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Assessing the Sensitivity of Transportation Assets to Climate Change in Mobile, Alabama

Results

Sensitivity Relationships in Mobile, Alabama

Local policymakers and transportation decision-makers can use the Sensitivity Matrix to target specific sensitivities in the transportation system with adaptive planning and design in order to decrease vulnerability. This work on the matrix revealed three main conclusions about the sensitivity of the transportation system to climate stressors.

It is important to bear in mind that sensitivity is only one component of overall vulnerability. So, even though a particular asset may be sensitive to a climate stressor, the consequence of that sensitivity is equally important. For example, the impact may be enormous (e.g., shut-in of offshore oil and gas production for several days), or it may be minor (e.g., a two-hour delay of commercial air flights). The integrated analysis of sensitivity together with criticality, exposure, and adaptive capacity will occur during the vulnerability assessment later in the Gulf Coast Study. Information on criticality of assets was previously assessed in an earlier stage of the study.2 Information on exposure is being drawn from the efforts discussed earlier in this report. These factors will be jointly and systematically considered in the upcoming vulnerability assessment.

Results by Climate Variable

Each of the four sections below details the sensitivities of transportation assets to stressors corresponding to the four main climate variables considered in this study. Note that more information on the thresholds and impacts mentioned below can be found in the Sensitivity Matrix.

Waves and Storm Surge

Many assets are sensitive to low levels of exposure to storm surge. Since sensitivity is so high, damage often begins when the asset is exposed to the storm surge and escalates when the asset is directly exposed to wave action. In some cases, once the storm surge is elevated to the point where it has overtopped the structure by a significant depth, the structure may actually be protected from wave damage. For example, during Hurricane Frederic in 1981, about 80% of the roads on the western side of Dauphin Island and 20% of the roads on the eastern side of the island were damaged. Three feet of sand covered Bienville Boulevard, the main east-west road on the island, and may have helped to protect it from the storm surge, since the road experienced only minor damage.3

The effects of storms including surge, high winds, and saltwater spray can severely damage electrical systems and electric parts of assets (e.g., control rooms of movable bridges, port machinery). For example, the majority of the bridges damaged in Hurricane Katrina were movable bridges, although in over one third of these cases, the damage was to a component of the bridge unrelated to the movable spans.4 Damages to electrical systems can also magnify the damage experienced by other transportation assets. For example, energy infrastructure is highly interdependent and the electricity outages from hurricanes often result in the closing of refineries, gas processes, pipelines, ports, and other facilities.5

Many coastal assets such as coastal bridges, floating piers, dry docks and offshore oil and gas infrastructure have been designed to withstand exposure to the 100-year storm. Engineers also use the 500-year storm as a "check storm" to check that a bridge can withstand scour created by an extreme event.6 The level of exposure to storm surge for most airports and heliports in the Mobile area is low, although facilities on Dauphin Island are already subject to flooding during extreme weather events, and are at high risk of exposure to relative sea-level rise.

Wind

As wind speed increases, damage to structures increases nonlinearly. Based on the analysis of historical damage to residential areas from storms, Powell and Reinhold (2007) assigned the following damage levels to the following wind speed ranges: between 22 and 91 mph (light damage), between 91 and 123 mph (moderate damage), and over 123 mph (severe damage). These damage thresholds correspond to loss levels of around 2%, 12%, and 60% of insured value in residential areas respectively.7 The destructive power of wind increases drastically upon reaching the threshold value of 123 mph - winds above the severe threshold produced about 30 times more loss to the insured value of residential areas than moderate winds.8 While this analysis was conducted using residential insurance losses as the measure of damage, it is a reasonable assumption that damage to transportation assets that have a large vertical profile (e.g., buildings, signs, signals, etc.) would follow a similar pattern; most flat profile roads and rails are not likely to exhibit a similar sensitivity. Damage to buildings increases in proportion to at least the second power of wind speed.9 The 3-second gust basic wind speed required by ASCE 7-05 is between 130-150 mph for majority of Mobile County. This is the wind speed that is used in ASCE 7-05 to determine the pressures that buildings and other structures must withstand, although other factors such as the Occupational Category of the building also influence design. ASCE 7-10 recommends a three-second gust wind speed of 140-175 mph, depending on the structure's risk category (ATC 2012).

