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Irish National Hydrology Conference 2010
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02 - COMMENT ON THE NOVEMBER 2009 FLOODING IN THE SHANNON AND CORRIB SYSTEMS
Conleth Cunnane 1 ,
Anthony Cawley 2 ,
1 Dept of Engineering Hydrology, NUI Galway
2 Hydro Environmental Ltd., www.HydroE.ie
ABSTRACT
This paper examines the notable flooding that occurred in November 2009 on four large
river catchments namely the Shannon, Suck, Fergus and Clare. The severity of the
November ‘09 event is examined in the light of previous flood events and regional flood
frequency statistics. Likewise the severity and prolonged nature of the rainfall that caused
these events is also examined. Climate change implications in respect to increased
frequency of flooding were examined on the four catchments, with both the River Fergus
and Shannon and showing definite trends of increased flooding since 1993.
Keywords: Flood Frequency Analysis, Flood Trend Analysis, Extreme Flood Flow and Extreme
Rainfalls
INTRODUCTION
Notable flooding occurred in the west and south of Ireland during the third and fourth week
of November 2009. This unprecedented flooding followed many weeks of persistent and
often heavy rain from Mid-October and throughout November and also and culminated in
the worst flooding in living memory. In affected areas, rainfall totals for November were the
highest on record at most stations, including the long-term stations at Malin Head and
Valentia Observatory, where records extend back over 100 years (Walsh, 2010). In the West
the catchments worst effected were the River Suck at Ballinasloe, the River Shannon from
Carrick-on Shannon through Athlone to Limerick, the River Clare at Claregalway, and the
River Fergus at Ennis. The Karst area of South Galway including a number of smaller karst
fed catchments such as the Clarin and Dunkellin river systems saw historically high flood
levels.
The four western catchments investigated in this paper namely the Fergus, Shannon, Suck
and Clare are reasonably large and sluggish catchments requiring persistent winter flood
conditions to produce notable floods. The rainfall characteristics leading up to the
November were conducive to flooding with persistent rainfall over the summer period, a
slight respite in September and October follow by 4 weeks of record rainfalls from the last
week of October to the 25 th November. This persistent rainfall resulted in completely
saturated catchment systems with the flood storage available in the lakes, floodplains,
turloughs and groundwater systems including the karst storage completely exhausted
resulting in elevated runoff rates and record flood levels and flow rates.
The November Flood event due to its long duration provided great opportunity for the
hydrometric teams to carry out the important flow metering at high flood stages on the
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subject catchments. This information for many gauges has resulted to significant changes to
the upper flood Rating and thus changes to the AM series.
RAINFALL CHARACTERISTICS NOVEMBER 2009
A series of fast-moving deep Atlantic depressions brought active frontal systems across
Ireland, bringing very wet and windy conditions. Rainfall totals for November were the
highest on record at most stations, including the long-term stations at Malin Head and
Valentia Observatory, where records extend back over 100 years. Valentia’s total of 360mm
was its highest of any month since observations began in the area in 1866, while its previous
highest annual total of 1923mm in 2002 was also exceeded during the last week of this
month, reflecting the persistence of wet weather throughout the year. More than twice the
average November amounts were measured at almost all stations and close to three times
the normal amount fell in some places. Heaviest daily falls at most stations were recorded
in the period 16 th to 19th, two-day falls of over 100mm were recorded in parts of the west
and southwest on the 18th/19th. (McGrath et al., 2010).
Weather in Galway in 2009
After a dry June the months of July and August 2009 experienced more than double (220%)
their normal rainfall. September and October were drier than normal (64% and 86%
respectively). The last week of October had 47.5mm of rain and this was followed by the
wettest November since recent records began in 1966 (329mm, 269% of normal). There
were 27 rain days in November with only one day without rain before the 28 th . A depth of
60.6mm fell on the 17 th which is the greatest one day rainfall recorded at Galway since
1966. Other significant falls included 30.2mm on the 15 th , preceded by a total of 64.9mm
during the preceding 7days, and falls of 28.7mm and 15.3mm on the 18 th and 19 th
respectively.
