Hurricane+Katrina+-+Overview+&+Lessons+Learned

Hurricane Katrina - Overview & Lessons Learned //Emily Wychock, BAE/MAE, Penn State University, 2012 // toc
 * Gulf Coast Region, United States - August 29, 2005 **

Introduction
Hurricane Katrina was one of the deadliest and devastating natural disasters to occur in the history of the United States. After billions of dollars in damage and over 1800 lives lost, Katrina ranks as the sixth strongest hurricane to ever form over the Atlantic Ocean. Wind speeds were recorded up to 140 mph and storm surges ranged from 25 to 30 feet across the gulf coast. With such extreme and unpredictable wind and water damage, hurricanes can leave a relentless path of destruction. Katrina formed over the Bahamas on August 23, 2006 and made landfall on several states over the course of the next week. The storm crossed the southern tip of Florida as a Category 1 storm and then strengthened to a Category 5 storm over the Gulf of Mexico. By the time it made landfall in southeast Louisiana on the morning of August 29th, Katrina was an extremely large Category 3 hurricane. Due to the failure of the levee and surge protection system in New Orleans, countless buildings suffered from severe water damage and over 75% of the city was flooded. This failure eventually led to a lawsuit against the U.S. Army Corps of Engineers, who designed and built the levee system. Although over six years have passed since Katrina made landfall, many people are still paying the price for the destruction caused by the hurricane.

Key Words
Hurricane, Katrina, New Orleans, Flood, Levee Failure, Wind Damage, Louisiana

Summary of Events
Katrina originally formed as a tropical depression roughly 175 miles southeast of Nassau in the Bahamas and then strengthened to a tropical storm as it traveled across the central Bahamas region. The next day the state of Florida issued a hurricane watch followed by a hurricane warning. On the evening of August 25th Katrina made its first landfall in the U.S. just north of Miami as a Category 1 hurricane. Wind speeds were estimated at 80 mph with gusts up to 90 mph, and significant flooding occurred in some areas. Overall the storm was only on land for around six hours and after traveling across the tip of Florida it quickly intensified with increased wind speeds over the Gulf of Mexico.

Between August 26th and August 27th Katrina continued to gain strength and nearly doubled in size. On the morning of August 27th the National Hurricane Center issued a hurricane watch followed by a hurricane warning for the north central Gulf region from Morgan City eastward to the Alabama and Florida state border. By the next day Hurricane Katrina had grown to a Category 5 storm with wind speeds measured over 160 mph, and extremely low pressure at the center of the storm as it headed north. The morning of August 29th Katrina made its second landfall near the Louisiana and Mississippi state border at the mouth of the Pearl River as a Category 3 storm. Intense wind and water forces pushed sea water up against the Mississippi river delta and the east-facing flood walls along the river. Storm surges reached a maximum height of nearly 30 feet, in most cases over-topping the flood walls and in many areas causing the walls to collapse under hydrodynamic forces. The failure of the Hurricane Protection System (HPS) in New Orleans along with excessive rainfall over a period of several hours lead to more than 75% of the city being under water. The hurricane eventually weakened as it traveled northward, but not without leaving a devastating path of destruction.

History & Background
//Hurricane Categories// The strength of a hurricane is measured by category, with one being the weakest and five being the strongest storm. This rating system, which is utilized by the National Hurricane Center, is known as the Saffir-Simpson Hurricane Scale. This scale was originally developed by wind engineer Herb Saffir and meteorologist Bob Simpson. This rating system provides an easy way to alert the public of possible severity of damages due to a storm. However the scale does not address the impacts of other hurricane-related issues such as severe rainfall and flooding.

[[image:failures/Saffir-Simpson Hurricane Scale.JPG width="506" height="153" caption="Figure 3. Saffir-Simpson Scale used to categorize the strength of hurricanes."]]
//Building Codes// In 2005 there was no statewide building code enforced in Louisiana, Mississippi, Alabama, or Texas. However, at the time several local governments had adopted model building codes, such as New Orleans which had adopted the 2000 edition of the International Building Code (IBC) and International Residential Code (IRC). (NIST, 2005) Most older structures were particularly vulnerable to damage because they were not up to date for hurricane-prone region requirements.

Damage Assessment
Hurricane Katrina was the most destructive and costliest natural disaster in the history of the United States since the 1928 Okeechobee Hurricane (DesRoches, 2006). Damage to buildings occurred from the initial storm surge, extreme winds up to 140 mph, and flooding as a result of excessive rainfall and the failure of flood walls. Due to the incredible impact this disaster had on the Gulf Coast region, many government agencies initiated investigations in order to assess damage and begin the recovery process. Several of these agencies include the Federal Emergency Management Agency (FEMA), the National Oceanic and Atmospheric Administration (NOAA), the National Institute of Standards and Technology (NIST), and the U.S. Geological Survey.

