Use of multi-proxy flood records to improve estimates of flood risk: Lower River Tay, Scotland

Catena 66 (2006) 107 – 119 www.elsevier.com/locate/catena

Use of multi-proxy flood records to improve estimates of flood risk: Lower River Tay, Scotland A. Werritty a,*, J.L. Paine a, N. Macdonald b, J.S. Rowan a, L.J. McEwen c a Environmental Systems Research Group, Department of Geography, University of Dundee, Dundee, DD1 4HN, UK School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK c School of Environment, University of Gloucestershire, Francis Close Hall, Swindon Road, Cheltenham, Gloucestershire, GL50 4AZ, UK b

Abstract Proxy flood records from sediment stacks in floodplain palaeochannels provide an opportunity to extend short instrumental records and thus improve current estimates of flood risk. The FBloody Inches_ (a meander cutoff on the lower River Tay, Scotland) has been infilling with flood deposits since c. 1761. Agricultural flood embankments locally breach with flows >850 m3 s 1 (introducing silts into the palaeochannel) and extensively fail with flows > 1200 m3 s 1 (which deposit sand). Repeated cores at the site (up to 1.4 m in depth) consistently reveal sand-rich flood units. In the upper section of the core, 137Cs dating enables these units to be correlated with clusters of floods in the post-1950 discharge record. Sand units in the lower part of the profile are correlated with major floods from 1814 onwards using a 210Pb-based chronology. These dates are independently corroborated by flood marks inscribed on Smeaton’s Bridge in Perth, 15 km downstream. Estimates of historic high flows back to 1814 can also be recovered from these flood stage levels at Smeaton’s Bridge. When combined with the 48-year-long flow record, these historic floods yield an augmented POT series extending back to 1847. Using a relatively high threshold of 1361 m3 s 1 and the Generalised Pareto Distribution to model this POT series, estimates of the 50-, 100- and 200-year floods are 1875, 2250 and 2050 m3 s 1, respectively. These results provide independent and robust confirmation of the flood risk at Perth using the standard statistical procedures advocated in the Flood Estimation Handbook. If successfully replicated at other sites, radiometric dating of sediment stacks in palaeochannels provides a new technique to further advance palaeohydrology and the recovery of historic floods. D 2005 Elsevier B.V. All rights reserved. Keywords: Proxy flood records; Palaeochannels; Sediment stacks; Radiometric dating; Flood risk analysis; Historic records

1. Introduction Many parts of north-western and central Europe have recently experienced large and widespread floods often attributed in the media to the early onset of climate change. These catastrophic floods have been especially severe in Southern and Eastern England (2000 and 2002), the Tizsa River, Hungary (2000) and in Prague and Dresden (2002). In Scotland, the late 1980s and early 1990s produced the wettest period since the 1750s (Smith, 1995) and for many rivers their highest recorded flows (Black and Burns, 2002). Such a pattern is consistent with the warmer and wetter

* Corresponding author. Tel.: +44 1382 345084. E-mail address: [email protected] (A. Werritty). 0341-8162/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2005.07.012

Scotland predicted by UK climate modellers (Hulme et al., 2002) but may also reflect a high degree of natural variability. Separating out natural variability from an upward trend in flood risk is crucial for an informed approach to future investment in flood defences (Werritty and Chatterton, 2004) but difficult with river flow records which rarely exceed 50 years in length (Black and Burns, 2002; Robson, 2002). Such short records also make it difficult to produce reliable and robust estimates for the return periods of recent large floods (e.g., the River Ness in 1990, the Rivers Tay, Earn and Helmdale in 1993 and the River Clyde in 1994: Werritty et al., 2002). But if these floods are to be regarded as potential analogues of a warmer and wetter Scotland, determining their frequency is also important for establishing the standards of protection for future or upgraded flood defences. A further complication is

