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21 Models and Scenarios Climate science has made significant advancements in the past couple of decades in the ability to model complex interactions occurring between dynamic factors. Global models, called general circulation models (GCMs), help explain at a high level the interactions between the atmosphere, the earth, and the ocean. Because greenhouse gas (GHG) emissions, particularly CO2, influence the models, they are an important part of each GCM. Scenarios downscaled to reflect regional conditions are input into the GCMs to predict future conditions for specific geographies at different points in the future. The scenarios have been developed by the World Climate Research Programmeâs Coupled Model Intercomparison Project (CMIP) (Box 2). C H A P T E R 3 Climate Considerations Box 2. What Is a Coupled Model Intercomparison Project? In support of the Intergovernmental Panel on Climate Changeâs (IPCC's) Assessment Report updates, the World Climate Research Programme created the Coupled Model Intercomparison Project (CMIP) in 1995 to study how changes in climate variables, such as the amount of CO2 in the atmosphere, result in changes to the climate in mathematical models. Assumptions about future GHG levels inform the scenarios used in CMIP3 (2007) and the trajectories used in CMIP5 (2014). CMIP3, which was used as the basis for the IPCCâs 4th Assessment Report in 2007, uses scenarios developed for the Special Report on Emissions Scenarios (SRES). The SRES assumes that changes in future emissions stem from changes in driving forces such as demographics, economic development, and technology. CMIP3 establishes four storylines that describe the relationships between emissions and their driving forces. Scenarios are derived from the storylines to project potential futures. Three storylines and scenarios are used frequently: 1. B1. This storyline is the most optimistic. It assumes that the world consistently chooses a development path that favors the efficient use of resources to support economic growth. Specifically, it assumes rapid social development and increases in education levels, high economic growth worldwide, a comparatively small increase in energy use, and a timely shift to non-fossil fuels. (continued on next page)
22 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook Box 2. (Continued) 2. A1B. This is a scenario characterized by two different storylines and is generally viewed as moderate to optimistic. It assumes rapid population growth to the mid-21st century that then slowly decreases, rapid introduction of new technologies, and a balanced use of fossil and non-fossil fuels. 3. A2. This is the current businessâas-usual path and assumes continuation in the future. Specifically, population growth continues at its current rate, regional patterns change little, and current economic development patterns change little. Transportation and electricity remain primarily powered by fossil fuels. Slow adoption of alternative fuel sources continues. A summary of the CO2 emissions assumed into the future for the various storylines in CMIP3 is shown in Figure 6. CMIP5 is the most current and extensive of the CMIPs. It uses representative concentration pathways (RCPs) in place of storylines and scenarios (i.e., SRES) to estimate future GHG concentrations in the atmosphere. There are four RCPs named after a possible range of radiative forcing values in the year 2100: 1. 2.6. Atmospheric GHG concentrations peak at 2.6 watts per square meter (W/m2) before 2100 and then begin to decline 2. 4.5. GHG concentrations stabilize at 4.5 W/m2 before 2100 and then begin to decline. Similar to B1 storyline in CMIP3. 3. 6.0. GHG concentrations stabilize at 6.0 W/m2 before 2100 and then begin to decline. Similar to B2 storyline in CMIP3. 4. 8.5. GHG concentrations reach 8.5 W/m2 by 2100. Similar to A1F1 scenario in CMIP3. Figure 6. CMIP3 CO2 emissions scenarios (IPCC, 2001).
Climate Considerations 23 Figure 7. CMIP5 CO2 emissions scenarios (van Vuuren et al., 2011). Box 2. (Continued) A summary of the radiative forcings over time for the CMIP5 RCPs is shown in Figure 7. The emissions from CMIP3 and the radiative forcings for CMIP5 have been converted to CO2 concentrations. Figure 8 compares the CO2 concentrations under the CMIP3 SRES with those from the CMIP5 RCPs. Figure 8. Comparison of CMIP3 and CMIP5 emissions scenarios (IPCC, Figure 1-4, 2014).
