Downburst dynamics and the implications for engineering structures

It is now an accepted fact that the disruption and economic losses arising as a result of extreme storms are increasing at a significant rate. There is also tentative evidence to suggest that these storms are increasing in frequency and magnitude due primarily to climate change effects, although it is acknowledged that such evidence is far from conclusive. Any increases in magnitude and frequency of extreme storms are likely to result in serious damage to the urban infrastructure, the world economy and society as a whole. In European terms, it has been suggested that by 2080, there will be an increase in wind-related insured losses from extreme European storms by at least....25-30bn Euro. However, it is perhaps worth noting that these estimates do not take into account society's increasing exposure to extreme storms, due to growing populations, wealthier populations and increasing assets at risk.

Over the last few years there has been renewed interest in the effects of extreme wind events, since in a number of cases these events are the most important with respect to wind loading (i.e., the design of buildings/infrastructure). One particular set of extreme wind events which has received little attention in the past are those associated with thunderstorm downbursts. During a downburst a column of air moves vertically downwards and impinges on the ground. This causes the resultant air to be displaced radially outwards from the point of impingement, with a ring vortex travelling away from the stagnation point. The effect of this is to alter the velocity field significantly. In other words, the velocity field which was assumed when the building was designed may no longer occur, and a new, very different field may exist. The effect that this new wind field has on typical structures has yet to be addressed.

Hence, there is a need to undertake a comprehensive examination of the structure of thunderstorm downbursts and to investigate the corresponding wind induced forces which can arise. The scarcity of full-scale data and the difficulty of predicting such events ensure that at present, modelling is a sensible way forward. Furthermore, the uncertainties associated with both physical and numerical modelling strongly suggest that a combined physical/numerically modelling programme supplemented by (limited) full-scale data is the best way forward. Without such an examination of the wind field associated with thunderstorm downbursts, the suitability of existing design methods remains an open question. This is of importance since in many parts of the world wind speeds of this origin constitute the design wind speeds. Even in areas where these events are not dominant, the continued investment and development in society and its related infrastructure ensures that society as a whole is more vulnerable to the effects of such an event irrespective of how frequently they current occur.

The potential impacts of this research are significant, yet, by their very nature are impossible to fully quantify at this stage. The proposal brings together leading academics and through the steering group and involvement of the Wind Engineering Society incorporates stakeholders from a broad range of disciplines. The proposal's deliverables will aid policy makers, planners and other high-level decision makers in ensuring more sustainable and resilient infrastructure in constructed, and ultimately benefit the societies that they serve, the UK economy and international research agendas. In the far future, the beneficiaries of follow-on research from the project are:

City Councils, who are responsible for managing the sustainable redevelopment of the areas under their control and who will be able to make direct use of the findings to deliver more resilient infrastructures.

The infrastructure owners, which is likely to include a public-private mix.

Practitioners ranging from Civil Engineers, Structural Engineers, developers, architects, consultants, contractors, planners, environmental scientists and meteorologists.

The international academic community, which will gain insight into the challenges associated with the designing for extreme storms.

EPSRC, EU and other research programmes, given that applied work of this nature which relies heavily on blue sky research can be agenda setting.

Policy makers, regulators and non-governmental bodies with interests in resilience.

The research staff employed on the project and their future employers, who will gain invaluable experience of this cross-disciplinary aspects research in this growing field.