Sea level rise can permanently inundate coastal transportation assets damaging infrastructure, leading to corrosion, and potentially rendering certain coastal infrastructure unusable without adaptation actions. Rises in sea level can also magnify the surge associated with storm events. Storm surges can cause immediate flooding and both horizontal and vertical coastal erosion.1 Damage to transportation infrastructure can be caused by the force of the water and from collisions with debris.2 Impacts from storm surge are discussed in greater detail in Section 7.
Over the 20th century, global average sea-level has risen by a total of 6.7 inches (0.17 meters) with recent observations suggesting an accelerated increase in the average rate of sea level rise.3 Along most of the Atlantic and Gulf coasts, relative sea-level has risen at a rate of 0.8 to 1.2 inches (2.0 to 3.0 centimeters) per decade during the 20th century. Along the Louisiana coast, relative sea level has risen at an even faster rate of a few inches per decade, due to relatively rapid land subsidence.4
As sea-level rises, the zone impacted by erosion and damage from inundation, storm surges, and waves expands inland. Since the 1970s, half of coastal Mississippi and Texas has experienced shoreline erosion of 8.5 to 10.2 feet (2.6 to 3.1 meters) per year.5 Louisiana has experienced even more significant erosion of 39 feet (12 meters) per year.6
In this section, the methodology for evaluating observed sea level measurements for the Mobile region and sea level rise projections is provided followed by a description of key findings.
Additional detail about the sea level rise analyses is available in the appendices.
This section describes the analysis of observed sea level rise based on observed data.
This section describes the methodology used to analyze observed sea level rise data in the Mobile region. The analysis of observed sea level is based on data from two regional stations. Data was collected from the NOAA Tides and Currents program for the Dauphin Island station8 and the Pensacola, FL station.9 Pensacola, FL is located about 60 miles (97 kilometers) from Mobile, AL, sea level data at Dauphin Island and Pensacola stations demonstrate similar trends of rising sea level over time. The Pensacola station was included as it provides a robust long-term data record from 1923 to present and is considered to be the most stable area along the Gulf.10
This section describes key findings from the analysis of observed sea level rise.
Key Findings for Historical Sea level Rise
Sea levels have been rising in the Mobile area. Based on observed data from 1966 to 2006, mean annual local sea level at Dauphin Island has increased 0.12 inches per year (2.98 millimeters/year) and mean annual local sea level at Pensacola has increased 0.08 inches per year (2.03 millimeters/year). For the entire Pensacola record (1923-2006), mean annual local sea level has risen at 0.084 inches/year (2.10 millimeters/year) (see Table 19). This locally observed rate of sea level rise is greater than the global average. Globally, sea level increased approximately 0.07 inches per year (1.7 mm/year) during the 20th century, and more than 0.14 inches per year (3.5 mm/year) since 1993.11
|1966 to 2006||1923 to 2006|
|Dauphin Island||0.12 inches/year||Not available|
|Pensacola||0.08 inches/year||0.084 inches/year|
Figure 51 illustrates the change in annual mean local sea level for Dauphin Island and Pensacola. Annual mean local sea level was estimated from monthly mean sea level data records with regular seasonal fluctuations removed. The general trends in mean annual sea level over time are consistent between the two stations, with the rate at Dauphin Island approximately 0.04 inches/year greater than at Pensacola. If one assumes a GSLR of 0.071 inches/year, then the local influence on RSLR is approximately -0.012 inches/year at Pensacola and -0.047 inches/year at Dauphin Island.12
Figure 51: Change in the Annual Mean Local Sea level for Dauphin Island, Alabama and Pensacola, Florida
Variability in local sea level is affected by many factors (see Factors Affecting Local Sea Level Rise text box).13 Many of the peaks shown in this time series likely reflect the inter-annual variation in sea level due to global ocean phenomena such as the El Niño-Southern Oscillation (ENSO).14 Peaks may also reflect unusual seasonal weather patterns (precipitation, temperature, and runoff), and years with increased storms and waves.15 The long-term trend that is visible through the "noise" in the data record provides a baseline of relative sea level changes experienced to-date.
Factors Affecting Local Sea level Rise
Sea level rise (SLR) does not happen uniformly across the globe. Since this study is focused on the local scale, it is important to consider potential local sea level rise (LSLR) scenarios for Mobile, AL. There are a number of factors that contribute to changes in local sea level including:
These latter five factors can vary at regional and/or local scales. In addition, inter-annual variability and episodic events such as storms and precipitation can affect year-to-year sea level.