108

A. Werritty et al. / Catena 66 (2006) 107 – 119

the identification of ‘‘flood-rich’’ and ‘‘flood-poor’’ epochs in the recent flood record (Werritty et al., 2002) sampling from which can generate strikingly different estimates of flood risk (see Kiem et al., 2003 and Kidson and Richards, 2005 for examples from SE Australia and northern Thailand, respectively). This adds another element within the natural variability of the flood record. One way of addressing these problems is to compile a flood chronology from documentary evidence which can extend the instrumental period back over several centuries. Many researchers have promoted the use of historical information in flood frequency analysis (Benson, 1950; Stedinger and Cohn, 1986; Hirsch, 1987; Wang, 1990; Bayliss and Reed, 2001) and in Britain the recent Flood Estimation Handbook (Institute of Hydrology, 1999) recommends this as one of several procedures for improved estimates of flood risk. In other parts of the world where rivers are incised into bedrock or cemented terraces, it has been possible to extend such analyses over millennia using palaeostage indicators such as slack water deposits and bedrock scour features (Benito, 2003). The ensuing reconstruction of a palaeoflood chronology provides a robust and credible addition to conventional flood risk analysis based solely on instrumental records (see Baker, 2003 for a recent review). In countries where rich documentary archives can be interrogated alongside these sediment archives, there has been a recent convergence of approaches with flood series being reconstructed using both documentary and sedimentbased sources. The recent SPHERE (Systematic Palaeoflood and Historical data for the improvement of flood Risk Estimation) programme funded by the European Commission has generated high quality flood series for the Rivers Tagus, Ter and Llobregat in Spain (Benito and Thorndycraft, 2004) and the Arde`che River in Southeast France using both documentary and sediment based sources (Sheffer et al., 2003). In humid temperate environments where slackwater deposits are less well developed, palaeoflood records are more commonly found either within overbank deposits along rivers (e.g., Vistula in Southern Poland, Starkel, 2002; the middle and lower South Tyne, Rumsby and Macklin, 1994), alluvial fans (Ballantyne and Whittington, 1999) or from boulder berms in small incising upland streams which can be dated by 14C or lichenometry (Macklin et al., 1992). Collectively these studies have significantly advanced our knowledge of wet periods and clusters of floods during the Holocene across much of Northern Europe, but their limited temporal resolution has made these methods less well-suited to identifying individual flood events within, for example, clusters of floods during Little Ice Age (Macklin et al., 1992). Sediment records have been analysed in a number of studies (e.g. Rowan et al., 2001; Winter et al., 2001) and in the context of floodplain sediment archives most extensively by Knox (1999). In this paper, we report on the development of a multi-proxy flood history for the River Tay, Scotland,

drawing on the sediment archive of a major palaeochannel complemented by archival, historical and epigraphic flood records from the city of Perth. The aims of the paper are as follows: 1. To recover reliably dated proxy flood records from sediment stacks in palaeochannels located in stable, lowland floodplains. 2. To check the resulting proxy record against the largest known floods independently derived from documentary and epigraphic sources. 3. To use the resulting extended flood record to improve flood risk analysis based on short instrumental records at a single station. This new approach to recovering proxy flood records can be applied to any river system where palaeochannels form part of a relatively stable floodplain and where the sediment supply and land use have been reasonably stable over the period of interest. Many floodplains in lowland Scotland provide sites where these conditions are broadly met and proxy flood records back to the late eighteenth and early nineteenth century can potentially be recovered (Werritty et al., 2003). If successful, this approach could assist in developing the next generation of flood risk maps which specify the outline of the 200-year flood (Scottish Executive, 2004).

