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

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??

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

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??

Extreme rainfall study - UK Observed daily rainfall data 1961-2000

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: 1961-70, 1971-80, 1981-90, 1991-2000 and for 1-, 2-, 5-, and 10-day events

Observed change in annual return period Fitted 10-day, 10-year return periods 1961-1990 1991-2000

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

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

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

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

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 (1961-2000) 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: 1961-70, 1971-80, 1981-90, 1991-2000.

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

Changes in frequency and timing Southeast England, 1-day

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

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

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; 1961-1990 Periods during change; 1981-90, 1991-00 Future periods including predictions: 2000-2030 Current work in SWURVE is looking at robust methods for deriving design storms in non-stationary climates