2013 State of Arizona Hazard Mitigation Plan Risk Assessment

Transcription

1 Subsidence Introduction/History Subsidence occurs when the established land surface elevation recedes due to changes in the subsurface. Causes of subsidence include, but are not limited to, removal or reduction of fluids (water, oil, gas, etc.), mine subsidence, and hydro compaction. Of these causes, hydro compaction and mine subsidence tend to be localized events, while fluid removal may occur either locally or regionally. The main cause for subsidence in Arizona is excessive groundwater withdrawal (i.e., discharge exceeds recharge). Once an area has subsided, the ground elevation will not rise again, even if the removed fluid, or earth is replaced. In the United States, more than 80% of the identified areas of subsidence are a result of our exploitation of underground water. The affects are compounded by the increasing draw of land and water resources which threatens to exacerbate existing land subsidence problems and initiate new ones. In many areas of the Southwest, and in more humid areas underlain by soluble rocks such as limestone, gypsum, or salt, land subsidence is an often-overlooked consequence of our land and water-use practices. Some subsidence is an often-overlooked consequence of our land and wateruse practices. Subsidence can cause regional drainage patterns to change, which effects flooding, backs up storm drains, and damages infrastructure both in the subsurface (water and electric lines, well casings, etc.) and surface (roads, canals, drainages, surveyed benchmarks, etc.). It aggravates riverine flooding, alters topographic gradients, and ruptures the land surface in addition to causing other hazards related to deterioration of land and water resources. Subsidence also causes fissures, which are discussed in this section as well. Land-use areas that are predominantly agricultural are at risk to experience the most intense subsidence because of irrigation. However, subsidence is not restricted to rural areas exponential population growth also places great demands on groundwater. The land subsidence areas in Arizona are the result of substantial groundwater withdrawal from aquifers in sedimentary basins. Subsidence normally results in bowl-shaped depressions, with loss of elevation greatest in the center and decreasing towards the circumference. Since 1900, the south-central Arizona s groundwater pumping for irrigation, mining, and municipal use has outpaced the recharge, in some areas by 500 times more than the recharge. (Schumann and Cripe, 1986). Over 3,000 square miles is affected by subsidence, including the surrounding and expanding areas of Tucson and Phoenix and rapidly growing northern Pinal County. Before many communities became established, agriculture was the driving force for groundwater pumping. In Arizona, groundwater accounts for 40% of all water use (Arizona Land Subsidence Group, 2007). Prior to 1980, water levels have declined up to 400 feet in some areas of southern Arizona, as illustrated below. Since the 1950s, the development of subsidence related earth fissures has greatly increased in Arizona, with hundreds now identified in the alluvial basin of southern Maricopa, western Pinal, eastern Pima and northern Cochise Counties. The majority of these fissures are forming in Pinal and Maricopa Counties. 279

2 Figure RA-13: Areas of Water Level Decline in Southern Arizona in 1980 Arizona Department of Water Resources (ADWR) has been analyzing and monitoring land subsidence across the State of Arizona since 2005 using Interferometric Synthetic Aperture Radar (InSAR) data. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface deformation or digital elevation using differences in the phase of the waves returning to the satellite, or aircraft. Sources for the SAR data include the ENVISAT, RADARSAT, and ERS-2 platforms. The technique can potentially measure centimeter-scale changes in deformation over time-spans of days to years. Areas currently being monitored by ADWR are shown above. Some of the more active areas of known subsidence include: Pinal County o Eloy 625 square miles subsided 15 feet between 1948 and 1985 o Stanfield 425 subsided 12 feet by 1977 o Apache Junction/Queen Creek 230 square miles subsided 3 feet by 1977 o Ak-Chin Indian Community 1998 through 2008, caused damages to sewer line which required jetting at a cost of $25,000. Repair of affected pipe cost $200,000. Maricopa County o Luke Air Force Base by 1992, ground-water level declines of more than 300 feet generated land subsidence of as much as 18 feet about 20 miles west of Phoenix on and near Luke Air Force Base (Carpenter, 1999). o Queen Creek by 1977, an area of almost 230 square miles had subsided more than three (3) feet (Carpenter, 1999). o Harquahala Plain subsidence of about 0.6 feet occurred in response to about 300 feet of water-level decline (Carpenter, 1999). o East Mesa/Apache Junction a total of 5.2 feet of subsidence was measured along the CAP near the Superstition Freeway, for the period of 1971 to 2001 (AMEC, 2006). o Paradise Valley between 1965 and 1982, over five (5) feet subsidence occurred (Carpenter, 1999). 280

