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Climate Variability and Change in Mobile, Alabama

Table of Contents

Executive Summary

Main Report

Executive Summary

1. Introduction and Background

1.1. Overview of Gulf Coast Project

Despite increasing confidence in global climate change projections in recent years, projections of climate effects at local scales remains scarce. Location-specific risks to transportation systems imposed by changes in climate are not yet well known. However, consideration of these long-term factors are highly relevant for infrastructure components, such as rail lines, highways, bridges, and ports, that are expected to provide service for up to 100 years.

To better understand climate change impacts on transportation infrastructure and to identify potential adaptation strategies, the U.S. Department of Transportation (USDOT) is conducting a comprehensive multiphase study of climate change impacts in the Central Gulf Coast region. This region was selected as the study's focal point due to its dense population and complex network of transportation infrastructure, as well as its critical economic role in the import and export of oil, gas, and other goods. The study is funded by the USDOT Center for Climate Change and Environmental Forecasting and managed by FHWA.

The Gulf Coast Study has two distinct study periods: Phase 1 (2003 to 2008) examined the impacts of climate change on transportation infrastructure at a regional scale; and Phase 2 (underway) is focusing on a smaller region, enhancing regional decision makers' ability to understand potential impacts on specific critical components of infrastructure, and to start evaluating adaptation options.

1.1.1. Gulf Coast, Phase 1 (Completed)

In the first phase, USDOT had four main objectives: (1) to gather data critical for analyzing the impacts of climate change on transportation infrastructure; (2) to determine whether climate data could be valuable in assessing vulnerability of infrastructure in the region; (3) to identify and implement an assessment approach; and (4) to then develop an overview of the potential impacts on infrastructure. The Phase 1 study utilized historical data on weather events, recent climate data, and projected changes in climate for the coming century.

Phase 1 study results indicate that the Gulf Coast region is particularly susceptible to climate change over the 21st century. Some of the changes projected for the region include the following:

The implications of projected changes in climate for regional transportation systems are significant. Increasing temperatures are likely to require modifications to system materials and maintenance. Increased severity of precipitation events could exacerbate incidents of flash flooding, threatening the stability of soils and foundational materials. The combined effects of land subsidence and absolute sea level rise (SLR) could permanently inundate existing infrastructure. Finally, an increase in severity of tropical storms could have significant impacts on coastal infrastructure. Damages due to storm surge, winds, and flying debris can be catastrophic, as has been seen with previous hurricanes.

1.1.2. Gulf Coast, Phase 2 (Underway)

While Phase 1 took a broad look at the entire Central Gulf Coast region (between Galveston, Texas and Mobile, Alabama) with a ‘big picture' view of the climate-related challenges facing infrastructure, Phase 2 is focusing on a single Metropolitan Planning Organization (MPO) region around Mobile, Alabama. The purpose of this phase is to evaluate which transportation infrastructure components are most critical to economic and societal function, and assess the vulnerability of these components to weather events and long-term changes in climate. Phase 2 will also develop tools and approaches that the Mobile MPO and other public and private system operators can use to determine which systems need to be protected, and how best to protect them. Through this study, USDOT intends to create a process that can be replicated in other MPO regions.

Phase 2 is broken down into the following tasks:

This report discusses the methodology and results from Task 2.

1.2. Overview of Task 2

This report, the Task 2 report, lays the climate data foundation upon which a vulnerability assessment will be conducted in the next task. In future steps of the project, a vulnerability screen will be conducted along with an assessment of the highly critical assets identified previously under Task 1, as reported in the Task 1 final report Assessing Infrastructure for Criticality in Mobile, AL.1

This report explores potential changes in five primary climate variables: temperature, precipitation, streamflow, sea level rise, and storm surge in Mobile, AL, the location selected as the study area for Phase 2. To do so, Task 2 characterizes the current climate conditions in Mobile, and then uses downscaled climate projection data, as well as sea level rise and storm surge modeling, to develop plausible climate futures. The climate information discussed in this report will be used to assess how the transportation system in Mobile might be affected by climate change.

Although this report does focus on Mobile, AL, the processes developed under this Task can be replicated by other transportation organizations across the country. The ultimate goal of this report is to not just identify how climate could change in Mobile, but also to develop robust methodologies, and identify existing datasets and tools, for developing these plausible climate futures. Furthermore, the work conducted under Task 2 will help inform the development of tools and resources to make these types of analyses easier for transportation agencies. To that end, the process of Task 2 is just as important as the results. Section 12 provides a discussion of how the lessons learned and information developed under Task 2 will be used in other products for different audiences.

Figure 1 below illustrates the components of this report and how they fit within the overall Gulf Coast

Figure 1: Roadmap for Phase 2 of the Gulf Coast Project

This figure shows a flow chart explaining how Task 2 fits in to the overall Gulf Coast study. Task 2, the development of climate information, involved understanding how climate may change in Mobile, AL. Task 2 also contains a sensitivity matrix and screen, which is not a part of this report. Findings from Task 2 will feed into the Task 3 vulnerability assessment of Mobile's transportation system, particularly the assessment of exposure and sensitivity. Tasks 1 through 3 will feed into Task 4, the development of tools and resources based on lessons learned from the study.

