An Overview of Extreme Rainfall Events from the British Rainfall Archive 1866-1968.
Source: environment.data.gov.uk (2018-06-19)

Flood Risk for Extreme Events (FREE): Historical Rainfall Data and Maps from the Quantifying Flood Risk of Extreme Events using Density Forecasts Based on a New Digital Archive and Weather Ensemble Predictions Project. The Quantifying Flood Risk of Extreme Events using Density Forecasts Based on a New Digital Archive and Weather Ensemble Predictions Project is a Natural Environment Research Council (NERC) Flood Risk for Extreme Events (FREE) Research Programme project (Round 1 - NE/E002013/1 - Duration January 2007 - December 2008) led by Dr Patrick McSharry, University of Oxford. The dataset contains a collection of rainfall depth maxima data, dating back to 1860, plus associated description documents and rainfall maps of extreme events across the UK, have been used. All of these products have been digitised from the paper version of the British Rainfall publication, and are now archived at the BADC to enable easy access for future use and the wider community.

Floods in the UK are often caused by heavy rainfall lasting from minutes to weeks. Efficient management and mitigation of flood risk, especially surface water flooding in urban areas, requires accurate and reliable precipitation forecasts as inputs to flood risk models. Houses in flat areas are particularly at risk and meeting the shortage of houses in the south-east requires building on these areas. To estimate the flood hazard risk in order to try to protect these buildings, accurate rainfall predictions are needed. However, the connection between record rainfall and flooding is highly nonlinear, so that rainfall predictions must also say how likely rainfall is at any time - calculating the probability of rainfall.

Extreme rainfalls caused devastating floods in Boscastle in 2004 and Lynmouth in 1952, but the causes and pattern of rainfall was different. Therefore, scientists also need to know what pattern of rainfall caused the flooding.

This research aims to get good quality predictions of the probability of rainfall by combining advanced methods from statistics, the output from a new supercomputer model of the weather, and a new computer archive of exteme rainfalls going back to 1866 (and up to 1968), provided by a specialist company Hydro-GIS Ltd. It also aims to produce an automatic system for discovering the most likely pattern in the predicted rainfalls. The new prediction system and data will be freely available over the internet for use by the government and universities.

Hydro-GIS Ltd
An Overview of Extreme Rainfall
Events from the British Rainfall
Archive 1866-1968.
Dr. Harvey J. E. Rodda, Dr. Rose G. Wood and Dr. Nina
MacDougall
Hydro-GIS Ltd.
10 Coles Lane
Chalgrove
Oxfordshire
OX44 7SY
UK
xxx@xxx.xx

Hydro-GIS Ltd
Acknowledgement
This presentation covers part of the work undertaken in the
project:
Quantifying the Flood Risk of Extreme Events using Density
Forecasts Based on a New Digital Archive and Weather
Ensemble Predictions
Funded by NERC as part of the FREE (Flood Risk from Extreme
Events) research programme.
A joint Hydro-GIS / Oxford University study with ECMWF and Met
Office as contributors
Project duration: January 2007 – December 2008

Hydro-GIS Ltd
Topics Covered
• British Rainfall
• Digital Archive
• Extreme Rainfall Observations
• Spatial Analysis and Classification
• Uses and Applications

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Started by George J. Symons in 1860 with
observations from 168 stations in the British Isles.
Was under the title “Symons British Rainfall”
until his death in 1900
Continued until 1991 as a Met Office publication,
with observations from approximately 6000 rain
gauges.
British Rainfall

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British Rainfall

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Gave a listing of various rainfall observations:
- annual totals
- monthly totals
- days on which rain fell
- maximum falls in 24 hours
- extreme falls in short periods
- droughts
- averages over number of years
Also included special sections on methods of rainfall measurement, notable events,
rules for observers, the staff of observers, evaporation and rainfall duration.
British Rainfall

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The chapter entitled “Heavy Rainfalls on Rainfall Days” or
“Heavy Falls in 24 Hours” provided detailed information on extreme rainfall events:
• 24hr observations from 9am-9am
• Listings of all depths over a specified threshold: > 2.5 inches (63.5mm) or >7.5% of
annual    total 1866-1961; >50mm or >4% of annual total 1962-1968
• Descriptions of the event from observers including a summary of the weather conditions,
the characteristics of the rainfall, ensuing floods and damage
• Isohyetal maps
• Photographs
The chapter was included in detail from 1866 -1968
Brief 1-page summaries included in editions from 1985-1991
This information has been compiled as a digital archive as part of the
FREE  research project
British Rainfall Extreme Rainfalls

