Climate Change Adjustments for Detailed Engineering Design of Roads

Experience from Viet Nam

,

• Publisher: Asian Development Bank
• Year: 2020
• Print ISBN:
  9789292622053
• eBook ISBN:
  9789292622060
• Topic: Technology & Engineering
• Subtopic: Global Warming & Climate Change
• Index: Technology & Engineering

This knowledge product explains the rationale and procedures for incorporating allowances for climate change in detailed engineering design, with a focus on credible adjustments to extreme rainfall and to mean and high-end sea-level rise. Highlighting worked examples drawn from Viet Nam's road transport sector and peer-reviewed research literature, it offers a point of departure for more sophisticated assessments of high-risk projects. It presents principles and approaches extendable to other design variables (extreme air temperature, evaporation, and wind speed) and transferable to other sectors, regions, and stages of the asset life cycle (from project concept to decommissioning). An accompanying step-by-step manual shows how each calculation is performed.

1 Introduction

Climate change is expected to intensify heavy rainfall and raise global sea levels. More intense rainfall occurs because a warmer atmosphere holds more water—at least ~6.5% more per degree Celsius, according to the laws of thermodynamics (Allen and Ingram 2002). Higher sea levels result from thermal expansion of the ocean, combined with ice melt from land, local gravity effects, vertical land movements, and changes in ocean currents. Without adaptation and resilience measures, heavier rainfall and higher sea levels are likely to increase river and coastal flood risk.

The Midterm Review of Strategy 2020 (ADB 2014c) of the Asian Development Bank (ADB) set out a vision for mainstreaming adaptation and climate resilience in project planning, design, and implementation. As a result, all ADB infrastructure projects now undergo mandatory screening to identify those at high or medium risk of being adversely affected by climate change. At-risk projects must then be “climate-proofed” and made resilient to identified climate change impact.

The ADB climate risk management framework (CRMF) (ADB 2014a; ADB 2014b) helps project teams identify climate change risks to project performance at the start of the project cycle, and then to incorporate adaptation measures in the design of projects that are deemed to be at high or medium risk. The CRMF has 20 steps spread over three project phases. These phases are (i) concept (with climate-risk screening); (ii) preparation (with assessment of climate risk and vulnerability); and (iii) implementation (with monitoring and evaluation).

Economic analysis of adaptation options lies at the heart of the project preparation phase and rests on the identification, then the valuation, of engineered and nonengineering adaptation options (ADB 2015). Engineered adaptation options might include levees or coastal defenses; nonengineering options could consist of early-warning systems, land use zoning, or environmental solutions (see ADB 2014a). In all cases, there should be explicit benchmarking of the economic analysis of the project design with and without climate change, as well as with and without climate adaptation measures (to demonstrate incremental costs and benefits).

National engineering design standards are routinely based on intensity–duration–frequency (IDF) tables for hydrologic events. These tables provide extreme-value estimates for design variables such as extreme rainfall, sea level, storm surge height, wind speed, wave height, and air and water temperature. Such values were previously estimated from statistical distributions fitted to historical records, with a stationary time series as the key assumption. Although most historical annual rainfall records may be stationary (Sun, Roderick, and Farquhar 2018), under climate change this position could become untenable, so other methods for estimating design values had to be invoked (Milly et al. 2002). Now, adjustments to extreme-value distributions can be based on expected changes in the same quantities shown in climate models.

This knowledge product describes the rationale and procedures for incorporating allowances for climate change in the detailed engineering design (DED) of a project. Attention is focused on credible adjustments to rainfall and sea level, but the same approaches can be applied to other design variables (such as extreme air temperature, evaporation, and wind speed). Ultimately, the confidence vested in adjustments depends on the uncertainty in climate-model simulations of design variables at the project scale (see Pol and Hinkel 2019). From relatively high to low confidence, the variables are global mean air temperature and sea level (high); regional sea level, rainfall, and monsoon systems (medium); and local rainfall, wind, and wave heights (low) (Flato et al. 2013). Dealing with climate-model uncertainty is therefore an important aspect of the procedures.

This knowledge product is part of broader international efforts to issue practical guidance on incorporating climate change in design standards and the economic appraisal of adaptation measures (Box 1). It was drafted within the framework of the guidelines for the Basic Infrastructure for Inclusive Growth (BIIG) projects of the Viet Nam road transport sector and the specific institutional context of those projects. As part of this work, the ADB project preparatory technical assistance (PPTA) team developed climate change–adjusted rainfall projections for use within hydrologic formulas for estimating future peak flows and flood levels (ADB 2018).

Box 1: National Guidance on Climate Change Adjustments for Project Design

There are surprisingly few examples of climate change guidelines issued by national governments. Their terminology and rationale also vary. Some refer to adjustments, others to allowances or risk reduction standards. Some guides are intended to shape asset design; others, for sensitivity (or stress) testing of the performance of options. The legal status of the guidelines may cover a broad range extending from mandatory design standards through to advice that supports investment decisions. However, all emphasize the need for underpinning with robust scientific evidence, with due acknowledgment of technical uncertainties. Early guidance invoked the precautionary principle to improve flood resistance. For example, MAFF (2001) prescribed a blanket 20% change in peak river flows for testing projects in the United Kingdom for effectiveness under climate change over an assumed 50-year lifetime. Subsequent advice for the country provides upper, central, and lower allowances that vary with region and period (2020s, 2050s, and 2080s). Other analyses advocate climate change allowances based on catchment type (e.g., Broderick et al. 2019). However, as guidance becomes more elaborate, there is a greater risk of inconsistent interpretation and implementation. See the Further Reading section of this knowledge product for other examples of guidance on climate change adjustments to project design.