24 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook Climate scientists use multiple consensus-based scenarios illustrating a spectrum of modeled changes in climate and weather over the 21st century, so selecting the appropriate climate scenarios is necessary to arrive at a useful CBA (Figure 9). What is considered an appropriate scenario may vary depending on many factors. Some states require consideration of a particular scenario or provide guidance about when to choose which scenario. The Massachusetts Department of Transportation (2017) has published statewide maps in which representative concentration pathways (RCPs) 4.5, 6.0, and 8.5 for four future periods (2030, 2050, 2070, and 2100) are developed for â¢ The projected percentage change of the 1 percent annual exceedance probability 24-hour precipitation event, â¢ The projected 1 percent annual exceedance probability 24-hour precipitation depth, â¢ The projected annual maximum number of consecutive days with temperatures over 95Â°F, and â¢ The projected number of days with temperatures over 95Â°F in the summer (https://gis.massdot. state.ma.us/cpws/). These maps allow planners to see and consider several planning scenarios over the short and long terms. Not all states have developed climate-planning scenarios; in these cases, transporta- tion agencies may be left to make their own determinations of which scenarios are appropriate. Until the United States departs from a business-as-usual path, an argument can be made that RCP 8.5 is the path to use in calculations, but as discussed in the following section, additional factors such as time frames for implementation, service life, and geographic context need to be considered as well when selecting a planning scenario. Some government agencies are starting to move away from the use of probabilistic scenarios toward non-probabilistic scenarios to manage uncertainty. For example, the Department of Defense and the National Park Service are starting to advocate the use of âwhat-ifâ scenarios for planning purposes. Under this approach, a planner asks questions such as â¢ What if extreme rain events increase surface water runoff flows by 100 cubic feet per second (cfs) per event in the next 20 years? How will that affect my culvert? What adaptation measures will we consider? â¢ What if extreme rain events increase surface water runoff flows by 1,000 cfs per event in the next 20 years? How will that affect my culvert? What adaptation measures will we consider? Figure 9. The selection of a climate scenario for planning will depend on the likelihood of occurrence and the severity of the consequences if the event occurs.
Climate Considerations 25 Non-probabilistic scenarios allow planners to ask questions based in probabilistic climate projections without relying on a specific SRES or RCP so that they can better manage risk. Plan- ners can then focus on the risk issues at stake rather than bring in climate change and the path under way. This approach helps planners and others involved to consider the impacts of climate change at the local level âin the context of physical, social, political, environmental, operational, and economic variables that strongly influence decision makingâ (NPS, 2013). The Maine Department of Transportation (MaineDOT) used non-probabilistic scenarios for its FHWA adaptation pilot. The department selected three modeled scenarios based on inundation mapsâno sea level rise, 3.3 feet of sea level rise (moderate projection), and 6.0 feet of sea level rise (business-as-usual projection)âand developed adaptation options for two bridges and one culvert, to which it then applied depth-damage functions to estimate construc- tion costs, damage and repair costs, and life-cycle costs (MaineDOT, 2014). If a DOT has not received guidance regarding climate scenarios to use in planning, a sensible approach is to consider relevant asset characteristics such as location (vulnerability and criticality), desired service life, capital and repair costs, and risk tolerance. Depending on these character- istics, the scenarios selected for CBA at the project level could vary from those that informed a climate vulnerability assessment of the entire asset catalogue. For example, vulnerable infra- structure with higher criticality may benefit from being resilient to current path projections and the business-as-usual/upper end of climate scenarios. It also makes sense to consider investing in greater resilience for assets with longer service lives, though resilience of the area served is also a consideration. If the surrounding area has become unlivable, that is a factor. The scenarios and time frames selected will have bearing on which alternatives are considered adaptive, their overall cost, and estimation of losses avoided. Ultimately, CBA will help distin- guish which alternatives are preferred based on performance over the range of time frames and scenarios examined. Considering Changing Climate in a Proposed Design Engineering design practice through much of the last century was to design for climatic phenomena, such as site hydrology and hydraulics, based on the assumption that the past accurately represented the future. Climate scientists refer to this assumption as stationarity, defined as unchanging mean, variance, and so on in climate-influenced design characteristics. If the past and future are similar, this assumption is reasonable, but even within the past 50 years many U.S. regions have seen changes that are significant for project design. In other words, stationarity can no longer be assumed. For example, from 1958 to 2012, the Northeast expe- rienced a 71 percent increase in the amount of precipitation falling in very heavy events (Figure 10), in which âvery heavy eventsâ are defined as âthe heaviest 1 percent of all daily eventsâ (USGCRP, 2014). Furthermore, the RCPs project that the frequency of extreme daily precipita- tion events in the Northeast will continue to increase on the business-as-usual path (extreme high) and in the extreme low scenarios (Figure 11), in which âextreme daily precipitation eventsâ are defined as âa daily amount that now occurs once in 20 yearsâ (USGCRP, 2014). Additional information regarding climate models and non-stationarity is included in Appendix C. Yet, many of the resources that constrain projects, such as design manuals and federal funding guidelines, assume stationarity. As a consequence, these resources have been slow to incorpo- rate both observed changes in design storms and projected changes based on climate models, leaving agency engineers with limited guidance and support for resilient engineering design efforts. Further, precipitation events as defined by climate scientists, for example, very heavy and extreme, do not correlate well to engineering design parameters, making design for future conditions challenging under non-stationarity.