Wind Damage
Unlike the coastal regions of Louisiana, in areas further away from the ocean wind was the prominent cause of damage to structures. Coastal areas in Mississippi fell victim to mostly wind damage as opposed to Louisiana which was in the direct path of Katrina and experienced mainly flooding and storm surge damage. Wind forces on low slopping roofs can create negative pressure on the building, resulting in uplift forces on the roof assembly. These forces are generated by wind that is deflected around and across the surfaces of buildings which leads to a drop in air pressure immediately above the roof surface. The air inside the building will flow to the underside of the roof deck creating a positive pressure, and combined with uplift forces may completely dislodge a roof assembly piece by piece (ANSI, 2011). Similar forces may also cause glazing on the exterior of a building to crack or fail due to extreme pressures. On top of these failure methods, penetrations to the enclosure can lead to further interior damage from rainwater infiltration if failure occurs during the peak of a storm such as Hurricane Katrina. Roof damage on several types of low slopped roofs during Katrina including spray polyurethane foam-based (SPF), built-up, and metal were consistent with damage from previous hurricanes of the same strength. The most common issues were roof outer membranes detaching from the structure as a result of not being properly secured according to fastener standards and assembly guidelines. Other fasteners that were properly installed were extremely corroded (Panasik, 2007). Buildings constructed without proper inspection were left vulnerable to winds over 100 mph if roof deck, insulation, or membrane coverings were improperly installed.

During the hurricane, the Louisiana Superdome was used as an emergency relief shelter for over 10,000 refugees. Due to extreme wind uplift forces, nearly half of the domed structure's roof membrane was detached from the metal deck and several sections of roof deck were dislodged from the structure. Fortunately the main structural system remained intact and no one was injured. The cause of this roofing failure was determined to be a combination of the lack of an air barrier, insufficient fasteners, and weak weld points on the metal roof deck. (Progar, 2011)

Damage to structures was not only caused by high wind forces alone, but by impact from airborne debris. Roof-mounted mechanical equipment that is not properly secured is one example of heavy equipment that could be destroyed and potentially cause damage to surrounding buildings. Debris much smaller than mechanical equipment is also capable of damaging the glazing of a building through impact with hurricane force wind speeds.

Water Damage
In coastal areas of Louisiana including New Orleans, storm surge was the prominent cause of damage (NIST, 2006). The height of a wave is determined by several factors including hurricane intensity, wind, and seafloor topography. High velocity water flow produces hydrodynamic forces which are capable of destroying solid walls and even entire buildings with insufficient foundation depth. Water forces due to storm surge depend on contact area, velocity, and approach angle. Surge heights from Katrina reached up to 30 feet in height miles offshore in the Gulf of Mexico. This type of force differs from a hydrostatic force which is the pressure exerted by a fluid due to gravity. Rising flood waters can cause hydrostatic pressure on exterior walls as well as uplift on structures that can lead to loss of foundation support and subsidence.

Flood waters are capable of destroying much more than structures. In a number of buildings in New Orleans critical equipment including electrical generators and chillers were not located on floors higher than expected flood levels and were damaged or left inoperable. Throughout coastal regions of Louisiana, entire residential neighborhoods remained underwater weeks after Katrina made landfall. Inadequate foundation depths lead to the collapse of many homes as a result of soil erosion and tidal scour. Other causes of structural failures in residential buildings included insufficient elevation of buildings exposed to wave forces, failure to provide a continuous load path from the roof to foundation, and failure to use corrosion-resistant structural connections (FEMA, 2011).

//Levee Failure// The Hurricane Protection System in Southeast Louisiana was comprised of a network of flood walls and levees with interior drainage. Pumping stations were also included in New Orleans where the land is below sea level. Flooding in New Orleans due to the levee system failure was a result of both breaching and overtopping. The additional problem with over topping of the flood walls was that resultant flooding lasted weeks in duration, as the water was unable to freely flow back to the ocean. Long term flooding lead to additional damage including mold growth, corrosion, and the deterioration of other building materials (FEMA, 2011). Due to excessive water volumes and flood elevations reached during the hurricane, many of the pumping stations included in the HPS became inaccessible or were without power. One third of the stations, which would be necessary to evacuate flood waters in the event of a levee system failure, were significantly damaged in the hurricane. Most stations were also unable to run without an operator (USACE, 2007).

Infrastructure
Throughout the coastal regions of Louisiana, Alabama, and Mississippi there was significant damage to many bridges, roads, and railroads. Among the approximate 45 bridges with significant damage, the majority was to the superstructure and was largely dependent on how the bridge decks were connected to the rest of the structure. According to an investigation report by the Technical Council on Lifeline Earthquake Engineering (TCLEE), bridges with decks that were rigidly connected to piers sustained more severe damage than those with simply supported decks without shear keys (DesRoches, 2006). Of all the bridges that suffered from 'significant' damage, most of the damage was due to spans shifting or collapsing, impact from barges, or eroded abutments. Of all the bridges that suffered from 'slight to moderate' damage, most were the result of high winds that damaged the operator house.