The findings in the Sensitivity Matrix identified two important sensitivities at relatively low wind speeds. First, there is evidence that electrical power systems may be significantly sensitive to damage from wind. For example, Reed et al. (2010) analyzed power outages during Hurricane Rita and found that outage fragility (defined as the ratio of outages in a parish to the total number of customers in the parish for the date of hurricane landfall) was 50% even when the wind speed was only around 51 mph, although some of these outages could be due to ancillary impacts of the hurricane. Similarly, lower wind speeds create dangerous conditions for road maintenance and truck operations. For example, at wind speeds between 45 and 60 mph, traffic limits on bridges are often imposed, electrical damage increases, and larger aircraft are prevented from taking off if winds have strong crosswind components.

In addition to direct damage, wind often serves as an ancillary stressor to storm surge and precipitation. Wind speeds increase wave action, are the major drivers of storm surge elevation, and can expose sensitive machinery and electronics to damage from precipitation, flooding, and storm surge.

Temperature - Heat Waves

Road pavement and steel railroad tracks are two transportation sub-modes that exhibit sensitivity to extreme heat events and diurnal temperature cycling. For paved roads, sensitivity to temperature depends in large part on the binder Performance Grade (PG) in use and the traffic loading experienced at that site. FHWA created a database tool called LTTPBind10 to help highway agencies select the most suitable asphalt binder PG based on historical temperature variation and an acceptable level of risk (usually 50% reliability or 98% reliability). PG 64-22 is the common asphalt grade recommended for Alabama and other states in the southeast. However, Alabama recommends the use of PG 67-22 for important roads in order to provide a higher margin of error against the possibility of rutting during particularly hot summers. Mobile County currently does not experience a lot of damage due to pavement softening. However, during extreme heat waves where the temperature can remain above 100° F, with relatively little cooling at night, the pavement can soften. Areas with high truck traffic (particularly areas where trucks stop) experience shoving during heat waves.11 Similarly, the risk of railroad track buckling increases significantly at around 110°F, though railroads may slow train speed when the temperature reaches 90°F to avoid buckling and derailments.12

Maintenance work is also sensitive to extreme heat events. For example, in addition to the aforementioned temperature limitations on operations, restrictions that limit the number of hours that road crew maintenance can work begin at 85°F.13 Concerning airport operations, higher temperatures also decrease air density, which may require cargo or passenger adjustments to lower plane weight for takeoff.14

Precipitation - Heavy Rain Events

Heavy rain events can flood local roads, wear out pavement, and scour bridge foundations. While extremely heavy rain events in combination with high wind speed or storm surge can rapidly damage assets, a series of moderately heavy precipitation events can also significantly degrade an asset over time. For example, precipitation will worsen existing pavement cracks and can expose the sensitive subgrade to moisture.15 Mobile County routinely experiences flooding issues during storms and heavy rainfall events. Due to a variety of factors, certain roads and bridges flood more easily than others. For example, county bridges that are elevated above the roadway can cause rainwater to runoff and flood either side of the road during heavy rain events.16

Stormwater drainage helps to protect road pavement and service from exposure to flooding. In Mobile County, stormwater drains are designed according to a 10-25 year storm depending on the drain type and road size. However, Mobile County engineers confirmed that flooding generally does not occur until a 50-100 year storm, indicating that stormwater drains may perform above their designed capacity.17 Debris can cause local flooding problems when small culverts or drains are blocked by tree limbs or other types of debris.18

Heavy precipitation events can increase stream flow velocity and width, which can erode supporting material from bridge foundations, resulting in scour-critical conditions. AASHTO LRFD specifications require that scour at bridge foundations be designed for the 100-year flood storm surge tide or for an overtopping flood of a lesser recurrence interval. The Alabama Department of Transportation has reported no serious problems with bridge scour in the area, even to bridges that are deemed scour-critical.19

Heavy rain events also impact road and airport services. For example, even light rain slows traffic and decreases the capacity of a road to handle traffic. Rain also increases safety risk on the road by impairing visibility and mobility and increasing the likelihood of hydroplaning.20 Heavy rain can flood runways, lower the crosswind takeoff and landing limits for aircraft, and thunderstorms can lead to flight delays or cancellations. Hail can cause significant damage to aircraft, hangars, and buildings.