26 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook FHWA has undertaken research to evaluate non-stationarity and associated potential impacts on transportation infrastructure, in particular through its Gulf Coast studies and climate- resilience pilot studies. These studies have enabled FHWA to identify different asset typesâ sensi- tivities to various climate stressors. A consolidated summary of some road networkârelated assets and stressors is summarized in Table 5. The full table is available at https://www.fhwa.dot. gov/environment/sustainability/resilience/ongoing_and_current_research/gulf_coast_study/ phasmod_task4/index.cfm. Additional information regarding the vulnerability of certain trans- portation asset characteristics to climate change is summarized in Table 6. These studies and data can be used to inform the development of design guidelines to account for expected future climate conditions. Figure 10. The number of very heavy rainfall events increased throughout most of the United States from 1958 to 2012 (USGCRP, 2014, Figure 2.18). Figure 11. The frequency of extreme daily precipitation events is expected to continue to increase into the future under all emissions projections (USGCRP, 2014, Figure 2.19).
Climate Considerations 27 Paved Roads Culverts Bridges Buildings Extreme Heat Sustained high temperatures can soften asphalt concrete pavement, resulting in rutting and shoving. Concrete pavement can heave at the joints. Thresholds for damage vary depending on pavement design; pavement binder may exhibit sensitivity starting at 108Â°F. While aggregate itself is not sensitive to temperature, its shape can influence the sensitivity of the overall hot-mix asphalt paving; angular aggregate may help prevent rutting. No documented relationship. Thermal expansion can expand road surfaces and bridge joints, increasing stresses on bridges. Research indicates that extreme heat results in temperature variations within girder sections, increasing stress in both tension and compression regions of the bridge (Hagedorn, 2016). Greater needs for cooling and increased stress on air conditioning systems are possible. Demand for water and energy usage may also increase. Precipi- tation- Driven Inland Flooding The velocity of water flowing over roadways can cause pavement and embankment failures. Multiple instances of complete pavement submersion are likely to damage pavement over time. Heavy precipitation can infiltrate cracks and leak under the Heavy precipitation can cause debris accumulation, sedimentation, erosion, scour, piping, overflow, and conduit structural damage. Increased flow velocities and depth beneath bridges can affect scour depth; if the stream elevation reaches the low chord bridge elevation, the local scour depths could be increased by 200%â300%. Overtopping can inundate the bridge, resulting in failure of the Flooding can inundate and damage buildings and building systems or components. pavement, damaging the subgrade. Sensitivity of the pavement depends on design and traffic loads; thinner pavements are more sensitive to water, and higher traffic loads increase stress, which can cause deformation. road surface (see Paved Roads column). Table 5. Summary of sensitivities of transportation asset types to climate stressors (adapted from FHWA, 2013b). (continued on next page)
Paved Roads Culverts Bridges Buildings Sea Level Change Hydraulically, sea level rise will reduce the 100-year return periods of flood- causing events because static water levels will be higher, so less rainfall and runoff will be required to achieve the same 100- year flood elevation (i.e., a smaller event will cause the same 100-year flooding). Tunnels may become more vulnerable both because the risk of their entrances and vents flooding will be greater and because the hydraulic pressure on the tunnel walls will increase as water tables rise. Combined with storm surge from hurricanes or norâeasters, gradual changes in sea level may be expected to damage or render In low-lying coastal areas, tidal flooding likely will become more and more frequent. As sea level rises, drainage systems become less effective as the relative elevation of the system outlet to sea level surface elevation becomes closer, resulting in more flooding. Sea level rise will decrease clearance under bridges. Combined with storm surge, sea level rise could increase erosion and scour damage to the abutment and cause slope failure. Sea level rise in combination with tidal actions, subsidence, or both can inundate low-lying buildings and structures in coastal areas. inaccessible low-lying coastal roads and tunnels. Storm Surge Pavements exposed to overwash can be damaged by the direct wave attack on the seaward shoulder of the road, the water flow across the road and down the landward shoulder (âweir flowâ), and the flow parallel to