Overview of the ARkStorm Scenario

Overview of the ARkStorm Scenario


This document summarizes the next major public project for MHDP, a winter storm scenario called ARkStorm (for Atmospheric River 1,000). Experts have designed a large, scientifically realistic meteorological event followed by an examination of the secondary hazards (for example, landslides and flooding), physical damages to the built environment, and social and economic consequences. The hypothetical storm depicted here would strike the U.S. West Coast and be similar to the intense California winter storms of 1861 and 1862 that left the central valley of California impassible. The storm is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years.

“Undoubtedly a repeat of the 1861-62 winter storm would have a significant impact not only on the California economy but on the U.S. economy as well.” (page 102 of the report)

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Resource Details



201 pages


Table of Contents

ARkStorm Meteorology (Flooding and Windspeed)
Coastal Inundation
Three Approaches to Estimating Damage
Lifeline Panel Process
Highway Damage
Wastewater Treatment
Water Supply
Agricultural Damages and Losses
Building and Content Repair Costs
Insurance Impacts
Business Interruption Costs
Truck Traffic Economic Impacts from Reduced Highway Capacities
Environmental and Health Issues
Public Policy Issues

Recording of January 8, 2020 Webinar on the ARKStorm Scenario

Excerpts From the Scenario Report

Historical Storms

  • “Winter storms of 1861-1862. Beginning in early December 1861 and continuing into early 1862, an extreme series of storms lasting 45 days struck California. The storms caused severe flooding, turning the Sacramento Valley into an inland sea, forcing the state capitol to be moved temporarily from Sacramento to San Francisco, and requiring Governor Leland Stanford to take a rowboat to his inauguration.” (page 2)
  • “The 1861-62 series of storms were the largest and longest California storms in the historic record, but were probably not the worst California has experienced. Geological evidence indicates that floods that occurred before Europeans arrived were bigger. Scientists looking at the thickness of sediment layers collected offshore in the Santa Barbara and San Francisco Bay areas have found geologic evidence of megastorms that occurred in the years 212, 440, 603, 1029, 1418, and 1605, coinciding with climatological events that were happening elsewhere in the world. There is no scientific evidence to suggest that such extreme storms could not happen again.” (page 2)


  • “Landslides on hill slopes damage buildings and other structures on those slopes, and in some circumstances can cause significant damage beyond the hills. Alluvial fans, which underlie many urban and suburban areas of California, are built up by repeated deposits transported from the mountains in floods. A significant part of those deposits may be the result of debris flows.” (page 17)
  • “While some landslides, triggered by the heavy rainfall season including the ARkStorm, will begin to move during or shortly after the storm they may continue to move for months or years. Furthermore, other deep-seated landslides may not begin to move until weeks or months following the storm.” (page 115)

Transportation Infrastructure

  • “The primary perils to highways are landslides either burying or undermining them; floods inundating them; and clogged culverts causing flooding and erosion where the water washes over the roadway and onto the soil beyond.” (page 24)
  • “The primary causes of bridge damage in severe winter storms are scour undermining the foundations of bridge piers or abutments and hydrodynamic pressure at the upstream edge of the bridge superstructure (the girders, driving surface, and crash barriers).” (page 24)
  • “Bridges with severe foundation damage or displaced spans can take months to repair or replace. Roads over deep-seated landslides can be partially restored quickly by regrading and reducing the speed limit, but permanent repair could take months.” (page 25)

Electric Infrastructure

  • “Hurricane-force winds (75 mph and higher) can cause transmission lines to sway and cause cross-phase shorting, or cause electrical transmission towers or poles to collapse.” (page 35)
  • “Power plants, high-voltage substations (also called bulk substations) and control facilities can be sensitive to flooding damage in at least two important ways. Flooding can damage control equipment. High-voltage substations and generating plants have high-voltage transformers (50 to 200 megawatts (MW) at high-voltage substations and 300 to 500 MW at generating plants) that also can be damaged by flooding, for example, by flood-borne debris impacting the transformer and ancillary equipment.” (page 37)
  • “The [large] transformers are not interchangeable and are too expensive to stockpile backups beyond those available for normal operational redundancy. If one of these large transformers were damaged, it could take 6 months or more to replace. There is typically some redundancy, enough so that at any given high-voltage substation, for example, one of these transformers can be inoperative and the substation can still operate.” (page 37)
  • “Flooding is a common-cause failure mode, the implication being that a flood can damage several components simultaneously, potentially damaging two or more of these transformers. Were this to happen, the utility would have to reroute power around the inoperative substation, which could take a few days, and immediately attempt to repair or replace the transformer once dewatering is completed. The reliability of the temporary grid layout would be reduced, meaning greater likelihood of power outages in the affected area.” (page 37)
  • “It is estimated that the material and labor required to repair power facilities could cost between $300 million and $3 billion. [year of report publication is 2011]” (page 39)