Table 2 : Sensitivity Screen developed during this project.

Italics indicates a sensitive relationship between the sub-mode and the climate variable. Non-italic indicates a non-sensitive relationship. Where available, information about thresholds is included.

Asset Categories   Sea-level rise and Storms Precipitation Temperature
Mode Sub-Mode Relative Sea-level rise (Gradual) Storm Surge (including increased wave action and sea-level rise impacts) Wind Incremental change in the mean (+/-) Increase in frequency or duration of heavy rain events Drought Incremental increase in the mean Increase in frequency or duration of heat events
Bridges Bridge (Superstructure)   Damage increases substantially when storm surge height equals low chord bridge elevation. At this point, the bridge is usually exposed to direct wave impacts on the superstructure. Recent guidance specifies that the vertical clearance of highway bridges should provide at least 1 foot of clearance over the 100-year design wave crest elevation. AASHTO LRFD specifications are based on a base design wind velocity of 100 mph, although the base design wind velocity investigated for tall structures to account for local wind speed conditions. ASCE 7-05 recommends a three-second gust basic wind speed of 130 to 150 mph in the Mobile area; ASCE 7-10 recommends 140 to 175 mph, depending on the structure's risk category.   Scour can make bridge more susceptible to collisions, wave action, and other impacts.     Bridge pavement is usually concrete and may exhibit similar sensitivities as road concrete pavement.
Bridge (Substructure, Abutment and Approach) Sea-level rise increases the base elevation of water during storm surge, thereby increasing damage due to scour, wave action, uplift and other stressors. Design standards require that bridge foundations withstand scour resulting from a 100 year storm. Strong winds create more powerful waves which can stress the bridge superstructure and substructure.   Scour at bridge foundations is generally designed to withstand the 100-year flood storm surge.      
Operator Houses (movable bridges) and electrical parts   If exposed, electrical components are very sensitive to low levels of salt water flooding. Movable bridges may begin to close operations at wind speeds of around 40 mph. Physical damage to operator houses has occurred historically at wind levels of 125 mph. Damage from wind tends to be minor.   Damage would require wind or storm damage to expose operator house and electrical equipment.      
Roads and Highway Paved roads (surface and subsurface) Sea-level rise increases the risk of erosion and flooding damage to coastal roads. Threshold depends on elevation of road, coastal protection, and other factors. Direct damage to road begins occurring once storm surge overtops road, particularly if waves are in direct contact with road structure. There is some protection from wave action if road is deeply overtopped or covered with sand.     While lower functional class roadways are typically designed for the 10-25 year storm, Mobile County roads tend to not experience damage from flooding until 50-100 year storms. No documented relationship, but some sensitivity may exist.   In Mobile, pavement may exhibit sensitivity at sustained air temperatures of 108 degrees F, particularly on routes with a high level of truck traffic.
Unpaved roads Most coastal roads do not have unpaved surfaces. However, if exposed, unpaved roads are more sensitive to erosion and damage caused by sea-level rise than paved roads. Most coastal roads do not have unpaved surfaces. However, if exposed, unpaved roads are more sensitive to storm surge damage than paved roads. Moderate winds stir up dust from unpaved roads, resulting in minor discomfort and damage. No documented relationship, but some sensitivity is likely. High sensitivity to washout from flooding. No documented relationship, but some sensitivity is likely.    
Stormwater drainage (culverts, side drains, etc) Sea-level rise increases potential for flooding of the stormwater drainage system. Storm surge can flood the stormwater drainage system beyond its design capacity. Damage from wind creates debris, which can clog stormwater drainage systems, exacerbating flooding damage. Culverts: 25 year storm capacity
Cross drains: 10 year storm capacity
Side drains: 10-25 year storm capacity (Mobile County design standards)
In Mobile, destructive flooding generally does not occur until around 50 to 100 year storm.
Culverts, cross drains, and side drains are designed to 25-, 10-, and 10 to 25-year storm capacity, respectively.
No documented relationship, but some sensitivity may exist.    
Highway, road and street signs and traffic lights   Flooding can damage electrical components, causing traffic lights and other signals to malfunction. Alabama AASHTO wind design speed is 140 mph; if signs are not buried deep enough, failure can occur at lower wind speeds (e.g., sign failures at 90 mph have been recorded in Miami-Dade county).   Heavy rainfall can impact visibility of signs.      
Road Work and Maintenance, Driver Safety, and Traffic and Service   If exposed to storm surge, road usually closed or rendered inoperable. Danger to road maintenance workers and road users at wind speeds of 40 mph; conditions become very dangerous at wind speeds of 75 mph. Very light rain reduces road capacity by 1 to 3% and roadway speed by 1 to 2%.
Light rain can reduce road capacity by 5 to 10% and roadway speed by 2 to 4%.
Heavy rain reduces roadway speed by 4 to 7%.
  No documented relationship, but some sensitivity is likely. Health and safety risk as well as possible engine/equipment heat stress begins at around 85F, but the situation becomes more critical at 105-110F. Restrictions limiting the number of hours that road crew maintenance can work begin at 85F. At 110F, operations are generally restricted.
Railroads Electrical Equipment (gates/flashers and signal bungalows) Equipment may become inundated. Exposure to storm surge can cause failure of electrical components, such as signals. Winds (head, cross, or tail) - 50 mph   If exposed, inundation of equipment can lead to electrical damages due to inundation of equipment.   Risk to electric rail components increases at temperatures equal to or exceeding 90 F.
Railroad Tracks, Ties, and Ballast Rail lines may become inundated Wave action can strip rail, ties, and ballast off of railroad bridges if they are exposed.     Immersion of wooden ties in water softens/expands the wood, weakening its ability to support tracks. Erosion of supporting systems can threaten track stability.   For steel railroad tracks: at 90 degrees F, rail speed is slowed to prevent buckling; buckling risks become pronounced above a threshold temperature of 110 degrees F.
Railroad services (i.e., operations) Service may be terminated if lines are inundated. Storm surge can scour the railbed, derail rail cars, and damage railway bridges over streams, all of which can disrupt service. Wind speeds greater than 50 mph can cause damage to electrical lines. Three-second gust basic wind speed used to determine the pressures that buildings and other structures must withstand according to ASCE 7-05 is 130 to 150 mph in the Mobile area. ASCE 7-10 recommends a three-second gust wind speed of 140 to 175 mph, depending on the structure's risk category.   Heavy precipitation or any flooding can cause damage. As little as two inches of flooding can short out locomotive motors.   Health and safety risks for workers at a heat index greater than 105 degrees F can lead to operational delays.
Airports and Heliports Runway and navigational aids Sea-level rise exacerbates storm surge elevation. Runways are sensitive to low levels of flooding; the storm surge threshold is therefore essentially equal to the airport elevation.     Heavy rain of 1 to 2 inches per hour can lead to standing water on runway, causing delays.   Low sensitivity - For range of expected temperature increases in Mobile area, effects on runway length requirements are negligible.
Aircraft     Cross-wind landing and take-off speed limit for most small aircraft is 23 mph; limit for larger aircraft can range from 46 mph in dry conditions to 20 mph on iced runways.   Wet runways can lower aircraft cross-wind takeoff/landing limits.
Hail greater than 1 inch in diameter can severely damage aircraft.
   
