Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
List of Figures
List of Tables
1. Introduction
2. Study catchments, data and methods
3. Variations in flood magnitude
4. Variations in flood frequency
5. Flood frequency analysis
6. Conclusions
Page ii ii
1
3
6
11
13
18 i

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
Page
Figure 1: Locations of study catchments and flow gauging stations 4
Figure 2: Annual maximum flood series for the four gauging stations 7
Figure 3: POT events exceeding thresholds giving 4.5, 2 and 0.2 events per year 12
Figure 4: Flood frequency analysis using partitioned flow records:
A. Dee at Manley Hall; B. Severn at Abermule,
C. Teifi at Glanteifi; D. Dyfi at Dyfi Bridge
16
Page
Table 1:
Table 2:
Table 3:
Table 4:
Table 5:
Comparison of recent trends in flood frequency and magnitude in selected British rivers
Characteristics of the four drainage basins
Inter-annual variability in flood magnitude, estimated using standard deviation
Statistical significance of the selected breakpoints in each record
Estimated flood magnitudes from frequency analysis on partitioned records
2
5
9
15
17 ii

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
1. Introduction
Temporal variations, or non-stationarity, in flood frequency and magnitude have been detected in both gauged flow records and longer reconstructed flood histories from many locations (e.g. Booy and Morgan, 1985; Rumsby and Macklin, 1994; Black, 1996; Longfield and Macklin, 1999; Benito et al.
,
2003; Mudelsee et al.
, 2003; Sheffer et al.
, 2003). Such non-stationarity in flood regime violates one of the principal assumptions (the stationarity of the mean) underpinning conventional flood frequency analysis (Beven, 1993;
Franks, 2002). The results of a flood frequency analysis depend not only on the length of record available but also on which particular years the flood record encompasses (Kiem et al.
, 2003). This will be most significant in cases where either trends or step-changes in flood frequency and magnitude are present within the available record (Reed, 1999). The recent occurrence of several major flood events in Welsh rivers (e.g. 1998, 2000, 2002) suggests that there may have been a sufficient change in recent flood regime to affect the results of flood frequency analysis. This could be manifest in two ways: first, the return period for a flood of a given magnitude could change, and second, the magnitude of a flood of a given return period could increase or decrease.
Changes in flood frequency and magnitude have been detected in gauged flow records from western Scottish rivers since 1988, including the occurrence of several maximum recorded floods and increases in flood frequency (Black,
1996). Peaks-over-threshold (POT) frequencies attained maximum recorded values in western Scottish catchments during the 1980s and 1990s (Black and
Burns, 2002). In England, the Easter 1998 flood event produced maximum recorded peak flows in many catchments in Warwickshire, Northamptonshire and Oxfordshire (Horner and Walsh, 2000). In the Tyne, in northern England, flood frequency was low between 1970 and 1979 and increased from 1980, with a sequence of major floods after 1986 (Rumsby and Macklin, 1994).
Similar results were obtained from the Ouse at York where high flood frequencies and magnitudes were recorded in 1944-1968 and 1978-1996 separated by a period of extremely low flood frequency and magnitude during
1969-1977 (Longfield and Macklin, 1999). Disastrous flooding also occurred in the 1980s and 1990s in south and west Ireland (Kiely, 1999).
An analysis of national changes in UK flood regimes revealed no continuous trends during the period 1940-1990 although systematic fluctuations, similar to those in precipitation values, were present (Robson et al.
, 1998). Subsequent work confirmed the absence of a long-term trend over the last 80-120 years but found evidence of an increase in protracted high flows during the last 30-
50 years (Robson, 2002). This was confirmed by Marsh and Dale (2002) who reported an increased occurrence of high long-duration flows in five large catchments in England and Wales from 1990. This reflects the increase in
1