3 Scottsdale/CAP o Canal subsided about 1 foot since construction (Carpenter, 1999). o Sections of the CAP canal in Scottsdale traverse an area that has subsided up to 1.5 feet over a 20-year period, threatening the canal s maximum flow capacity. In response, CAP raised the canal lining 3 feet over a one-mile segment of affected area at a cost of $350,000. A second and much larger subsidence area was later identified near the Scottsdale Airpark. Plans for raising the canal lining will cost an estimated $820,000. Recently, a third subsidence area has been identified east of the Scottsdale Airpark in the Scottsdale West World area. This happened in spite of the fact that during the original design phase, CAP Engineers showed considerable foresight in mapping a route to minimize the likelihood of encountering zones of subsidence (Gelt, 1992). Cochise County o Willcox areas to the northwest and southeast o Bowie o San Simon o Tombstone Pima County o Avra Valley northeast of Tucson 7 La Paz County o Harquahala Plain An example of the damage potential at least partly attributable to subsidence occurred on September 20, 1992, when a storm that generated four inches of surface runoff occurred north of Luke Air Force Base. Subsidence in the area caused a flow-reversal in the Dysart Drain, which instead of conveying flows away from the base as designed, actually directed flows onto the base forcing a three (3) day closure. Damages included flooding of the runways and 100 homes and were estimated to total over $3 million. Potential Secondary/Cascading Effects Basin subsidence, the slow but persistent sinking of the basin floor, is heterogeneous, expressed over a large area and capable of effecting small but critical changes in the gradient. Rapid subsidence, associated with sinkhole development in the substratum evaporates (gypsum, salt, anhydrite) or carbonates (limestone or dolostone). Carbonates are not considered here, but is a concern in parts of central and north-central Arizona, where carbonates either crop out or occur in the shallow subsurface to depths of several hundred feet. The sedimentary rocks surrounding Sedona, as one example, are dotted with at least seven sinkholes, two of which actively failed since Sinkhole formation near Sedona is attributed to collapse in the Redwall Limestone (Lindberg, 2009), nearly 500-feet below the ground surface. Secondary or cascading events associated with basin subsidence, include: gradient changes in drainage leading to localized flooding and ponding of flood waters; gradient changes leading to negative effects on water and sewer systems; tilting of agricultural fields which could requiring expensive re-leveling of fields; 7 USGS Circular No. 1182, South-Central Arizona 281

