Insights into Storm Surge Vulnerability
Apr 23, 2015
Editor's Note: Many building features contribute to the vulnerability or resilience of an exposure to storm surge. In this article we discuss the main determinants of a building's vulnerability.
Understanding the complex nature of storm surge damage is a multifaceted task. Many building features contribute to the vulnerability or resilience of an exposure. Regional differences in building practices and design codes also play a role in how much damage storm surge will inflict. Further understanding of storm surge damage can be gained through post-disaster damage surveys, such as those AIR conducts after every major U.S. hurricane—most recently, after Hurricane Sandy—and by accounting for secondary risk characteristics. In this article we explore the many aspects of building vulnerability across lines of business.
Storm Surge Vulnerability of Residential Buildings
In broad terms, storm surge can cause either structural or nonstructural damage to buildings. Structural damage occurs when the main load resisting system of the building, which includes the building envelope, framing, or foundation, is damaged, leading to a compromise in the structural integrity of the building. Nonstructural damage, on the other hand, comprises damage to all other components, such as interiors, mechanical, electrical and plumbing (MEP) systems, that are not part of the main load resistive system of the building.
Foundation type and first floor elevation are the main determinants of a residential building's vulnerability. Buildings with an elevated first floor are less vulnerable to damage than buildings with the first floor on street level; however buildings with an elevated first floor could still sustain significant damage, depending on the foundation type. The house in the center of Figure 1 suffered no damage because it was sufficiently elevated above the surge from Hurricane Sandy. The houses on either side suffered damage, as the one on the left was founded on a crawlspace and the one on the right was founded on a basement.
Generally, houses built on stilt-type foundations with their first floors elevated above the base flood elevation (BFE) experience minimal damage except in very extreme events. But houses founded on slabs or shallow spread foundations, or on basements, can experience significant damage. In brief, slab foundations are made of concrete poured into a mold set in the ground, offering little or no elevation; shallow spread footing foundations are made of concrete blocks, support a single column or wall, and are slightly below ground or below the basement. In general, the presence of a basement exacerbates the level of damage to contents and the structure including floors, interior walls, and finishes.
During AIR's damage survey after Sandy, engineers found that houses in the coastal communities of New York and New Jersey not elevated typically had slab foundations or shallow spread foundations and experienced extensive damage. (See Figure 2.) Instances of collateral damage can be expected in houses with closed foundations, such as center-block and perimeter wall systems, due to differences in hydrostatic pressure between the inside and outside walls. The vulnerability of a particular foundation type is related to the elevation of the lowest floor in the building. Houses founded on open foundation types, such as short wooden stub columns, can be subject to uplift due to the oncoming surge, resulting in dislocation and collapse.
Older buildings sometimes lack a continuous load path, which is a way of constructing a building so that a series of solid connections within the structure can transfer all types of loads, including lateral and uplift, to the foundation of the building and then the ground. A continuous load path enables a residential building to resist a load, or force, such as storm surge; the absence of such a load path makes a building more vulnerable. (See Figure 3.)
Losses from storm surge also depend on the location of utilities and service equipment such as mechanical, electrical, and plumbing (MEP) systems. If located in the basement or on insufficiently elevated first floors, surge inundation can lead to total MEP systems failure, thereby severely inhibiting the usability of the building. In houses without basements, MEP systems are typically located on the lowest floor. If the lowest floor is not elevated above the BFE, significant damage or near-total destruction of these systems can be expected. (See Figure 4.)
Residential buildings also often have appurtenant structures such as garages, carports, decks, and porches. Appurtenant structures increase a residential building's vulnerability, especially if they are not properly attached to the main structure and lack a continuous load path to transfer the force of storm surge to the main structure.
Commercial Building Vulnerability to Storm Surge
Surge damage to low-rise, non-engineered commercial buildings is similar to the kinds of damage inflicted on residential buildings. Mid- and high-rise buildings typically have robust structural systems; however, good structural performance alone does not ensure adequate protection from flood damage. Hurricane Sandy demonstrated that mid- and high-rise buildings do not have to be severely damaged to be rendered inoperable.
Although severe structural damage is rare in well engineered commercial buildings, it can happen. Figure 5 shows structural damage to the floor slab of a 12-story, reinforced, concrete-framed, high-rise condominium building in Jersey City, New Jersey. The building's two levels of underground parking garage were completely inundated by Hurricane Sandy's storm surge, and there was about 4 feet of surge above the first floor slab. The unbalanced flood loads acting on the floor slab caused the slab to crack near the columns.