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments total winter precipitation between 1961 and 2000 reported by Osborn and
Hulme (2002). There was also an increase in the intensity of winter precipitation between 1961 and 1995 (Osborn et al.
, 2000).
In Welsh rivers changes in flood frequency and magnitude were first reported by Howe et al.
(1966; 1967), who found a substantial increase in flood heights at Welsh Bridge, Shrewsbury, on the Severn during 1940-1964 compared with the period 1911-1940 (Table 1). A similar pattern was found on the Wye
(Howe et al.
, 1966), although Walsh et al.
(1982) suggested that the increase there occurred in the late 1920s. A later analysis of flooding in the Severn found that the highest frequency of floods occurred in the late 1940s and early to mid 1960s, followed by a decline after 1968 and an increase in the late
1970s and early 1980s (Higgs, 1987). On the Tawe, in south Wales, a much greater frequency of high-magnitude floods was found during 1929-1981 than in the period 1875-1928, with distinct clusters of major floods occurring in
1929-1933, 1957-1967 and 1979-1981 (Walsh et al.
, 1982). Similar results were reported from the Ebbw. Black (1996) found no maximum recorded floods in the Teifi, upper Severn and Dee to parallel those which occurred in western Scottish catchments after 1988. However, recent work in the Trannon subcatchment of the Severn found an increase in flood magnitude and frequency since 1988 (Mount et al.
, 2005).
Table 1: Comparison of recent trends in flood frequency and magnitude in selected British rivers
River Period of record
Date of change
Increase or decrease in flood frequency and magnitude
Reference
Howe et al.
(1966; 1967)
Wales
Severn
Wye
Tawe
Ebbw
Trannon
England
Tyne
Ouse
1911-
1964
1953-
1983
1911-
1964
1875-
1981
1908-
1978
1969-
2002
1700-
1992
(1875-
1992 shown here)
1878-
1996
1940
1968
1978
Decrease
Increase
1940 Increase
1929
1929
1988
1875
1895
1920
1940
1955
1970
1980
1904
1944
1969
1978
Increase
Increase
Increase
Increase
Increase
Decrease
Increase
Decrease
Increase
Decrease
Increase
Increase FM
Increase FF
Decrease
Increase
Notes: FM = flood magnitude; FF = flood frequency
Higgs (1987)
Howe et al.
(1966)
Walsh et al.
(1982)
Walsh et al.
(1982)
Mount et al.
(2005)
Rumsby and Macklin
(1994)
Longfield and Macklin
(1999)
2

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
The effects of natural climatic variation on the results of flood frequency
(return period) analysis were first demonstrated by Howe et al.
(1966; 1967) for Welsh Bridge on the upper Severn. Howe et al.
(1966; 1967) split the flood record into two parts (pre-1940 & post-1940) and plotted flood height against recurrence interval. The gradient of the fitted frequency curves, and their position relative to the flood height axis, on the graph increased from the
1911-1940 period to 1940-1964. Higgs (1987) found that there was a significant decrease in the gradient of curves fitted to return period analyses on records from Caersws and Montford on the upper Severn which were divided at 1968/1969. Comparable analyses from other British rivers are lacking. However, Dunne and Leopold (1978) performed flood frequency analysis on a 36 year flood record (1934-1970) from the Tana River, Kenya, partitioned at 1961. The resulting frequency curves had markedly different gradients. Booy and Morgan (1985) divided a long flood record from the Red
River at Winnipeg, Canada, into three equal parts and obtained a similar result. More recently Franks (2002) has shown the effect of a marked change in climate on flood frequency curves using flow data from New South Wales while Kiem et al.
(2003) examined the effect of climatic oscillations (ENSO,
IPO) on flood frequency curves using the same data. Jain and Lall (2000) found systematic variations in the magnitude of the “100 year flood” when estimated using a 30 year moving window on the 82 year flood record from the Blacksmith Fork River.
The aim of this Technical Appendix is to (1) use gauged flow records to determine whether there has been a recent change in the frequency and magnitude of flooding in Welsh rivers, and (2) investigate the potential effects that any change might have on the results of flood frequency analysis. First, the study catchments, data and methods used are outlined. Second, the annual maximum flood records are examined for changes in flood magnitude.
Third, the peaks-over-threshold records are analysed for changes in flood frequency. Finally, the records are partitioned to gauge the effect of the observed changes on the resulting flood frequency curves.
This work was undertaken as part of the Predictive and Investigative
Modelling of Flood Risk within Welsh River Catchments, a multi-partner project funded by the Welsh Assembly Government, Environment Agency
Wales, Countryside Council for Wales, British Geological Survey and
University of Wales, Aberystwyth. The data used were provided by the
Environment Agency.
2. Study catchments, data and methods
Four Welsh rivers, the Dee, Dyfi, upper Severn and Teifi, were selected for this study, of which two drain to the west and two to the east of the Cambrian
Mountains (Figure 1). These rivers have relatively large catchments (> 400 km 2 ), long-standing flooding problems, particularly in their lower reaches and comparatively long flow records. Very few Welsh rivers were gauged prior to
3

