Staged Adaptation Planning

Pavement structural modifications can be relatively simple through surface-layer rehabili- tation, or complex through base-layer rehabilitation/reconstruction. Short-term adaptation strate- gies can consist of relatively low-cost HMA overlays, the timing and frequency of which can be estimated using pavement simulations. When a pavement is projected to fail, simulated pavement life can be extended by simulating direct HMA overlay or mill and overlay [AASHTO, 2008;

Tanquist, 2012]. Staged adaptation planning with changing environmental/climate conditions must answer three questions. (1) When should the existing pavement structure be replaced? (2) What should the new pavement structure look like? (3) What is the right balance between pave- ment performance and costs over stakeholders’ time period of interest? In other words, what is the most cost-effective combination of HMA maintenance/overlays and base-layer rehabilita- tion/reconstruction with changing environmental/climate conditions?

The DAPP approach introduces the concepts of adaptation pathway mapping, tipping points, and an adaptation scorecard [Haasnoot et al., 2013; Kwakkel, Haasnoot et al., 2016;

Kwakkel, Walker et al., 2016]. Pathway mapping and tipping points can be used to address ques- tion (1). Adaptation pathway mapping provides a dynamic mechanism for systematically evalu- ating adaptation strategies, or pathways including multiple adaptation actions, throughout the pavement management period (60 years). Adaptation tipping points are points along the path- way where the current strategy is no longer effective and a new action is warranted [Haasnoot et al., 2013; Kwadijk et al., 2010; Kwakkel, Haasnoot et al., 2016; Kwakkel, Walker et al., 2016].

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An adaptation tipping point in pavement management may occur when the groundwater moves into the base layers of the pavement structure resulting in pavement life reduction [Knott et al.,

2017]. Other tipping points include road-surface inundation with groundwater rise or surface- water flooding requiring radical adaptation actions such as raising the road, road abandonment, or active pumping. Temperature non-stationarity may introduce the need to repave with a differ- ent asphalt binder grade [Chinowsky et al., 2013]. The combined effect of changes in two or more environmental/climate parameters may also affect the nature and timing of adaptation tip- ping points.

The PCSC and Optimal PCSC described above are powerful tools to help stakeholders answer question (2): “What should the new structure look like?” The Optimal PCSC (Figure 6- 2) can be used to identify pavement structural designs that maintain the desired service life with projected environmental/climate change when the existing pavement structure ceases to perform according to historical norms. Some structural designs will be optimal early in the century, oth- ers later in the century, and some may be optimal for a wide range of climate-variable combina- tions.

An adaptation pathways map and scorecard [Haasnoot et al., 2013; Kwakkel, Walker et al., 2016; Kwakkel, Haasnoot et al., 2016] can be used to answer question (3): “What is the right balance between pavement performance and costs over the time periods of interest for stakehold- ers?” A total of 10 hypothetical adaptation pathways are illustrated on a pathways map (Figure 6-3) and described in an adaptation score card (Table 6-1) [Haasnoot et al., 2013]. The pave- ment designs used in the adaptation pathway map are chosen from the Optimal PCSC (Figure 6- 2) and include the existing pavement structure and structural designs that perform well for the plausible environmental/climate combinations projected to occur during the pavement

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management period. The options presented in Figure 6-3 and Table 6-1 are used to illustrate the methodology and are not based on a real analysis.

Figure 6- 3.Example of an adaptation pathways map, modified from [Haasnoot et al., 2013], showing maintenance paving actions (X) and adaptation tipping points (red circles) where maintenance paving with the current base layer is no longer effective. Note: This figure is for illustration only and is not based on real data.

Pavement performance simulations are run for each optimal pavement design with cli- mate-variable parameters changed in time steps based on the pavement-life sensitivity and/or stakeholder preference. The climate-variable parameters are determined using downscaled cli- mate models for two or more RCP scenarios. The timing of adaptation actions is performance- based and not tied to a pre-determined rehabilitation cycle. For this example, adaptation strat- egy 1 begins with the existing structure (ES) as illustrated by the top horizontal line on the adap- tation pathways map (Figure 6-3). The points at which maintenance paving (MP), or HMA over- lay, is needed to avoid simulated pavement failure are illustrated with “X”. The road is aban- doned when inundated at year 50. Adaptation strategies 2 through 4 begin by modifying the ES

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to achieve each new structural design. Adaptation strategies 5 through 7 begin with the existing structure and investigate changing structures when the existing structure is projected to fail. Table 6- 1.Adaptation score card showing adaptation pathways, the costs per pavement cycle, and the 50-year total cost assuming the road is abandoned once inundated. ES= existing structure, Sd= structure d, CC(d) = capital cost to construct structure d, MP = maintenance paving cost (modified from [Haasnoot et al., 2013]). Note: This table is to illustrate the methodology and is based on Figure 6- 3 not real data.

The present-value costs of base-layer reconstruction (CC) and maintenance paving (MP) are tallied in the adaptation scorecard (Table 6-1). For example, adaptation strategy 5, ES to structure d (Sd), represents constructing Sd from ES when the pavement model predicts ES fail- ure at the first X on the ES pathway. The costs include the capital costs of constructing Sd (CC(d)) from the ES in the first pavement cycle, 4MP in the second cycle, and 2MP in the third cycle. The total cost for adaptation strategy 5 is CC(d) + 6MP. The remaining strategies investi- gate implementing more than one structural modification in addition to maintenance paving over the pavement management period.

Adapt. Strategy No. Path Actions Adapt. Capital Cost (CC) Maint. Paving Cost (MP) Adapt. Capital Cost (CC) Maint. Paving Cost (MP) Adapt. Capital Cost (CC) Maint. Paving Cost (MP) Total Cost Road Elevation in 2070 Side Effects 1 ES 0 2MP 0 4MP 0 3MP 9MP - - 2 Sd CC(d) MP 0 4MP 0 2MP CC(d)+7MP - - 3 Sg CC(g) MP 0 2MP 0 2MP CC(g)+5MP - - 4 Sk CC(k) MP 0 2MP 0 2MP CC(k)+5MP - - 5 ES to Sd CC(d) 0 0 4MP 0 2MP CC(d)+6MP - - 6 ES to Sg CC(g) 0 0 2MP 0 2MP CC(g)+4MP - - 7 ES to Sk CC(k) 0 0 2MP 0 2MP CC(k)+4MP - - 8 ES to Sd to Sg CC(d) 0 CC(g) MP 0 2MP CC(d)+CC(g) +3MP - - 9 ES to Sd to Sk CC(d) 0 CC(k) MP 0 2MP CC(d)+CC(k) +3MP - - 10 ES to Sd to Sg to Sk CC(d) 0 CC(g) MP CC(k) MP CC(d)+CC(g) +CC(k)+2MP - -

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