2. Recovering proxy flood records from sediment stacks in palaeochannels Floodplains adjacent to Scotland’s larger rivers often display well-developed palaeochannels which have developed as active rivers migrate across their valley floors (Werritty and Leys, 2001). Once abandoned, these cutoff channels become significant sediment sinks and, having been slowly infilled by overbank sediments, eventually become annealed into the floodplain. Cores extracted from these channel infills typically yield a basal gravel (recording the former channel bed) with alternating silty and sandy facies comprising the remaining sediment stack. Silty facies within a typical core record moderately sized, overbank floods capable of conveying silts and clays in suspension across the floodplain. Sandy facies within a core register rarer, larger floods capable of transporting fine- to mediumsized sands in suspension across the floodplain followed by deposition in palaeochannels and elsewhere (Nicholas and Walling, 1997). The resulting pattern in the cores is akin to that displayed in bar codes used for retail goods: the thin black lines represent rare, major floods and the remaining white areas lesser, more moderate floods. The sediment stack recovered in the core is thus a potential flood archive with the sandy facies registering the signature of rare, large floods. If individual sandy facies can be dated, a proxy record of floods can be reconstructed back to when the

A. Werritty et al. / Catena 66 (2006) 107 – 119

palaeochannel was cutoff from the main channel. The reconstructed flood record reports frequency but not magnitude and thus does not depend on the grain size of the suspended sediment transported in flood being a function of discharge. Given that the conveyance process is selective across the floodplain, it is important to stress that preferential sedimentation of the coarser fractions will occur in palaeochannels during extended periods of inundation. This simple conceptual model of proxy flood archives contains a number of key assumptions. The most important of these are (i) steady sediment supply and minimal land use change upstream plus and (ii) stable local flood hydraulics which control overbank flows and the ingress of flood waters into the palaeochannel. Ideally, these upstream and local site controls should be invariant throughout the record. However, given the possibility of recovering proxy flood records in Scotland back to the late eighteenth and early nineteenth centuries, it is unlikely that either assumption can be fully met. Deforestation, the draining of wetlands, channelisation and the development of hydropower have all impacted on river regimes and sediment fluxes throughout the uplands and lowlands (Smout, 2000). The most important impacts are changes in flow regime, especially where hydro-schemes have been developed, and reduced sediment yields in the lower reaches of Scotland’s major rivers (Gilvear and Winterbottom, 1992). Whilst these changes are problematic, they are not fatal to the recovery of flood archives provided that sediment-rich waters still reach specified palaeochannels throughout the record and any changes in land use and flow regime can be well documented. Potentially more problematic are changes in the local hydraulic controls which determine the rate and pattern of overbank flows across the floodplain. The most significant of these controls are agricultural flood embankments designed to protect against the 5- to 10-year flood, some of which date back to the early nineteenth century (Werritty et al., 2005). Providing the history of their construction and maintenance is reasonably well known, robust proxy flood records can be recovered from palaeochannels in floodplains protected by flood embankments designed to provide low standards of protection. A final assumption implicit in the conceptual model developed above is that the floodplain has not been subject to vertical change (incision or aggradation) which, during the period of the proxy record, has altered the threshold for floodplain inundation. Investigation of fluvial geomorphology Sites of Special Scientific Interest during the 1980s and 1990s (see Gregory, 1997) has yielded many potential sites for retrieving proxy flood records across Scotland. Of these sites, the Derry Burn (McEwen, 1997), the River Clyde and Medwin Water (Rowan et al., 1999) and the FBloody Inches_ on the lower Tay (Paine et al., 2002) have already been investigated and the expected alternation of silty and sandy facies within the channel fill reported. In this paper we concentrate on the FBloody Inches_ because its proxy flood record can be

109

independently checked against a very detailed flood chronology derived from documentary and epigraphic sources at Perth, 15 km downstream (Macdonald, 2004).