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
Figure 1: Locations of study catchments and flow gauging stations.
1960, when a major expansion of the gauging network began. Most records are therefore around 30 to 40 years in length (Table 2). The catchments are similar in having relatively high relief and high mean annual rainfall. Rainfall is lower in eastward draining catchments, as would be expected. The large difference in mean annual rainfall between the Dyfi and Teifi, which both drain west, is due to the difference in relief and catchment configuration. Mean and maximum daily flow data were available for the four gauges. A difference of opinion exists over the suitability of mean daily flow data for analysis of flood frequency and magnitude. For example, Robson and Reed (1999) regard daily mean flow data as unsuitable for flood frequency analysis due to the short response times of many UK catchments while Svensson and Jones
(2004) consider daily mean flows to be indicative of the magnitude of the maximum daily flow. For two of the gauges, Abermule and Glanteifi, the
4

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
5

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments maximum daily flow records began in 1990. The mean daily flow records began in 1962 and 1959, respectively and these were used in all analyses.
Since these gauges record runoff from a relatively large catchment area daily mean flows approach maximum daily flow values. Greater differences may occur at very high flows due to uncertainty in the upper part of the rating curves. Some caution should therefore be exercised when interpreting these results. For Dyfi Bridge maximum daily flow data were available from 1979.
The mean daily flow record from this site began in 1962 but four years of data were missing during the early 1970s. For this reason the maximum daily flow record was chosen for analysis. For Manley Hall on the River Dee mean and maximum daily flow data were available from 1970. Mean daily flow data from
1937-1968 were available from the Erbistock gauge, the predecessor of the current Manley Hall gauging station. The two stations are less than 1 km apart. However, preliminary analysis showed the Erbistock data to be of significantly lower quality then the Manley Hall data. These data were therefore excluded from the analysis and the maximum daily flow data from
Manley Hall were used.
Analysis was based on water years (starting October 1 st ) since winter flooding dominates the regime in these largely un-urbanised catchments. Water years were labelled using the calendar year in which they begin, following Robson and Reed (1999). Part-years of data at the beginning and end of the record were excluded. All records end on 30 th September 2004. Annual maximum and peaks-over-threshold series were extracted from the record for each gauge. Independence of peaks was established using the methods outlined by Robson and Reed (1999). Annual frequencies were derived from the peaks-over-threshold series based on three thresholds set to give 4.5, 2 and
0.2 floods per year over the period 1979-2003.
3. Variations in flood magnitude
The annual maximum flood series have been plotted for each of the four gauging stations and the pattern of variation smoothed using a 5-year moving average (Figure 2). Temporal variations in the magnitude of the annual maximum flood are apparent at all four stations. The four types of temporal behaviour associated with climatic variability recognised by Reed (1999) can all be identified in the annual maximum series (trend, step change, periodicity, and quasi-random behaviour). However, long-term trends, persisting through the entire period of record, are notably absent at these sites. In the latter part of the Abermule record (after 1987) a trend of increasing flood magnitude is apparent, but this appears to be the only example in the four records. It coincides with a period of high inter-annual variability where there is no apparent trend in the lower-magnitude floods. Therefore, the apparent trend is primarily the result of the increasing frequency of high-magnitude flood events. Step changes in flood magnitude are more common. The abrupt increase in flood magnitude at Manley Hall in the period 1998 to 2000 and the sudden decrease in flood magnitudes from 1967 to 1968 at Abermule are particularly pronounced examples of this type of behaviour. There is some
6

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
400
350
300
250
200
150
100
50
500
Dee at Manley Hall 1970-2003
450
400
350
300
250
200
150
100
50
0
500
1960 1965 1970 1975 1980 1985 1990 1995 2000
Dyfi at Dyfi Bridge 1979-2003
450
200
150
100
50
400
350
300
250
400
350
300
250
200
150
100
50
0
1960 1965 1970 1975 1980 1985 1990 1995 2000
500
Severn at Abermule 1962-2003
450
0
1960 1965 1970 1975 1980 1985 1990 1995 2000
500
Teifi at Glanteifi 1959-2003
450
0
1960 1965 1970 1975 1980 1985 1990 1995 2000
Water year
Figure 2: Annual maximum flood series for the four gauging stations.
7