Water & Wastewater Infrastructure

  • “Water-supply and wastewater treatment panels noted that they rely on supplies of chemicals that are carried on railway and trucks every few days. In the Sacramento panels, for example, it was observed that most of the chemicals used by water and wastewater treatment facilities are carried over I-80, which would be temporarily cut off.” (page 34)
  • “Sewer flow is typically driven by gravity and aided by lift stations. Sewer pipes can be damaged by landsliding and in some cases by scour, especially external scour to shallow cut-and-cover tunnels. Sewer pipes also can be damaged by ground settlement and other movement aggravated by soil becoming saturated after multiple storms.” (page 42)
  • “The primary cause of damage to pumping stations and wastewater treatment plants discussed in panels was flooding damage to electrical equipment and sediment getting into pumps.” (page 42)
  • ” The degree of damage to electrical equipment depends on whether the equipment is deenergized before flooding. The damage is worse if the equipment is not deenergized before being powered down, which can happen if flooding occurs without sufficient advanced warning or at night when minimal crews are on hand to shut down equipment. When a wastewater treatment plant or lift station is flooded and shut down, untreated sewage may emerge from nearby maintenance holes and wetwells, and flow by gravity overland to nearby rivers and shorelines, contaminating a radius of up to 1/2 to 1 mile around the point of sewage discharge if the flooding reaches that far. Once the sewage enters a stream, creek, or river, the distance of contamination can far exceed the 1/2 to 1 mile distance. The contaminated area potentially can require evacuation of homes and businesses.” (page 42-43)
  • “Flooding also can carry large amounts of sediment into sewer pipes; one panel estimated that 10-15 percent of pipes in flooded area will have large amounts of sediment that will need to be cleaned out. Sewer pipe damage will continue to emerge for several months after the storm.” (page 44)
  • “In Los Angeles, one important point of significant damage is the north outfall sewer, which zigzags under the Los Angeles River. This aging structure is partly brick-lined, partly lined with unreinforced concrete. It is a shallow, cut-and-cover structure, and it seemed possible to some panelists that the sewer could be damaged by external scour near the river.” (page 44)
  • “It is uncertain how long repairs would take to restore damaged electrical equipment. One panel felt that, if the equipment is deenergized before being wetted, the equipment can be dried and reenergized within a day of floodwaters receding; otherwise short-circuited equipment might take weeks or more to replace. Another panel disagreed with the notion that deenergized equipment could be dried and quickly restored to service, and felt instead that flooded electrical equipment could be contaminated with silt and have to be replaced, which might take months (one panel suggested 3-6 months).” (page 44)
  • “Since sewer lines are co-located with streets, where lift stations and WWTPs cease to function, untreated sewage can emerge onto streets, causing hazmat conditions that cause road closures and hinder repairs to other lifelines.” (page 47)
  • “About 95 percent of Californians get their water from a public, municipal source. About 324 water districts in California and more than 8,000 small public water systems serve 6 to 7 percent of the population.” (page 48)
  • “Within one basin, it seemed realistic that a limited plume of untreated sewage could contaminate a significant part of the aquifer in one basin, interrupting water-supply service to perhaps 25 percent of the utility’s customers. Restoration would involve repairing damaged electrical equipment and cleaning the wells. Repairs to that system could cost $100 million and take 2 to 4 weeks if the system were properly shut down before electrical shorts could occur; otherwise repairs could plausibly cost $1 billion and take up to 6 months. … Two northern California water utilities that rely on ground water echoed the same concern, suggesting that it was realistic for half of their wells to be impacted. Representatives from one of the two utilities felt that its wellheads could be disinfected and water supply restored approximately 3 days after floodwaters receded and power was restored. Its wellheads are supplied by backup power—emergency generators powered by natural gas with some onsite storage—although because the electrical equipment is located at ground level, the generator and its electrical equipment would be damaged, rendered nonfunctional, and have to be replaced.” (page 48-49)
  • “Loss of water transmission from northern to southern California because of damage from overtopping of levees in the Sacramento Delta or aqueduct damage caused by flash flooding. (For example, flash flooding in the Arroyo Pasajaro could disable the California Aqueduct between Tracy and Coalinga, as happens elsewhere periodically.) About half of southern California’s water comes from the Delta. Panel participants felt that it was realistic for levee repairs necessary to restore conveyance to southern California to take 3 months. (Not the same as the amount of time required to repair all levee breaches and dewater flooded islands, which would probably be several years.)” (page 49)
  • “Greatly increased turbidity in surface water because of runoff carrying sediments into reservoirs and because of erosion of the banks of reservoirs. Panelists concluded that in southern California at least, water quality would be a far more significant problem than quantity, primarily in that filters would have to be flushed frequently, and that there would be concerns of contaminants from runoff potentially requiring extended boil-water orders.” (page 49)
  • “Treatment facilities isolated by roadway or rail damage or flooding can run out of chlorine; some facilities receive chlorine shipments every 3-4 days.” (page 56)
  • “Pumps in wellfields, however, are generally not equipped with emergency generators, so water supply to communities that rely on ground water or on water that requires pumping may be susceptible to loss of water resulting from power failure.” (page 56)