Hydro-GIS Ltd
6.39
4.15
2.24
Patterdale Hall
X
6.96
3.43
3.53
Grasmere, Pavement End
X
5.99
3.05
2.94
Skelwith Bridge
X
5.94
3.40
2.54
Grasmere, High Close
X
6.57
3.50
3.07
Elterwater
X
5.37
2.78
2.59
Ambleside, Lesketh Howe
X
5.12
2.68
2.44
Ambleside, Nook Cottage
X
7.66
3.41
4.25
Dungeon Ghyll
X
5.38
1.77
3.61
Kendal, Natland Park
X
5.28
3.01
2.27
Keswick, Barrow House
X
9.16
4.31
4.85
Buttermere, Hassness
X
8.26
3.85
4.41
Borrowdale Vicarage
X
6.93
1.65
5.28
Wyhtburn Vicarage
X
10.24
4.10
6.14
Seathwaite
X
5.07
2.49
2.58
Duddon Valley, Ulpha Vicarage
X
5.29
2.18
3.11
Sedbergh, Brig Flatts
IX
5.92
2.88
3.04
Skelwith Fold (Ambleside)
VIII
5.95
2.7
3.25
Monk Coniston Park
VIII
5.93
2.99
2.94
Duddon Valley, Seathwaite Vic.
VIII
5.17
2.46
2.71
Hawkshead, Eastwaite Lodge
VIII
Total in 48 hrs
in.
Rainfall 25th
in.
Rainfall 24th
in.
Station
Division
Table for 24th and 25th August, 1891

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“During the next few days the passage of vigorous secondaries
maintained unsettled stormy weather. On the 28th a very deep
depression west of Ireland caused widespread gales, which in
some districts were of exceptional violence and gave gusts
exceeding 100 miles an hour at Dunfanaghy in north-west
Ireland, at Tiree to the west of Scotland, at Paisley and at
Renfrew. Entries in the table are confined entirely to Dartmoor
where the fall exceeded 3 inches on that day.”
A sample of text from January 1927

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Isohyetal Map from 22nd July 1907

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Flooding in The Rhondda Valley 11th November 1929

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Quality Control
Every effort was made to ensure the depths entered in British Rainfall
were of good quality:
• Correspondence between BR editor and observers
• Unlikely depth observations compared with observations from nearby gauges
• Observations noted when gauges overflowed (“+” included)
• Averages taken to replace spurious values which could not be supported by
other evidence
1884
“Nor does a shadow of suspicion rest upon the record; the gauge is a good one,
the observations have always been carefully made, and the above figures are
thoroughly supported by the surrounding stations.”

Hydro-GIS Ltd
An Overview of Extreme Rainfall 1866-1968
Numbers of Observations per Year
0
500
1000
1500
2000
2500
1866
1876
1886
1896
1906
1916
1926
1936
1946
1956
1966
Year
Number of Entries
Minimum 24 (1868, 1878)
Maximum 2237 (1968)
Average
280
Total
28,223
0
1000
2000
3000
4000
5000
6000
7000
1866
1876
1886
1896
1906
1916
1926
1936
1946
1956
1966
Year
Number of Gauges
Met Office
British Rainfall
Numbers of rain gauges per Year

Hydro-GIS Ltd
Maximum Observed 24hr Rainfalls
0
50
100
150
200
250
300
1866
1876
1886
1896
1906
1916
1926
1936
1946
1956
1966
Year
Maximum 24hr fall (mm)
Maximum 279.4 (1955)
Minimum  91.9 (1871)
Average   149.8

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Maximum Observed 24hr Rainfalls
16 Observations  over 200mm (128> 150mm; 1616 >100mm)
Dorset
Wynford House
7.9
200.7
18/07/1955
Somerset
Bruton (Pitcombe Vicarage)
7.9
200.7
28/06/1917
Cornwall
Camelford (Roughtor View)
8.0
203.2
08/06/1957
Cumberland
Seathwaite
8.0
204.0
12/11/1897
Inverness-shire
Loch Quoich (Kinlochquoich)
8.2
208.3
11/10/1916
Dorset
Upwey (Elwell)
8.3
211.1
18/07/1955
Glamorgan
Rhondda (LluestWen Res.)
8.3
211.1
11/11/1929
Somerset
Aisholt (Timberscombe)
8.4
213.1
28/06/1917
Somerset
Bruton (King's School)
8.5
215.4
28/06/1917
Caernavonshire
Snowdon (Llyn Llydaw Copper Mill)
8.6
217.9
22/11/1908
Dorset
Upwey (Higher Well)
9.0
228.6
18/07/1955
Devonshire
Longstone Barrow
9.0
228.6
15/08/1952
Somerset
Cannington (Brymore)
9.4
238.8
18/08/1924
Dorset
Upwey (Friar Waddon)
9.5
241.3
18/07/1955
Somerset
Bruton (Sexey's School)
9.6
242.8
28/06/1917
Dorset
Martinstown (The Chantry)
11.0
279.4
18/07/1955
County (pre
1974)
Location
Depth
(in)
Depth
(mm)
Date
9 observations from just 2 events – July 1955 and June 1917