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments evidence for periodicity in the data, particularly in the early part of the Glanteifi record, where there appears to be a cycle of six to eight years in the magnitude of the annual maximum flood. Certain parts of the times series do not conform to any of these patterns of variation and may represent the fourth type of behaviour identified by Reed (1999), that is, quasi-random behaviour.
There are considerable differences between the annual maximum flood series from the four catchments. This reflects the different locations of the catchments with respect to the tracks of flood-producing weather systems and varying catchment characteristics such as relief (Figure 1 and Table 2). Floodproducing storms tend to cross Wales in either an easterly or a north-easterly direction. This means that areas to the east of the Cambrian Mountains, in parts of the Dee and Severn catchments, receive significantly less rainfall than those further to the west. The latitude of the catchment, with respect to storm tracks, may also be a factor determining the amount and intensity of precipitation received. Orographic enhancement of precipitation amounts will be greatest in areas of high catchment relief and slope, for example the Dyfi catchment. Differences in the size, and the inter-annual variability, of annual maximum floods are also dependent on basin size and shape. This affects the total amount of river flow and the arrival times at the gauging station of peaks from the various sub-catchments. Differences in geology, soils and land cover, the proportion of catchment runoff which is regulated through flood control and other reservoirs, and the capacity for floodwater storage on the floodplain also affect the magnitude of peak river flows. The differences between the four records provide some evidence for an east-west divide in patterns of variation of annual maximum flood magnitude in Wales. There are some striking similarities between the Manley Hall and Abermule records, which come from the rivers which drain to the east of the Cambrian Mountains. The Dyfi Bridge and Glanteifi series, from the west-flowing rivers, also show some similarities.
The differences between the records from the east-flowing and west-flowing rivers are particularly marked although the pattern is complicated by the other factors mentioned above.
During the mid-1960s a series of large magnitude floods occurred in the
Severn, followed by a sudden large decrease in flood magnitude from 1967 to
1968. The record from the Erbistock gauging station, predecessor of the current Manley Hall gauge on the Dee, shows that particularly large floods occurred there during 1964 and 1965. The two records were similar during the
1970s, with low-magnitude floods occurring on both rivers. Inter-annual variability was also low at both stations during this period. However, the 1972 flood on the Severn stands out in the Abermule record as particularly large.
The peak flow was recorded on 6 th August 1973. This summer flood was caused by widespread intense rainfall associated with the passage of a cold front falling on an already wet catchment (Newson, 1975). Summer floods are unusual but not unknown: of the 42 annual maximum floods in the Abermule record all except three occurred between October and March. The period of low-magnitude flooding continued at both sites through the early 1980s. A large flood occurred on both rivers in 1987. At Abermule this represented a pronounced change in flood regime. A step-change in magnitude occurred, followed by a trend of increasing flood magnitude and increasing frequency of
8

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments high-magnitude floods. At Manley Hall the large flood of 1987 appears to be more of an isolated event as floods during the late 1980s and early 1990s were no larger than those which occurred during the 1970s and early 1980s.
A step-increase in flood magnitude occurred during the late 1990s at Manley
Hall. Particularly large floods occurred during 2000 and 2003 at this site. At both gauges a simultaneous increase in inter-annual variability accompanied the increase in flood magnitude (Table 3). Lower-magnitude annual maximum floods continued to occur but were, in both cases, much rarer than formerly.
Table 3: Inter-annual variability in flood magnitude, estimated using standard deviation
Catchment Gauge Period of record
Annual maximum flood series
Selected periods of record
Mean
(m 3 s -1 )
Standard deviation
Time period Mean
(m 3 s -1 )
Standard deviation
Teifi
Dee
Dyfi
Severn
Manley Hall 1970-2003 230.6
Dyfi Bridge 1979-2003 307.8
Abermule 1962-2003 141.7
Glanteifi 1959-2003 185.7
72.0
48.8
50.1
53.7
1970-1997 214.2 49.6
1998-2003 306.9 111.6
1979-1983 351.5
1984-1997 290.1
1998-2003 312.5
1962-1969 171.1
40.7
52.0
15.9
68.3
1970-1986 113.7
1987-2003 156.0
1959-1982 179.7
1983-1996 185.5
32.6
42.9
46.3
70.4
1997-2003 206.4 27.8
The context provided by the record from the Erbistock gauging station, where clusters of particularly large floods were recorded during the mid-1940s and the early to mid-1960s, shows that the occurrence of two such large flood peaks as those recorded at Manley Hall in 2000 and 2003 is not exceptional.
Similar clustering of large magnitude floods reported by Walsh et al.
(1982) suggests that this behaviour is not unusual in Welsh rivers. At Abermule the magnitude of floods from the recent period of high-magnitude flooding have not attained those observed during the 1960s. The operation of the Clywedog dam, which regulates a small part of the catchment, may have a minor influence on the magnitude of flood peaks in the Abermule area. However, differences in the characteristics of flood-generating storms, such as location
9