the road as the storm surge recedes and water settles on lower spots in the road. There is evidence that the âweirâ flow might be the primary asset- failure type. Storm surge can cause debris accumulation, sedimentation, erosion, scour, piping, inundation, and conduit structural damage. Powerful storm waves can stress both the superstructure and the substructure of a bridge. Stress can damage or destroy the connection between the superstructure and the substructure, leading to the bridge span being shifted or even unseated completely. Shifting of the spans damages other parts of the bridge, including abutments, caps, and girders. Storm surge can also wash large debris, such as barges, into bridges, causing impact. Storm surge can also result in scour and erosion damage to the bridge. Storm-surge forces acting directly on a building can cause it to collapse. Flooding from storm surge can inundate a building, damaging the building, its systems, and its contents. Erosion caused by storm surge can undermine foundations, resulting in structural damage and collapse. Storm surge can also carry debris, which can affect structures, causing damage. Table 5. (Continued).
Climate Considerations 29 Paved Roads Culverts Bridges Buildings Wind Wind does not directly damage pavements, but can disrupt traffic. Debris generated from a wind event can clog the stormwater drainage system, resulting in localized flooding. Wind stresses bridges with horizontal loading. Strong winds can create high flow velocities and high wave impacts, which can stress the bridge superstructure and substructure, and can also lead to scour and erosion. High winds can blow construction materials loose. Airborne debris can strike buildings and structures. Drought Drought can contribute to the cracking and splitting of pavement. Sedimentation can occur during periods of low flow. No documented relationship. No documented relationship. Dust Storms No documented relationship, but can disrupt traffic. No documented relationship, but could result in sediment deposition Potential impact on mechanical and electrical systems used to operate the bridge. No documented relationship, although could influence performance of mechanical systems. at the entrance to and within culverts. Wildfire Asphalt can ignite during tunnel fires. Research has shown that asphalt can ignite at temperatures between 896Â°F and 986Â°F, and degradation can begin at 572Â°F. Even without igniting pavement, high temperatures can soften it. Concrete is unlikely to ignite but can experience expansion around 1,112Â°F. Debris flows following wildfires can flood and damage roads. Wildfires can denude slopes and change soil properties, affecting the watershed hydrology and sediment transport processes and increasing overland runoff. The increased runoff can lead to destructive debris flows, blocking and damaging culverts. Post-wildfire debris flows can damage bridges via drag, buoyancy, impact, or burial. Bridges can be displaced, lifted off their foundations, or damaged from debris flow. Buildings could burn. Winter Storms Issues are related primarily to freeze- thaw cycles. See Changes in Freeze/Thaw row. Culverts can become blocked by snow and debris, resulting in localized flooding. Increased water flows around culverts can result in erosion around culverts. Increased precipitation (snow or rain) can increase soil saturation, decreasing lateral soil resistance of piers and making the bridge susceptible to greater movement. Decreased incidents of winter storms could mean less use of salt deicers, which could decrease corrosion rates. Excessive snow or ice loads can cause roof collapses. Ice infiltrating cracks in bricks and mortar or other exterior coatings can cause spalling. This moisture can also rot wood framing materials. The weight of snow and ice on trees and poles can topple them, damaging buildings and structures. Table 5. (Continued). (continued on next page)
30 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook Paved Roads Culverts Bridges Buildings Changes in Freeze/ Thaw Pavement reaction to freeze/thaw cycles depends on the paving mix (e.g., aggregate, air voids). Water seeps into cracks in the roadways, and during freeze/thaw cycles, the water freezes and expands, exerting pressure underneath the pavement surface. When the ice thaws, a void forms and vehicles driving over the pavement cause it to weaken and collapse over the void, forming potholes. This same phenomenon can cause cracks, deformations, and wheel ruts. No documented relationship between freeze/thaw and metal culverts, although soil upthrust could result in displacement or loss of foundation support. Freeze/thaw could cause joint separation in concrete culverts. Water that seeps into fissures in the bridge deck can result in cracking, eventually reaching the road surface. Concrete bridge