Lecture 18: Hurricanes and Storm Surges Ch. 14: p.416-­‐435; 439-­‐440; 449-­‐452 Hurricanes: Hurricanes kill more humans per year on average than any other type of natural hazard. They are regions of very low pressure, with inward-­‐moving winds that can attain speeds in excess of 260 kph/162 mph. The winds rotate CCW in the N.H. and CW in the S.H. They form over warm ocean water (at least 25°C/77°F) at 5-­‐20° latitude. In the western Pacific and SE Asia, they are called typhoons. In the Indian Ocean and Australia, they are called cyclones. Hurricanes start as low pressure cells called tropical depressions then become tropical storms (80-­‐90 each year). If the sustained wind speed exceeds 74 mph, it becomes a hurricane (~45 per year; average of 6 per year in the Atlantic -­‐ two are Cat. 3 or higher). They affect 44 million people and causing an average $700 million in damage each year. Saffir-­‐Simpson Hurricane Scale: The severity of a hurricane is measured using the Saffir-­‐Simpson scale, which varies from a Category 1 to a Category 5 (see Table 14-­‐1). Record low pressure: 882 mb (Hurricane Wilma, Oct. 19th, 2005, Category 5; 185 mph winds). Hurricane Naming Convention: Hurricane and tropical storm names are maintained by the World Meteorological Organization. For the North Atlantic hurricanes, there are 6 lists of names that are re-­‐used every 6 years (alternating female and male names). Some names get retired in the case of particularly prominent or damaging events. If all 21 alphabetical names get used up (e.g. the 2005 season), Greek letters are used (Alpha, Beta, Gamma…). There are different lists of names for cyclones and typhoons in other parts of the world. 2010 Hurricane Season: As of today, there have been 12 hurricanes in the 2010 season, and 19 named storms. One of these was a CAT 5 (Igor). Hurricane Recurrence Intervals: From 1851-­‐2008, the U.S. had direct hits by only three Category 5 storms: the Labor Day hurricane in Florida (1935), Camille (1969) and Andrew (1992). The most expensive was Katrina (2005 – Category 3) at $91 billion*, followed by Andrew (1992) which caused $52 billion* in damages, and Ike (2008) at $27 billion* (*2010 adjusted). Hurricane Formation: Hurricanes form over warm ocean water. Moist rising air develops anvil clouds around a central convective chimney (the low pressure eye). To sustain a strong circulation, there must be little variability in wind speed at different elevations (i.e. wind shear). Such conditions occur in the Atlantic from late June through November, but are less likely in El Niño years. The rising air moves fastest around the edge of the eye, called the eye wall. More air moves in from the side to replace it, producing a storm 160-­‐800 km wide. The eye is 20-­‐50 km across, with little wind, warmer temperatures, and the lowest pressure. The motion of the entire storm is driven by the prevailing trade winds at about 25 kph. Hurricane Motion Paths: Hurricanes move westwards driven by the trade winds (0-­‐30°) before moving away from the equator and veering in response to the Coriolis effect and the transition to the mid-­‐latitude westerlies (30-­‐
60°). In the N.H., they first track westwards, then to the NW before moving back towards the N or NE. Hurricane Duration: Hurricanes last longer over a warm open ocean (e.g., following the Gulf Stream). Such storms create a threat to the NE seaboard and Canada. Storms may strengthen when moving over patches of warmer 1 water (e.g., Katrina). A storm dissipates rapidly after coming onland due to the loss of its energy source but continues to produce heavy rainfall for days. Hurricane Vulnerability: The most vulnerable locations to hurricanes are Florida, the Carolinas, and the Gulf coast. The worst storms are typically in August or September when ocean surface temperatures are warmest. Nor’easters: Nor’easters should not be confused with hurricanes, even though they can be just as damaging. Nor’easters occur from October through April (i.e., winter storms), but especially in February. They are low pressure areas typically lacking a distinct eye or circular shape. Nor’easters get their name from the strong winds (up to 90 mph) that blow from the northeast. These winds are usually related to the forward left portion of the storm, where the counterclockwise winds around the low pressure area blow onto the coastal regions from the NE. Storm Surges: A storm surge is a rapid rise in sea level often many meters high (sometimes >10 m). It results from the low atmospheric pressure of a hurricane. With less pressure pushing down on the ocean, it rises up. Strong winds exacerbate the problem by pushing a mound of water towards the coastline, often with high waves superimposed. Storm surges are responsible for 90% of all deaths in hurricanes (by drowning). Barrier islands are particularly at risk, especially where coastal dunes have been removed for construction. Houses in coastal regions prone to storm surges are