This section describes the methodology and key findings for the analysis of future sea level rise in the Mobile region.
The approach first identified possible levels of global sea level, and then adjusted these levels based on local subsidence and uplift of land to estimate changes in relative sea level.
This approach relied on selection of multiple plausible future scenarios of sea level, as precise levels of sea level rise cannot be predicted. Increases in atmospheric greenhouse gas concentrations are linked to future changes in global sea level. However, there is a large amount of uncertainty associated with quantitatively estimating those changes. Therefore, a set of plausible sea level rise scenarios for Mobile, Alabama were explored. A scenario-based analysis is a standard approach in the face of "deep uncertainty" associated with environmental or other challenges relating to future conditions. The scenarios used in this analysis, which are reflective of the state-of-the-science, are not predictions. Rather, the scenarios represent conditions that may occur, thereby encompassing a representative range of possible future conditions.
Once global sea level rise scenarios were selected, they were adjusted to reflect local uplift and subsidence, and then state-of-the-art quantitative models were used to assess the inundation of Mobile under each scenario.
Two of the six factors affecting local sea level rise (see text box above) were considered in this study: global sea level rise and changes in local land elevation. These two factors are likely to have a strong influence on local average sea level by the end of the century. Local sea level rise (LSLR) was estimated by simply adding the current rates of subsidence or uplift to each global sea level rise scenario.
The other four factors were not considered in this study because they were not considered to likely significantly impact the results, or due to resources constraints. For more information on why other factors were not considered, see Appendix D.1.This section describes the methodology used to characterize future sea level rise in Mobile. This analysis included a literature review and selection of global sea level rise scenarios, adjustment of those GSLR scenarios for local subsidence and uplift to estimate LSLR, and in-depth inundation mapping for each of the LSLR scenarios.
To identify plausible future levels of global sea level, a literature review was conducted. The findings of this literature review are discussed below.
Climate change may increase global sea level through two dominant pathways: by the melting of land-based ice caps and glaciers and by thermal expansion of ocean waters due to increasing temperatures. By 2100, the IPCC projects an increase in sea level of 0.6 feet to 1.9 feet (0.18 to 0.59 meters) in response to rising temperatures. This projection accounts for thermal expansion and the melting of glaciers and ice caps but what is now recognized as a low rate of loss16 for the ice sheets. However, satellite observations suggest ice sheets are already becoming affected and recent studies suggest that sea level rise could be much greater than projected by the IPCC in 2007.17
According to the National Research Council, land ice loss is expected to accelerate as temperature increases, leading to a GSLR of 1.6 to 3.3 feet (0.5 to 1.0 meter) by 2100, with the possibility of up to 5.3 feet (1.6 meters).18 This range is conservative compared to other recent studies that estimate sea level rise of up to 2.0 meters (6.6 feet) by 2100. Other estimates of sea level rise range from 2.6 to 6.6 feet (0.8 to 2.0 meters),19 1.6 to 4.6 feet (0.5 to 1.4 meters),20 and 3.2 to 5.1 feet (0.97 to 1.56 meters).21 This large range is indicative of the considerable scientific uncertainty associated with estimating GSLR.
These studies, viewed collectively, demonstrate both the large potential for sea level rise in the future and the large uncertainty associated with current understanding of ice dynamics.
Global Sea Level Rise
Based on these recent estimates of global sea level rise, this study uses a sea level rise estimate that falls in the middle of the NRC estimates, i.e., 0.75 meters (2.5 feet) by 2100. Using the precautionary principle22 as a guide, this study also explores the implications of sea level rise of 2 meters (6.6 feet) by 2100. In addition, the study considers a rise of 0.3 meters by 2050, which corresponds approximately to the scenario of 0.75 meters (2.5 feet) by 2100, assuming a linear trend. Due to the significant, aforementioned uncertainty, relative probabilities are not assigned to these values.
If, at a later date, the science indicates that GSLR may be above or below these values, the findings presented below are still useful. Different dates can be roughly assigned to each GSLR scenario. For example, if sea level rises much more slowly than anticipated, the 0.3 meter (1.0 foot) scenario could be assumed to occur in 2100.23 Similarly, if sea level rises more rapidly than anticipated, the 0.75 meter (2.5 feet) scenario could be assumed to occur in 2050.24 The main caveat with doing so would be that the rates of subsidence/uplift would be mismatched with the years. However, subsidence/uplift in the study region is anticipated to have a less significant effect on SLR over the 21st century than other factors, so this mismatch would not constitute a major problem, and the use of these scenarios would still provide a general sense of the assets that would be exposed to LSLR.