3. Flood record recovered from the FBloody Inches_ on the lower River Tay The River Tay is Britain’s largest river with an average flow of 160 m3 s 1 at its tidal limit in Perth (Fig. 1) and a documented flood history back to 1210 (Britton, 1937). The nearest gauging station to the FBloody Inches_ is 5 km upstream at Caputh (drainage area 3210 km2, period of record 1947 to present, and average flow of 139.2 m3 s 1, Centre for Ecology and Hydrology, 2003). High runoff ratios are generated by low evapotranspiration and thin soils on impermeable substrates, although significant storages in Lochs Tay, Tummel and Rannoch moderate the resulting flows and sediment fluxes (Fig. 1). Hydro-electric schemes dating back to the 1930s control the outflows from Lochs Tummel and Rannoch and flow within the River Garry with the result that 62% of the drainage of the River Tay is now regulated (Payne, 1988). The tidal limit of the Tay is reached at Perth (Fig. 1) whose flood history has been reconstructed by McEwen and Macdonald in great detail back to the early 1700s (see extract in Table 1) and in less detail back to 1210. The oldest extant river crossing is Smeaton’s Bridge, Perth opened in 1771 and the survivor of many catastrophic floods evident by its epigraphic flood record back to 1814 inscribed into the western bridge pier (Fig. 2). 3.1. The history and development of the FBloody Inches_ The FBloody Inches_ is the largest oxbow lake within the whole of the Tay drainage, its separation from the current main channel being reported in a series of maps spanning the period 1720 to 1864 (Fig. 1C – E). Although the planimetric accuracy of eighteenth century maps and estate plans can be questioned, the overall history of channel change at this site can be reconstructed with considerable confidence. Thus, The New Statistical Account of Scotland (1845, Volume X, p. 1130) reports the following entry for the parish of Kinclaven: ‘‘About eighty years ago, in consequence of an extensive flood from the Highland mountains, the Tay was swollen to a great height, and forced out to itself a new channel on the north side of the parish: for instead of flowing in a circuitous course along the south side of the farm of Haugh of Meikelour, then a peninsula, it burst through the narrow isthmus in a straight form and course. . . .. The old course of the river was about three miles, whereas its new course does not exceed half a mile in extent’’. This implies that the process of abandonment of what is now the FBloody Inches_ began around 1761 – a year with

110

A. Werritty et al. / Catena 66 (2006) 107 – 119

Fig. 1. Location map: (A) River Tay drainage basin within insets for panel (B) and location within Scotland; (B) Location of FBloody Inches_, Caputh gauging station and Meikleour estate; (1C) River Tay: highly sinuous channel but undivided (maps 1720 – 50); (D) River Tay: cut-off channel developed through neck of downstream meander (maps 1783 – 1827); (E) River Tay: FBloody Inches_ now an abandoned channel (OS map 1864) and flood embankments in place.

three major floods (Table 1). The detailed chronology begins with James Stobies’ map of 1783 which reported the existence of two channels and, according to John Thompson’s map, this pattern continued up to 1827 (Fig. 1D). By the first edition of the 1:10,560 Ordnance Survey map in 1864 the FBloody Inches_ had become detached from the main channel and was by then an oxbow lake only inundated by the occasional flood (Fig. 1E). Flood embankments on the lower Tay are reported as early as 1733 (Gilvear and Winterbottom, 1994) and Stobie’s map records what appear to be embankments on the southern side of the FBloody Inches_ in 1783. The New Statistical Account of Scotland (1845, Volume X, p. 679) reports flood embankments being constructed at Delvine following a large flood

in October 1831 and these are clearly visible on the 1864 OS map. Flood embankments designed to control inundation by events with return periods of 5– 10 years have been maintained in the vicinity of the FBloody Inches_ up to the present day. However, as reported by Gilvear and Black (1999), they are subject to local failure at flows > 850 m3 s 1 and widespread failure at flows > 1200 m3 s 1 resulting in extensive inundation of low lying areas downstream of Caputh including the whole of the FBloody Inches_. 3.2. Data capture: field and laboratory methods Given the well-documented history of the FBloody Inches,_ plus information on the building and maintenance

