement. Measurement Method(s) Define precision, accuracy, time-averaging, thereby establishing what processes can be measured. Monumentation Determines how much of the total subsidence signal you can measure; e.g., benchmarks and CORS do not “see” shallow processes. Sampling Strategy Determines what processes can be isolated and measured. Subsidence Monitoring Basics
Let’s review the basic geological reality about modern landscape change in south Louisiana The modern Mississippi River delta has evolved over last ~8,000 yrs. Landscape is due to the interplay between subsidence, accretion, and global sea level rise. Flooding builds land by sediment deposition and stimulates wetlands biologic processes. Deltas cannot grow much above sea level. The delta is composed of lobes that formed as the river shifted position with time. As a lobe was abandoned, accretion ceased yet the lobe continued to subside. Over time, the lobe is slowly inundated by the Gulf. A load of ~120m of delta sediments and ocean have been added to the Earth’s crust since 18K years ago and would be expected to cause down- warping and gravitational instability. Because such a landscape is not favorable for human habitation, levees were built by order of Congress to prevent flooding and to maintain Miss. River navigation. It has also had the unintended consequence of halting accretion. Subsidence, however, continues.
Deep Processes: Sediment and water load induced flexure of the lithosphere: constant 0 to -7mm/yr Faulting: variable up to -40mm/yr Salt evacuation: variable 0 to -?? mm/yr Shallow Processes: Natural consolidation and compaction Desiccation by urbanization (behind levees) Organic soil oxidation Oil & gas extraction: variable 0 to -3 mm/yr Water pumping: variable up to -65mm/yr Natural and Anthropogenic Processes that Result in Subsidence
Loading explains some of the observations, but not all. Something else is going on. Predicted present-day vertical motions in mm/yr from delta & ocean loads GULFNet station at Cocodrie, LA sinking 6.3 mm/yr Ivins et al (2006)
Geodetic Leveling-Biloxi to NOLA BiloxiNOLA Major regional subsidence begins near 1966 and ends at ~2000. Occurs south of Tepatate-Baton Rouge fault. Lesser subsidence occurs to Pensacola. Note arching of coastal Mississippi. Subsidence centered on Michoud fault, but can be mapped southwest to the coast. Waveland
Benchmarks do not record all subsidence. However, an ensemble of geodetic measurements can. ~11mm/yr GULFNet station at Cocodrie, LA sinking 6.3 mm/yr Total = ~3/4 in/yr
GPS monitoring shows that SE La moves differently than the rest of stable North America Dokka et al. (2006) GRL HorizontalVertical
Hot colors = high; cold colors = low. Notice that medium blue marks sea level. Areas in dark blue within the flood protection levees are below sea level. Flood protection levee Build a levee in the delta plain and the land subsides below sea level. This has already happened in lower Plaquemines, Mississippi River South NOAA LIDAR.. and behind almost every levee In SE Louisiana.
Subsidence is causing change to: Natural systems: everything is sinking. Built environment; Levees are sinking. Bridges are getting lower and the water is getting higher. Evacuation roads are getting lower. Differential subsidence is causing infrastructure failures. Geodetic networks are being ruined.
Corroborates geodetic studies. Shows that slow (7mm/yr) subsidence gave way at 1966 to high rates ~40 mm/yr. Followed by 30 year exponential decay. Sinking of the Paris Road Bridge The Paris Road bridge has sunk ~0.7 m (2.7ft) since late 1950s! It is due to deep processes, not compaction, fluid extraction or levee building. Another reality: the surrounding reclaimed wetlands has subsided several additional feet due to desiccation and oxidation.
GrandIsle Raceland La Hwy 1 sank ~1 foot between 1982 and 2002 Subsidence has lowered evacuation roads throughout coastal Louisiana 2 4 6 8 10 12 B 358 F 220 G 358 Q 359 H 359 N 221 11 Benchmark Orthometric Height (feet) 2002 PREDICTED 2002 GPS 1993 1982 2002
NOAA told the U.S. Congress that Louisiana in 2001 that the system used to measure elevations was, Subsidence has ruined the Geodetic Networks along the Gulf Coast Subsidence has ruined the Geodetic Networks along the Gulf Coast “inaccurate and obsolete and unable to support public safety.”