The literature does not provide estimates of local sea level rise projections, specific to Mobile. Therefore, GSLR rates were modified to account for vertical land motion, as discussed below. As already noted, LSLR is determined not only by global changes in the ocean's volume, but also by local changes in land elevation due to geological plate movement, extraction of underground water and resources, and other factors. For example, if global sea level rises 0.1 meters, and the land elevation also rises 0.1 meters over the same time period, then the LSLR would be zero. Conversely, if the land were to subside by 0.3 feet (0.1 meters) while global sea level rose by 0.3 feet (0.1 meters), the LSLR would be 0.7 feet (0.2 meters).
Many areas of the Gulf coastal zone are subsiding due to geological faulting and compaction of sediment resulting in part from groundwater withdrawal.25 However, the rate of subsidence is not uniform. In some places, uplift is occurring (see below). To better evaluate the impacts of sea level rise, the US Geological Survey (USGS)26 assisted with an analysis of the added effects of vertical land motion to the GSLR scenarios, to provide a more accurate estimate of projected local sea level rise in Mobile.
USGS estimated subsidence and uplift rates using Interferometric Synthetic Aperture Radar (InSAR) data together with a series of stable survey benchmarks and tide gages. The approach is discussed in more detail in Appendix D.2. In summary, the InSAR data provided vertical movement data for most of the study area, while the benchmark data helped to augment the InSAR data outside the spatial domain of the InSAR data.27 InSAR data were used where possible, because they are spatially continuous and possess relatively high accuracy. A spatially complete data set of vertical motion from these two datasets was arithmetically added to a high resolution Digital Elevation Model based on LIDAR data28 to estimate the vertical position of the land surface out to 2050 and 2100.
A Geographic Information System (GIS) was used to map all locations below the prescribed GSLR scenarios that are subject to potential inundation from LSLR.29 Then, GIS was used to overlay inundation under each of the sea level rise scenarios on top of the critical assets defined in Task 1 of the Gulf Coast Study. This analysis considered the bare earth elevation of assets that is, the elevation of the land on which the assets sit. It did not consider the height of the assets themselves.
This section presents the results of the sea level rise analysis, including inundation maps for the three selected scenarios. The LSLR results shown here are relative to Mean Higher High Water (MHHW).
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.
Vertical change rates for the 75 benchmarks considered by USGS ranged from -0.08 to 0.02 inches per year30 (-1.9 to 0.5 millimeters per year) with a mean of -0.03 inches per year (-0.75 millimeters per year) and a standard deviation of 0.02 inches per year (0.42 millimeters per year) (see Appendix D.2 for details). The 198,129 InSAR data points plotted in Figure 5231 had vertical change rates ranging from -0.2 to 0.2 inches per year (-5.1 to 3.6 millimeters per year) with a mean of -0.02 inches per year (-0.60 millimeters per year) and standard deviation of 0.03 inches per year (0.82 millimeters per year). Due to the large number of InSAR data points, each individual value is not shown in this report (see Figure 52). Table 46 in Appendix D.3 provides the vertical land surface rates for benchmark surveys and corresponding InSAR data in Mobile and Baldwin counties.
Figure 52: Vertical Land Motion for Mobile and Baldwin
To place these vertical land surface change estimates into context, the maximum subsidence rate from the benchmark data (-1.9 millimeters per year) would produce a total subsidence of 0.17 meters (6.6 inches) by 2100. With the exception of Dauphin Island, which has the study area's greatest subsidence rate, the contribution of subsidence and uplift to LSLR in Mobile County are relatively minor in relation to the effect of GSLR (e.g., the scenarios of 0.3 – 2.0 meters explored in this study).
Although the results presented here are preliminary, underlying analysis indicates that they are more accurate than prior analyses. However, it would be useful in subsequent scientific studies to check and revise these estimates as future benchmark survey data, Continuously Operating Reference Stations (CORS) data, and InSAR or other satellite data become available.
Figure 53, Figure 54, and Figure 55 illustrate the inundation resulting from the three inundation scenarios. Under the scenario of 0.30 meters (1.0 foot) GSLR by 2050, LSLR inundates the lowest lying land in the Mobile region (see Figure 53). These areas include wetlands associated with some of the creeks that feed into Mobile Bay. This includes wetlands in the Chickasaw area, to the east of Tillman's corner, and near Fowl River, as well as the lowlands along the southern mainland coast of the county. Low-lying areas also include 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.