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments of storm tracks and rainfall intensity, may also be responsible. The most striking difference between the Abermule and Manley Hall records is the earlier onset of the recent period of high-magnitude flooding recorded at the former station. This may be due to the locations of the two catchments with respect to storm tracks, differences in catchment characteristics and the larger percentage of the Dee catchment which is subject to flow regulation. Gradual changes in the location of common storm tracks may also have affected one catchment before another. Furthermore, flood control reservoirs in the Dee catchment may have been used to limit the amount of water reaching the gauge in the downstream part of the catchment for floods below a certain magnitude.
The similarities between the flood record in the rivers draining to the west of the Cambrian Mountains include a decrease in flood magnitude from 1979 through the early 1980s, a period of relatively high inter-annual variability from the mid-1980s to the mid-1990s, and a period of moderate magnitude floods with low inter-annual variability at the end of the record. Comparisons between the two sites are more difficult due to the short record length at Dyfi
Bridge. The most notable difference is the absence of particularly large floods at Dyfi Bridge during the period of high variability from the mid-1980s to the mid-1990s. This period had different characteristics at the two sites. At
Glanteifi it was a period of generally low-magnitude flooding punctuated by large floods while at Dyfi Bridge it was a phase of usually moderate floods interrupted by two larger and three particularly small annual maximum floods.
The low variability which characterises the last years of the record at each gauge is where the highest level of similarity is observed between these two sites (Table 3).
The differences between the annual maximum flood series from the rivers draining to the west and to the east may be due in part to the rain-shadow effect caused by the presence of the Cambrian Mountains. This explains why annual maximum floods in the smallest catchment, the Dyfi, may frequently be larger than those which occur in the Dee catchment, the largest catchment in the current study. The difference in catchment relief and basin shape partly explains differences in annual maximum flood magnitude between the Dyfi and Teifi catchments. The location of the gauging station may also have an influence on the results. For example, gauges in the Dyfi and Teifi catchments are located at less than 6 m above mean sea level. Although the tidal limit is downstream of the gauges extreme high tides in the estuary may possibly impede the progress of flood water and enhance peak stage at the gauging station. However, it should be noted that Spencer (2006) does not record any tidal influence at these sites. The Manley Hall and Abermule gauges are both located at significantly higher altitudes and are thus unaffected by tidal processes.
10

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
4. Variations in flood frequency
Peaks-over-threshold series were used to calculate annual frequencies for three nested thresholds set to give an average of 4.5, 2.0 and 0.2 events per year during the period 1979-2003 at each gauging station (Figure 3). The use of three nested thresholds allows changes in the frequency of minor, moderate and major flood events to be examined. Thresholds selected are those commonly used by previous researchers (e.g. Longfield and Macklin,
1999; Black and Burns, 2002). The data plotted show the total number of flood peaks exceeding each threshold in each water year. Temporal variations are apparent in the annual frequencies of floods exceeding each of the three thresholds at each site. These changes are superimposed on a high level of inter-annual variability in the data. The east-west divide is less apparent in temporal patterns of flood frequency than of flood magnitude. Clustering of flood peaks above the highest threshold is seen in all four records as would be expected from the clustered occurrence of high-magnitude annual maximum floods. In addition, it is evident that it is not uncommon for two highmagnitude floods to occur during the same water year. Variations also exist between the temporal patterns of floods of different magnitudes. For example, an increase in the occurrence of minor floods (those exceeding the lowest threshold but not reaching the middle threshold) at Abermule precedes an increase in the frequency of moderate and major floods around 1987. At
Manley Hall the increase in the frequency of moderate floods occurs during the 1980s, preceding the increase in flood magnitude in the late 1990s. This suggests a gradual change, as opposed to a step change, in the characteristics, such as location of the storm track or intensity of rainfall, of flood-producing storms. A greater apparent difference between flood magnitude and flood frequency is found in the record from Glanteifi. A similar periodicity is present in the record but continues through the whole of it; superimposed on this periodicity is a trend of increasing flood frequency.
Similarities in the temporal patterns of the two data series suggest that there is a relationship between flood magnitude and flood frequency. For example, flood frequency on the Severn at Abermule reflects the magnitude of the annual maximum flood with high numbers of floods during the 1960s, low numbers during the 1970s and an increase in the mid to late 1980s. Pearson product-moment correlation coefficients between annual maximum flood magnitude and the annual frequency of floods above the middle and high (2.0 and 0.2 events per year) thresholds are strong and positive for all sites. The dominant flood-producing weather type in these catchments is heavy orographic precipitation associated with frontal systems which occur in winter.
Years in which high-magnitude floods occur are, therefore, usually characterised by the frequent occurrence of flood-producing weather systems.
This is important to consider when interpreting the results of flood frequency analyses, since they are frequently performed on the annual maximum data series.
11

