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Demographic and climatechange influences on future

pluvial flood risk

Prof Alan Werrity and Prof Donald Houston (University of St Andrews)

Introduction

• What is pluvial flooding?

• Appraisal of current pluvial flood risk difficult

• Appraisal of future risk even more difficult!!

• Hydrological and climate change influences

• Demographic and social influences

The determinants of flood risk (based on McLaughlin, 2011)

Appraisal of Current flood risk

1. Where is the hazard?

2. Where is the exposure?

3. What receptors are most vulnerable?

Most effort has gone into 1 & 2

Appraisal of Future flood risk

1. Change in hazard

2. Change in exposure

3. Change in vulnerability of receptors

Most effort has gone into 1

Current flood risk (SEPA, 2011)

• Hazard + Exposure = – 133k properties at flood risk (5%)

• Fluvial: 64%

• Coastal: 23%

• Pluvial: 13% (but probably a lot higher)

• UNCERTAINTY in extent of pluvial hazard

• What about population?

• What about vulnerability?

Vulnerability and current exposure (Walker et al 2006; Werritty et al 2007; Houston et al 2011)

• River flooding:

– Socially deprived groups slightly under-represented

• Coastal flooding:

– Socially deprived groups over-represented

• Surface water flooding:

– Socially deprived groups slightly over-represented

• UNCERTAINTY: Social deprivation is a crude proxy for flood vulnerability

Future pluvial flood risk

• Change in hazard

– UK uplift in extreme daily rainfall: 8%

– UK uplift in extreme hourly rainfall: ???

• Change in exposure:

– UK population growth 1990-2050: 48%

– 1.2 million additional people at pluvial flood risk (Houston et al 2011):

• Climate change: 300k

• Population growth: 900k

• Change in vulnerability: ???

Increase in pluvial flood risk

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

Climate change 1990-2050

Demographic change

2001-33

Level of certainty in pluvial flood risk appraisal (Low, Moderate, High)

Current Future

Hazard Moderate

Moderate

Exposure Moderate

Low

Vulnerability Moderate

Low

Vulnerability and future risk

• Population ageing

• Insurance withdrawal/higher premiums?

• Flood risk areas blighted?

• Developed for social housing?

Conclusions

• More emphasis in flood risk appraisal required on: – Population – Population growth – Demographic change

• More research required on

– Extreme rainfall under climate change – Surface water flooding modelling – Population change in flood risk areas – Social vulnerability to flooding

References

• Houston, D., Werritty, A., Bassett, D., Geddes, A., Hoolachan, A. and Macmillan, M. (2011) The Invisible Hazard: pluvial flood risk in urban areas. York, Joseph Rowntree Foundation. (Due for publication summer 2011).

• Walker G, Burningham K, Fielding J, and Smith G (2006) Addressing Environmental Inequalities: Flood Risk. R&D Technical Report, SC020061/SR1, Bristol, Environment Agency.

• McLaughlin, M. (2011) Nation Flood Risk Assessment. Presentation to SNIFFER Flood Risk Management Conference, Edinburgh, March 2011.