Subsidence mechanics Case study Lecture 30 Subsidence mechanics Case study

San Joaquin Valley The Central Valley of California includes both the Sacramento Valley in the North and the Southern two thirds is composed of the San Joaquin Valley The Central valley is 400 miles long and 20 to 70 miles wide and extends over 20,000 square miles The valley is surrounded by mountains which include the Sierra Nevadas (East), Diablo and Temblor Ranges (West) and the Tehachapi Mountains (South)

San Joaquin (sediment rich) The trough of this region is filled with sediments: Marine sediments from inundation by ocean Overlain by continental sediments from erosion of high mountains brought to the valley floor by streams More than half of the thickness is composed of fine grained sediments such as clay and silt as well as fluvial and lacustrine deposits These sediments are highly susceptible to compaction. These sediments also help to form a large and important aquifer system surrounded by mountains to deliver water to recharge the system

Mining ground water Mining ground water for agriculture in the San Joaquin Valley has enabled the valley to become one of the most productive agricultural regions in the world. California ranks the largest agricultural producing state in the nation It produces 25 percent of the nations table food on only 1 percent of the country’s farmland. Ground water mining has led to land subsidence in the valley on an unprecedented level.

Aquifer two parts The Aquifer used to supply irrigation through ground water pumping is composed of two parts: Shallow aquifer system which was used pre-development for early agriculture Deep aquifer, which has a layer of Corcoran Clay, which confines it from the shallow aquifer and was used more in development

Four main causes of subsidence in San Joaquin Valley 1)Subsidence caused by aquifer system compaction due to the lowering of ground-water levels by sustained ground-water overdraft 2) Subsidence caused by the hydrocompaction of moisture-deficient deposits above the water table 3) Subsidence from oil and gas field withdrawal 4) Subsidence related to crustal geotectonic movements *The first is by far the most prominent, followed by hydrocompaction (these are the two discussed here)

Land Subsidence by aquifer system compaction Caused when hydraulic head declines Or basically water is over drafted More water is taken out than is put back into the groundwater system. = a reduction in pore pressure and draining the pore space in an aquifer. Once pore pressure declines the aquifer’s mineral matrix takes on a greater portion of the geostatic stress.

Reversible deformation Stress once taken by the pore fluid is now taken by the mineral matrix, which will compress to a degree If the mineral matrix only compresses slightly and can support the above stress, then we see reversible deformation Basically the mineral matrix is allowed to rebound with increasing fluid pressure only resulting in small scale subsidence

Irreversible Deformation Inelastic The other alternative is that water is taken out and the mineral matrix reaches its maximum level of stressing Resulting in a permanent rearrangement and irreversible compaction or subsidence The mineral matrix collapse has happened on a large scale in the deep confined aquifer. Before Subsidence Mineral Matrix collapse

Groundwater mining changed the flow path Left shows pre-development, when the groundwater flowed from the mountains to the center of the valley and then discharged through either the streams or through evapo-transpiration from plants Right shows post development, when groundwater flows generally downward and towards the pumping centers Most of the subsidence measured in the valley was correlated with groundwater pumping in the deep confined aquifer

Problems caused by subsidence 1.Damage to bridges, canals, roads, storm drains, sewers, canals and levees 2.Damage to buildings 3.Failure in well casings In low lying areas, subsidence can result in tides moving into areas that were once above sea level Large economic costs

Subsidence Realized Due to the many benefits of agriculture, total groundwater pumping in the San Joaquin aquifer went up dramatically to sustain the agriculture. By 1955, about ¼ of the total ground water extracted for irrigation in the U.S. was pumped in the San Joaquin Valley Until 1968, water used for irrigation was supplied almost entirely by groundwater The large increase in post-development groundwater overdraft has had large implications on subsidence. Subsidence reached 9 meters in some areas of the San Joaquin Valley by 1970 after nearly a half century of groundwater extraction. About 1.5 million ha of land are subsiding

Reclamation Projects By the late 1960’s, large federal and state reclamation projects were begun to fix the large overdraft/subsidence problem Water began to be diverted to agriculture by the Sacramento-San Joaquin Delta and San Joaquin River. Under these reclamation projects there was a dramatic period of recovery where the aquifer recovered as much as 200 feet between 1967 and 1974.

Severity realized again Severe droughts in California between 1976-77 and 1987-91 caused a diminishing recharge to the aquifers and rivers. To recover, old pumping plants were opened and new wells were drilled. In 1977, there was 0.5 ft of subsidence. Similar drops were seen in 1987-1991. This scenario showed that a small increase in the pumping of the area has large implications Reduced ground water storage capacity due to lost pore space from compaction shows that a small increase in groundwater mining has large costs and the effects are irreversible.

Management Today The environmental risks posed by ground water extraction and the vagaries of the regional climate have led to ingenious approaches for conjunctive surface water-groundwater management. (Zektser et al 2004, 402) Water banking is an often used strategy, in which calculated amounts of water are diverted during wet years and recharged in large alluvial aquifers. The water is then kept underground with small amounts of losses. During dry years the water can then be used.

Hydrocompaction- “near surface subsidence” Hydro compaction is compaction near the surface due to wetting. Entirely different from subsidence caused by compaction of deep over pumped aquifer systems, although it still has large implications. Mainly seen with the expansion of irrigated soil onto the arid, gentle slopes of the alluvial fans along the west side and south end of San Joaquin Valley.