components are also susceptible to freeze/thaw cracking. As temperatures increase, some geographies could experience an increase in freeze/thaw cycles, resulting in more damage to bridges than previously experienced when temperatures remained below freezing for long periods. Increases in freeze/thaw will increase stresses to exterior coverings, possibly resulting in increased spalling of brick and delamination of roofing materials. Heave associated with freezing and subsequent re- settlement associated with thawing can crack concrete foundations. Perma- frost Thaw Permafrost is defined as any ground that remains frozen year- round for 2 or more consecutive years. As temperatures rise, the active layer of permafrost (the surface layer) becomes thicker, and the ice in the active layer melts. As the ice melts, the ground surface subsides, resulting in thaw settlement. This thaw settlement occurs unevenly, which can pose a threat to any infrastructure built Road slope sloughing can fill ditches and plug culverts with sediment. Permafrost thaw could weaken foundation soils, resulting in culvert settlement. Bridge superstructures are not directly impacted by permafrost thaw. As global temperatures rise and permafrost begins to thaw, the pilings of bridges constructed on permafrost can settle, resulting in bridge collapse. Permafrost thaw can undermine foundations, causing differential settlement and buildings sinking into the ground. on top of the permafrost. Table 5. (Continued).
Topic Design or Regulatory Considerations Example Guidance FEMA Floodplain Practical alternatives to locating within the floodplain for the 100-year event Increases in the 100-year water surface elevation of an established regulatory floodway Increases in the water surface outside the regulatory floodplain (less than 1.0 ft) and impact on additional property Backwater limitations Title 23, Section 650, Subpart A: Location and Hydraulic Design of Encroachments on Flood Plains of the Code of Federal Regulations FHWA Non-Regulatory Supplement Attachment 2 Local jurisdiction drainage design criteria (e.g., Virginia DOT Drainage Design Manual) Hydraulics Crown elevation Embankment elevation Setback and right-of-way elevations Flood frequencyâbased risk of traffic interruption U.S. DOT âClimate Adaptation Plan 2014 Ensuring Transportation Infrastructure and System Resilienceâ AASHTO (2007) Highway Drainage Guidelines FHWA (2016) HEC-17: Highways in the River Environment: Floodplains, Extreme Events, Risk, and Resilience Drainage Superelevation transitions of zero cross slope away from sump/sag area of vertical curves Cross slopes identified to ensure positive drainage toward outer edges of travel lanes Ditch shapes, depths, lining materials, and grades designed to minimize erosion Appropriate inlet/catch basin spacing, subbase drainage, including underdrains and cross drains; inlet and storm sewers âover designedâ in areas where overland relief is not available Drainage design accounts for partial clogging of inlets; combination analysis for throat and grate inlet configurations Riprap sized for velocity and outlet configuration of stream bank FHWA Highway Subdrainage Design AASHTO (2007) Highway Drainage Guidelines AASHTO Model Drainage Manual AASHTO (2011) A Policy on the Geometric Design of Highways and Streets Local jurisdiction Drainage Design Criteria (e.g., Virginia DOT Drainage Design Manual) Local jurisdiction roadway design manuals for establishment of superelevation placement and rate standards (e.g., Virginia DOT Road and Bridge Standards) Stormwater detention design and placement (e.g., detention basins) Materials Gradation options: impervious (dense and compacted), low permeability (gap graded, e.g., stone matrix), permeable (open graded) High-viscosity binder preferred Pavement additives resistant to moisture damage 1993 AASHTO Guide for Design of Pavement Structures Local jurisdiction material design requirements (e.g., Virginia DOT Materials Manual of Instructions and 2014 Secondary and Subdivision Pavement Design Guide) AASHTO (2011) A Policy on the Geometric Design of Highways and Streets Table 6. Design or regulatory considerations regarding climate impacts on some transportation assets. (continued on next page)