Airfield buildings and structures (e.g., terminal buildings, hangers, air traffic control tower)   Airport buildings are sensitive to low levels of flooding; the storm surge threshold is therefore essentially equal to the airport elevation. Basic wind speed used to determine the pressures that buildings and other structures must withstand according to ASCE 7-05 is 130 to 150 mph in the Mobile area. ASCE 7-10 recommends a three-second gust wind speed of 140 to 175 mph, depending on the structure's risk category. Certain airport buildings are considered "essential facilities" and are designed to withstand greater pressures.       Increased temperatures will increase cooling requirements in terminals and buildings  
Services and airport/ heliport operations   Airport services are sensitive to low levels of flooding; the storm surge threshold is therefore essentially equal to the airport elevation. Airports close in hurricane conditions (i.e., wind speeds greater than 74 mph)   Thunderstorms with lightning within 3, 5, or 17 nautical miles and heavy rain of 1 to 2 inches per hour can cause delays.     Higher temperatures may limit the types of aircraft that can take off on certain days, or pilots may have to adjust cargo or passengers to lower weight.
Oil and Gas Pipelines Pipelines, aboveground   Sensitivity generally low Wind speeds above 60 mph can damage pipeline systems.   Damage caused by weakened soil structure due to precipitation or inundation from storms.      
Pipelines, underground   Sensitivity generally low     Damage caused by weakened soil structure due to precipitation or inundation from storms.
Pipeline may be unearthed from flooding.
     
