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Extreme Rainfall and Flood Forecasting in the UK
Neil Fox
Dept. of Atmospheric Science
University of Missouri – Columbia
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With thanks to
Professor Chris Collier – University of Salford, Manchester, UK
UK Department of Environment, Food and Rural Affairs (DEFRA)
Will Hand – Met Office
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Outline
UK Flood Forecasting Responsibility Climatology Extreme rainfall Extreme floods
From extreme rainfall forecasting to flood forecasting
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UK Flood Warning procedures
Responsibility of Environment AgencyHydrologists receive rainfall forecasts from
Met OfficeVarying levels of
meteorological training Automation Data acceptance
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Extreme UK rainfall of the 20th Century
All events surpassing a given depth-duration curve (Bilham, 1935) are classified as “extremely rare”
We use the record of these events to construct the climatology
Note problems: Gage locations and sampling
• (spatial and temporal) Events split over sampling periods
• E.g. 2 rain days
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Bilham, 1935, rainfall classification
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Data Sources
Non-peer-reviewed publications!
British RainfallMeteorological MagazineWeather
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Climatology
There are very few extreme rainfall events over the UK
Developing common features to aid forecasting relies on a small sample
The most extreme rainfall events (associated with MCS’s occur less than once a decade!?)
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Number of events by month
05
1015
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Numb
er of
events
All Frontal Convective
Seasonal Variation In Extreme Rainfall
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Clipart Map of Britain
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Hindolveston(1959)
Walshaw Dean (1989)
Brymore (1917)Bruton (1924) Martinstown
(1955)
The most extreme events
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Where’s Kinlochquoich?
Second most extreme event in the century ran over 9am so rainfall split between two observing periods!
Also missing is Louth (1920) a real flash flood which killed 20 people Shows the problem with listing extreme events
–you may miss some.
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Extreme Rainfall Locations
Kinlochquoich
Brymore
Bruton
Martinstown
Walshaw Dean
Hindolveston
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So which were the worst floods?
Bradford (1956)
Lynmouth (1956)
Camelford (1957)
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Worst Flood Locations
Bradford
Lynmouth
Camelford
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Getting from Extreme Rainfall to Extreme Flood
What are the important factors? About the precipitation? About the catchment? About the conditions?
Can we base warnings on this? How?
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Factors Contributing To Flood The likelihood that the heavy rain area will become stationary
and long-lasting. Availability of significant precipitable water in the lower
atmosphere. The likelihood that heavy cells embedded in the main area
will move parallel to the main watercourse. If this happens the flood peak is likely to be enhanced.
The steepness of the catchment leading to a short time to peak.
The soil moisture condition of the catchment. The likelihood of unimpeded flow to the main watercourse;
whether or not significant vegetation and channel debris are likely to be problematic and whether or not there are constrictions in the channel that will facilitate a build-up of water for later release as a ‘wave’.
Snowmelt.
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Catchment Size and Shape (relative to storm)
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Baseline Susceptibility to Flooding
We can define characteristics of a catchment that either promote or restrict the likelihood of flooding
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Combining Area and stream length Extreme flood probability as a function
of approx. channel capacity
0
0.2
0.4
0.6
0.8
0 5 10 15
Area / DPLBAR
Prob
abili
ty
AREA/DPLBAR
6-12 km2/km
or >30km2/km – 1pt
15-30 km2/km – 2pts
1- 6 km2/km – 3pts
<1 km2/km – 4pts
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Flood Indicators
Catchment characteristics can be found for many UK rivers in the Flood Studies Report (FSR, 1975) and Flood Estimation Handbook (FEH, 2000)
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Assessment level
0 1 2 3 4
Steep catchment?(DPSBAR)
<1:50(<0.02)
1:50 - 1:20(0.02-0.05)
1:20 - 1:10(0.50-0.10)
1:10 - 1:50.10.0.20
>1:5(>0.20)
Land use/Veg.Type (URBEXT)
EssentiallyRural <0.05
0.05 – 0.125 0.125-0.250
0.250-0.5
ExtremelyUrban >0.5
Snow depth 0mm <10mm 10-50mm 50-100mm >100mm
Debris? No dry spell - fast flow
No dry spell - slow flowing river
dry spell before - slow flowing rural or fast urban
short dry spell - urban med to slow flow
long dry spell before - urban med. to slow flow
SMD >100mm 50-100mm 15-50mm 5-15mm <5mm
Channel constrictions
None Some constrictions
Soft constrictions Solid constrictions or soft ones that may act temporarily
Major, solid constrictions
Heavy Rain?(Peak hourlyCatchment rainfall)
<4mm/h 4-10mm/h 10-20mm/h 20-40mm/h >40mm/h
Long lasting? v.short <15 mins
short 15-45 mins
medium 45-90 mins
long 90-180 mins v.long >180 mins
Rain stationarity Rapid >20m/s Fast 10-20m/s medium 5-10m/s slow 1-5m/s stationary <1m/s
Direction of motion 80-90 degs 60-80 degs 20-60degs 5-20 degs 0-5 degs - parallel
AREA/DPLBAR
6-12 km2/km or >30km2/km
12-30 km2/km 1-6 km2/km <1 km2/km
PercentageRunoff
<20 20-40 40-60 60-80 80-100
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Case study Data
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Score for Chew Stoke Storm, 1968River
Chew Brue Frome Midford
DPSBAR 0.05 0.07 0.03 0.08
URBEXT 0.0089 0.0065 0.0713 0.0301
Snow depth 0 0 0 0
Debris? n/a n/a n/a n/a
SMD 17 23.4 22.9 22.1
Heavy Rain 50mmh-1 20mmh-1 25mmh-1 35mmh-1
Direction ofmotion
270-85=185 270-70=200 270-180=90 270-180=90
RainStationary?