32 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook Topic Design or Regulatory Considerations Example Guidance Erosion and Sediment Control Flow depth Flow direction Velocity Discharge Width Presence of debris Use of geotextiles Landscaping and slope planting Temporary measures during construction AASHTO Guide for Transportation Landscape and Environmental Design Department of Environmental Quality Erosion and Sediment Control Handbook Local jurisdiction guidelines and standards Overtopping Assumes weir flow Velocity Head (elevation of overtopping water minus road-surface elevation) Flood frequency at which overtopping occurs FHWA (2016) HEC-17: Highways in the River Environment: Floodplains, Extreme Events, Risk, and Resilience Table 6. (Continued). Some states and regions are undertaking efforts to develop design guidelines to account for non-stationarity and the associated potential impacts. For example, California is using geographic information systems to conduct a comprehensive vulnerability assessment of its transportation network. The state aims to identify âhot spotsâ that could be particularly vulner- able to climate change based on the National Academy of Sciencesâ Sea Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future (National Research Council, 2012), as well as two 2009 research reports by the California Energy Commissionâs Public Interest Energy Research programâThe Impacts of Sea Level Rise on the California Coast (2009b) and Climate Change Scenarios and Sea Level Rise Estimates for the California 2009 Climate Change Scenarios Assessment (2009a). New York City is developing climate-resiliency design guidelines for publicly funded buildings and infrastructure, including transportation, based on climate projections developed by the New York City Panel on Climate Change. Evaluating If Adaptation Is Needed When Guidelines Are Not Available Where climate change design guidelines do not exist, agencies can ask questions such as those following and depicted in the decision tree in Figure 12 to determine whether design more resilient to extreme weather and changing climate is likely to be desirable from an economic loss-avoidance perspective. These questions consider the interactions between climate, infra- structure, land use, and population changes. Changes to timing, frequency, and magnitude of design-relevant events need to be considered as well. â¢ Does the historical record show changes relevant to the design of the assets under consider- ation (i.e., is the asset or corridor under consideration climate-sensitive; will it experience higher levels of damage when subjected to small climate variations)? â¢ Do climate scenarios show changes relevant to the design of the assets under consideration (i.e., is climate a dominant risk factor; at what point does an agency want or need to take action to prevent or diminish climate impacts)? Transportation agencies may need to develop their own definitions of âdominant risk factorâ based on their own criteria, such as repair costs exceeding X percent of the inflation-adjusted original investment, an asset failing X years before the end of its useful life, an increase by X percent or number of traffic accidents, and so on.
Climate Considerations 33 Figure 12. A decision tree can help inform decisions about whether to adapt to climate change and extreme weather.
34 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook â¢ Do the scenarios agree, or mostly agree, on the direction of those changes (i.e., all show either an increase or a decrease)? â¢ If the changes are expected in the future, are they expected within the service life of the asset? â¢ Do either or both the historical record and climate scenarios show changes in a direction that would require more robust design (i.e., can the existing asset or system cope with the projected climate changes and is the existing system sufficiently robust?) Have or will factors such as land use changes exacerbate observed or predicted changes (e.g., to runoff)? â¢ Have or will factors such as population growth increase the number of users who are or will be affected by observed or predicted changes? â¢ Are there long-term trends, especially inundation vulnerabilities, which suggest that asset maintenance or construction in this area may not be viable (e.g., Zillow, the National Oceanic and Atmospheric Administration [NOAA], regional or state climate impact studies)? Transportation agencies can use the listed authoritative references to work through these questions, representing the best available science and engineering guidance as of this writing. Useful FHWA references for evaluating if and how to incorporate climate change adaptation into planning, design, and O&M include â¢ HEC-25, volume 2 (https://www.fhwa.dot.gov/engineering/hydraulics/pubs/nhi14006/ nhi14006.pdf), for assessing extreme events in the coastal environment; â¢ HEC-17 (https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif16018.pdf) for assessing extreme events in riverine environments; and â¢ HOP-15-026 (https://ops.fhwa.dot.gov/publications/fhwahop15026/fhwahop15026.pdf) for adapting transportation systems management, operations, and maintenance to climate change. Incorporating Climate Change into Cost-Benefit Analysis For assets or systems deemed critical or long-lived, as well as vulnerable to climate change scenarios selected by the DOT, a range of adaptation strategies may be developed. For planning a