The presence of basements can increase a commercial building's vulnerability, exacerbating storm surge damage to both structural and nonstructural components. Basements can fill up rapidly and violently with water, causing significant damage to interior building elements such as floors, walls, and basement ceilings. In rare cases, structural damage is sustained; basement walls can cave in, or basement ceilings/first floors can be damaged.
Damage to nonstructural MEP systems is the most common type of damage that commercial buildings experience, the failure of which can cripple building operations. The associated business interruption (BI) costs can be high. In some cases, BI can have larger ramifications for the general public, as in the case of hospitals that must shut down temporarily and relocate patients. The cost of returning building functions back to normal includes both the direct costs of repairing damaged equipment and contents, and the costs of lost rent and business income.
For smaller businesses, such as restaurants, groceries, and retail stores, basements are often used for storage and storm surge can cause contents damage. In these cases, the contents may be foodstuffs, refrigerators containing perishables, and overstock, all of which are likely to be a total loss.
The contribution of contents to total damage can be significant depending on the level of inundation and the damageability of contents, which in the AIR model depends in turn on the occupancy of the building as certain types of contents are more vulnerable than others.
Regional Differences in Vulnerability
Engineering studies, claims data, and damage surveys indicate that there can be significant variation in building vulnerability by region and year built. This variation is due to changes in building codes and regional differences in construction practices, such as the prevalence of basements or elevated first floors. Reliable loss estimation depends on accurately capturing the spatial and temporal differences in vulnerability.
First floor elevation is typically not addressed in building codes, but rather is left to community floodplain management. Floodplain management guidelines are usually tied to base flood elevations (BFEs) as specified by the Federal Emergency Management Agency (FEMA) through Flood Insurance Rate Maps (FIRMs) that are part of the National Flood Insurance Program (NFIP). The specified BFEs are then adopted by local communities. FEMA BFE is the elevation of surface water resulting from a flood that has a 1% chance of being equaled or exceeded in any given year. Some communities adopt design flood elevations (DFEs) that include "freeboard," which is additional height above the BFE required for the lowest habitable floor and utilities.
The NFIP, with its specification of BFE requirements for new construction (Post-FIRM structures), is relatively new when compared to the age of many buildings located along the coastline. Older, Pre-FIRM structures have been grandfathered into the NFIP program and frequently have the lowest habitable floors and utilities below the BFE. Consequently, when a flood occurs that is anywhere close to the BFE, these older buildings suffer tremendous damage and disruption for occupants, as was the case with Hurricane Sandy. Therefore, the age of a building (Pre- vs. Post-FIRM) is an important determinant of vulnerability.
The prevalence of certain foundation types in specific regions results in regional variation in storm surge vulnerability. Basements, for example, are widespread in the Northeast in both residential and commercial construction, but much less common in the Southeast. (See Figure 7.)
Specialty Lines—Automobiles and Pleasure Boats
Auto losses are often difficult to estimate due to their mobility and uncertainty about public response in advance of hurricane landfall, which can vary by region. Car dealerships can generate large losses simply due to the accumulation of value in one location and difficulty of moving hundreds of automobiles to a surge-safe area. Individual auto owners can, and do, move their cars in advance of a storm, but in the case of Hurricane Sandy, for example, many did not; worse, many kept them in underground garages, where some were completely submerged. In that sense, the loss potential represented by automobiles is regional and highly dependent on the population density and presence of underground parking garages.
Storm surge generally causes the majority of severe damage to pleasure boats during a hurricane. Rising water levels can push docks and pilings beyond their capabilities, resulting in boats being impaled by the pilings themselves or becoming free of their moorings. These free-floating boats can collide with other crafts, docks, pilings, or the ground, which can result in damage to the hulls, engine, and other components. If the hull has been significantly compromised, the boat can sink. In addition, pleasure boats can experience damage from debris impact.
The insights into storm surge vulnerability shared here underscore the importance of having detailed location-level exposure information, such as distance from the coast, type of foundation, first floor elevation, compliance with FEMA guidelines, and year built. Having this type of data will go a long way toward helping companies assess and manage their storm surge risk.
FEMA P-942, Mitigation Assessment Team Report: Hurricane Sandy in New Jersey and New York, Federal Emergency Management Agency, November 2013.