Under the scenario of 0.75 meters (2.5 feet) GSLR by 2100, LSLR exacerbates the impacts noted for the 0.3 meters (1.0 foot) scenario (see Figure 54). One particularly dramatic change is the extensive flooding that occurs across most of the wetlands at the head of the Bay and as far north as the wetlands to the east of Satsuma. However, this result hinges on the assumption of no change in vertical accretion32 by the wetland. Regardless, the exposure of the area's roads and rail to short-term storm-related flooding will increase since the still-water33 table will be closer to the elevation of current road surfaces.34 The area at risk of flooding under this scenario would also include low-lying areas north of downtown, west of the CSX rail yard, and east of Route 45.
Inundation from sea level rise under the scenario of 2.0 meters (6.6 feet) GSLR by 2100 significantly shifts the southern Mobile County shoreline northward and inundates most of Dauphin Island (see Figure 55). This finding assumes no natural or human-generated vertical accretion. 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. This scenario may also cause the shoreline to migrate north of I-65, if there is no vertical accretion from either natural or human sources. The approximately 0.8 inches per year (20 millimeters per year) rate of LSLR implied by this scenario would make it more difficult for natural vertical accretion to keep pace than under the 0.75 meter (2.5 feet) scenario. It would also lead to inundation of some of the lowest areas in the downtown and port waterfront.
Figure 53: Potential Inundation with Global Sea level Rise of 0.3 meter by 2050 36
Figure 54: Potential Inundation with Global Sea level Rise of 0.75 meter by 2100
Figure 55: Potential Inundation with Global Sea level Rise of 2.0 meter by 2100
The approach to sea level rise mapping used here is appropriate for initial exposure assessment. However, several factors affecting sea level rise were not taken into account. For example, vertical addition or subtraction of sediment through coastal engineering, changes in the vertical accretion rate of wetlands, and small-scale protective barriers were not taken into account. For a more thorough account of the caveats, gaps, and replicability of this study, see Appendix D.5.
Sea level rise can permanently inundate certain coastal assets, rendering them unusable without adaptive measures. Inundation of transportation assets was computed by overlaying the sea level rise estimates onto the elevation of each asset. A summary of the inundation of critical transportation assets is provided in Table 20. A more detailed summary is provided in Table 47 of Appendix D.4. Except for ports, Mobile's critical transportation assets are minimally exposed to sea level rise in the low- and mid-range scenarios of 0.30 meters (by 2050) and 0.75 meters (by 2100) of global sea level rise, respectively. In both of these scenarios, only 0 to 4% of critical assets of each mode are exposed. Under the highest scenario of 2.0 meters of sea level rise by 2100, transit have the highest fractional extent of exposure, with 50% of facilities exposed. Pipelines have the lowest fractional extent of exposure, with 3% of pipeline-miles exposed.
Across all sea level scenarios, critical roads and rails are most exposed linear assets to sea level rise in terms of the fractional extent of inundation. Exposure of critical roads ranges from 4% of linear extent inundated under the lowest sea level rise scenario up to 13% under the highest. The area's critical rail lines are similarly exposed to sea level rise, with 20% of kilometers exposed under the highest sea level rise scenario.
In contrast, pipelines have the lowest fractional extent of exposure to sea level rise for linear assets. One percent of critical pipeline-kilometers are exposed under the mid-range scenario, while 3% are exposed under the highest scenario.
Port facilities are significantly exposed to sea level rise, with 46% of the 26 critical ports exposed under the lowest scenario, and 92% exposed under the highest scenario.
One of the two critical transit facilities, the GM & O Transportation Center, is inundated under the highest sea level rise scenario.
Of the two critical airports, only Mobile Downtown Airport experiences any inundation under the sea level rise scenarios. One percent of the airport's area is inundated under the lowest scenario and 3% is inundated under the highest sea level rise scenario. These relatively minor effects impact wetlands at the edge of the airport.
|Scenario||Roads (miles)||Rail (miles)||Pipelines (miles)||Ports (#)||Transit Facilities (#)||Mobile Downtown Airport (mi2)*|
|9 of 209
|2 of 196
|3 of 426
|12 of 26
|0 of 2
|0 of 3
|11 of 209
|2 of 196
|7 of 426
|18 of 26
|0 of 2
|0 of 3
|2100 2.0m||26 of 209
|40 of 196
|13 of 426
|24 of 26
|1 of 2
|0 of 3
Note: The "highly critical" asset list was revised after the criticality report was completed to include parts of CR188, CR59, and the Cochrane Bridge in response to comments received from local stakeholders. Therefore, the total km presented here may differ from that reported in the Criticality Assessment report.