Note: The components covered by this report are indicated with blue shading. The gray shading indicates other components of the Phase 2 study that are covered under other tasks and reports.

1.3. Report Roadmap

The main body of this report is organized by climate variable, with one section dedicated to each of the following variables:

Within each of those sections, this report first characterizes the current climate situation in Mobile, AL, and then discusses potential climate futures. Both the methodology used and the results of the analyses are presented. In addition to the key findings of the analysis, each section includes a discussion on the general implications of the potential climate futures for the transportation sector. The specific impacts that the climate projections may have on particular transportation assets in Mobile will be investigated in the next stage of this project.

The final section of this report includes a discussion of how the information developed in this report will be used for later activities under this project, and how it will inform the work of activities beyond this project.

2. Setting the Stage for Climate Research

2.1. Selection of Climate Variables

Task 2 included an assessment of the climate variables that have the greatest potential to impact transportation assets and operations: temperature, precipitation, streamflow, sea level rise, and storm surge. Wind was also calculated as part of the storm surge modeling, although it was not a specific focus of this study.2

An important part of this work was determining the appropriate format for communicating results for each climate variable. For example, this report goes beyond a generic exploration of projected changes in "temperature", looking instead at specific changes to both long-term averages (e.g., change in average annual temperature or average monthly temperature) as well as short-term extreme events (e.g., number of days above 95°F (35°C)). The decisions on the format used to express climate information were vital in making this work relevant to the transportation community. Attention was focused on identifying the climate effects that have the most potential to have an impact on transportation. The appropriate formats used for a transportation perspective may be quite different than the formats appropriate for other sectors, such as human health, ecosystem services, or other economic sectors.

2.2. Methodology Overview

For each climate variable, this report first characterized the current (or recent historical) situation in Mobile, and then evaluated how that variable could change based on published literature, prevailing assumptions of future emissions of greenhouse gases, and a variety of modeled data. Table 1 provides an overview of the methods used in this report.

Table 1: Overview of Analytical Methods Used
Climate Variable Methods Used to Analyze Current/Historical Situation Methods Used to Develop Future Projections Methods Used to Evaluate Exposure under Potential Future Scenarios*
Temperature Historical data from 5 NOAA GHCN weather stations in the Mobile Region. The start of the data record varied by station, ranging from 1915 to 1956. Data was collected through September 2010 for all stations. Downscaled daily global climate model data for B1, A2, and A1FI emission scenarios.
Timeframes: 1980-2009 (hist.), 2010-2039 (near), 2040-2069 (mid), and 2070- 2099 (long)
To be addressed in Task 3 (vulnerability assessment)
Precipitation Historical data from 5 GHCN weather stations in the Mobile region. The start of the data record varied by station, ranging from 1912 to 1956. Data was collected through September 2010 for all stations. Downscaled daily global climate model data for B1, A2, and A1FI scenarios, 1980-2099
Timeframes: 1980-2009 (hist.), 2010-2039 (near), 2040-2069 (mid), and 2070- 2099 (long)
To be addressed in Task 3 (vulnerability assessment)
Streamflow Historical data from five stream gages in the Mobile region through the USGS Surface Water Database. The start of the discharge data record varied by station, ranging from 1951 to 1995. Data was through September 2010 for all stations. Modeled using USGS modified Thornwaite monthly water balance model, fed by projected temperature and precipitation
Timeframes: 2010-2039 (near), 2040-2069 (mid), and 2070- 2099 (long)
To be addressed in Task 3 (vulnerability assessment)
Sea Level Rise Historical data collected from two NOAA tidal gages. Dauphin Island data were available from 1966-2009. Pensacola data were from available from 1924-2009. Review of recent scientific literature GIS mapping of inundation areas, assuming 30 cm (by 2050),and  75 cm and 200 cm (by 2100) of global sea level rise, and accounting for local subsidence and uplift
Storms and Storm Surge Case study analysis; storms selected through discussion with local experts and literature review Review of recent scientific literature Use of ADCIRC and STWAVE models to simulate two historical hurricanes (Georges and Katrina) assuming different levels of intensity and sea level rise

* A review of the scientific literature helped in the selection of plausible scenarios of sea level rise, and then mapping was used to show how Mobile would be inundated under those scenarios. Similarly, the scientific literature and discussions among the research team and with local stakeholders aided in the selection of storm scenarios, and mapping was used to show the inundation of Mobile under those scenarios.

2.3. Dealing with Uncertainty

Information provided on future climate in this report represents plausible projections, but not predictions. The information developed was based on a variety of assumptions, including the rate at which greenhouse gases are emitted into the future. The assumptions are based on recent and widely-accepted knowledge within the scientific community; however, there is a certain degree of uncertainty surrounding these assumptions. There is also uncertainty inherent in the various models that lay the basis of the analyses. Furthermore, there is natural variability in climate, which causes, for example, some winters to be much colder than the previous winter, or for some years to be wetter than others.

The climate futures described in this report are all plausible, but are not certain to occur. Additionally, none of the projections are considered more likely to occur than the others. The uncertainty around each of these components should be considered when conducting vulnerability assessments and implementing risk reduction measures. There are various techniques used to address uncertainty, including probabilistic approaches to quantify uncertainty, modeling various emission scenarios to produce a wide range of future possibilities, comparing present-day model results with observations, and engaging expert judgment to express uncertainty based on level of agreement and amount of evidence.

