Engineering Methods for Precipitation under a Changing Climate

This chapter summarizes federal agencies’ known efforts to translate climate projections into usable design parameters to guide future infrastructure design in the water resources arena, starting with assessing the impact of climate change in their areas of jurisdiction. In addition, the US Army Corps of Engineers, Environmental Protection Agency, FEMA, and US Geological Survey are developing methodologies to integrate the Intergovernmental Panel on Climate Change Fifth Assessment Report climate projections—specifically those for precipitation and temperature—to assess their impact on the design parameters used to manage the water resources projects under their authority. Finally, an exploration at the national scale into the performance of climate models and available design guidance over an observation period, say 1980 to 2015, is recommended. This exploration may result in a more unified approach that is acceptable to the water resources engineering community.

A common basis for comparison among extreme storm events is the average recurrence interval (ARI), in years. Quantitative analysis of the rainfall time series aims to summarize the variables of interest (depth, duration, intensity, and time between storm events) and to assess the probability that storm events of various magnitudes will occur based on the rainfall record. Such analysis shows that the ARI of storm events and the subsequently generated floods downstream are not of the same magnitude. The floods generated by Hurricanes Fran and Alice were influenced by concentrated areal precipitation, particularly for Hurricane Fran. In the case of Hurricane Alice near Pandale, Texas, the storm event point precipitations were extreme, but much less concentration and contribution of areal precipitation occurred upstream of the Pecos River USGS streamflow gauging station near Shumla, Texas, than occurred for Hurricane Fran upstream of the Raleigh-Durham area streamflow gauging stations.

This chapter summarizes methods for incorporating risk and climate change into hydrologic design procedures based on the new guidance document from the Federal Highway Administration: Hydraulic Engineering Circular #17 (HEC-17), “Highways in the River Environment—Floodplains, Extreme Events, Risk, and Resilience.” The document includes suggested levels of analysis for risk and vulnerability assessments of riverine transportation infrastructure. The guidance draws on the best actionable engineering and science methods, as well as data found in technical publications and reports, accepted manuals and standards, and federal and state agency reports. HEC-17 encourages the use of confidence limits whether or not climate change, or other sources of potential nonstationarity, is explicitly considered. It introduces a quantitative indicator—the climate change indicator—to assess whether projected climate change might be significant compared with the underlying variability and uncertainty in the historical record.

Clearly, nonstationarity in future precipitation regimes is an important topic that requires increased attention. This chapter presents advances in extending stationary concepts and methods to evaluating the performance of hydraulic projects under a nonstationary paradigm. First, the chapter briefly sums up stationarity concepts. Then, it summarizes extensions to the concept of expected waiting time, which is the same as the return period, expected number of events, and risk, suggesting how to use them for hydrologic design under nonstationarity. Whether even the high-resolution climate models have the capacity to simulate potential changes in precipitation extremes at local scales is a concern. The chapter covers important situations of increasing floods and extreme sea levels, and the methods described apply to many extremes including wind, precipitation, floods, sea level rise, and other hydrologic variables.

This chapter focuses on nonstationary models, where precipitation is assumed to differ from previous years, making them the most representative and suitable for addressing needs relating to a changing climate. Whether serial dependency is considered yields two different models: nonstationary model without serial dependency and nonstationary model with serial dependency. Nonstationary models with serial dependency describe the effect of climate change better than those without serial dependency, which means higher accuracy. The chapter also presents a comparative case study of extreme precipitation in three selected areas: New York City, the Washington, DC, metropolitan area, and San Francisco. The comparison focuses on studying the differences in extreme precipitation among the three cities, analyzing new extreme precipitation events with a focus on climate change, and studying the effect of serial dependency on different events over varying periods.

This chapter identifies key research priorities for fulfilling a subset of the work under the Climate-Resilient Buildings and Core Public Infrastructure project that relate to the health and safety of building occupants under projected changes in climate. In particular, (dis)comfort and mortality of occupants because of future projected overheating of building enclosures and the performance of common wall assemblies used in Canada under extreme wind-driven rainfall events are of interest. These climate hazards were selected because Canada is projected to undergo large increases in temperature in the future. Premier health agencies have identified future increases in temperature as a potential health risk for Canadians. Similarly, wind-driven rain has been identified as a critical climate load for the assessment of hygrothermal performance of building envelopes.

On May 30, 2017, the ASCE Subcommittee on Hydroclimatology and Engineering Adaptation held a workshop on Engineering Methods for Precipitation under a Changing Climate in Reston, Virginia. The workshop gathered professionals from academia, public agencies, and the private sector to discuss engineering methods for management and mitigation of precipitation extremes and floods throughout the life cycle of projects; approaches to uncertainty and how acceptable risk is determined; communication with climate scientists and engineers on engineering needs for climate information; and status of climate trends, climate projections, and uncertainty. Two breakout sessions followed a series of presentations by hydroclimatology experts. The chapter synthesizes ideas from the breakout sessions and discusses how the civil engineering community might carry them forward.

The ASCE Committee on Adaptation to a Changing Climate (CACC) Subcommittee on Hydroclimatology and Engineering Adaptation organized the workshop on Engineering Methods for Precipitation under a Changing Climate. The CACC’s purpose is to identify and communicate the technical requirements and civil engineering challenges involved in adaptation to climate change. Results from the breakout sessions were intended to provide information to inform standard bodies that are intent on ensuring infrastructure remains functional and durable over its service life while adapting to and/or rapidly recovering from changing extremes of climate and weather, such as droughts, floods, and precipitation extremes, including those that accrue from ice and snow events. The results would apply to engineering practices and standards intended to provide acceptably low risk of failure regarding functionality, durability, and safety of infrastructure systems and facilities over their service life.