A detailed assessment of the Athenry gauge in Co. Galway is presents in Table 1 below:
Duration rainfall mm
Rainfall
mm/day Date
Return
period
1 47.1 47.1 17-Nov-09 5
2 84.4 42.2 17 to 18-nov-09 66
3 106.4 35.5 17 to 19-Nov-09 140
4 120.9 30.2 15 to 18-Nov-09 167
6 142.9 23.8 15 to 20-Nov-09 170
8 178.2 22.3 15 to 22-Nov-09 356
10 205.4 20.5 15 to 24-Nov-09 472
12 223.3 18.6 15 to 26-Nov-09 460
16 252.6 15.8 11 to 26-Nov-09 378
20 291.9 14.6 8 to 27-Nov-09 456
25 323.9 13.0 31Oct to 24-Nov-09 377
30 361.0 12.0 29Oct to 27-Nov-09 273
Table 1: Rainfall Statistics for Athenry Gauge Co. Galway
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Comparison with 1994-1995
Extensive flooding occurred in south Galway in winter 1994-1995 as a result of prolonged
rainfall that amounted to a 3 month total of more than 550mm over the area. The
corresponding 3 month rainfall total in Galway city was 600mm. In comparison during 2009,
the following 3, during monthly totals were recorded in Galway. Thus the 3 month winter
rainfall total in Galway during the 1994-1995 flooding was not exceeded during any of these
three month periods in 1999, although the successive 3 month totals were consistently high.
The 1 month duration rainfall in 2009 considerably exceeded any previous 1-month totals
including 1994-1995 event.
3 Month Period Total Rain (mm) in Galway
July – Sept 2009 441
Aug-Oct 2009 393
Sept-Nov 2009 507
Oct-Dec 2009 558
Table 2: 3-month Rainfall Amounts 2009 – Galway City
Figure 1: November 25day duration Rainfall at 1km Grid (copied from Met Eireann Climatological
Note No . 12, Feb 2010) referenced against the 1941-2004 period
Trend in Rainfall
McGrath et al, (2010) reported findings of basic trend analysis by Met Eireann on a number
of their high quality rainfall stations. Some stations showed an increase in the frequency of
wet (> 10m) and very wet (>20mm) days over the past decades but found large regional
variation with conflicting trends at some stations that are geographically very close. Trend
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analysis of rainfall data is notoriously difficult due to the large temporal and spatial variation
involved and research by the WMO, (2009) suggest long records greater than 100 years may
be required to detect such trends.
Walsh (2010) concluded that the rainfall of November 2009 was notable for the number of
stations which recorded their highest ever November monthly rainfall; for the number of
wet days and the number of heavy precipitation days. The return period analysis indicates
that the rainfall totals over the longer durations (8 days or more) in the midlands, and parts
of the southwest and northwest, were extremely rare events.
FLOODING ON THE RIVER FERGUS
The River Fergus at Ballycorey gauge (27002) located immediately upstream of Ennis Town
peaked on the 24 th November 2009 and exceeded the previous historical maximum flood
(observed in1995 and 1999) for over 10 days between the 20 th and 30 th November.
A continuous flood flow series is available for the Ballycorey Gauge from 1954 to present
which provides a 56 year annual maximum (AM) flow series. No arterial drainage or
significant changes to the catchment have taken place over this sample period and the
gauging station is located on the outlet channel from Ballyalla Lake in a relatively canalised
section of river and is considered to be a very stable and stationary site. A crump weir was
installed in 1972 for improved low flow measurement but this structure does not affect the
flood flow range. The Ballycorey station under recent review as part of the flood studies
update has an A1 flood rating classification, which is considered the top classification.
The rating relationship for the entire AM series was reviewed for this paper and included
the more recent flow measurements carried out by the OPW at Ballycorey during the
November Flood event. This flood rating essentially consists of flow measurements take for
the 1959 flood, the 1995 flood and the recent 2009 flood (the 1999 event was not measured
by current meter). The review showed the flow measurements for the 1995 flood to be
inconsistent with both the 1959 or the 2009 measures, which show good agreement, refer
to Figure 3. Given the stable morphology of the gauging station and which is not drowned
from downstream the 1995 flow estimates were considered to be questionable and
eliminated from the revised rating.