3. Temperature

3.1. Observed Temperature

Located on the United States' Gulf Coast, Mobile, Alabama is characterized by a very warm climate, with temperatures typically ranging from the high-50s to high-70s Fahrenheit. Overall, average annual temperatures have been relatively constant over the past 50 years in the Mobile region. While average annual temperatures have remained relatively constant, average minimum temperatures in March and September have decreased over the past century.

Table 2: Historical (1912 to 2009) Average for Temperature Variables
Temperature Variable Historical Average
(1912-2009)
Average annual temperatures
Average annual mean temperature 66.9°F
Average annual minimum temperature 56.1°F
Average annual maximum temperature  77.5°F
Hot and Cold Days
Hottest day of the year 97.9°F
Number of days above 95°F 12 days
Coldest day of the year 19.3°F
Number of days below freezing 23 days
Summer and Winter Temperatures
Average Maximum Summer Temperature 90.0°F
Average Mean Summer Temperature 80.5°F
Average Minimum Winter Temperature 41.7°F
Average Mean Winter Temperature 52.5°F

3.2. Projected Temperature

3.2.1. Methodology

Climate projections of temperature were statistically downscaled from a number of models and analyzed to project how annual, seasonal, and monthly-average weather conditions, specific weather thresholds, and extreme conditions relevant to Mobile, Alabama, may change in the future.

Projections were modeled for each of the following emission scenarios and time frames:

To account for local influences, large-scale global climate model data were downscaled to individual local observation stations in the Mobile region. Projections of daily maximum and minimum temperatures were statistically downscaled from up to ten climate models housed in the World Climate Research Program (WCRP) Coupled Model Intercomparison Project (CMIP3) multimodel data set. This downscaling produced climate projections for each emission scenario and time frame, relative to one climate baseline (1980 to 2009).

To focus the study on climate projections that represent a robust projected change from baseline conditions, a statistical test (a paired t-test) was used to identify significant (p<0.05) changes, i.e., climate projections that are statistically different from simulations of today's climate. This test helps identify which of the climate projections show a significant amount of change.

3.2.2. Key Findings
General

Temperature is projected to increase over time. The farther out in time, the greater the amount of temperature increase.

Overall, the amount of temperature increase is directly proportional to the increase in emissions—that is, the high (A1FI) emission scenario is associated with greater overall temperature increases than the low (B1) emission scenario. However, the increase in seasonal and monthly means is more variable across the emission scenarios. For example, under the low (B1) and moderately high (A2) emission scenarios, seasonal average temperatures are projected to increase the most in the fall season, with monthly average temperatures increasing the most in October. Under the high (A1FI) emission scenario, seasonal average temperatures are projected to increase the most in the summer season, with monthly average temperatures still increasing the most in October. As emissions increase there may be a tendency for peak warming to shift from fall to summer seasons.

Average Annual Temperatures

Average annual maximum, minimum, and mean temperatures are projected to increase significantly. Average annual mean temperatures increase steadily with each 30-year time period, by approximately 1°F (0.6°C), 2°F (1°C), and 3°F (2°C) for the low (B1), moderately high (A2), and high (A1FI) emission scenarios, respectively. Minimum temperatures are projected to increase more than maximum temperatures.

By the end-of-century, average annual mean temperature may increase to 70.5°F (21.4°C) under the low (B1) emission scenario, 73.8°F (23.2°C) under the moderately-high (A2) emission scenario, and 74.8°F (23.8°C) under the high (A1FI) emission scenario. Average annual maximum temperatures are projected to increase to as high as 84°F (29°C) by the end-of-century under the high (A1FI) emission scenario.

Seasonal and Monthly Mean Temperatures

Average seasonal and monthly mean temperatures are projected to increase significantly.3 The largest average seasonal mean temperature increases are projected to occur in the fall (particularly in October) and are largely dependent on changes in average minimum temperatures. The range of daily temperatures is projected to decrease. Lower temperatures benefit pavement and other infrastructure from reduced softening or expansion of materials, which are correlated with high temperatures. However, as the range of daily temperatures decreases, there may be less cooling relief overnight.

Extreme Temperature Events

The number of heat events above 95°F (35°C) and 100°F (38°C) are projected to increase dramatically. By mid-century, projections indicate there will be 2 to 5.5 additional weeks above 95°F (35°C). By end-of century, projections indicate there will be 3 to 11 additional weeks above 95°F (35°C). The number of days above 105°F (41°C) and 110°F (43°C) are not projected to change significantly.

The length of the longest heat wave (defined as consecutive days over 95°F (35°C)) is also projected to increase. By mid-century, the longest heat wave is projected to lengthen by about 1 to 2 weeks. By end-of-century, the longest heat wave is projected to lengthen by between 1 week and 1 month.

The average coldest four days in winter are projected to be nearly 3 to 6°F (2 to 3°C) warmer by end-of-century. Projections of the coldest day of the year suggest that the extreme cold day in a 30-year time period will warm substantially more than the average cold day in the same time period.