A. Werritty et al. / Catena 66 (2006) 107 – 119

111

Table 1 Extract from the flood chronology for the River Tay 1761 – 1814 Year

Date

Cause

1761

5 March

Atholl Muniments

1761 1761 1768 1772 1772 1773

18 October 29 October August 7 February 31 July 11 February

Thaw Rain Thaw

Atholl Muniments Atholl Muniments Annual Register, 1768 Atholl Muniments Atholl Muniments Atholl Muniments

1774

12 February

Thaw

1774

Autumn

Statistical Accounts, 1791 – 1799

1780 1789 1790 1790

February 23 August

1791 1794 1814

25 January March 12 February

Statistical Accounts, Statistical Accounts, Statistical Accounts, Atholl Muniments Statistical Accounts, Atholl Muniments

Rain

Thaw Thaw

Primary source

Secondary source

Cant, 1806; Statistical Accounts, 1791 – 1799

Notes ‘‘Greater destruction than known for 20 years’’ Bridge over Almond washed away Third time in last 12 months

Cowan, 1904; Marshall, 1849; Peacock, 1849; Coates, 1916

Hutton, 1995; Ove Arup et al., 1994

1791 – 1799 1791 – 1799 1791 – 1799

Almond in spate, Tay rose very fast River choked with ice. Smeaton’s newly completed bridge (1771) came through the severest test unscathed. Terrible inundation ‘‘In a short time the town was an island, the water ran with a great current. . . within end of the Skinnergate and laid many houses in the New Row’’ Cant, 1806 Isla rose two feet higher than ever before Overflowing of Isla banks Flood at Perth Spate and stormy weather

1791 – 1799

Perthshire Courier, 1814; Atholl Muniments

Marshall, 1849; Coates, 1916 Ove Arup et al., 1994 Marshall, 1849; Peacock, 1849

River choked with ice like 1774. Both Inches flooded, 26.6 ft AOD at Smeaton’s Bridge

Source: McEwen (in press) and Macdonald (2004).

of flood embankments which serve as hydraulic controls on inundation of the palaeochannel, a programme of coring was undertaken at the site. Eight sediment cores, typically 1.4 m in length, were obtained from the FBloody Inches_ meander using a vibro-corer, a 1-m gouge auger and a 0.5-m Russian auger. The presence of a gravel – sand boundary within individual cores enabled the effective separation of the palaeochannel from the current main channel to be determined. Each core was logged, discrete sedimentological units identified and sampled at an interval of 1 cm. Following drying and gentle disaggregation, the granulometry of each core was determined using a Coulter LS230

laser granulometer on the chemically dispersed mineral fraction. In order to establish the rate of sedimentation and thus the dates of individual flood units within each core, an independent chronology is required. This was achieved in this study using 137Cs and 210Pb radiometric dating techniques. 210Pb is a naturally occurring radionuclide with a half-life (t 1/2) of 22.2 years produced from the 238U decay series. The decay of radon gas (222Rn, t 1/2 = 3.8 days), which diffuses from lithospheric 226Ra (t 1/2 = 1602 years), results in atmospheric 210Pb which returns to the land surface mainly during precipitation. The activity level of 210Pb

Fig. 2. Westernmost pier of Smeaton’s Bridge, Perth. Post-1814 flood levels inscribed into the buttress (Macdonald, 2004).

112

A. Werritty et al. / Catena 66 (2006) 107 – 119

within the oxbow lake sediments thus comprises a Fsupported_ fraction 210Pbsupp, supplied by 226Ra, and an atmospheric fallout equivalent known as the Funsupported_ fraction 210Pbunsupp. If one assumes that annual fluxes of 210 Pb from the atmosphere are constant over time, levels of 210 Pbunsupp within an aggrading sediment stack will vary as a function of age (Oldfield and Appleby, 1984). Having sealed the sediment samples for 30 days to achieve equilibrium with 226Ra, gamma spectrometry was used to measure supported 210Pbsupp (via 214Pb at 352 keV) and direct measurement of total 210Pb at 46.5 keV. In addition to measuring 210Pb levels throughout each core, 137Cs levels were simultaneously determined for the upper 30 cm. 137Cs is an artificial radionuclide produced by