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Seasonality
All
0.0
5.0
10.0
15.0
20.0
25.0
1
2
3
4
5
6
7
8
9
10
11
12
Month
% Frequncy
>100mm
0.0
5.0
10.0
15.0
20.0
25.0
1
2
3
4
5
6
7
8
9
10
11
12
Month
% Frequncy
>150mm
0.0
5.0
10.0
15.0
20.0
25.0
1
2
3
4
5
6
7
8
9
10
11
12
Month
% Frequncy
All observations
Observations > 150mm
Observations > 100mm

Hydro-GIS Ltd
Spatial Distribution of Extreme Rainfalls
All Observations
0
5
10
15
20
25
30
35
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
XXIII
Division
% Frequency
> 100mm
0
5
10
15
20
25
30
35
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
XXIII
Division
% Frequency
> 150mm
0
5
10
15
20
25
30
35
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
XXIII
Division
% Frequency
British Rainfall
Divisions

Hydro-GIS Ltd
Spatial Distribution of Extreme Rainfalls
All observations were listed by division, not
ideal for a proper spatial analysis
Distribution strongly affected by the
numbers of gauges in each division.
Fewer gauges in upland, sparsely
populated areas.
E.g. 1899  had data from 3528 gauges
only 446 in Scotland and 188 in Ireland
Observations from the Republic of Ireland
stopped in 1939

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Spatial Distribution of 24 hr Rainfalls Over 150mm
> 150mm
0
5
10
15
20
25
30
35
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XXI
XXII
XXIII
Division
% Frequency
57 observations >150mm
(45%) are from just six events
Some gauge grid references could be
obtained from the Met Office

Hydro-GIS Ltd
A classification was required for more refined rainfall forecasting so that
events with similar characteristics could be treated as specific set.
Classification based on information available in British Rainfall:
• Spatial distribution and patterns from maps
• Seasonality
• Maximum observed rainfall depths
• Information from observers such as descriptions of the rainfall intensity, duration
presence of thunder
Initial classification applied to mapped events, further work will cover all events
and include synoptic meteorology indices such as Lamb and GWL.
Event Classification

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Mesoscale Convective Complexes (MCC) – small depressions characterised
by intense precipitation from convective cells within a larger area of continuous rain. Occur in
summer affect S and SW Britain, falls of over 200mm in 24 hrs can be observed. Also known as
Mesoscale convective systems (MCS).
East Coast (EC) – depressions where the eastwards progression stalls over the UK bringing
moist air and rainfall from the North Sea to affect areas of the east coast. Occur in summer and can
bring continuous rain for up to 4 days.
Thunderstorms (T) – isolated occurrences or progressions of convective cells, occur in
summer but lacking the structure of an MCC.
Orographic (O) – rainfall associated with the normal west – east movement of Atlantic
depressions which is enhanced over the mountainous areas and occur throughout the year.
Depression (D) – rainfall associated with a depression but not showing the features of the
other classes. Can occur throughout the year.
Types of Events

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Event Classification
1
2
3
1) MCC
2) East Coast
3) Thunderstorm

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Event Classification
5
4
4) Orographic
5) Depression / other

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Class Frequency
Classification of Rainfall Maps
22
28
39
68
60
0
10
20
30
40
50
60
70
80
MCC
EC
T
O
D
Class
Frequency

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Class Seasonality
0
10
20
30
40
50
1
2
3
4
5
6
7
8
9
10
11
12
Month
% Frequency
MCC
EC
T
0
10
20
30
1
2
3
4
5
6
7
8
9
10
11
12
Month
% Frequency
O
D

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Spatial Patterns
Required a revision of British Rainfall
Divisions
Simple E/W split
Ireland not included due to lack of maps

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Spatial Patterns
Divisional location
of maximum isohyet
0
1 – 10
11 – 20
21 – 30
31 – 40
41 – 50
>50
% Events
MCC
EC
T
O
D

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Potential Uses of the British Rainfall Archive
There are a wide range of research applications in meteorology, climatology,
hydrology and other fields:
• Studies of extreme weather;
• Design rainfall estimation;
• Long term climatic trends;
• Flood studies;
• Drainage design
The digital archive will be made freely available for research purposes in the UK

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Applications
Information compiled in the digital archive is being used in the current study:
• As input, calibration and verification data for extreme rainfall forecasting;
• The classifications enable analysis to be undertaken for specific event types;
• Spatial prediction of rainfall;
•Application of computer learning and shape recognition for an automated classification.
Other practical uses include:
• Scenarios for flood forecasting;
• Emergency management planning;
• Assessments of historical flood risk;
• Thames EA GIS interface.

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Comparison of 20th July 2007
with events from BR archive
200+ rainfall maps – potential scenarios for flood forecasting/emergency planning
20th July 2007
10th July 1968

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Historical Flood Records
Photos, descriptive text and rainfall observations are useful for historical flood studies
and flood risk assessments
Flood damage in Somerset following the Cannington storm,
18th August 1924.

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