3.3. Implications for Transportation

These projected changes in temperature have some notable implications for transportation infrastructure and services. In general, higher temperatures result in more rapid deterioration of pavements that could require changes in repair and maintenance schedules (although in the longer term, newer and more durable pavement designs could reduce this impact). In addition, longer growing seasons due to longer periods of warmer temperatures could require more attention to mowing in rights of way, thus affecting maintenance budgets. An increase in the duration and frequency of extreme temperature events can result in increased buckling of rail and rutting and shoving of pavement. These impacts could be exacerbated by reduced potential for cooling relief overnight for pavement and other infrastructure. Excessive heat can contribute to equipment failures and more frequent vehicle breakdowns. Energy requirements for air conditioning of buildings, equipment, transit facilities, and freight are likely to increase. Ports, in particular, may see increases in energy costs to meet air conditioning and refrigeration requirements.

Extreme heat events also have health and safety implications for transportation agency personnel. In particular, maintenance and construction schedules may need to be adjusted to avoid health risks to workers.  Further, the costs of ensuring the comfort and safety of passengers – particularly of train and bus travelers – are likely to increase.

The implications of the temperature findings detailed in this report on transportation assets and services in Mobile will be investigated in the next task of this study (Task 3: Vulnerability Screen and Assessment).

4. Precipitation

4.1. Observed Precipitation

Based on observed data from five observation stations, total annual precipitation in Mobile averaged 65.3 inches (165.9 centimeters) from 1912 to 2009. This makes Mobile one of the rainiest cities in the United States. Annual precipitation can vary by as much as 13.4 inches (34 centimeters) or 20%. Total annual precipitation has not changed significantly over the past century.

The maximum 24-hour precipitation event recorded each year has fluctuated over the historical record, exhibiting no significant trends. Precipitation associated with the average maximum 24-hour precipitation event across all stations from 1912 to 2009 was 5.2 inches (13.2 centimeters).

Precipitation amounts tend to be evenly distributed throughout the year, with July being the rainiest month and October being the driest. Over the historical period, monthly precipitation has increased significantly over the past century in January, October, and November. Summer precipitation has also exhibited an increasing trend.

4.2. Projected Precipitation

4.2.1. Methodology

Precipitation projections for Mobile were statistically downscaled using the same methodology as was used for temperature projections (see Section 3.2). Statistically downscaled data were analyzed to project changes in annual, seasonal, and monthly-average weather conditions, specific weather thresholds, and extreme conditions relevant to the study area.

4.2.2. Key Findings
General

Total annual precipitation is not projected to change significantly in the near-term, regardless of emission scenario. By mid- and end-of-century, total annual precipitation is projected to increase under the low (B1) emission scenario. Under the moderately-high (A2) and high (A1FI) scenarios, annual precipitation totals are projected to remain statistically similar to the baseline.

Seasonal and Monthly Precipitation

With very few exceptions, future seasonal and monthly precipitation totals are not projected to differ significantly from current climate conditions. Under the low (B1) emission scenario, winter precipitation is projected to increase significantly in the near-term and by mid-century, and fall precipitation under the low emission scenario is projected to increase significantly by mid-century.

Precipitation Events

Maximum seasonal three-day precipitation is projected to increase across all seasons, emission scenarios, and time frames, though not all increases are statistically significant.

For all time periods under all emission scenarios, precipitation during high-probability/low-impact 24-hour storms is projected to increase by 1 to 3 inches (3 to 8 centimeters), an increase of more than 60%. Meanwhile, precipitation during low-probability/high-impact 24-hour storms is projected to increase by 4 to 8 inches (10 to 20 centimeters), an increase of more than 65%. This suggests extreme storms will become more intense and potentially damaging.

Under the low (B1) and moderately-high (A2) emission scenarios, the storms experienced today across all return intervals are projected to occur more frequently in the future.

Two-day and four-day precipitation events that are currently uncommon in the Mobile region will become more frequent by mid- and end-of-century, particularly under the low (B1) and moderately-high (A2) emission scenarios. The precipitation associated with these events is projected to increase significantly over time under all emission scenarios.

4.3. Implications for Transportation

While minor changes in the total annual levels of precipitation are not likely to affect transportation, increases in the magnitude and frequency of precipitation events can have significant local impacts.  These include the near-term consequences of heavy downpours as well as the longer-term damages associated with these events. More frequent and intense heavy precipitation events can cause flooding, mudslides, landslides, soil erosion, and result in high levels of soil moisture. These hazards can cause immediate damage during a rainfall event, necessitating emergency response. They also can undermine the structural integrity and maintenance of roads, bridges, drainage systems, and tunnels, necessitating more frequent repairs and reconstruction. The design of culverts and water receiving areas in vulnerable locations may need to accommodate greater capacity than current designs. Interestingly, an intense rain event after a period of very dry conditions can cause as much damage to assets and services as an intense rain event following a period of very wet conditions.  In the first case, the dry ground cannot absorb the water quickly enough and it runs off or pools, while in the second case, the ground is already saturated and cannot absorb additional precipitation, so the water again runs off or pools.

Flooding can render a route temporarily impassable, and require maintenance to clear mud and debris. The connectivity of intermodal systems – including goods movement to and from ports – can be disrupted even if short segments of roadways are flooded.  Severe precipitation can cause delays in air travel as aircraft are grounded or rerouted. Transportation agencies may need to fortify their emergency management and traffic management capabilities in anticipation of more frequent instances of heavy rainfall and associated response measures.