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
6
4
2
10
Dee at Manley Hall 1970-2003
8
0
10
1960 1965 1970 1975 1980 1985 1990 1995 2000
Dyfi at Dyfi Bridge 1979-2003
8
6
4
2
4
2
0
10
1960 1965 1970 1975 1980 1985 1990 1995 2000
Severn at Abermule 1962-2003
8
6
0
1960 1965 1970 1975 1980 1985 1990 1995 2000
10
8
Teifi at Glanteifi 1959-2003
4.5 events per year
2.0 events per year
0.2 events per year
6
4
2
0
1960 1965 1970 1975 1980 1985 1990 1995 2000
Water year
Figure 3: POT events exceeding thresholds giving 4.5, 2 and 0.2 events per year.
12

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
5. Flood frequency analysis
Flood frequency analyses were carried out using the basic method introduced by Gumbel (1958; Dunne and Leopold, 1978) in order to demonstrate the possible effects of non-stationarity in the record, rather than attempt to provide a definitive flood frequency analysis for each site according to the method recommended in the Flood Estimation Handbook (Robson and Reed, 1999).
Following the approach used in previous studies, the annual maximum flood record for each site was first divided into periods characterised by differing flood magnitude, (e.g. Howe et al.
, 1966). Based on analysis of annual maximum and peaks-over-threshold series the Manley Hall, Abermule and
Glanteifi records were divided into two 17-year periods, 1970-1986 and 1987-
2003, and earlier data were discarded. The t -test was used to determine whether there was a statistically significant difference in flood magnitude between the individual time periods (Table 4). This division did not suit all three gauges equally well but facilitated comparison between them. 1970 represents the start of the Dee record at Manley Hall. It was also considered a suitable point to partition the Severn record after the period of high-magnitude flooding during the 1960s. Flood magnitudes were significantly greater during the period 1962-1969 than in 1970-1986 at Abermule (Table 4). There were insufficient data for the earlier period to include it in the analysis. 1987 was considered an appropriate point at which to separate the more recent period of high flood frequency and magnitude from the earlier period of lower flood frequency and magnitude. Flood magnitudes were significantly greater during
1987-2003 than in 1970-1986 at Manley Hall and Abermule (Table 4). This division also supplies two series of equal length for each of the three gauging stations, which provides a good basis for comparing the results from the two periods. This division of the period of record was unsuitable for the shorter record from Dyfi Bridge This record was therefore partitioned at 1991, this year being included in both analyses to give records of equal length. The flood frequency analysis for the Dyfi catchment is therefore not directly comparable with the those for the other three catchments. Each data series was plotted on log-normal axes using the Weibull formula to determine plotting positions.
Straight lines were fitted to each data set using linear regression.
This analysis of the partitioned record is subject to limitations (see Kidson and
Richards, 2005, for a discussion) in addition to those arising from the method of flood frequency analysis chosen. The annual maximum flood records used are relatively short, particularly when estimates of the magnitude of rare floods are required. Partitioning the record shortens the series further, reducing the confidence that can be attached to the results. The actual divisions between periods of high- and low-magnitude flooding appear to be slightly different for each of the gauges here. These divisions have been standardised to facilitate comparisons between gauges but this means that somewhat artificial divisions are imposed at some of the sites. The exact position of a division within the record may not always be obvious, particularly when a gradual change in flood magnitude occurs. The period of record is also a limiting factor when attempting to define periods of high- and low-magnitude flooding, since portions of a particular period may fall outside the available record. The small number of different periods defined within the partitioned record (two) cannot
13