Pipelines, offshore   Offshore oil and gas infrastructure is designed to withstand the 100 year storm. Historically in the Gulf Coast, damage to pipelines has drastically increased at around a Category 4 hurricane (Cat 4 storms are characterized by 130-155 mph winds and 13-18 foot storm surges).          
Aboveground infrastructure (e.g., compressor stations, metering stations, other buildings, structures)   No documented relationship, but some sensitivity is likely. Wind can damage buildings at speeds greater than 30 to 40 mph; basic wind speed used to determine the pressures that buildings and other structures must withstand according to ASCE 7-05 is 130 to 150 mph in the Mobile area. ASCE 7-10 recommends a three-second gust wind speed of 140 to 175 mph, depending on the structure's risk category.          
Electric Power Systems Electric Power Systems   Wind, storm surge, and waves damage essentially every component of electric power systems (transmission lines, towers, insulators, generating plants) through direct flooding impacts, damage from debris, and other effects such as salt spray contamination. Risk of damage from wind increases around threshold of 45 mph.   Low sensitivity; soil moisture effects electrical power system components, such as poles. Drier conditions can increase the likelihood that trees will snap and break in storms, damaging power lines.   Increased demand for air conditioning can stress electric power systems during heat events.
Marine Ports, Terminals, and Waterways Electrical Equipment   If exposed, high sensitivity. No documented relationship, but some sensitivity is likely.   If exposed, some sensitivity.      
Terminal Buildings   Damage is likely to occur when wave height overtops elevation of port. Design wind speed for Mobile port structures ranges from 130-140 mph (3-second gust). ASCE 7-10 recommends a three-second gust wind speed of 140 to 175 mph, depending on the structure's risk category.          
Channels No documented relationship, but some sensitivity is likely. Storm surge can wash debris and sediment into the shipping channels, necessitating dredging following the storm.       Lower water levels due to drought decrease the cargo limits for shipping, especially for barges.    
Piers, wharves, and berths   Damage is likely to occur when wave height overtops elevation of port. Indirect impacts due to increased wave height.         Paved piers could be susceptible to surface buckling
Port services (i.e., operations) No documented relationship, but some sensitivity is likely. Storms cause damage to marine port services by disrupting the power and communications networks, displacing port workers, washing away channel buoys, and submerging debris in ship channels. Berthing large vessels is affected when wind speeds exceed 23 mph; high-speed ferries stop operating at wind speeds of roughly 46 mph; container and gantry-type cranes at affected by sustained wind speeds greater than 29 mph.   No documented relationship, but some sensitivity is likely. No documented relationship, but some sensitivity is likely.   No documented relationship, but some sensitivity is likely.

1 OFCM (2002)

2 U.S. Department of Transportation (2011)

3 USACE (1981)

4 Padgett et al. (2008)

5 U.S. DOE (2009)

6 See Sensitivity Matrix for specific design specifications and references.

7 Data were from zip code locations during Hurricanes Andrew, Hugo, and Opal.

8 Powell and Reinhold (2007)

9 Pielke and Landsea (2002)

10 Federal Highway Administration (2012)

11 Mitchell (2010)

12 Aggarwal and Wickersham (2010); OFCM (2002)

13 OFCM (2002); NRC (2008); US CCSP (2008)

14 Ang Olson (2009)

15 The sensitivity of pavement depends on the pavement design. For example, in Mobile County, most county roads are thin bituminous pavements, which are more sensitive to water than other types of roadway. If moisture breaches the subgrade from the pavement shoulder, it deforms the subgrade which is then subjected to high stress loads during traffic. In thick bituminous pavements, the thicker pavement layers mean that less stress is transmitted to the subgrade. As a result, the pavement is less sensitive to moisture in the subgrade (Dawson, 2008).

16 Mitchell (2010)

17 Mitchell (2010)

18 Mitchell (2010)

10 Powell and Reach (2010)

20 OFCM (2002)

Updated: 03/27/2014
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