While these impacts are not new to transportation agencies, the frequency and severity of these problems are likely to increase as the incidence of extreme precipitation events rises. Managing damage and service disruption in real time may take more agency resources and require new communication channels and coordination protocols. Transportation agencies may need to consider preventive adaptation measures to increase the resilience of infrastructure (e.g., through design, operational improvements, and/or altered maintenance practices) and to prepare for additional emergency response needs associated with projected changes in precipitation patterns.

The implications of the precipitation findings detailed in this report on transportation assets and services in Mobile will be investigated in the next task of this study (Task 3: Vulnerability Screen and Assessment).

5. Observed Streamflow

Based on observed data from five stream gage sites in Mobile County, there is large variability in year-to-year peak streamflow events in the Mobile region. Over the past 58 years, annual peak streamflow at Chickasaw Creek has demonstrated a positive (increasing), but statistically insignificant, trend.

For the past 20 years, all Mobile area stream gage locations have demonstrated a similar pattern of annual mean stream discharge, similar to the pattern of annual peak streamflow. Average annual discharge at Chickasaw Creek from 1952 to 2010 has not changed significantly, suggesting the general characteristics affecting annual discharge have not changed.

Monthly mean discharge in the Mobile area is highest from February to April and lowest from October to November.

5.2. Projected Streamflow

5.2.1. Methodology

Monthly streamflow projections were developed using a monthly water balance model (WBM) driven by Mobile-specific information. The model estimates monthly runoff, evapotranspiration, and soil moisture within a basin or sub-basin using user-provided monthly precipitation and temperature data. The model was calibrated using streamflow data from three stream gage sites and meteorological data from the Coden and Mobile observations stations. The monthly runoff projections were translated to monthly discharge projections using the basin area for each stream gage.

5.2.2. Key Findings

During the summer months, monthly stream discharge is projected to decrease while actual evapotranspiration is projected to increase. During much of the winter and early spring months, monthly stream discharge is projected to increase.

Soil moisture is projected to decrease during the summer, particularly by the end of the century.

5.3. Implications for Transportation

It is unclear whether the projected changes in streamflow and soil moisture will have any significant impact on the vulnerability of transportation in Mobile.  The impact of these changes will be evaluated in the Task 3 vulnerability assessment.

More generally, the transportation implications of changing streamflow patterns are similar to those resulting from severe precipitation events, discussed above. Streamflow changes are likely to have the most significant effects on roadways, but may also impact rail lines; landside operations at ports; and facilities at airports, bus stations, and train terminals.  Changes in seasonal and monthly hydrology could require consideration of wetland performance. Erosion patterns may also be affected, necessitating more frequent maintenance and changes in vegetation management.

The implications of the streamflow findings detailed in this report on transportation assets and services in Mobile will be investigated in more detail in the next task of this study (Task 3: Vulnerability Screen and Assessment). At this point, additional analysis may be done to consider the effects on peak flow events, which can affect performance of culverts, ditches, and water runoff collection and treatment systems.

6. Sea Level Rise

6.1. Observed Sea Level Rise

Sea levels have been rising in the Mobile area. Based on observed data between 1966 and 2006, local sea level4 near Dauphin Island, Alabama rose approximately 0.12 inches (0.30 centimeters) per year while relative sea level in nearby Pensacola, Florida rose approximately 0.08 inches (0.20 centimeters) per year.

6.2. Projected Sea Level Rise

6.2.1. Methodology

To characterize future sea level rise in Mobile, a literature review of state-of-the-science studies was conducted to understand how global sea levels could change in the future. The estimates in the literature vary, and it is not possible to definitively say the extent to which the sea level will rise; however, using the range of projected changes in global sea level, three plausible global sea level futures were selected: 0.3 meters (1.0 foot) by 2050, 0.75 meters (2.5 feet) by 2100, and 2.0 meters (6.6 feet) by 2100.

These global sea level rise levels were then adjusted for local land subsidence and uplift rates in the Mobile area to determine the local sea level rise in Mobile. Then, a Geographic Information System (GIS) was used to map the inundation of Mobile County under each of these sea level rise scenarios, which include the vertical land surface change and the global sea level rise (GSLR) for 2050 and 2100.5 The analysis does not take into account vertical protective structures such as sea walls and levees, nor does it take into account pumping systems, since there are relatively few such structures and systems in Mobile County.

6.2.2. Key Findings

The analysis indicates modest subsidence over most, but not all of southeastern Mobile County, which will amplify the impact of projected global sea level rise. The magnitude of the rates of subsidence in Mobile County is generally expected to be less than the magnitude of global sea level rise. In turn, the amount of land inundated by sea level rise in the study region is expected to be much less than that temporarily inundated by hurricanes that occasionally strike the area.

The scenario of 0.3 meters (1.0 foot) of global sea level rise by 2050 could inundate the lowest lying land in the Mobile region. These areas include wetlands associated with some of the creeks that feed into Mobile Bay, as well as low-lying areas such as Gaillard Island, Terrapin Island, and parts of Dauphin Island. This level of inundation implies that short-term surges in water elevation due to relatively minor storms could lead to over-washing of the lowest lying coastal roads.