nuclear weapons testing and is measured at 661 keV. It is widely used to determine post 1950 sediment fluxes and has proved highly successful in determining patterns of floodplain aggradation (Walling and He, 1997). Another use is to verify 210Pb chronologies because well-defined marker horizons can be identified at 1954, 1963 and 1986. These dates correspond to the initial stratospheric dispersion of weapons-testing in the early 1950s, the fallout peak in 1963 (thereafter followed by a test-ban treaty) and a major peak in 1986 which corresponds to Chernobyl-derived fallout (Rowan et al., 1993). These analyses were carried out using a low background Ortec HPGe co-axial detector at the University of Dundee. Count times were typically in the order of 2 days giving analytical errors of 5– 8% (1r).

Fig. 3. Granulometric analysis of core T5 from the FBloody Inches_ cross-referenced to the major floods recorded at Smeaton’s Bridge, Perth and catchment influences on flow regime and sediment supply.

A. Werritty et al. / Catena 66 (2006) 107 – 119 Table 2 Stage, discharge and selected return periods for the 20 largest ranked floods at Smeaton’s Bridge, Perth 1814 – 2000 (Macdonald, 2004) Rank

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Date

1993 1847 1950 1951 1868 1868 1990 1853 1928 1849 1912 1913 1851 1852 1894 1903 1910 1974 1947 1909

Stage at Smeaton’s Bridge (m)

Discharge (m3 s 1)

Return period (GPD: 1361 m3 s threshold) (years)

6.48 6.11 6.03 5.97 5.90 5.85 5.85 5.79 5.77 5.70 5.68 5.66 5.65 5.64 5.64 5.64 5.61 5.61 5.55 5.52

2269 1961 1899 1853 1799 1745 1745 1715 1700 1646 1630 1615 1607 1600 1600 1600 1577 1577 1531 1508

250 75 57 46 38 31 27 27 26 21

1

3.3. Results from core T5 The results for the core T5 are summarized in Fig. 3 which reports the percentage of coarse, medium, fine and very fine sands throughout the core. The very fine and fine sand fractions are only reported after 1870 and 1825, respectively, when they become significant components in the grain size distribution. Depth has been converted to age

113

using the 210Pb dating approach of Jetter (2000), which divides the log 210Pbunsupp profile into different segments reflecting different sedimentation rates over time. This approach was adopted because of the acknowledged limitations of the CRS and CIC dating models to floodplain systems which experience highly variable rates of sedimentation (He and Walling, 1996; Appleby, 2001). Massspecific concentrations of 210Pbunsupp ranged from 181.4 T 47 to 23.4 T 17 Bq kg 1. The 210Pb chronology was extended back to c. 1860, and interpolated thereafter to the basal date of c. 1780 established from archival and map sources as the channel cut-off date. An attempt to independently date the sand units within the sequence using OSL yielded inconclusive results requiring further analysis (Robinson, personal communication). Core T5 was extracted using a vibrocorer which resulted in compaction of the highly organic upper units (0 –32 cm). As a result, there are only 16 dates for the post 1900 samples (compacted organic-rich units) but 60 dates for samples during the period 1800 – 1900 (units dominated by denser minerogenic units). The arrow heads record the dates for floods reported on Smeaton’s Bridge (see Table 2 for a list of flood levels above 5.49 m – equivalent to flows > 1485 m3 s 1). Visual inspection shows that the majority of the floods are represented by localised coarser-grained facies. Initially, these facies occur in the coarse sand fraction (up to 1830), thereafter the medium and fine sands register these coarser facies. The occurrence of these facies is especially clear for the floods in 1814, 1839, 1847, 1853, 1868 (twice) and 1894 where the temporal resolution in the core is optimal. Indeed, detailed analysis of the medium- and finesized sands curves reports perfect correspondence between

Fig. 4. Core T6 (0 – 30 cm) from the FBloody Inches_ dated by 137Cs. Minerogenic-rich units associated with the 1947 – 1951 and 1989 – 1993 flood clusters. Error bars for each of the 11 samples reported with 1986 and 1963 peaks clearly differentiated.