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DIOSCHARGE (cumec)
90
80
70
60
50
40
30
20
10
Gauging
Pre 1972
1976 to 2000
most recent
recommended
0
0.5 1 1.5 2 2.5 3
STAGE (m)
Figure 2: River Fergus Rating Data at Ballycorey Gauge
c)
e
m
(cu
e
a
rg
isch
D
80
70
60
50
40
30
20
10
Gauging 2009
Gauging 1995
Gauging 1959
0
0 0.5 1 1.5 2 2.5 3
STAGE (m)
Figure 3: Flow gauging for the 1958, 1995 and 2009 flood events
The total catchment area to the gauging station is 562km 2 and has a mean annual maximum
flow rate QBAR of 39.5cumec. This represents a QBAR rate of 0.07cumec per km 2 which is
extremely low suggesting a highly attenuated catchment with possible significant
groundwater flows (karst) bypassing the gauging site. The estimated 100year flood for this
gauge fitting GEV distribution by the method of L-moments (NERC, 1999) gives a Q100 of
72cumec representing a Growth Factor (Q100/QBAR) of 1.82 which is lower than the FSR
National Growth Factor of 1.96 (NERC, 1975) but is reasonable consistent with west of
Ireland growth factors (Cawley and Cunnane, 2003). The November 2009 flood of 79.3
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cumec has a growth factor of 2.0 and based on the statistical analysis has an estimated
return period of 220years. This estimate of return period is reasonably consistent with the
range of return period estimates by Met Eireann of rainfalls for 12 to 25 day durations
(Walsh, 2010).
Over a 60 year record period a total of 7 flood events exceeded 50cumec with 6 of these
occurring since 1993. A 5-year moving average of the Am Series also suggests a trend
towards increased runoff in latter decades.
The majority of AM floods (87%) of all significant floods occurred between the months of
November to February with the Fergus catchment requiring reasonably long duration wet
antecedent conditions to fill the large catchment storage available both within the karst
bedrock and within the lake storage prior to generating the peak flows.
ref
Gauge
Name
AREA
km 2
AM
Years
QBAR
(cumec)
Q2009
(cumec)
Growth
Factor
Ret
Period
Date of
Peak
27003 Corofin 168 53 24.97 60.85 2.44 300 24-Nov
27002 Ballcorey 562 56 36.8 79.3 2.15 220 26-Nov
Table 3: Flow gauges on the River Fergus and flood statistics.
90
80
BALLYCOREY
70
Discharge (cumec)
60
50
40
30
20
10
0
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Figure 4: Annual Maximum Flood flow Series for the River Fergus at Ballycorey Ennis.
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90.000
80.000
70.000
Frequency Analysis of flood Flows- Ballycorey (27002)
GEV - lmoments
AM Flood Level Data
Flood Flow (cumec)
60.000
50.000
40.000
30.000
20.000
10.000
0.000
-2.000 -1.000 0.000 1.000 2.000 3.000 4.000 5.000 6.000
Yvariate
Figure 5: Flood frequency Analysis of Ballycorey AM Series fitting a GEV distribution by l-
moments.
FLOODING ON THE RIVER SHANNON
The River Shannon saw historical maximum flood levels from Lough Allen downstream to
Lough Derg in November 2009. All gauges including a number of ESB gauges which are in
operation for 80years showed the November 2009 flood to be the largest flood recorded
substantially exceeding the next largest flood event. Statistical analysis of eight gauges
estimated return periods varying from 140 years to in excess of 500years. The gauges
examined on the River Shannon are summarised below in Table 4. The date of peak flow
varied between the sites but generally occurred from the 24 th to the 27 th November and was
almost flat between these dates. It is important to note that Lough Derg is controlled at
Parteen Weir. Nevertheless the statistics suggest a notable flood event and certainly in
excess of 100-year return period.
