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

Methodology

Developing the Sensitivity Matrix

The Sensitivity Matrix was developed in a six-step process:

Step 1: Define the climate variables. The climate variables included in the Sensitivity Matrix were selected based on a general understanding of potential future changes in climate in the Mobile region. The five climate variables that were initially considered are: incremental relative sea-level rise, waves and storm surge, wind, precipitation (incremental changes in precipitation, heavy rain events, drought), and temperature (incremental changes in temperature, extreme heat events). However, based on the early analyses, the initial interviews with Mobile County transportation experts, and the results of the Gulf Coast Study Phase I, this set was narrowed to: waves and storm surge (including consideration of relative sea-level rise), wind, precipitation, and temperature. All climate variables were included in the Sensitivity Screen.

Step 2: Define transportation modes and sub-modes. There are several sub-modes defined within each of the six transportation modes (bridges; roads and highways; railroads; airports and heliports; oil and gas pipelines; and marine ports, terminals, and waterways) plus electricity. These sub-mode categories were broad enough to cover the wide range of assets and components within each category, but at a level of detail sufficient to identify climate stressor relationships on specific classes of assets. These modes and sub-modes included the following:

Step 3: Establish impact-asset relationships. This step identified the relationships between climates stressors and their impacts on specific assets. For each of the sub-modes isolated in Step 2, relevant relationships with each of the four climate variables were qualitatively described. This information was populated in the Matrix, using a combination of historical data, case studies, literature, design standards and guidelines, and expert consultations.

Step 4: Identify thresholds. Although the relationships between climate stressors and sub-modes are described qualitatively in the Sensitivity Matrix, sub-modes often have thresholds above which climate stressors are likely to cause significant damage. Drawing upon information from design standards and guidelines, and historical data and case studies from previous weather events, known thresholds were identified.

Step 5: Develop region-specific detail. In this step, information from historic data and case studies in the Mobile region was incorporated into the Matrix. This information allowed the evaluation of the sensitivity and, in some cases, exposure of specific transportation in Mobile to the impact-asset relationships described in the Matrix. For example, using data on the elevation of airport runways in Mobile airports, it was possible to evaluate the exposure of local airports to impacts from storm surge and waves. Similarly, based on damage assessments from Hurricanes Andrew, Ivan, and Katrina, the historical sensitivity exhibited by bridges, highways, and oil and gas pipelines in the Mobile area to these events was assessed.

Step 6: Identify indicators of sensitivity. Finally, the information summarized in the Matrix was evaluated to identify potential indicators of sensitivity. These indicators are design-related features that are relevant to the sensitivity of an asset.2 In combination with information on thresholds, indicators of sensitivity could help assess whether assets within a specific category may be more or less sensitive to projected climate stressors.

Steps 3 through 5 relied heavily on three primary sources of information: expert consultations, design standards, and analogues from historical data and case studies. Each data source is described in sub-sections below.

Expert Consultations

The sensitivity analysis began by identifying and consulting key transportation mode experts inside Mobile County and elsewhere. These meetings proved to be integral to the rest of the analysis since the modal experts were able to quickly identify locally important hazards and also describe the nature of the causal relationships. Many of the state and county engineers consulted seemed to be most concerned and informed about the hazards posed by heavy precipitation events. In contrast, there was very little concern expressed about increased temperatures or heat waves.

Design Standards and Guidelines

Transportation infrastructure is designed according to standards and guidelines that are based on detailed sets of empirical tests, asset-specific quantitative models, and other engineering analyses. Engineers use these documents to ensure that a design meets the functional specifications of a project within accepted limits and factors of safety. Design standards and guidance consider relevant climate variables-both typical climate characteristics in which the design will operate, as well as infrequent events such as violent storms, floods, and hurricanes, whose occurrence is estimated through design return periods for each hazard. As a result, design standards and guidelines can be used to isolate specific climate stressors relevant to a particular asset. The relationships provided within design standards can also be used to provide quantitative indicators of an asset's sensitivity to a particular climate stressor. For example, the ASCE 7-05 3-second gust basic wind speed for the Mobile region is between 130 and 150 mph, depending on proximity to coastal areas; ASCE 7-10 recommends a three-second gust wind speed of 140-175 mph, depending on the structure's risk category (ATC 2012). Although other parameters influence building design-such as the Occupational Category of the building-this wind speed threshold provides an indication of the operating conditions that are used in the design of buildings and other structures, and can be used to identify whether there may be potential sensitivities that should be investigated in more detail.

Standards and guidelines can be useful in identifying the thresholds or limits of a design, beyond which damage is likely to occur. For example, when determining the low chord elevation of bridges, engineers use numerical and computational tools that roughly estimate storm surge and wave height associated with a storm of a particular return period (e.g., a 50- or 100-year storm). While assets tend to have a factor of safety built into the design, climate hazards that exceed the design limits can cause damage. In addition, climate change may change the probabilities of the design return periods for hazards; for example, the probability of what is currently considered a 50-yr storm could change.

Analogues

Analyzing historical data and case studies from other areas along the Gulf Coast provided some of the most-detailed information on sensitivity for this analysis. For example, a significant body of research exists on the impacts of Hurricane Katrina on transportation in the Gulf Coast. During the sensitivity assessment, the existing literature on hurricane damages to Mobile and surrounding regions was analyzed in order to detect any patterns in damage. For example, studies investigating the performance of the I-10 Twin Bridge as compared to the neighboring U.S. 11 and railroad bridges over Lake Pontchartrain during Hurricane Katrina provided evidence that certain features of the superstructure (girders and diaphragms) can influence the sensitivity of bridges to storm surge. The impacts documented for Hurricane Frederic-the most devastating hurricane to have struck Mobile in the past several decades-were also important in the development of the Sensitivity Matrix.

