1. IHP-HELP Centre for Water
Law, Policy & Science
UNESCO
Demographic and climate
change influences on future
pluvial flood risk
Prof Alan Werrity and Prof Donald
Houston (University of St Andrews)

2. 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

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

4. 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

5. 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

6. 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?

7. 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

8. 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: ???

9. Increase in pluvial flood risk
45%
40%
35%
30%
25%
Climate change 1990-2050
20%
15%
10% Demographic change
2001-33
5%
0%

10. Level of certainty in pluvial flood
risk appraisal
(Low, Moderate, High)
Current Future
Hazard Moderate Moderate
Exposure Moderate Low
Vulnerability Moderate Low

11. Vulnerability and future risk
• Population ageing
• Insurance withdrawal/higher premiums?
• Flood risk areas blighted?
• Developed for social housing?

12. 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

13. 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.