4 reduced freeboard of levees leading to exceeded design limits, such as overtopping, and localized flooding; formation of earth fissures; Infrastructure damage notably to roads, railroads, earthen dams, and water and sewer systems; damaged water casing and disrupted well heads which could result in broken piping impacting water delivery and quality. Probability and Magnitude The complexity of the factors associated with subsidence areas make it difficult to recommended procedures that quantifiably determine the probability and magnitude of future subsidence. It is reasonable, however, to anticipate that currently active subsidence areas are also considered susceptible to future subsidence. As long as groundwater discharge continues to exceed recharge, subsidence will occur. Even once water table equilibrium is reached, a period of 5-10 years will pass before the ground finishes subsiding. Accordingly, areas defined by ADWR as active subsidence areas were mapped as High hazard zones and all other areas were assigned a Low hazard. The high hazard subsidence zones for the State are presented on the following map. The Arizona Department of Water Resources has been monitoring land subsidence throughout Arizona since 2002 using NASA s satellite and aerial synthetic aperature radar data (InSAR and UAVSAR). The Department has identified land subsidence features that cover more than 1,100 square miles. Vulnerability Most of the significant damages associated with subsidence are typically related to the causal effects of subsidence as it relates to flooding events and fissure development. Other attributable costs and damages include: Uneven or differential subsidence across agricultural fields requiring expensive re-leveling efforts. Agricultural fields in the western Salt River Valley, lower Santa Cruz basin, and Willcox basin have all required re-leveling. Well damage and protruding well casings in both agricultural and urban areas. Replacement of gravity based irrigation systems due to flow reversal. The increased incidence of local riverine flooding caused by reduction of elevation and changes of topographic gradients are the most costly impacts of land subsidence. No estimates of loss to state-owned critical and non-critical facilities have been estimated. Instead, an estimate of exposure to the subsidence hazard areas is estimated. In summary, about $1.28 billion in state-owned critical and non-critical facilities are exposed to an active subsidence area. Regarding human vulnerability, approximately 139,588 persons, or about 16% of the total 2010 state population, are potentially exposed to an active subsidence area. The potential for deaths and injuries are negligible as the hazard of subsidence does not pose an extensive direct threat to human life. The compilation of risk assessment data from local plans indicates that approximately $55.7 billion in locally identified critical and non-critical facilities are exposed to a subsidence hazard area. 282

5 Map RA-15: Subsidence Hazard Zones 2013 State of Arizona Hazard Mitigation Plan Areas of Land Subsidence in Arizona Observed as of April, Source: ADWR ADWR has been monitoring land subsidence throughout Arizona since 2002 using NASA s satellite and aerial synthetic aperature radar data (InSAR and UAVSAR). The dept has identified land subsidence features that cover more than 1,100 square miles. Table RA-80: State Owned Asset Inventory Loss Estimates to Subsidence Percentage of Statewide Exposure Estimated Replacement Cost (x 1000) County Facilities Estimated Structure Loss Apache % 0 None Estimated Cochise % 1,989 None Estimated Coconino % 0 None Estimated Gila % 0 None Estimated Graham % 0 None Estimated Greenlee % 0 None Estimated La Paz % 214 None Estimated Maricopa % 20,234 None Estimated 283

6 Percentage of Statewide Exposure Estimated Replacement Cost (x 1000) County Facilities Estimated Structure Loss Mohave % 0 None Estimated Navajo % 0 None Estimated Pima % 932,683 None Estimated Pinal % 328,269 None Estimated Santa Cruz % 0 None Estimated Yavapai % 0 None Estimated Yuma % 0 None Estimated Statewide % 1,283,389 None Estimated Table RA-81: State Facilities Located in the Subsidence High Hazard Area State Facilities in the High Wildfire hazard Area Apache Cochise Coconino Gila Graham Greenlee La Paz Maricopa Mohave Navajo Pima Pinal Santa Cruz Yavapai Yuma Total Critical Facilities Banking and Finance Institutions Communications Infrastructure Electrical Power Systems Emergency Services Gas and Oil Facilities Government Services Transportation Networks Water Supply Systems Non-Critical Facilities Businesses Cultural Educational Recreational/Leisure Residential Total