The scenario of 0.75 meters (2.5 feet) of global sea level rise by 2100 would exacerbate the impacts noted for the 0.3 meters scenario. Under this scenario, extensive flooding could occur across most of the wetlands at the head of the Bay and as far north as the wetlands to the east of Satsuma. The exposure of the area's roads and rail to short-term storm-related flooding will increase. The area at risk of flooding under this scenario would include low-lying areas north of downtown, west of the CSX rail yard, and east of Route 45.

Under the scenario of 2.0 meters (6.6 feet) of global sea level rise by 2100, coastal inundation would significantly shift the southern Mobile County shoreline northward and would inundate most of Dauphin Island. While parts of Dauphin Island are at an elevation above 2.0 meters (6.6 feet), these areas would still likely be at significant exposure to storm surge and may not survive severe storms. Sea level rise under this scenario would also lead to inundation of some of the lowest downtown and port waterfront areas.

6.3. Implications for Transportation

Sea level rise can permanently inundate certain coastal assets, rendering them unusable without adaptive measures. With the exception of ports, Mobile's critical transportation assets, as detailed in Task 1, are minimally exposed to sea level rise in the low- and mid-range scenarios of 0.30 meters (1 foot) and 0.75 meters (2.5 feet) of global sea level rise, respectively. In these scenarios, only 0 to 5% of critical assets of each mode are exposed. Under the high-range scenario of 2.0 meters (6.6 feet) of global sea level rise, exposure of critical assets of each mode ranges from 3 to 92%. A summary of the inundation of critical transportation assets is provided in Table 3.

Table 3: Percent of Critical Transportation Assets Inundated under Each Sea Level Rise Scenario
Sea Level Rise Scenario Roads (miles) Rail (miles) Pipelines (miles) Ports (#) Transit Facilities Mobile Downtown Airport (mi2)*
0.3 meters by 2050 4% 1% 1% 46% 0% 1%
0.75 meters by 2100 5% 2% 2% 69% 0% 2%
2.0 meters by 2100 13% 20% 3% 92% 50% 3%

*The other highly critical airport, Mobile Regional Airport, is not inundated under any sea level rise scenarios.

Inundation of small segments of coastal infrastructure can have broader implications if those segments are critical to the connectivity of the overall system.  Further, coastal assets that are not fully inundated could be affected by rises in sea level. For example, higher sea levels can increase the amount of shoreline erosion, thereby threatening coastal assets. Furthermore, higher groundwater levels can adversely affect pavement subgrade stability and stormwater system performance.

The interaction between sea level rise and storm surge is a critical consideration. Sea level rise exacerbates the vulnerability of infrastructure to storm surge, as higher water levels permit storm surge to travel farther into the County.

In addition to the direct effects of sea level rise on transportation infrastructure, the ecological impacts of sea level rise may have implications for transportation. The inundation of wetlands, for example, can destroy wetland mitigation efforts in which transportation agencies have invested. Further, inundation of natural coastal areas reduces the amount of ecological barriers - wetlands and marshes that absorb energy from tropical storms and hurricanes – that serve as buffer zones protecting populated areas.

Sea level rise is expected to be gradual, allowing time for assets to be protected or relocated. Dikes and levees, for example, can help protect transportation assets, and many assets can be completely relocated over time. However, such adaptive measures may require significant long-term planning and financial resources.

More information on the implications of the sea level rise findings detailed in this report as they relate to Mobile-specific transportation assets and services will be provided in the next task of this study (Task 3: Vulnerability Screen and Assessment).

7. Storm Events

7.1. Observed Storm Events

A variety of storm events affect Mobile including summer-time air-mass thunderstorms, tropical storms and hurricanes, and mid-latitude storms which may cause severe thunderstorms and/or heavy rains.

Mobile experiences frequent severe thunderstorms in the spring and fall, often accompanied by tornadoes. Key ingredients for these storms include a strong jet stream and warm, moist surface air.

Alabama experiences a storm originating in the tropics (e.g., tropical storm or hurricane) approximately every 1.5 years. Hurricanes strike the state about every 7.5 years, with a direct hit by a hurricane occurring approximately every 16 years.

These storm events can be destructive, causing flooding, downed power lines, and other infrastructure damage. To better understand the characteristics of recent extreme storms in Mobile, a case study analysis was conducted. The case studies provide an understanding of the current weather hazards that affect transportation planning and design.  The case study analysis identified the key ingredients responsible for fueling each storm, serving as a basis for understanding how changes in climate may alter these ingredients and thereby influence future storm development. Information on reported damage from each storm was also recorded in the case studies; this information will help inform the vulnerability assessment in later stages of the project.

7.2. Projected Storm Events

7.2.1. Methodology

In this study, information on future storm events focused on projected changes in hurricane activity. Projected changes in hurricanes were developed using two techniques:

A literature review was conducted to investigate scientific projections of storm-related atmospheric phenomena known to be important for storm development in Mobile. For example, this literature review investigated projected changes in the frequency and intensity of tropical storms and hurricanes.

A scenario-based analysis of storm surge from hurricanes was also conducted; this analysis sought to answer two main questions:

To answer these questions, the storm surge inundation from 11 storm scenarios was modeled. These 11 scenarios were developed using Hurricane Georges and Hurricane Katrina—two damaging storms that affected Mobile in recent history—as base storms, and then adjusting certain characteristics of the storm parameters to simulate what could happen under alternate conditions. For the Georges simulations, all three sea level rise scenarios were examined. For the Katrina simulations, the modeling considered different adjustments, including shifting the path of Katrina so that it hit Mobile directly, intensifying the storm, and adding in 0.75 meters (2.5 feet) of sea level rise.