114

A. Werritty et al. / Catena 66 (2006) 107 – 119

the highest floods at Smeaton’s Bridge (1814 – 1868) and local coarsening. The lower temporal resolution after 1900 means that individual samples are integrating over larger time intervals. Despite this, local coarsening within the medium- and fine-sized sand curves corresponds well to clusters of floods for 1928 –1931, 1947– 1951, and 1989– 1993. The cluster for 1909 –1913 is clearly registered in the re-emergence of the coarse sand fraction. Individual floods in 1962 and 1974 also register localised coarser signatures. Confirmation of the existence of sand-rich flood facies in the compacted upper part of core T5 (1 –7 cm) was sought by extracting an immediately adjacent core (T6) using a Russian auger to minimise compaction. The upper 30 cm of T6 was dated by 137Cs, based on a 1 cm sampling interval. This resulted in a higher resolution than in T5, the minerogenic-rich units associated with the 1947 –1951 and 1989 – 1993 flood clusters being clearly visible within predominantly organic sediments (Fig. 4). More generally, the channel fill reports a steady upward fining above the basal gravels which, it is inferred, corresponds to the bed of the River Tay in the 1780s. Visual inspection and laboratory analyses suggest that five distinct units can be identified within the channel infill (Table 3). The lowest unit (56 – 95 cm, immediately above the basal gravels) is dominated by medium- to fine-sized sands (125 – 500 Am). Dated between 1780 and 1830, this unit represents the period when both channels had active sandy beds (Fig. 1). The second unit (34 – 55 cm : 1830 –70) is dominated by fine and very fine sands (63 – 250 Am). The declining proportion of fine sand during this period (Fig. 3) is consistent with the development of embankments following the 1831 flood. The third unit (33 – 17 cm : 1871 to 1903) is initially dominated by very fine to medium sands but these give way to silts which eventually comprise the modal fraction for the unit. This unit corresponds to the period after the final separation of the FBloody Inches_ from the main channel (Fig. 1) during which inundations capable of transporting the finer sand fractions would have declined. The fourth unit (18 –33 cm: 1909– 1950) is dominated by silts and clays (67%) but with occasional discrete sand-rich units (Fig. 3). It corresponds with a period of increased river engineering and regulation probably resulting in lower sediment yields. The final unit (1– 7 cm: 1950 – 2000) continues this dominance of silts and clays and, from 1960 onwards, registers virtually no sediments > 250 Am).