7 Table RA-82: County Population Sectors to Subsidence County Total Population Population Percentage Over 65 Total Population Exposure to Earthquake Over 65 Percentage Over 65 Apache 71, % 8, % Cochise 131,346 4, % 22, % Coconino 134, % 11, % Gila 53, % 12, % Graham 37, % 4, % Greenlee 8, % 1, % La Paz 20,489 1, % 6, % Maricopa 3,817, , % 462, , % Mohave 200, % 46, % Navajo 107, % 14, % Pima 980, , % 151,293 12, % Pinal 375, , % 52,071 11, % Santa Cruz 47, % 6, % Yavapai 211, % 50, % Yuma 195, % 30, % Statewide 6,392, , % 881, , % Population counts based on census 2010 Table RA-83: Local & Loss Estimates Based on Subsidence County Total Estimated Asset Value (x $1,000) Asset Value to Hazard (x $1,000) Estimated Potential Losses (x $,1000) Statewide Totals $382,041,435 $55,722,337 $0 Apache $11,101,665 No Data No Data Cochise $10,615,770 No Data No Data Coconino $22,517,439 No Data No Data Gila $6,811,526 No Data No Data Graham $2,999,628 No Data No Data Greenlee $6,747,353 No Data No Data La Paz $2,359,292 No Data No Data Maricopa $189,975,238 $28,859,746 No Data Mohave $15,521,558 No Data No Data Navajo $11,908,834 No Data No Data Pima $50,584,821 $22,836,910 No Data Pinal $14,610,551 $4,025,681 No Data Santa Cruz $3,044,947 No Data No Data Yavapai $18,491,858 No Data No Data Yuma $14,750,955 No Data No Data NOTE: No Data denotes lack of available information for assessment. 285

8 Risk & Vulnerability Based on the Index Values and Assigned Weighting Factors determined in the CPRI table discussed at the beginning of this section and updated by the Planning Team, the results based on Subsidence are shown below. County CPRI average values are also given below. These figures are based on information provided in their current respective mitigation plans. Table RA-84: State CPRI Results for Subsidence Risk Due to Subsidence Hazard Probability Magnitude/ Severity Warning Time Duration CPRI Score (max:4) Likely Negligible >24 Hours >1 Week Subsidence CPRI Score = (Probability x.45)+(magnitude/severity x.30)+(warning Time x.15)+(duration x.10). 2.2 Table RA-85: County CPRI Results for Subsidence County/Community CPRI Maricopa 1.85 Avondale 2.50 Buckeye 1.00 Carefree 1.00 Cave Creek 1.00 Chandler 1.00 El Mirage 1.75 Fountain Hills 2.50 Fort McDowell Yavapai Nation 1.30 Gila 1.00 Gilbert 2.85 Glendale 2.05 Goodyear 1.45 Guadalupe 1.45 Litchfield Park 1.45 Unincorporated Maricopa County 2.95 Pima 2.18 Marana 2.35 Oro Valley 2.35 Pascua Yaqui Tribe 1.00 Sahuarita 2.30 Tucson 2.80 Unincorporated Pima County 2.30 Source: Arizona county hazard mitigation plans. 286

9 Environmental Risk & Vulnerability 2013 State of Arizona Hazard Mitigation Plan Based on the Index Values and Assigned Weighting Factors determined in the Environmental Risk CPRI table discussed at the beginning of this section and updated by the Planning Team, the results based on Subsidence are shown below. Table RA-86: State Environmental CPRI Results for Subsidence Environmental Risk Due to Subsidence Component Probability of an Impact Magnitude/ Severity Duration of Impact/Damage CPRI Score (max: 3.6) Air Unlikely Negligible < 1 month.90 Water Unlikely Limited 6 months+ 2.1 Soil Unlikely Limited 6 months+ 2.1 Average CPRI Environmental Risk Rating: 1.7 (max 3.6) Consequences / Impacts Public There are no obvious direct impacts to public health and safety due to the effects of subsidence conditions. Indirect/secondary impacts are more likely and are typically seen in the form of fissure and flood damage. Inconvenience may result from down utilities during the restoration process. Responders to the Incident Similar to the impact to the public, there should be no threat to responders as this is not considered an incident response type of hazard. Continuity of Operations / Delivery of Services Subsidence is not a threat to the state s ability to continue effectively functioning. Environment Due to the surface elevation drops caused by subsidence, the resulting environmental threat is generally associated with flooding and potential contamination due to entry of floodwaters directly into groundwater through fissures. See the Flooding/Flash Flooding profile in this section for more information on its impacts. Subsidence can also cause fissures which will render the land/area unusable for development and agriculture. The long-term threat is the elevation dropping and reducing or compressing the aquifer holding capacity permanently for the given area. This would impact as a water resource for sustainability of vegetation and wildlife. Economic / Financial Condition of Jurisdiction Subsidence can affect a jurisdiction s economy by causing flood-prone areas, backing up storm drains, infrastructure damage and fissures. Although flooding in this case would be a secondary impact, response and recovery to floods are very costly. Flooding can significantly impact a jurisdiction s economy through residents and businesses. Overall, subsidence alone does not have the potential to cause great economic stress. Public Confidence in Jurisdiction s Governance Although response is not generally an issue directly related to this hazard, taking a proactive approach by addressing subsidence through mitigation/prevention as opposed to only when a critical point is reached should help the public s confidence level. Lack of situation knowledge 287