Simulations of storm-induced water levels (i.e., storm surge) were performed using the ADvanced CIRCulation model (ADCIRC). The ADCIRC storm simulations were driven by meteorological forcing data extracted from six-hour advisory forecast and observation reports issued by the NOAA National Hurricane Center (NHC). The wave characteristics accompanying each of the storm surge scenarios were simulated using STeady State spectral WAVE (STWAVE).

7.2.2. Key Findings
Literature Review Findings

The literature review suggests that Mobile may experience less mid-latitude cyclonic activity (e.g., severe thunderstorms) as the jet stream moves northward in response to a warming climate, but that this decrease in activity may be compensated by an increase in the intensity and/or frequency of extreme localized convective activity.

Based on the literature, it is difficult to predict the impacts of climate change on tropical cyclone activity as increasing vertical wind shear would reduce the development of tropical cyclones, while increasing sea surface temperatures could increase their intensification. Though this is an active area of debate among scientists, a scientific consensus report suggests that the future may bring a reduction in the frequency of hurricanes but an increased intensity of those hurricanes that do form.

Storm Surge Analysis Findings

The general magnitude of flooding from storm surge, even by the "natural," unadjusted Hurricanes Georges and Katrina, exceeds the inundation from even the most extreme sea level scenario (2 meters) considered in this report. In other words, the land area temporarily affected by the surge from even moderate hurricanes is greater than the land area affected by the upper bounds of likely sea level rise over the 21st century.  Flooded areas under these natural storm scenarios include all of the coastal wetlands in Mobile County; low-lying areas along the waterfront and ports; as well as Gaillard Island, Terrapin Island, and nearly all of Dauphin Island.

The analysis of a strike by a larger hurricane than the region has experienced in recent history produced significantly increased storm surge. If Hurricane Katrina both shifted so that it hit Mobile directly and sustained its maximum wind speed through landfall, the surge at the Mobile Docks is estimated at 27.65 feet (8.38 meters). In this case, nearly all of the land to the east of I-65 would become flooded. Moreover, waves could affect structures more than 10 meters above sea level, including the downtown airport runways and hangars. In the most intense scenario ("shifted" Katrina, 0.75m SLR, MaxWind), the surge at Mobile Docks is estimated at 31.02 feet (9.40 meters) and the inundation impacts would be correspondingly greater.

Relatively speaking, sea level rise made a modest impact on the degree of inundation. Increased intensification of storms appears to be a much more significant driver in terms of amount of land inundated. However, inundation from sea level rise is permanent and affects groundwater levels, causing lasting effects. In contrast, inundation from storm surge, though damaging, tends to be temporary and repairable.

See Table 54 and Table 55 in Appendix D.9 for supplemental statistics of transportation modes inundated under these scenarios, including number of transit stops, miles of evacuation routes, and other metrics.

7.3. Implications for Transportation

Storm surge can have very significant impacts on transportation systems, rendering them unusable for the duration of the surge (lasting several hours or more). Critical facilities – including roads, bridges, rail lines, airports and ports – may be unusable, or exhibit reduced capacity, even after the waters recede due to damage to transportation assets, supporting infrastructure (e.g., utilities and telecommunications), or access routes. Damage can range from debris removal to complete destruction of certain assets. The direct costs of clean up, repair, and replacement can be high, and the secondary implications of disrupted transportation networks and supply chains can have widespread impacts on community life, and on the local and regional economy.

Table 4 shows the percent of critical transportation assets inundated under each storm surge scenario. Based on fractional extent of exposure, critical port facilities are most exposed to storm surge, with 92% to 100% of critical port facilities inundated, depending on the scenario. Critical rail lines are also highly exposed due to their coastal location, with 57% to 80% of critical rail-miles inundated. In contrast to the port facilities, pipelines have the lowest fractional extent of exposure, with 3% to 16% of pipeline-miles exposed.

Exposure varies for critical roadways. In the lowest surge scenario, 27% of the critical roadway length is exposed, whereas in the most extreme scenario, 75% of the critical roadway length is exposed. Importantly, even in the lowest scenario, many of the key evacuation routes are affected.

One of the two critical transit facilities, the GM&O Transportation Center, is inundated under all storm scenarios. Of the two critical airports, only Mobile Downtown Airport is inundated under any of the storm surge scenarios. Under the lowest storm surge scenario, 4% of the airport's surface area is inundated. Under the highest storm surge scenario, the entire airport is inundated. Only when the track of Katrina is shifted would key aspects of the airport's operations be exposed to inundation.