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments be used to define the possible limits of flood magnitudes associated with particular exceedence probabilities at each site. Earlier, and future, periods of flooding may fall between or outside the range of results from the current analysis.
Table 4: Statistical significance of the selected breakpoints in each record.
Catchment Gauge First time period Second time period Significance level
Years Mean
(m 3 s -1 )
Years Mean
(m 3 s -1 )
(one-tail test)
Dee
Dyfi
Severn
Manley Hall 1970-1986
Dyfi Bridge 1979-1990
Abermule 1962-1969
207.5 1987-2003
317.4 1991-2003
171.0 1970-1986
253.7 0.05
298.9 Not significant
113.7 0.05
Teifi Glanteifi
1970-1986
1959-1969
113.7 1987-2003
172.4 1970-1986
156.0 0.01
179.5 Not significant
1970-1986 179.5 1987-2003 200.6 Not significant
N.B. Not significant refers to non significant differences at a significance level of 0.1.
Flood frequency analysis on the partitioned annual maximum flood record produced different results at each of the four gauging stations (Figure 4). At three of the sites, Manley Hall (Dee), Abermule (Severn) and Glanteifi (Teifi), there was an increase in the magnitude of floods associated with small exceedence probabilities. However, there was a decrease in the magnitude of floods associated with small exceedence probabilities at Dyfi Bridge. This is consistent with the pattern of temporal variation in annual maximum flood series noted above. Differences between the first and second periods occurred in both the gradient of the distribution and the position of the data with respect to the y -axis (annual maximum discharge). At Manley Hall on the
Dee, the gradient of the regression line is much steeper in the later period than the first. There is a 51 % increase in the estimated magnitude of the flood with an exceedence probability of 0.05 from the 1970-1986 period to the
1987-2003 period (Table 5). The same effect is visible in the record from
Glanteifi, although the difference in gradient is much less and there is only a
20 % increase in the estimated magnitude of the flood with an exceedence probability of 0.05 between 1970-1986 and 1987-2003. At Abermule there is a relatively small increase in the gradient between the first and second periods but the magnitude of floods of all exceedence probabilities were significantly greater in 1987-2003 than in 1970-1986. This equates to a
46 % increase in the estimated magnitude of the flood with an exceedence probability of 0.05 from the earlier to the later period. However, this is in spite of the very similar magnitude of the largest flood in each part of the partitioned data series. The partitioned record from Dyfi Bridge shows a decrease in the gradient of the fitted line between the 1979-1991 and 1991-2003 periods. This
14