10 will lead to a misunderstanding of the situation and result in frustration and a negative attitude toward those that are perceived as responsible, which in most cases would be the government. Resources Definitions: Sources: Resources for additional information regarding Arizona subsidence hazards include the following: Arizona Geological Survey, Geologic Hazard Center: U.S Geological Survey, USGS Groundwater Programs: The Geological Society of America: AZ Dept of Water Resources, Interferometric Synthetic Aperture Radar, 2009, (Used in time series monitoring of subsiding basins in south-central Arizona) References: Baum, R., Landslide and Land Subsidence Hazards to Pipelines, Open-File Report , USGS. Carpenter, M.C., 1987, Water-level declines, land subsidence, and specific compaction near Apache Junction, south-central Arizona: U.S. Geological Survey Water-Resources Investigations Report , 22 p. Carpenter, M.C., Land subsidence in the United States [Galloway, D., Jones, D.R., and Ingebritson, S.E., editors], South-Central Arizona: Earth fissures and subsidence complicate development of desert water resources, USGS Circular Conway, B.D., What is land subsidence?, presentation to Arizona Planning Association Conference, Arizona Department of Water Resources. Gelt, J., Land subsidence, earth fissures change Arizona s landscape, Arroyo, Summer 1992, Vo 6 No. 2, University of Arizona. Holzer, T.L., 1980, Research at the U.S. Geological Survey on faults and earth fissures associated with land subsidence: Engineering Geologist (Newsletter of the Engineering Geology Division, Geological Society of America), v. 15, no. 3, p Holzer, T.L., 1981, Preconsolidation stress of aquifer systems in areas of induced land subsidence: Water Resources Research, v. 17, p Land Subsidence and Earth Fissures in Arizona, Arizona Geological Survey, Arizona Land Subsidence Group (ALSG), 2007 Leake, S.A., Land subsidence from ground-water pumping, Impact of climate change on the southwestern United States workshop, USGS. Pewe, T.L., 1984, Land subsidence and earth fissures, in Pewe, T.L., and Kenny, R., Geologic hazards of Arizona: Tempe, Arizona State University, Department of Geology, p Pewe, T.L., Land subsidence and earth-fissure formation caused by groundwater withdrawal in Arizona; a review, in Groundwater geomorphology; the role of subsurface water in earth-surface processes and landforms [Higgins, C.G., and Coates, D.R., eds.], Geological Society of America Special Paper 252, Boulder, CO. Sandoval, J.P., & Bartlett, S.R., undated. Land subsidence and earth fissuring on the Central Arizona Project, Arizona, US Bureau of Reclamation, Arizona Projects Office. Schumann, H.H., & Genauldi, R., Land subsidence, earth fissures, and water-level changes in southern Arizona, Arizona Bureau of Geology and Mineral Technology Map 23. Slaff, S., Land subsidence and earth fissures in Arizona, Down-to-Earth Series 3, Arizona Geological Suvey, Tucson, AZ. US Geological Survey, 1999, Land Subsidence in the United States. Circular 1182, P (South-Central Arizona). 288