Table 4: Percent of Critical Transportation Assets Inundated under Each Storm Scenario
Storm Scenario* Roads (miles) Rail (miles) Pipelines (miles) Ports (#) Transit Facilities** Mobile Downtown Airport (mi2)
Georges-Natural 27% 57% 3% 92% 50% 4%
Georges-Natural-30cm SLR 28% 59% 3% 92% 50% 5%
Katrina-Natural 28% 60% 3% 92% 50% 5%
Georges-Natural-75cm SLR 30% 62% 6% 92% 50% 7%
Katrina-Natural-75cm SLR 33% 66% 10% 92% 50% 9%
Katrina-Shift 46% 72% 12% 92% 50% 65%
Georges-Natural-200cm SLR 40% 68% 12% 92% 50% 15%
Katrina-Shift-75cm SLR 55% 74% 13% 96% 50% 90%
Katrina-Shift-ReducedPress-75cm SLR 60% 76% 13% 96% 50% 98%
Katrina-Shift-MaxWind 67% 78% 15% 100% 50% 100%
Katrina-Shift-MaxWind-75cm SLR 75% 80% 16% 100% 50% 100%

*Scenarios are presented in the order of least to greatest inundation of critical roads.
** Only two transit facilities were identified as critical. The GM&O facility downtown is inundated under all storm scenarios, while the Beltline facility is not inundated under any, leading to a 50% exposure statistic for all scenarios.

The extent of inundation of critical transportation assets from storm surge is much greater than exposure from long-term sea level rise. While potentially highly destructive, the duration of the exposure to surge is limited, whereas sea level rise is more likely to be gradual and more widespread. Sea level rise compounds the severity of storm surge. The prospect of more frequent and more extreme storm events increases the adaptation burden on transportation.

Additional information on the implications of the storm surge findings detailed in this report on Mobile-specific transportation assets and services will be provided in the next task of this study (Task 3: Vulnerability Screen and Assessment).

8. Applications of the Information in this Report

8.1. Assessing Vulnerability in Mobile

Understanding Vulnerability

Vulnerability = f(exposure, sensitivity, adaptive capacity)

  • Vulnerability is the degree to which an asset is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes
  • Exposure is the nature and degree to which an asset is exposed to significant climatic variations;
  • Sensitivity is the degree to which an asset is affected, either adversely or beneficially, by climate-related stimuli
  • Adaptive capacity* is the ability of a system (or asset) to adjust to climate change to moderate potential damages, to take advantage of opportunities, or to cope with the consequences

Source: IPCC, 2001

* “Resilience” is sometimes used interchangeably with the term “adaptive capacity;” but is also used in the adaptation literature as a term related to, but distinct from, adaptive capacity; and sometimes as a concept representing the opposite of vulnerability. To avoid the confusion associated with the term “resilience,” this study exclusively uses the term “adaptive capacity.”

The climate information developed in this report will inform a climate change vulnerability assessment. Looking at the transportation assets deemed "Highly Critical" in the first task of Phase 2 of the Gulf Coast Study, the exposure to future climate effects will be considered, using the climate information developed in this report. Then, sensitivity of assets to those exposures will be considered. Adaptive capacity will also be addressed during the vulnerability assessment, but was not addressed in this report.

Together, the evaluation of exposure, sensitivity, and adaptive capacity of the critical transportation assets will provide insight into the larger scale vulnerabilities of Mobile's transportation system to climate change. The study aims to both identify highly vulnerable assets on an individual level, as well as develop an overarching understanding of the vulnerabilities of the transportation system as a whole.

8.2. Informing Similar Work Elsewhere

There are a number of other transportation climate change vulnerability assessments underway across the nation. As this work is among the earliest and most in-depth, the findings and lessons learned may help inform those efforts going forward.

For example, the USDOT has recently funded two sets of climate change vulnerability assessment pilots. The Federal Highway Administration (FHWA) funded the first set of five pilots. The pilot studies were designed to test and improve a draft framework for conducting vulnerability assessments of transportation assets and services, with a primary focus on highway assets. The Federal Transit Administration (FTA) is funding a second set of pilots aimed at transit assets and services. These pilot studies will build upon lessons learned through the FHWA pilots and findings in Phase 2 of this study.

There are a number of organizations and partnerships underway in the Gulf Coast area that are aimed at understanding the impacts of climate change to Gulf Coast communities, and promoting ways to increase the resiliency of the communities. The project team is actively engaging with these organizations to encourage information sharing and to leverage local knowledge.

Finally, an important goal of Phase 2 of the Gulf Coast Study is to develop tools and resources that will assist MPOs generally and other transportation agencies in conducting additional analyses. The processes and lessons learned throughout this report will help inform development of these resources. The tools and resources ultimately developed under this project will reduce the barriers to conducting similar analyses at local scales across the US.


1 Available at http://www.fhwa.dot.gov/environment/climate_change/adaptation/ongoing_and_current_research/gulf_coast_study/ 

2 There are other climate and weather effects that can be affected by climate change, and that may even have the potential to affect transportation, but were not included in this study because their anticipated effect on transportation is relatively low, or because of resource or technical limitations.

3 "Significant" changes were identified using a statistical test (a paired t-test). See Appendix C.3.2 for a description of the paired t-test.

4 Local sea level rise is due to local or regional factors such as land uplift and subsidence from shifting local tectonics and changes in the amount of fluid in sediment pores; sedimentation and erosion adding or subtracting the amount of sediment at a particular location; gravitational changes; changes in oceanic and atmospheric circulation patterns; and changes in ocean density due to changes in salinity and temperature, in addition to global sea level rise.

5 Potential inundation due to long-term sea level rise is presented relative to Mean Higher High Water.

Updated: 10/31/2014
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