Developing the Sensitivity Screen

The Sensitivity Screen is a complementary tool developed from the Sensitivity Matrix. Although the Matrix contains a wealth of information on relationships, thresholds, Mobile-specific details, and indicators of sensitivity, it is difficult to quickly apply this descriptive information to evaluate which assets may exhibit sensitivity to climate stressors. The Sensitivity Screen extracts information from the Matrix to show two layers of information in an accessible, easy-to-use format. The Sensitivity Screen consists of:

  1. A grid listing the climate variables in columns, and the transportation modes and sub-modes in rows;
  2. A color-coded mapping of the sub-modes that were found to exhibit sensitivity to specific climate stressors in the Sensitivity Matrix, and
  3. A layer of quantitative information on climate variable thresholds above which impacts may be more severe (also extracted from the Sensitivity Matrix).

Together, these elements allow planners and decision makers to: (i) screen out sub-modes that were not found to exhibit sensitivity to certain climate stressors in the Sensitivity Matrix, and (ii) layer climate projections onto the screen to identify where future climate variables are likely to exceed current thresholds.

The method used to assemble the Sensitivity Screen is depicted in Figure 3. The color-coded mapping of sub-modes that exhibit sensitivity to certain climate stressors distinguishes between two levels of sensitivity: (i) sensitive (color-coded in orange), and (ii) not sensitive. Sub-modes that have orange-shaded cells were affected-either adversely or beneficially-by the stressors corresponding to each climate variable. Grey-shaded cells denote sub-modes that were not affected by the climate stressor. On top of this information is a layer of quantitative data on climate thresholds above which impacts may become more pronounced.

Figure 3 : Illustrative Framework for Developing the Sensitivity Screen

This figure illustrates the framework for developing the Sensitivity Screen. The first component of the screen is the background sensitivity matrix grid, with the modes and sub-modes of transportation assets forming the rows and climate stressors as the columns. Layer 1 flags with a color code if a given sub-mode is sensitive to a given climate stressor. Layer 2 is the climate stressor thresholds, showing quantitative data on climate thresholds above which impacts may become more pronounced.

Applying the Sensitivity Screen and Matrix: Practical Tools for Planners

Understanding which asset types and subtypes are sensitive to certain climate stressors is essential to helping planners evaluate and prepare for the risks of future climate changes. The Sensitivity Screen was developed to convey the results from the Sensitivity Matrix in a format that can be used as a tool to support rapid screening and vulnerability assessments. The Sensitivity Screen is shown in Table 2.

Once planners have identified the critical transportation assets under their jurisdiction, they use the Sensitivity Screen to preliminarily identify which critical assets are not only important, but also sensitive to particular climate stressors. This step will screen out those assets that are not sensitive, thereby leaving a more manageable list of assets. This screening step can also be used to identify cases where individual assets are sensitive to multiple climate stressors, indicating the potential for compounding effects. The assets that are screened out using the Sensitivity Screen and Matrix should be monitored and revisited over time as resources allow.

Next, planners can use the Sensitivity Matrix to better determine which of the preliminarily identified assets may be impacted by climate change in a subsequent, rigorous vulnerability assessment. The Matrix provides information at a level of detail intermediate between the Screen and detailed design, performance, and maintenance information possessed by individual modal engineers. One of the particular advantages of the Matrix is that it provides cross-modal information to help focus subsequent detailed engineering analyses of potential impacts. This focusing can be done in part through the Matrix's information on indicators of sensitivity that can be used to bore down from sub-mode classes to individual assets to assess their sensitivity. The indicators information can also be used to help initiate consideration of adaptation measures that might be taken to reduce sensitivity and thus vulnerability. The threshold information contained within the Matrix also provides a resource that can be used by transportation planners as they review their design standards, particularly the "100-year storm" in light of changing climate conditions and consequent exposure of transportation assets.

In a comprehensive climate change vulnerability assessment, the critical assets that passed through the Screen should subsequently be assessed for exposure to climate stressors, and their adaptive capacity. Assessments of this type are being carried out in other parts of the Gulf Coast Phase 2 project. The previous section of this report discussed the assessment of the exposure of critical and sensitive assets to storm surge, waves, and long-term inundation due to sea-level rise and subsidence/uplift. The upcoming vulnerability assessment is combining the exposure, sensitivity, and adaptive capacity of critical assets to qualitatively assess vulnerability. These analyses will determine conditions under which the thresholds identified in the Sensitivity Screen are crossed, and the conditions under which other key aspects identified in the Matrix are encountered. The results from these steps will be used to prioritize a short list of assets/stressors for detailed engineering analysis in later efforts under this project.


1 Electrical power systems and services are a separate sector from transportation modes. Electrical power systems were included in this analysis because the reliance of transportation modes on electrical systems can be a determining factor for sensitivity; however, this sector was not analyzed in sufficient detail to distinguish between sub-categories.

2 In some cases, features were included that influence the exposure of assets to climate stressors. For example, the use of flood protection (e.g., dikes, retaining walls) to shield runways from waves and storm surge.

Updated: 03/27/2014
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