Hydrocompaction Conditions are needed in which there are flash floods of sediment into San Joaquin valley. Then these sediments are dried out with high temperatures followed be relatively low rainfall. These sediments are often mixed in with clay, which is a strong bonding agent when dry. These high porosity sediments are often covered with 100 feet or more of overburden These factors create the special conditions needed.

Hydrocompaction Realized 1940s and 50s, farmers used flood irrigation Flood irrigation caused pooling of water. This was just the wetting that these areas needed to reduce these bonds and cause hydrocompaction. Pooling

Hydrocompaction Large Implications because it damaged pipelines, power lines roadways airfields and buildings. This kind of compaction can continue for months or years as the slowly descending water front slowly seeps in and weakens deeper and deeper deposits.

What has been done to resolve hydrocompaction Realization resulted in this problem being taken into account in the engineering of the California Aqueduct. Construction of the Aqueduct was preceded with wetting and thus compacting to bring the land to a stable state before construction was undertaken.

Measuring Land Subsidence Different techniques used Traditionally, land subsidence was measured using spirit leveling, Vertical Extensionometers and more recently Global Positioning System. Even more recently a new tool is used called InSAR.

Vertical Extensionometer Device commonly used Pipe with cable inside a casing Pipe extends from surface to some distance below and using instruments at the top are able to measure subsidence

InSAR Interferometric Synthetic Aperture Radar Powerful new tool that is critical in the mitigation of subsidence today Remotely senses small changes in land surface elevation at an unprecedented level of spatial detail Is now being used by a large number of scientists including the USGS to map and monitor subsidence in the San Joaquin valley and elsewhere

How InSAR works Uses radar which transmits a signal of electromagnetic energy This one stream of pulsed train microwaves reflects off the ground and are again received by the satellite Produces a solid image of continuous ground that is 60 miles wide The subsidence is able to be recorded when the Satellite makes successive tracks over the same orbital position at different times. Then these two image are compared to each other. .4 to .2 inch resolution is possible using this technique

InSAR example: Las Vegas Valley, Nevada Shows subsidence in the Las Vegas Valley between April 1992 and December 1997. Compiled using three successive interferograms Discovered that subsidence is caused by aquifer-system compaction and controlled in part by the surface faults in this area.

Subsidence Across America Shading represents all the aquifer systems. The lists under each state name represent all the valleys that have considerable subsidence. Shows that there are many problems with subsidence as we increase agriculture across America.

Examples across America Lower Santa Cruz Basin, Arizona Large scale groundwater extractions lowered the water table by as much as 150 m in various parts Cause in subsidence that reached 4.5 feet San Francisco Extraction has caused subsidence of up to 2.4 m which has caused the necessitation of levies to avoid the infiltration of seawater Gulf Coast Texas 244,900 ha of land have subsided an average of 30 cm since 1943 In Galveston Bay the estimated losses to property damage estimated at $32 million dollars In addition there has been a loss of 9960 ha of marshes due to the subsidence. Los Angeles Occurring at an annual rate of 0.7 m over the last several decades Subsidence caused by oil and gas pumping

In Conclusion Subsidence is a major problem due to overdraft from ground water pumping to provide for large scale agriculture The San Joaquin Aquifer has some of the most productive agriculture in the U.S. yet also some of the biggest subsidence at up to 9m New techniques are being used to show the extent to which subsidence is occurring and InSAR will be a valuable tool in the future It will be important to mitigate and find the correct ways to deal with these growing problems as there will be an increasing demand for agriculture and engineering solutions If we do not, subsidence in the San Joaquin Valley and other areas will continue and the results will not be reversible.

Cited References Zektser, S., loaiciga, H., & Wolf J. (2004). Environmental impacts of groundwater overdraft: sesected case studies in the southwestern united states. Environmental Geology, 2005(47), 396-404. Galloway, D. (2001). San Joaquin Valley California. Retrieved Apr. 10, 2006, from Land Susidence in the United States Web site: http://pubs.water.usgs.gov/cir1182 . Ruud, N., Harter, T., & Naugle A. (2004). Estimation of groundwater pumping as closure to the water balance of a semi-arid, irrigated agricultural basin. Journal of Hydrology, 297(1-4), 51-73. Schoups, G., Hopmans J. W., Young C. A., and Varugt, J. A. (2005). Sustainability of irrigated agriculture in the san Joaquin valley, California. PNAS, 102(43), 52-58. Umbach, K. W. (1997). Retrieved Apr. 10, 2006, from A Statistical Tour of California’s Great Central Valley Web site: http://www.library.ca.gov/CRB/97/09. (n.d.). Ground water atlas of the united states. Retrieved Feb. 20, 2006, from http://capp.water.usgs.gov/gwa/ch_b/B-text3.html. Ireland, R, Poland, J.F and Riley F.S. (1984). Land subsidence in the san juoquin valley as of 1980. USGS Professional Survey, Paper 437-I, p B54-B58 United States Geological Survey, (2000).Measureing Land Subsidence from Space. Fact Sheet . 051-00, Leake, S.E. (2004).Land Subsidence from Groundwater Pumping. USGS. <http://geochange.er.usgs.gov/sw/changes/anthropogenic/subside/>