Flood flow estimates were available for Parteen Weir from the ESB and Banagher Gauge
from the OPW. The flow estimate at Parteen weir represents the combined flow through
Ardnacrusha and the flow over the weir to the River Shannon. The Parteen gauge gave a
flood peak magnitude of 842.3 cumec (0.08cumec per km 2 ) for the November Flood event
which represents a growth factor of 1.58 over the QBAR of 532 cumec (0.051 cumec per
km 2 ). The previous highest flood flow was 749 cumec recorded in December 1959. The
November 2009 flood flow was estimated to be a 172 year return period event by fitting a
GEV distribution to the series (refer to Figure 6). At Banagher the Nov 09 flood peak flow
was estimated to be 735 cumec (0.092 cumec per km 2 ), a growth factor of 1.75 over the
QBAR of 430.5 (0.052 cumec per km 2 ) and a return period of 300 years (refer to Figure 8).
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Ref
Gauge Name
Area
km 2
AM
years Ret Period date of peak
25075 Parteen Weir 10480 79 flow & W.L. 172 26-Nov
25017 Banagher 7989 60 flow & W.L. 300 27-Nov
26028 Shannonbridge 4999 56 W.L. 175 27-Nov
26027 Athlone 4703 58 W.L. >150 25-Nov
26091 Thatch (L Ree) 4695 70 W.L. 140 28-Nov
26085 Jamestown 1364 53 W.L. >500 24-Nov
26030 Lough Allen 425 W.L. 22-Nov
Table 4: Hydrometric Gauges and Flood statistics for River Shannon
ref
Gauge
Name
AREA
km 2
AM
Years
QBAR
cumec
Q2009
cumec
Growth
Factor
Ret
Period
date of
Peak
25017 Banagher 7,989 60 420.5 735 1.75 300 27-Nov
25075 Parteen 10,480 79 532.1 842.3 1.58 172 26-Nov
Table 5: Flood Flow Gauges
The EPA Hydrometric team obtained a flood flow measurement of 648.4cumec and
701cumec at Portumna Gauge on the 24 th and 27 th Nov and 643 cumec at Banagher on the
25 th .
35.0
Flood Level m OD Poolbeg
34.8
34.6
34.4
34.2
34.0
33.8
33.6
33.4
33.2
33.0
Portumna
1931
1933
1935
1937
1939
1941
1943
1945
1947
1949
1951
1953
1955
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
Figure 6: Annual Maximum Flood level Series of River Shannon at Portumna
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34.4
Flood Level m OD Poolbeg
34.2
34.0
33.8
33.6
33.4
33.2
Kilalloe
33.0
1931
1933
1935
1937
1939
1941
1943
1945
1947
1949
1951
1953
1955
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
Figure 7: Annual Maximum Flood levels Series of River Shannon at Killaloe
900.000
Frequency Analysis of flood Flows- Parteen Weir (25075)
800.000
Frequency Analysis of flood Flows- Bannagher (25017)
800.000
GEV - lmoments
Series3
700.000
GEV - lmoments
AM Flood Level Data
700.000
600.000
Flood Flow (cumec)
600.000
500.000
400.000
Flood Flow (cumec)
500.000
400.000
300.000
300.000
200.000
200.000
100.000
100.000
0.000
0.000
-2.000 -1.000 0.000 1.000 2.000 3.000 4.000 5.000 6.000
-2.000 -1.000 0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000
Yvariate
Figure 8: Parteen Weir Frequency Analysis Figure 9: Banagher Frequency Analysis
Yvariate
FLOODING ON THE RIVER SUCK
The River Suck saw notable flooding in Nov 2009 resulting in serious flooding of Ballinasloe
town. Three long duration gauges were examined on the River Suck and are summarised
below in Table 6. Bellagill the gauging station just north of Ballinasloe has an A1 flood flow
rating classification and flow measurements were carried out by the OPW during the
November flood thus improving reliability of the peak flow estimate. Using the new Rating
information a peak flow of 216.5cumec (0.18 cumec per km 2 ) was estimated for the
November 2009 flood (OPW estimate of 224 cumec) representing a growth factor of 2.306
times the QBAR estimate of 93.9cumec (0.08cumec per km 2 ). The runoff rates for the
more upstream gauges of Derrycahill and Rockwood are 0.17 cumec per km 2 and growth
factor rates of 1.96 and 1.82 respectively. The estimated return periods for the gauges are
490 years at Bellagill, 300 years at Derrycahill and 140 years at Rockwood. The very high 1
and 2 day rainfalls on the 17 th to 19 th November were the catalyst for such extreme flooding
at Ballinasloe.