15ms-1 15ms-1 15ms-1 15ms-1
Long lasting? 9 hours 5 hours 5 hours 5 hours
Channelconstrictions
Some some some n/a
AREA / DPLBAR
129.1/15.42 139.52/13.46
150.61/13.17
147.4/13.76
SPRHOST 28.9 36.4 43.5 29.1
Score 22 19 16 17
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No cheese today Gromit
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Poor Gromit – didn’t check the forecast
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Scoring for Walshaw Dean Storm River
Hebden Water Hebden Beck Calder
DPSBAR 0.12 0.10 0.11
URBEXT 0 0 0.0856
Snow depth 0 0 0
Debris? yes yes some
SMD n/a n/a n/a
Heavy Rain? 100 70 3
Direction ofmotion
300-270=30 360-300 = 60 300-260=40
Rain stationary? Yes Yes Yes
Long lasting? 120 mins 120 mins 120 mins
Channel constrictions Yes (solid culverting) Some
AREA / DPLBAR 36.0/5.39 22.25/6.04 905.16/43.10
SPRHOST 56.3 57.4 30.1
Score 25+ 25+ 18+
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Lynmouth, 15th August 195230 DeadSuccessive flood wavesCaused by temporary damming of debris under
stone arch bridges
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Situation on 15th August, 1952
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River
East Lyn Barle Exe West Lyn
DPSBAR N/A 0.14 0.15 0.30
URBEXT 0.0004 0.0001 0.0004
Snow depth 0 0 0 0
Debris? Yes No No Yes
SMD
Heavy Rain? 141mm 203mm 170mm 149mm(228mm)
Direction of motion 160-157=3 300-157=143 280-157=113 180-157=23
Rain stationary?
Long lasting? 10 hours
Channelconstrictions
Yes No No Yes
AREA / DPLBAR 76/ 128.01/21.81 120.87/26.49 23.5/9.2
SPRHOST (24.8) 42.8 34.6 24.8
Score 30+ 28 27 38
Score table for Lynmouth Flood, 1952
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Contributions to Lynmouth Flood
Exceptional local rainfallSteep catchmentsDebrisOld stone bridges
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To do Flood Forecasting
Need:
1. Catchment Conditions
2. Forecast Severe storm indices and rainfall forecast
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Future Directions
More flexible scoring Less discrete (more levels or continuous
functions) Local climatologies
Weighting factors
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Need to Know More about:
Causes of the rainfallAtmospheric ConditionsOr indicators of extreme rainfall potential
E.g. Boyden Index – as used by Collier and Lilley (1994)
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Classification by Storm Type
Depth-duration relationships appear to fall on certain ‘lines’
Can speculate that these different lines correspond to different storm types
Have some evidence in observationsBUT:
Very few storms Old storms not well observed
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Differentiation of events by storm type Magenta – MCS? Blue –Isolated convection?
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Analysis Problems
Small number of eventsSome of which were a long time agoNo upper airNo radarNo satelliteVery difficult to determine storm type
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Avoid circular arguments
“The rainfall was so extreme it had to be an MCS”
Then we find that all extreme rainfall events are caused by MCS!!
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Need More Data
Drawing conclusions from recorded observations
Cloud motion and developmentRainfall time series
This is what we will be doing
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Where this leads…
In doing this analysis we hope to find:
Common attributes of extreme events
• E.g. Storm type Common preconditions
• Vertical profiles
• Lifting indices
• Water Vapor
• Trigger mechanisms
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