Wind-driven rain and future risk to built heritage in the United Kingdom: Novel metrics for characterising rain spells

Scott Allan Orr,Maureen Young,Dawson Stelfox,Joanne Curran,Heather Viles

doi:10.1016/j.scitotenv.2018.05.354

Abstract

Wind-driven rain (WDR) is rain given a horizontal velocity component by wind and falling obliquely. It is a prominent environmental risk to built heritage, as it contributes to the damage of porous building materials and building element failure. While predicted climate trends are well-established, how they will specifically manifest in future WDR is uncertain. This paper combines UKCP09 Weather Generator predictions with a probabilistic process to create hourly time series of climate parameters under a high-emissions scenario for 2070–2099 at eight UK sites. Exposure to WDR at these sites for baseline and future periods is calculated from semi-empirical models based on long-term hourly meteorological data using ISO 15927-3:2009. Towards the end of the twenty-first century, it is predicted that rain spells will have higher volumes, i.e. a higher quantity of water will impact façades, across all 8 sites. Although the average number of spells is predicted to remain constant, they will be shorter with longer of periods of time between them and more intense with wind-driven rain occurring for a greater proportion of hours within them. It is likely that in this scenario building element failure – such as moisture ingress through cracks and gutter over-spill – will occur more frequently. There will be higher rates of moisture cycling and enhanced deep-seated wetting. These predicted changes require new metrics for wind-driven rain to be developed, so that future impacts can be managed effectively and efficiently.

Highlights

The IPCC was virtually certain1 in 2013 that the troposphere has warmed since the mid-20th century and had high confidence2 that global precipitation patterns have changed – trends that are likely to continue over the twenty-first century (IPCC, 2013)
An important part of the latter is to highlight aspects of wind-driven rain that are not captured by the current standard metrics – imperative to characterising future risk if they are predicted to significantly change in frequency of occurrence and/or severity
This study has evaluated the projected characteristics of Wind-driven rain (WDR) spells and exposure for eight UK sites towards the end of the twentyfirst century under a high-emissions scenario

Summary

Introduction

The IPCC was virtually certain in 2013 that the troposphere has warmed since the mid-20th century and had high confidence that global precipitation patterns have changed – trends that are likely to continue over the twenty-first century (IPCC, 2013). The presence and movement of water contributes to the weathering and deterioration of porous materials and other aspects of building performance. Traditional building materials such as stone and mortar, as well as concrete, are affected by many weathering processes in which water is implicated, such as chemical weathering (Charola and Ware, 2002), freeze-thaw weathering (Hall, 1999), salt weathering (Doehne, 2002) and biological weathering (Crispim and Gaylarde, 2005; Warscheid and Braams, 2000). WDR represents the main moisture source and cause of deterioration on most building façades (Erkal et al, 2012), as it is implicated in both long-term deep-set wetting (McCabe et al, 2013; Smith et al, 2011) and short-term risks of building element failure (e.g. rain penetration through edges of doors or windows (ISO, 2009), and junctions and gaps)

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Discussion

Conclusion