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ref
Gauge
Name
AREA
km 2
AM
Years
QBAR
cumec
Q2009
cumec
Growth
Factor
Ret
Period
date of
Peak
26002 Rockwood 626 58 57.6 104.9 1.82 140 21 Nov
26005 Derrycahill 1050 56 89.3 175.2 1.96 300 21 Nov
26007 Bellagill 1184 58 93.9 216.5 2.306 490 21 Nov
Table 6: Flood Flow Gauges – Suck Catchment
Figure 10: Annual Maximum Flow Series River Suck at Bellagill
FLOODING ON THE RIVER CLARE
The River Clare is gauged at three locations, Claregalway (insufficient record length of only
14 years), Ballygaddy and Corofin. The AM Series available at the Corofin and Ballygaddy
sites is post arterial drainage having series length of 46 and 36 years respectively. The
November 2009 flood peak rate at Corofin was measured at 192 cumec (0.276 cumec per
km 2 ), at Ballygaddy it was measured at (0.172 cumec per km 2 ) and at Claregalway it was
measured at 165 cumec (0.154 cumec per km 2 ).
ref
Gauge Name
AREA
km 2
AM
Years
QBAR
cumec
Q2009
cumec
Growth
Factor
Ret
Period
date of
Peak
30007 Ballagady 632 36 63.2 108.9 1.72 90 21-Nov
30004 Corrofin 695 46 99.2 192 1.94 140 21-Nov
30089 Claregalway 1073 14 116 165 1.42 > 100 22-Nov
Table 7: Flood Flow Gauges – Clare Catchment
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ASSESSMENT OF CHANGE IN FREQUENCY OF LARGE FLOODS IN WESTERN RIVERS
The question "Are floods getting bigger or more frequent?" is another way of asking if there
is an increasing trend in flood magnitudes over time. An inspection of a time series of
annual maximum flood data ending in 2009-10 is inclined to suggest that flood activity has
been more intense in recent years than in past decades not just in Ireland but in UK and all
over Europe. As well as 2009, the years 1990, 1994, 1995, 1999, 2005, 2006, 2007 have seen
large floods occurring in parts of Ireland and in Europe.
There are a number of forms of trend analysis which can be employed to test whether what
has been observed rejects the hypothesis of no trend, i.e. in the absence of trend what is
the probability of observing what has occurred. If this probability is very small e.g. less that
5% or 1% the hypothesis of no trend could be rejected. A range of up to 5 standard non
parametric tests and one parametric test of trend have been applied to the systematically
gauged annual maximum flow series at approximately 100 gauging sites in Ireland up to the
year 2006 as part of the Flood Studies Update programme and have been reported
elsewhere (Mandal, 2009). These detected some degree of trend, not always upwards, in
about 10% of stations, although the Mann Whitney test detected a far higher percentage of
stations with trend than the other tests.
For this paper a split sample non-parametric test has been applied to the annual maximum
water level records of the rivers Fergus, Clare, Suck and Shannon where record lengths vary
between 36 and 80 years, mostly in the range 50 to 60 years. Basing a test on the water
levels removes any uncertainty in the conclusions which may be dependent on the rating
curve, especially where rating curves have been adjudged to have changed over time. This
test consisted of considering the 6 largest floods on record and counting how many of
these, x, occurred over the last 20 years and how many of them, y = 6-x, occurred during the
earlier period of record. The probability of this division of the 6 occurring on the assumption
that conditions remain constant throughout is calculated. If this is very small then it can be
suggested that conditions are not constant throughout the period of record.