Lessons Learned
The widespread destruction and millions of dollars in damage from Katrina lead to many code revisions and updates for both new construction and existing structures in hurricane-prone areas. Many of these changes in design standards are important parts of modern day building codes.

Factors Contributing to Disaster
Homes in New Orleans were usually not constructed on stilts or significantly above sea level under the impression that the levee system would remain intact and protect against a storm surge. Although the flood walls would not protect against flooding due to heavy precipitation only, they were expected to be more than enough to protect against a storm surge. The hurricane protection system had no redundancy or backup if a levee wall was breached, which is one reason these failures lead to such significant damage in residential areas from both high velocity water and elongated flooding. Figure 6 illustrates an attempted temporary fix of a breached flood wall which was taken six days after Katrina made landfall.

According to the investigation lead by the U.S. Army Corps of Engineers, levee and flood walls failed in three different mechanisms:
 * 1) Rotational failure of the flood wall-sheet pile system triggered by soil erosion due to overtopping
 * 2) Massive erosion and scour of the earthen levee at the levee and flood wall junction (with water overtopping)
 * 3) Sliding instability of the flood wall-levee system due to foundation failure (without water overtopping)

The third failure mechanism is illustrated in Figure 7 which shows a section of flood wall being exposed to hydrostatic forces from rising flood waters. Unstable foundation soils allowed the wall to lean under pressure which creates a gap between the wall and levee fill material. As the gap fills with water, hydrostatic pressure on the wall increases and water seeps into lower soil levels. During post-storm investigations of the levee system in New Orleans, this gap was visible in many areas close to where the wall was completely breached.

Another factor that contributed to widespread damage in New Orleans, was corrosion of building materials. Over time building materials are inevitably subject to weather damage and decay and will lose their original strength. Coastal areas are specifically vulnerable to moisture and salt in the air, which can speed up the corrosion of metal connectors and fasteners. As salt from ocean spray accumulates on metal surfaces, it accelerates the electrochemical process that causes corrosion. High humidity in the Gulf Coast region also increases the rate of corrosion, and can lead to weakened connections.

Preventative Solutions & Code Revisions
//Levee System// Specific improvements were recommended for the hurricane protection system in New Orleans, and upgrades needed to be completed in a timely fashion in order to prepare for the following year's hurricane season. It was suggested that pump stations which would be used in the event of a flood wall failure be designed to prevent reverse flow, be able to withstand hurricane force winds without damage, and to improve the quality of maintenance at stations. The U.S. Army Corps of Engineers also recomended a backup system and to improve emergency access and communication (USACE, 2007).

//Enclosure// The current ASCE-7 requires that glazing in wind-borne debris regions must be protected by either temporary or permanent storm shutters or impact-resistant glazing. Windows and doors can be selected with high design pressure ratings that can withstand impact from large objects as well as extreme pressure differences. Walls below the lowest elevated floor may also be constructed as breakaway walls. These walls are designed to resist wind forces under normal conditions, but to collapse under lateral forces during a hurricane or extreme weather without affecting the integrity of the remaining structure. By acting independently, the failure of a breakaway wall will not directly lead to the total collapse of a building. Another possible solution is the use of precast concrete panels, which have been shown to perform above average compared to other cladding materials under hurricane conditions. Architectural precast panels and precast concrete hollow-core roofs are gaining popularity in residential construction, especially with apartment complexes and condominiums in coastal regions (Shutt, 2006, 24)

//Building Codes & Standards// Storm surge heights that were recorded during the hurricane exceeded levels defined by existing flood hazard maps as well as historical data in certain locations. Inaccurate risk-based storm surge maps were updated in order to be used for modern flood-resistant design of structures. One reason these maps were inaccurate was that storm surge heights associated with specific hurricane categories did not necessarily coincide. The five categories of the Saffir-Simpson scale are mainly determined by wind speeds. Although the height of a storm surge is partly dependent on wind, it is also affected by other factors including the geography of the coast line and slope of the ocean floor. Storm surge heights recorded during Hurricane Katrina show values that far exceed those associated with a category 3 storm (9 to 12 feet). Katrina is not the only example of this discrepancy between storm surge and wind speed; Hurricane Rita which made landfall less than a week later in Texas showed the same correlation.

Today in New Orleans
Over five years have passed since one of the largest natural disasters to strike the United States made landfall in New Orleans. Today around one third of the cities total properties still remain vacant. However, the city recently approved a new master plan and comprehensive zoning ordinance with the goal of completely transforming the city economically, environmentally, and politically by 2030. This new long term plan extends far beyond simply recovering from Katrina, and includes detailed zoning documents about future land use, planned developments, and the preservation of historic neighborhoods. More information about the newly approved master plan prepared by the City Planning Commission can be found here.

Conclusion
The devastation of Hurricane Katrina left a lasting impact on New Orleans as well as the nation as a whole. Failures in the hurricane protection system led to important lessons learned and necessary changes that will help prevent another disaster from causing the same magnitude of damage in the future.