required to be built on posts or pilings several meters high. These posts must have deep foundations due to high amounts of erosion by the waves of the storm surge. Many coastal regions in the United States prone to hurricanes are at low elevations for great distances from the beach, so flooding damage can extend far inland (e.g. Houston area). Storm surge flooding extended 51 km (32 mi) inland at Baytown, TX (Hurricane Alicia, 1983). Hurricane Ike Storm Surge (2008): Coastal Texas (Galveston and Houston areas) was flooded by a 3.6 m high storm surge from Hurricane Ike in September 2008 (CAT 2 at landfall). Although 1 million people were evacuated, 13,000 did not and the storm killed 48 in Texas. Beware the Northeast Quadrant!: The most severe storm surge typically occurs in the NE quadrant of a hurricane, where the CCW moving winds are blowing directly onshore. This part of the storm is also on the side in which the storm itself is moving, which causes the strongest winds and thus the highest storm surge. Where winds blow offshore in the SW quadrant, storm surge effects are minimal. A Catastrophic Storm Surge Scenario: Storm surges are most severe in narrow bays or inlets where water must rise up higher within a narrow space. A potentially catastrophic scenario would have the NE quadrant of a hurricane hitting the western corner of Long Island, NY, at high tide. Manhattan would flood completely. The Biggest Catastrophe in American History: The people of Galveston, TX, discovered the perils of building on a barrier island when a Category 4 hurricane hit the city on September 8, 1900. A 6-­‐meter storm surge covered the city, which had a maximum elevation of only 2.5 m after all the dunes had been removed. Over 3600 homes were destroyed and 8,000-­‐12,000 people died. The cost: $14.4 billion. Wind Damage: By far the greatest amount of damage done by hurricanes is due to wind. Winds cause 1000s of pounds of force on building walls and create a suction above a roof like an airplane wing, often pulling it off entirely. Any loose objects become dangerous projectiles in a hurricane. Rainfall and Flooding: Heavy rainfall over prolonged periods can result in flooding due to saturated ground and increased surface runoff (e.g. Houston in 2001 due to Hurricane Allison). 2 Hurricane Fatalities: See Table 14-­‐4. The greatest death toll due to an Atlantic hurricane was 10,000 in Nicaragua and Honduras due to Mitch (1998 – Category 5). Other than the 1,833 people killed by Hurricane Katrina in 2005 (Aug. 2006 statistics), note that no hurricanes on this list occurred after 1957. Hurricane Costs: Current cost estimates for Hurricane Katrina are of the order of $91 billion, the costliest disaster in U.S. history (almost twice that of the second placed Hurricane Andrew). In contrast to fatalities, 14 of the 15 costliest hurricanes all occurred after 1960, with 11 of the 15 occurring after 1989? Why is this?* Hurricane costs are dramatically linked to population surges in coastal areas. Other socio-­‐economic factors include the effects on offshore oil platforms and ultimately gas prices. Hurricane Damage Mitigation: The National Flood Insurance Program (NFIP) also applies to coastal regions. The National Flood Insurance Reform Act (1994) requires that flood prone areas be mapped in order for FEMA to implement a Flood Insurance Rate Map (FIRM). In the case of coastal areas, a designation was applied to regions that have at least a 1% chance of being flooded in any one year. The location of structures controls insurance rates (with a 30-­‐day waiting period on any policies). Coastal building standards for the NFIP include: • all new construction must be landward of high tide • new structures on pilings; lowest floors must be above the 1% flood probability level • areas below the lowest floor must be open or have breakaway walls Hurricane Prediction: The paths of hurricanes are generally unpredictable. Likely paths have a cone of uncertainty. Storm motions are driven by trade winds and are affected by the locations of high and low pressure areas. Coastal regions are given <24 hours of advance warning of the most likely landfall location. Most evacuations take at least 30 hours. Complicating matters, 80% of the 40 million people prone to hurricanes have never been through an evacuation. See http://www.nhc.noaa.gov/archive/2007/graphics/al04/loop_5W.shtml (Hurricane Dean – Aug. 2007) Hurricane Alerts: In the event that a hurricane is expected to affect a coastal region, an alert system is implemented with two levels of severity: HURRICANE WATCH: a hurricane is possible within 36 hours. HURRICANE WARNING: a hurricane is expected within 24 hours; evacuate if advised. Typically, about 400 miles of coastline receives a hurricane warning. Ultimately, 125 miles of coastline is badly affected. Evacuation costs are generally about $1 million per coastal mile. So about $275 million is spent on evacuating people who are ultimately not at great risk. 3