water resources project, for example, the final decision to incorporate adaptation measures for either gradual changes in more frequent events or changes in the magnitude of extreme events is determined by â¢ Funds available, â¢ The vulnerability and criticality of the asset or corridor affected, and â¢ Whether the benefits outweigh the costs for continued maintenance in certain areas (and obligations for the public disclosure of such situations) on the business-as-usual path and for each adaptation option. Limited funds may dictate that only adaptive measures related to gradual changes or smaller but more frequent events can be implemented, while additional funding may allow the incor- poration of adaptive measures related to extreme events. Either way, a transportation agency may consider whether the benefits outweigh the costs for continued maintenance in certain areas on the business-as-usual path, then communicate this to the legislature and public in the agencyâs funding and policy capacity. Thus, the public and their representatives have some of the data they need to consider whether the business-as-usual path is delivering what they want (Figure 13). The public can also discern if it is worthwhile and a better fit with their values to invest in another path that would prevent climate change, such as transitioning away from fossil fuels, though certain adaptations may still be desirable to manage risk unless or until the transition is accomplished.
Climate Considerations 35 Figure 13. Decision makers need to determine if they want to remain on a business-as-usual path or want to pursue other policies (USGCRP, Figure 2.7, 2017). Update to the Scenario The replacement culvert will have an expected useful life of 30 to 50 years, depending on the design. The replacement is expected to be accomplished in the next 2 years at the same location as the existing culvert; therefore, designers should consider expected precipitation conditions between 2049 and 2069. Conservative assumptions would use a 50-year useful life and precipitation conditions around 2069; however, designing for climate conditions at the end of the assetâs useful life could be overly conservative, and often in practice 60 percent of the design life is used. In this case, that would be 30 years, which equates to 2049. Designers should also consider current precipitation conditions; some areas of the country are predicted to become drier than they are currently. The more conservative (i.e., higher-flow) condition will be used to design a new structure. A spreadsheet-based tool was developed to predict the changes in peak dis- charges (Qp) and the risk associated with climate change. Data input to the tool include current rainfall frequency-depth-duration curves (from NOAA Atlas 14 or similar sources), the location zip code, drainage area, curve number, and time of concentration (Tc). The tool evaluates a rainfall temporal distribution for each recurrence interval (Tr). The changes in 24-hour design storms caused by climate change were obtained from EPAâs SWMM-CAT (Climate Adjustment Tool). A climate change outcome of warmer, wetter conditions for the far term (2045â2074) was selected for analysis, as this is consistent with predictions for the Chesterfield, Virginia, area. Peak discharges for current and future conditions were calculated (continued on next page)
36 Incorporating the Costs and Benefits of Adaptation Measures in Preparation for Extreme Weather Events and Climate ChangeâGuidebook and adjusted to be statistically consistent with their corresponding recurrence intervals. Discharges for future conditions were found to be higher than those for current conditions, so future discharges were used in the design calculations for the new structure. Data needed at this stage include â¢ Expected useful life of current facility, â¢ Expected useful life of replacement facility, â¢ Anticipated time frame for implementation of adaptation strategies, â¢ Scenarios to be used for analysis, and â¢ Recurrence interval of the design event. Data needed for flood events include â¢ Flood discharge/flow rates (or other parameters of interest) for events with recurrence intervals that exceed the design event recurrence interval (i.e., if the design event recurrence interval is 50 years, you will also need data for the 100- and 500-year events). â¢ 24-hour precipitation data for the design event recurrence interval plus recurrence intervals that exceed the design event recurrence interval. â¢ Flood discharge/flow rate (in cubic feet per second or other parameter of interest) of the design event. Optional information needed includes â¢ Tools or software that could be used for analysis of future hazard conditions based on the selected scenario (to ensure the selected scenario and related data are compatible with the tools and software to be used).