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 (described in detail in Section 7.2).
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 extra-tropical storms35 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.
The implications of the sea level rise 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).
1 NRC, 2008; USCCSP, 2008a
2 NRC, 2008; USCCSP, 2008a
3 IPCC, 2007a; DOT FHWA, 2010; National Science and Technology Council, 2008
4 National Science and Technology Council, 2008.
5 Nicholls et al., 2007
6 Nicholls et al, 2007
7 Based on the preliminary analysis provided by K. Van Wilson, USGS.
8 Station ID: 8735180.
9 Station ID: 8729840.
10 Personal communication with Dr. Scott Douglass of the University of South Alabama's Marine Sciences program.
11 IPCC 2007a; NOAA 2012c
12 Based on a preliminary analysis by USGS provided to FHWA.
13 Thompson et al., 2008. Local sea level rise as discussed in the Climate Projections section of this report describes changes in sea level due to global sea level rise, uplift, and subsidence of land.
14 Parker, 1992
16 Ice loss can be due to ice sliding directly into the ocean, from melting of the ice, and from direct evaporation of the ice into the atmosphere.
17 A 20-year study funded by the National Aeronautics and Space Administration (NASA) suggests that ice sheets in Greenland and the Antarctic are melting at an increasing rate with each passing year. This trend is thought to be directly correlated with warmer summer temperatures.( Rignot et al., 2011; Gardner et al., 2011) Total losses from both ice sheets averaged roughly 475 billion metric tons (534 billion short tons) of ice each year, enough volume to increase average global sea levels by 1.3 millimeters (0.05 inch) per year. The same study proposes that if current ice sheet melting rates continue, average total sea level rise could reach 32 centimeters (12.6 inches) above current averages by 2050 from melting ice sheets, glacial ice caps, and thermal expansion. Another study projects mountain glaciers and ice caps around the world could lose up to 75 percent of their present ice volume by 2100. (Radic and Hock, 2011)
18 NRC, 2010b
19 Pfeffer et al., 2008
20 Rahmstorf, 2007
21 Vermeer and Rahmstorf, 2009
22 Principle 15 of the Rio Declaration of 1992 states: "In order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation."
23 This scenario could manifest itself if there is little to no acceleration in the transfer of water from ice sheets to the ocean over the next century.
24 This scenario could manifest itself if the loss of ice sheets is at the upper end of what has been reported in the literature.
25 Dokka, 2006
26 The analysis of vertical land motion was led by K. Van Wilson, USGS MS Water Science Center, 308 South Airport Road, Jackson, MS 39208. He gratefully acknowledges the following at the National Geodetic Survey who were very helpful in providing technical assistance and requested benchmark information: Denis Riordan, State Geodetic Advisor for Mississippi, Jackson, MS; Jim Harrington, State Geodetic Advisor for Alabama, Montgomery, AL; and Vasanthi Kammula, Chief, Project Analysis Branch, Silver Springs, MD. The author also acknowledges Zhong Lu, USGS, Cascades Volcano Observatory, Vancouver, WA, for his technical assistance in adjusting the InSAR data using the updated benchmark results. Original ERS-1/2 SAR data are copyrighted by the European Space Agency (ESA). Original ALOS/PALSAR data are copyrighted by the Japan Aerospace Exploration Agency (JAXA) and Japan Ministry of Economy, Trade and Industry (METI). More details about the USGS analysis are available from FHWA in the form of three technical reports that were submitted by USGS.
27 In the western-most areas of Mobile County and the western end of Dauphin Island, actual data values were extended outward into data voids to build the interpolation surface.
28 LIDAR data provided by the City of Mobile, 2010
29 A new file delineating the shoreline at high resolution was generated. This file was used in this analysis as well in the storm surge analysis.
30 In referring to vertical change rates, subsidence is expressed as a negative number, and uplift as a positive number.
31 Figure provided courtesy of Van Wilson, USGS.
32 Vertical accretion refers to the upward growth of the top level of sediment due to the accumulation of both inorganic sediment and organic matter.
33 The still-water level refers to the elevation of the water surface in the absence of waves.
34 This vulnerability may be partially mitigated by increasing the elevation of road surfaces during the routine recapping of the asphalt roads in the area.
35 Lacy and Hoover, 2011
36 Subsidence and uplift are accounted for in the three scenarios using InSAR and benchmark data, as described in the text.