Dams & Levees

  • “Release of large quantities of water from reservoirs through valves and spillways could damage roads and bridges, and any other lifelines such as water-supply pipelines or telecommunication cables carried on dams.” (page 59)
  • “The expert panel convened in Sacramento to discuss damages to levees for the ARkStorm scenario felt that urban levees might be threatened or overtopped at 60 to 75 critical sites, and that 15-20 breaches might realistically occur.” (page 59)
  • “Time to close all breaches: 440 days. Time to dewater all islands: 580 days” (page 64)


  • “Many of the components of the telecommunication system are susceptible to physical damage in a severe storm, damage that can slow or interrupt voice or data service. Some of the mechanisms of storm damage are: flooded manholes, toppled poles, misaligned microwave dishes, severed cables, inundated buildings, and damaged antennas. Two of these damage modes are illustrated in figure 39, which contains images from Hurricane Katrina. In one, wind damage to rooftop equipment interrupted microwave communications. In the other, flooding to the central office ground floor damaged power equipment and other central office components. (Many power systems of central offices are installed in the lower part of the building because of the weight of the power equipment. A flooded power room will shut down the facility.) Similar damage has been observed in other storms, such as the December 2007 storm in the Pacific Northwest.” (page 66)
  • “Soil failure and flooding also can damage cables. For example, in the same 2007 Pacific Northwest storm, an optical fiber cable was damaged by soil failure and a number of fibers were severed. The result was 3 days of internet congestion between Australia, New Zealand, and North America. Figure 40 shows damage to fiber optic cable. In one, water in a flooded utility tunnel entered fiber optic cable at a splice, causing signal degradation and transmission capacity reduction. In the other, cable laid along a railbed was damaged when the railbed washed out.” (page 66)
  • “The direct loss to service providers is estimated to be on the order of $100 million, including costs of material, logistics, and technical personnel. [report published in 2011]” (page 67)
  • “Telecommunications recovery can be limited by restoration of roads, bridges, and power. The most important among the three is electric power, because the equipment does not operate without power and backup power is limited.” (page 70)
  • “Loss of telecommunications hinders restoration of other lifelines such as water and wastewater because the lifelines can rely on cellular phones to dispatch and coordinate service calls.” (page 70)
  • “It is very common for collateral damage to occur to telecommunications cables because of damage to bridges on which the cables are collocated.” (page 70)