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments gives a reduction of 12 % in the estimated magnitude of a flood with an exceedence probability of 0.05. There is greater uncertainty associated with this estimate then those for the other sites due to the shorter record length.
1970-1986
Linear fit 1970-1986
1987-2003
Linear fit 1987-2003
1000
800
600
400
200
100
80
60
40
20
1000
800
600
400
200
100
80
60
40
20
0.999
0.99
0.95
0.8
0.6
0.4
0.2
Exceedence probability
0.05
0.01
1E-3
1970-1986
Linear fit 1970-1986
1987-2003
Linear fit 1987-2003
0.999
0.99
0.95
0.8
0.6
0.4
0.2
Exceedence probability
0.05
0.01
1E-3
Figure 4: Flood frequency analysis using partitioned flow records: A. Dee at
Manley Hall; B. Severn at Abermule.
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Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
1000
800
600
400
200
100
80
60
40
20
1970-1986
Linear fit 1970-1986
1987-2003
Linear fit 1987-2003
0.999
0.99
0.95
0.8
0.6
0.4
0.2
Exceedence probability
0.05
0.01
1E-3
1000
800
600
400
200
100
80
60
40
20
1979-1991
Linear fit 1979-1991
1991-2003
Linear fit 1991-2003
0.999
0.99
0.95
0.8
0.6
0.4
0.2
Exceedence probability
0.05
0.01
1E-3
Figure 4 (continued): Flood frequency analysis using partitioned flow records:
C. Teifi at Glanteifi; D. Dyfi at Dyfi Bridge.
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Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments
Table 5: Estimated flood magnitudes from frequency analysis on partitioned records
Catchment Gauge
Dee
Dyfi
Severn
Teifi
Time period Estimated magnitude of flood with exceedence probability of 0.05
Manley 1970-1986
Hall
1987-2003
Dyfi Bridge 1979-1991
1991-2003
Abermule 1970-1986
1987-2003
Glanteifi 1970-1986
1987-2003
297
449
433
382
175
255
280
335
Percentage change
+51
-12
+46
+20
The results obtained from the Severn at Abermule show a change in the gradient of the flood frequency curve similar to that reported by Howe et al.
(1966) from analysis of flood height data from the Severn at Welsh Bridge,
Shrewsbury, from 1911 to 1964, divided at 1940 based on an observed change in flood magnitude. The use of flood height data mean that comparison of the relative change in flood magnitude in the two studies was not possible. The observed pattern of variation at Glanteifi, Manley Hall and
Dyfi Bridge, altering the gradient of the flood frequency curve, is similar to that found by Dunne and Leopold (1978) for the Tana River in Kenya and Booy and Morgan (1985) for the Red River at Winnipeg. Both types of change, shifts in the position of the data and changes in the gradient of the flood frequency curve, also occur in the data presented by Franks (2002) for gauges in New South Wales where the gauge records were divided on the basis of an observed change in flood magnitude which occurred in 1945.
If the data for the two periods are combined at each site, the resulting regression line falls between the two plotted for each gauge in Figure 4, leading to underestimation of flood magnitudes in one period and overestimation in the other period. The degree to which flood magnitudes associated with a particular return period are over- or under-estimated will depend on the degree of displacement between the two data sets. In the case of the Manley Hall and Abermule records this difference is considerable. The inclusion of the pre-1970 portion of the record in the analysis would complicate matters further. The proportions of “wet” and “dry” periods in a gauged record will have a strong influence on the parameters of the resulting flood frequency distribution. Where flood frequency analysis is performed on a relatively short gauged record the year in which the gauge was established will be significant. For example, in certain catchments a gauge which became operational in 1970 would have yielded very different results if flood frequency analysis was performed on the available data in 1979 than if the gauge was commissioned in 1960 and the analysis performed in 1969. This highlights the need to review flood frequency analyses periodically as more data become available at a site. The broad synchronism between changes in flood
17

Technical Appendix 6: Recent changes in flood frequency and magnitude in Welsh river catchments frequency and magnitude at gauges in different catchments supports the conclusion of Jain and Lall (2000) that pooling data from records of differing length, and therefore incorporating unspecified proportions of “wet” and “dry” periods, in flood frequency analysis may significantly affect regional flood frequency estimates.
6. Conclusions
The results presented here show that flood magnitude and frequency have varied significantly during the last 40 years in Welsh river catchments. The assumption of stationarity must therefore be rejected at this temporal scale.
The data suggest an east-west spatial divide in the temporal variation of flood magnitude within Wales, probably due to the presence of the Cambrian
Mountains. Further work is needed to determine how temporal changes in flood magnitude vary spatially in other parts of Wales. The results confirm that there has been a recent increase in flood magnitude in eastern Welsh rivers and an increase in flood frequency at three of the four study sites. Recent flood magnitudes do not appear to be exceptionally large when compared with the magnitude of floods during the 1940s and 1960s. The results show that there is a relationship between the magnitude of the annual maximum flood and the annual frequency of moderate and major floods. The relatively frequent occurrence of more than one major flood within a water year suggests that exceedence probabilities based on the analysis of annual maximum flood series alone in Welsh catchments should be treated with some caution. The clustering of high-magnitude floods also suggests that the assumption of independence of annual maximum flood peaks is invalid and that further work is necessary to determine how the exceedence probability of high-magnitude floods varies depending on their previous occurrence.
The observed changes in flood magnitude and frequency can have a marked effect on the results of flood frequency analysis. Analysis of gauged records partitioned on the basis of variations in flood magnitude and frequency show that changes in flood regime may alter the gradient of the flood frequency curve and the position of the data set relative to the y -axis (flood magnitude).
These results are consistent with earlier findings from the River Severn and with results from catchments in other parts of the world. Further work is required to determine how the different methods and models employed in flood frequency analysis are affected by temporal variations in flood frequency and magnitude such as those observed in this study.
18