1. EGS-AGU-EUG Joint Assembly Nice, France, 7th April 2003
New Developments in Flood Risk Estimation
Session 11: Revised FSR Rainfall-Runoff Method
EGS-AGU-EUG Joint Assembly Nice, France, 7th April 2003
Seasonal changes in UK extreme rainfall
Hayley J. Fowler and Chris G. Kilsby
WRSRL, University of Newcastle, UK

2. Future predicted increases – GCMs
Introduction
SWURVE project (Sustainable Water : Uncertainty, Risk and Vulnerability in Europe) funded under the EU Environment and Sustainable Development programme
Past decade, perceived increase in rainfall intensities – widespread flooding and landslides
Future predicted increases – GCMs
Changes have huge economic and social implications in terms of increased flooding
Design of structures depends on accurate return period estimation – recent spatial/temporal changes in these estimates at daily/multi-day level??

3. UK 2000 Evidence for change There is considerable
apparent direct evidence
of increased flooding:
Meuse/Rhine 1996
Oder 1998
Easter 1998 UK
Honduras 1999
Mozambique 2000
Autumn 2000 in UK
Summer 2002 central Europe
UK 2000

4. Future predicted increases – GCMs
Introduction
SWURVE project (Sustainable Water : Uncertainty, Risk and Vulnerability in Europe) funded under the EU Environment and Sustainable Development programme
Past decade, perceived increase in rainfall intensities – widespread flooding and landslides
Future predicted increases – GCMs
Changes have huge economic and social implications in terms of increased flooding
Design of structures depends on accurate return period estimation – recent spatial/temporal changes in these estimates at daily/multi-day level??

5. Extreme rainfall study - UK
Observed daily rainfall data

6. Growth curve production
Standard regional frequency analysis
Based on :
Extraction of seasonal maximum series
standardisation by RMED (median seasonal maximum event) at each station
Pooling of data by region
Single-site L-moment ratios then combined - weighted regional averaging by record length
GEV distribution fitted using L-moments
Growth curves produced for each decade: , , , and for 1-, 2-, 5-, and 10-day events

7. Observed change in annual return period
Fitted 10-day,
10-year return periods

8. Seasonal change – autumn
Increases in rainfall for estimated return period
5- and 10-day mainly Scotland and East England
1- and 2-day – across the UK
Due to increases in RMED

9. Seasonal change – spring
Picture of change similar
Increases most prominent at 1- and 2- day durations but also 5- and 10-day durations in northern and western regions

10. Seasonal change – winter
Changes spatially variable across the UK
Increases in Scotland and particularly at 5- and 10-day durations
Little or no change in southern regions

11. Seasonal change – summer
Consistent downward trend in estimated rainfall for short (2y) to long (100y) RP events
Particularly at 1- and 2-day durations and most prominently in south and east UK
Significant decrease in RMED since 1970s

12. Changes in timing of extreme rainfall events
POT events considered – defined to occur if total daily rainfall exceeds two standard deviations above the long-term ( ) mean wet day at a specific location
Standardised data pooled regionally and used to examine changes in timing and frequency of extremes: modal POT month and annual POT frequency
Decadal variability examined using fixed decades: , , ,

13. Changes in timing
Important - may determine return of soils to near field capacity (timing flood events)
Autumn typically dominant season for flooding in northern UK – until recently winter dominant season in southern UK
Little change to timing / frequency in north & west
South and east – significant changes. Over 1990s, annual frequency of POT events increased substantially, by up to 50% and changes to timing with much greater concentration in autumn months

14. Changes in frequency and timing
Southeast England, 1-day

15. Changes in frequency and timing
Central and East England, 10-day

16. Summary: 1
Significant changes in rainfall extremes are predicted globally and in the UK
Increasing annual RP thresholds, particularly for long duration events in north and west & eastern Scotland
Changes in seasonal extremes - increases in heavy rainfall events in winter/autumn - reductions in summer. Likely to also be the case under climate change
Seasonal effects are important, since the timing of extreme rainfall together with antecedent wetness (or snowmelt) can be crucial for flood generation
Increases in frequency and severity of flooding are therefore very likely

17. Summary: 2
A dilemma is therefore posed to the practising engineer - what period to use to define design storms?
Compromise between:
Length of record
Stationary period of climate
Choices include:
Historic period, pre-change;
Periods during change; ,
Future periods including predictions:
Current work in SWURVE is looking at robust methods for deriving design storms in non-stationary climates