A Compound Flooding Manual of Practice for Engineers

Compound flood events result from a combination of multiple meteorological, hydrologic and oceanographic drivers occurring either in close succession or concurrently. These events often lead to greater impacts on flood magnitude, volume and duration compared to isolated events driven by individual factors. Key contributing drivers include heavy precipitation, storm surges, river floods, seiches, waves and tides, and high groundwater levels. These drivers may be statistically interdependent, and the processes that cause them may be non-stationary. For example, as global temperatures rise, tropical storms could intensify, and sea level rise is likely to accelerate. However, the rate and magnitude of these changes are subject to significant uncertainty due to a lack of understanding of model physics and the natural variability in the climate system at decadal to multidecadal time scales. Traditional engineering practices have generally used univariate approaches that assume stationary conditions. To address the challenge of compound flood analysis in a changing climate, the American Society of Civil Engineers (ASCE) is writing a Manual of Practice on Compound Flooding. The purpose of this Manual of Practice is to provide guidelines for the practicing engineer on how to analyze compound flooding events.

The presentation will provide an overview of the Manual of Practice. The following topics are included in the Manual of Practice:

• Definitions and typologies of compound flooding and the methods of their characterization in statistical and process-based frameworks
• Hydrodynamic process-based models, including rainfall-runoff, riverine, and coastal models
• Statistical models of compound flooding, including measures of correlation, multivariate statistical methods particularly the use of copulas, non-stationary analysis, and clustering in a compound perspective
• The joint probability method (JPM): a probabilistic modeling framework for estimating the frequency of exceedance of coastal hazards by integrating probabilistic information about storm recurrence rates, joint distributions of tropical cyclone parameters, and resulting coastal storm responses, as simulated by atmospheric and hydrodynamic numerical models
• Linking statistical and process-based models to create hybrid models of compound flooding
• Analysis of the conditions affecting compound flooding and how they are changing, including sea level change, vertical land motion, precipitation extremes, tropical and extratropical storms, and land-use changes; review of methods for analyzing projections of future compound floods
• Risk and uncertainty analysis: definition of risk in the compound flooding framework, uncertainty associated with compound flooding, and methods for incorporating the uncertainty in engineering planning, design, and risk assessment
• Case studies to illustrate how the models and methods can be applied to practical problems

The Manual of Practice is an example of how scientific methods can be transferred to practicing engineers for the analysis of real-world problems.