The probability is calculated as
P =
where n=20 is the length of recent period considered, x is the number of the entire record's
top 6 values which occurred in this recent period, M = N – n where N = total record length
and y is the number of the record's top 6 values which occurred in earlier period of M years.
The results are shown in detail in Table 8.
These show that the greatest departure from uniformity occurs in the Fergus and Shannon
sites where x = 4 or 5. Depending on record length the associated P value in these rivers
ranges from approximately 0.017 to 0.029, that is chances ranging from 1 in 60 to 1 in 33
approximately. These figures suggest that the more recent 20 years has been "floodier" that
the earlier 30 to 60 years. On the other hand the results for the Clare and Suck are less
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dramatic where x values are mainly 3 and hence P values are mostly of the order of 0.1 to
0.3 which cannot be regarded as exceptional.
Thus the evidence is of some trend in the Shannon and Fergus catchments with no
discernable trends in the Clare or Suck.
Of course this analysis is just one of many which could be done in this way since selection of
the n=20 most recent years and the highest 6 values is somewhat arbitrary. This selection
was guided by the fact that the level of flood being considered is approximately that of the 1
in 10 year flood for the majority of the stations used where record lengths are of the order
of 50 to 60 years. A threshold of 1 in 10 excludes consideration of most of the medium and
insignificant events, which if included might play the role of the "tail wagging the dog". The
choice of n=20 is based on the fact that the perception the beginning of increased
"floodiness" is associated with the early 1990s, especially post 1993. It was decided to use 2
whole decades rather that just 17 years for n.
It is worth noting that the highest water level occurred in 2009 in all but 1 of the 14 stations
examined. The exception was the River Clare at Corofin having the highest flood level
recorded on 2nd November 1968. There is some uncertainty as to the accuracy of this level
as it was queried on the water level recorder chart in a contemporaneous note written by a
senior engineer of the OPW Hydrometric Section.
CONCLUSIONS
The November 2009 flood event in the west of Ireland was an exceptional event in terms of
long duration rainfall that extended over much of the West, South and Midlands and in
particular covered significant portion of the Shannon System which includes the River Suck
and River Fergus. Analysis of rainfall amounts by Met Eireann indicated extreme rainfall
totals for 8, 16, 25 days and monthly durations over these catchments with return periods
generally in excess of 200 years. The November 2009 flood event produced the maximum
recorded flood at all but one of 14 hydrometric gauges analysed for this paper.
Flood frequency analysis of long duration gauges on the subject catchments (50 to 80 year
series) using EV1 and GEV probability distributions fitted by L-moments (NERC, 1999)
produced return periods significantly greater than 100 years. For a number of gauges
estimates in excess of 300 years and in some cases 500 years return period were obtained.
Such estimates must be taken with a degree of caution as the extrapolation length from a
60 and 80 year series is very large.
The majority of the hydrometric gauges analysed on the Fergus and Shannon System have
shown significantly greater flood activity since 1993 with 4 and 5 out of the six largest floods
occurring in this latter period based on 60 and 80 years of records. The River Suck and River
Clare have not shown any definite trend towards increased floodiness.
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REFERENCES
Cawley A.M. and Cunnane C. (2003) “Comment on Estimation of Greenfield runoff Rate”,
Irish National Hydrology Conference Tullamore.
Cawley AM, Fitzpatrick J., Cunnane C. And Sheridan T. (2005) "Selection of Extreme Flood
Events – The Irish Experience", Irish National Hydrology Conference Tullamore.
Mandal, U., (2009), Trend Analysis, Part of Final Report of WP2.2 Flood Studies Update
Programme, OPW Technical Report.
McGrath R. Fealy R. and Sheridan T. (2010) “Recent Irish weather extremes and climate
change”, 9 th Scientific Statement, RIA Climate Change Science committee.
NERC(1975) “ Flood Studies Report, Volume I"
NERC(1999) “UK Flood Estimation Handbook”
Walsh S. (2010) “Report on Rainfall of Nov 2009” –Climatological Note No. 12, Met Eireann,
Feb 2010.