Oil Platforms & Coastal Infrastructure

  • “The summary map shows that severe wave damage potential is predicted on the mostly west-facing beaches in Los Angeles and northern San Diego Counties, and the oil platforms in the western part of the Santa Barbara Channel. The coastal infrastructure that appears most at risk of severe wave damage includes the Manhattan, Hermosa, Venice and Imperial Beach piers, as well as coastal structures (for example, groins, jetties, seawalls) in the Los Angeles International Airport (LAX) region, and along Highway 1 in northern San Diego County. Sewage infrastructure near LAX (for example, Hyperion Treatment Plant) also appears vulnerable.” (page 13)
  • “Drastic shoreline change (beach erosion) induced by the ARkStorm conditions could lead to significant damage to public and private infrastructure, including the following regions: Imperial Beach, La Jolla, Del Mar, Solana Beach, Carlsbad, Malibu, Santa Clara River mouth (for example, McGrath State Park), Rincon Parkway, Carpinteria, and Isla Vista (for example, University of California at Santa Barbara). The cliff failure pilot project in Santa Barbara only identifies a few sites with major cliff failure potential, but one of those sites is immediately adjacent to the Summerland Water Treatment Facility.” (page 13-14)

Agricultural Sector

  • “Longer periods of inundation occurring in the autumn and winter may prevent spring planting altogether. Spring and summer flooding, regardless of the duration of inundation, will destroy most field and row crops in the ground.” (page 75)
  • “The ARkStorm winter storm agricultural damages are an order of magnitude greater than 1997 flood damages. Statewide damages to crops, livestock, and fields were estimated to range between $3.7 and $7.1 billion.8 Destruction of perennial orchard and vineyard crops accounted for most of the estimated agricultural production losses. We note that these perennial crop losses may have been underestimated because of the use of old survey information that in some areas may be more than 10 years old.” (page 87)

Building Impacts & Insurance

  • “Nearly one-quarter of the total building square footage in California is affected by flooding in ARkStorm, with little variation of this ratio between occupancy classes (fig. 55) Most flooded buildings are not a total loss, but rather experience damage requiring repair costs between 10 percent and 50 percent of replacement cost. Residential buildings dominate the flood-related building repair costs.” (page 94)
  • “The damaging effects of flood are generally not covered under residential or nonresidential property policies and must be purchased separately, in the case of residential coverage, or in addition to the coverage offered under the standard commercial property policy.” (page 99)


  • “We estimate that 1.5 million people13 reside in the flooded areas of the ARkStorm scenario. Most of these people are concentrated in Sacramento and San Joaquin Counties, but Sutter County has the highest percentage (97 percent) of population living in a flooded area.” (page 104)
  • ” ‘If Katrina exposed what happened when many people have no cars to evacuate; Rita seemed to show the other side of the coin’- (Victoria Transport Policy Institute, 2006). … during Hurricane Rita, emergency managers in Texas expected people in coastal areas who owned boats to take the boats with them by trailer and cause additional congestions on the roads. However, they had not anticipated families using all their cars to evacuate (rather than take one car per family) and in some cases residents took trailers and horse trailers as well. This evacuation led not only to very heavy congestion on the road but also to a shortage of gas (Cabinet Office Civil Contingencies Secretariat, 2006).” (page 113)

Public Health & Environmental Challenges

  • “Some rock types, and the soils developed on them, may contain naturally elevated levels of potentially toxic metals (such as selenium, zinc, copper, arsenic, and lead), environmentally deleterious minerals (such as iron sulfides that generate acid rock drainage when weathered), mineral toxicants (such as asbestos), or pathogens (such as Coccidioides Immitis, the soil fungus that causes Valley Fever). Storm-induced erosion or landslides affecting these rock types have the potential to disperse these materials in the environment.” (page 148)
  • “Erosion or flooding of agricultural lands could lead to extensive loss or contamination of arable soils. Storm runoff from agricultural lands, residential areas, and urban areas could release a variety of sediment-borne or water-borne anthropogenic contaminants into the environment. Water supplies used for human consumption, livestock consumption, or agricultural irrigation, including surface water and shallow groundwater, could become contaminated by a wide variety of contaminants or pathogens. Following the storm, contaminated sediments and debris redistributed by landslides or floodwaters could then dry out and become available for further redistribution by human disturbance and (or) wind transport.” (page 149)
  • “In addition to the acute physical threats to safety posed by the storm, the possibility exists for adverse health effects on humans and ecosystems. These effects could include, for example, potential outbreaks of infectious disease from exposure to contaminated floodwaters, consumption of contaminated drinking water, or exposure to dusts from landslide or flood deposits containing soil pathogens such as Coccidioides Immitis.” (page 149)

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