Monitoring Subsidence— Reference Perspective Build a network based on NGS CORS. They will maintain coordinates for you Densify with additional reference stations. Establish and maintain coordinates with TIM. Keep in step with NGS through MAD (Mutually Assured Discussion). Strengthen network integrity by establishing Gulf Coast-wide super-network consisting of other Trimble networks.
Monitoring Subsidence— User Perspective The issue you want resolved will determine your measurement needs. There are two issues to be resolved in coastal Louisiana: Designing, building and maintaining the Hurricane Protection System (HPS) to protect coastal communities against relative sea level rise and storms. Planning for the future and monitoring coastal restoration.
The HPS issue is straightforward. Design of the HPS is based on desired outcomes and modified by reality. Politics shape the desired outcomes. Technical issues reveal the reality. Storm surge and wave analysis are major tools for design. Design requires accurate vertical control for: Accurate topo/bathy for modeling. Construction requires access to vertical control. Annual measurement of HPS to prove viability. Status: Vertical control is provided by LSU GULFNet CORS and GULFNet RTN. GULFNet RTN is used by USACE to measure the HPS each year.
Horiz. Error =1 cm, Vert. = 2 cm, EVERYWHERE! When NGS Grav-D completed, users can get 2 cm orthometric heights LSU GULFNet RTN Based on National CORS, the most reliable component of the National Spatial Reference System in Louisiana. Implemented in all of Louisiana.
RTN Real-Time Network Based on Trimble VRS technology “Positioning anywhere, any time” Cellular device to connect user to Internet LSU Internet and LSU GULFNet RTN Kinematic mode: + 0.3 ft (1 sec.) Relative vertical: + 2 cm (3.) + 2 cm (3 min.) Native output in “CORS 2002” but can easily. be put in 2004.65 context through calibration.
The Louisiana Legislature requested LSU measure the height of the South LA HPS in 2006-2007. N = 50,000 points Note: Does not reflect post 2007 changes Due to construction by USACE
Building levees speeds up compaction, but may be the only way to provide protection Cloverly Plantation ~8 ft of subsidence in 75 years Golden Meadow
Operational NHC Katrina SLOSH model using outdated or assumed elevation data Katrina SLOSH using accurate topography Note: The so-called “funnel” is a modeling artifact caused by the use of authorized elevations instead of valid measurements along south levee of MRGO.
Planning and Coastal Restoration will be greatly impacted by subsidence The Big Question: Will Louisiana be viable for habitation after the combined relative sea level rise effects of subsidence and eustacy? System to Support Planning (next 100 years): –Where and how much is the coast likely to subside? Processes operate at mm/yr level. –How much will sea level rise? Occurs at mm/yr level. –How fast can the wetlands accrete? System to Support Monitoring and Management –Need: mm/yr resolution. –Need: Everywhere and 4-D capability. –Need: Process identification.
How do you Maintain a Geodetic Network in a Area of Deformation? -- Issues to Remember The datums in which we are usually interested are national or continent-scale in dimension. Deformation zones are regional to sub-regional. Thus, you need to be able to connect your network to a POB(s) whose: motion is independently known and, coordinates are known. VTDP and HTDP are models, not observation. Validation by monitoring is preferred. Monitoring of network integrity is essential: Trimble Integrity Monitor, frequent network adjustments.
Take Aways Very few places are stable. Deformation is everywhere in some form or another. Represents a hazard to public safety, infrastructure, and geodetic networks. Deformation can be natural or anthropogenic. Monitoring is thus essential. Monitoring requires knowledge about the cause(s), an appropriate strategy, and the right measurement/monitoring tools.