EPA (2010) “Report on Hydrometric Activities undertaken by Environmental Protection
Agency from 21 Nov 2009 to 3 Dec 2009".
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27002 Fergus 27003 Fergus 30004 Clare at 3007 Clare at 26002 Suck 26005 Suck
at Ballycorey at Corofin Corrofin, Galway Ballygaddy, Tuam at Rockwood at Derrycahill
1954-2009 1957-2009 1964-2009 1974-2009 1952-2009 1954-2009
2009 9.75 2009 20.864 1968 30.00 2009 35.11 2009 50.62 2009 45.90
1994 9.35 1994 20.541 2009 29.84 1989 34.94 1954 50.44 1968 45.39
1999 9.35 1968 20.499 2006 29.21 1999 34.92 1968 50.43 1999 45.29
2007 9.24 1999 20.471 1974 29.01 2006 34.78 1965 50.18 1990 45.24
1959 9.24 2005 20.401 1999 28.93 1986 34.66 2006 50.16 1989 45.22
1993 9.15 1959 20.369 1994 28.88 1985 34.61 1990 50.12 2001 45.21
Total N 56 53 46 36 57 56
Recent n 20 20 20 20 20 20
M = N - n 36 33 26 16 37 36
No. Top k 6 6 6 6 6 6
p = k/N 0.1071 0.1132 0.1304 0.1667 0.1053 0.1071
Recent x 5 4 3 3 3 4
Earlier y 1 2 3 3 3 2
nCx 0.039994 0.1164 0.23508 0.23789 0.2007 0.10415
MCy 0.073052 0.16325 0.23179 0.24231 0.20649 0.15341
Cond Prob = 0.002922 0.019 0.05449 0.05764 0.04144 0.01598
Sum Probs 0.169286 0.17053 0.1722 0.17587 0.16978 0.16996
P = 0.017259 0.11143 0.31644 0.32776 0.2441 0.09401
1 in 57.9 9.0 3.2 3.1 4.1 10.6
Table 8: The six largest annual maximum water level events on record and their years of occurrence for the stations listed and calculation of the
probability that these six are divided between the early period of record and the last 20 year period in the manner which occurred (x,y).
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26007 Suck Shannon at Shannon at Shannon at Shannon at Shannon at
at Bellagill Bridge Shannonbridge Portumna Killaloe Athlone Jamestown
1952-2009 19547-2009 1931-2009 1931-2009 1952-2009 1957-2009
2009 43.29 2009 38.749 2009 34.76 2009 34.33 2009 39.094 2009 45.02
1968 42.92 1999 38.27 1994 34.40 1994 34.01 1954 38.64 1999 44.52
1954 42.86 2006 38.22 1989 34.38 1989 33.94 2006 38.6 1964 44.35
2006 42.82 1994 38.21 1959 34.33 1959 33.94 1999 38.59 1974 44.32
1999 42.78 1954 38.19 1999 34.30 2006 33.91 1994 38.57 1983 44.31
1989 42.76 2001 38.19 2006 34.33 1999 33.90 2001 38.56 2006 44.31
Total N 58 56 79 79 58 53
Recent n 20 20 20 20 20 20
M = N - n 38 36 59 59 38 33
No. Top k 6 6 6 6 6 6
p = k/N 0.1034 0.1071 0.0759 0.0759 0.1034 0.1132
Recent x 3 5 4 4 5 3
Earlier y 3 1 2 2 1 3
nCx 0.19718 0.039994 0.045554 0.045554 0.035702 0.214538
MCy 0.20438 0.073052 0.109385 0.109385 0.06915 0.215358
Cond
Prob = 0.0403 0.002922 0.004983 0.004983 0.002469 0.046202
Sum
Probs 0.16961 0.169957 0.167079 0.167079 0.169957 0.170534
P = 0.2376 0.01719 0.029823 0.029823 0.014526 0.270929
1 in 4.2 58.2 33.5 33.5 68.8 3.7
Table 8: (continued) The six largest annual maximum water level events on record and their years of occurrence for the stations listed and calculation of the probability
that these six are divided between the early period of record and the last 20 year period in the manner which occurred (x,y)
22