Source: Consultant’s formulation.

The following procedures for road-design practice are intended for other sectors and stages of the asset life cycle (Table 1). Efforts to reduce climate risk to assets should begin at the concept stage, when managing exposure may be more effective than incorporating risk-reduction measures or retrofitting at the operating phase (Hallegatte 2009). For this reason, there will always be a place for hazard mapping and risk zoning. As will be shown later, identifying potential climate threats at the site selection (concept) stage could preempt the need for costly adaptations at the design stage. Climate risks at the construction stage include the impact of extreme weather on occupational health and the safety of workers, and disruption to supply chains. These risks are not discussed in this knowledge product.

Table 1: Infrastructure Categories and Asset Life Stages

o = applicability of aspects of the climate change adjustments to the concept, construction, operation, and decommissioning stages of an asset; x = applicability of adjustments for extreme rainfall and sea level for major infrastructure types.

Source: Adapted from LCE (2015) and ASCE (2017).

Infrastructure operations may be affected by a host of climate risks. For instance, an intake to a water treatment plant sited near an estuary may be affected by future sea levels and storm surges leading to saltwater ingress. More stringent allowances for climate change may be required for the operational safety of assets, such as design floods for reservoir spillways (Veijalainen and Vehviläinen 2008). Some very long-lived utilities, like power plants in the coastal zone, must consider safe decommissioning, site remediation, and security for hazardous waste, perhaps in the context of sea-level rise over centuries (Wilby et al. 2011).

All asset categories listed in Table 1—whether an airport, bridge, mass transit system, port, power plant, or wastewater treatment facility—involve provisions for rainwater drainage or for fluvial and coastal flood defense systems. The procedures in this knowledge product for estimating extreme rainfall and sea-level rise are therefore widely applicable. However, some categories of infrastructure require adjustments to other climate variables such as extreme air temperatures (for estimating future crop water needs to size irrigation systems) and wind gusts (for bridge and airport design). Much rarer events than the 25-year return period considered here must be factored into design and operating procedures where there are safety standards (e.g., dam and bridge safety). These special cases require bespoke assessment and fall outside the scope of the present knowledge product.

2 Purpose and Scope

This knowledge product describes the rationale and procedures for incorporating climate change allowances in project design. The workflow generates climate change adjustments to extreme rainfall, channel discharge, and coastal erosion used in DED of roads and associated infrastructure. The various steps are demonstrated with worked examples drawn from the Viet Nam road transport sector (ADB 2018) and from peer-reviewed research literature. But the principles and practices adopted here are intended to be transferable to other sectors, regions, and stages of the asset life cycle.

The term adjustment factor is used throughout to mean the change in a design variable (e.g., 25-year return period, 1-day maximum rainfall) for a specified time horizon (e.g., 2030s), with respect to a defined baseline period (e.g., 1986–2005). Where feasible, some indication of the range of scientific uncertainty is given. All tables of adjustment factors presented here are based on Global Climate Model (GCM) output from the Coupled Model Intercomparison Project, Phase 5 (CMIP5) (Taylor, Stouffer, and Meehl 2012). Unless stated otherwise, a high-emission scenario—Representative Concentration Pathway RCP8.5—is assumed. (Other climate-model products, such as the National Aeronautics and Space Administration’s Earth Exchange Global Daily Downscaled Projections (NEX-GDDP¹), at 25-kilometer-resolution daily temperature and precipitation, are available but require file conversion.)

The scope of this knowledge product is restricted to adjustment factors for (i) 1-day annual maximum rainfall total (Rx1day); (ii) mean sea-level rise (SLR); and (iii) high-end water level. Tables of factors and consequences for design, using sites in central and northern Viet Nam, are presented. For an earlier set of extreme-rainfall and sea-level tables, see MONRE (2016). Other knowledge products may be issued later for variables such as sub-daily rainfall intensity, extreme air temperature, evaporation, and drought index.

Although climate hazards are treated separately here, it is important to recognize that they can be concurrent. For instance, within the Asia and Pacific region, tropical cyclones typically bring heavy rainfall, with high wind speeds, waves, and storm surges. Therefore, the DED should reflect the possibility of climate-driven changes in multi-hazards at a site.

The next section describes the steps in, and general features of, the adjustment procedure developed by the PPTA team for extreme rainfall, with entry points to mandatory formulas and design standards (ADB 2018). The section after that deals with mean sea and high-end water levels. Two worked examples show how tables of climate change adjustment factors are applied in practice. The final section identifies some key knowledge gaps and opportunities for further development of the guidance.

¹ https://cds.nccs.nasa.gov/nex-gddp/.

3 Assumptions and Procedures for Extreme Rainfall Estimates

The following procedures are based on two premises. First, historical hydrometeorological records do not adequately represent the extreme climate conditions lying in store for long-lived structures. Second, climate models yield rel...

Table of contents

• Front Cover
• Title Page
• Copyright Page
• Contents
• Tables, Figures, and Boxes
• Acknowledgments
• Abbreviations
• Units of Measure
• Executive Summary
• 1 Introduction
• 2 Purpose and Scope
• 3 Assumptions and Procedures for Extreme Rainfall Estimates
• 4 Procedures for Estimating Mean Sea-Level Rise and High-End Water Levels
• 5 Worked Examples
• 6 Concluding Remarks
• Appendixes
• References
• Further Reading
• Back Cover