4. Revised flood risk analysis incorporating known historic floods The high degree of correspondence between flood signatures recorded in the FBloody Inches_ channel infill and the documentary and epigraphic records for Perth suggests that the above analysis has captured an unusually detailed and complete proxy flood record on the lower River Tay back to 1814. This proxy record is now explored in more detail and a revised flood risk analysis presented for Perth based on an augmented flood series in which historical floods are added to those captured in the post-1952 instrumental record at Ballathie. Perth has an exceptionally rich set of documentary and epigraphic sources for reconstructing the flood history on the lower River Tay (McEwen, 1993; Macdonald, 2004). Written records include manuscripts in the Atholl Muniments (estate archives), the The Statistical Account of Scotland (1791 – 1799), Cant’s Memorabilia of the City of Perth (1806), Peacock’s Perth: Its Annals and Its Archives (1849) and, from 1814 onwards, the Perthshire Courier newspaper. These sources yield a remarkably detailed flood chronology for Perth (see Table 1 for an extract over the period 1761 – 1814). To this can be added flood levels inscribed into the westernmost pier of Smeaton’s Bridge (Fig. 2) adding at least a further 20 flood levels (Black and Anderson, 1994). Documentary sources imply that the Tay’s cross-section at Perth has experienced minimal lateral or vertical change since the early years of the nineteenth century. Since none of the floods since 1814 have involved an ice jam, the flood levels at this stable cross-section can readily be converted into flows using a rating relationship based on the stage at Smeaton’s Bridge and measured flows at Ballathie (the river gauging station 4 km upstream of Perth) for 1948, 1950, 1951, 1962, 1974, 1989, 1990 and 1993. Although the continuous flow record at Ballathie did not commence until 1952, the Scottish Environment Protection Agency has been able to recover the peak flows for 1948, 1950 and 1951 based on contemporary manual stage board readings and subsequent gaugings. As a result of these historical reconstructions, the twenty largest floods on the River Tay at Perth can be reconstructed with considerable confidence in terms of both level (stage) and flow (see Table 2). It is noteworthy that 17 of the 20 largest floods since 1814 are historic floods predating the instrumental record.

Table 3 Grain size data for the five sedimentation units reported in Fig. 3 for core T5 at the FBloody Inches_ 210

Pb time period

Depth (cm)

Clays <2.01 Am (%)

% Silts 2.01 – 62.5 Am (%)

Very fine sands 62.6 – 125 Am (%)

Fine sands 125 – 250 Am (%)

Medium sands 251 – 500 Am (%)

Coarse sands >500 Am (%)

1950 – 2000 1900 – 1950 1870 – 1900 1830 – 1870 1780 – 1830

1–7 8 – 17 18 – 33 34 – 55 56 – 95

4.11 3.84 1.90 0.95 0.08

66.81 62.81 38.90 13.88 1.98

22.90 22.58 36.28 27.59 3.08

6.11 7.59 20.07 46.86 28.29

0.06 2.72 2.85 10.58 51.97

0.00 0.46 0.00 0.14 14.61

A. Werritty et al. / Catena 66 (2006) 107 – 119 Table 4 Return periods for the 2-, 5-, 10-, 20-, 50-, 100-, 200- and 500-year floods on the River Tay at Perth using the Generalised Logistic (Fsingle site_ and Fpooled_ methods) and the Generalised Pareto Distribution on Faugmented historical_ data with a threshold of 1361 m3 s 1 Return period (years)

FSingle site_ (m3 s 1)

FPooled_ (m3 s 1)

FAugmented historical_ (GPD) (m3 s 1)

500 200 100 50 25 10 5 2

2650 2275 2025 1810 1610 1375 1220 975

2700 2300 2050 1750 1625 1475 1275 1150

2450 2250 2050 1875

Given the relatively high threshold, return periods for the Generalised Pareto Distribution are only listed for the 50-, 100-, 200- and 500-year floods.

Having reconstructed the most extreme floods back to 1814, it is now possible to compare estimates of flood risk based on the post 1952 annual maxima series at Ballathie with the historically augmented record derived from Smeaton’s Bridge and accompanying documentary accounts. Using Flood Estimation Handbook software (Institute of Hydrology, 1999) Fsingle site_ and Fpooled estimates_ of flood frequency have been derived for the 1952 –2000 annual maxima series (AM) using the Generalised Logistic distribution with parameters fitted by L-moments. The results are reported as estimates of the 2-, 5-, 10-, 20-, 50-, 100-, 200- and 500-year floods (Table 4). Estimates of flows up to the 100-year flood are rather variable, with neither method consistently higher or lower than the other. However, there is a rapid convergence (to within 50 m3 s 1) for estimates of the 100-, 200- and 500-year floods. The instrumental record can be augmented by incorporating historic floods using the methods recommended by Bayliss and Reed (2001) and a threshold of 1361 m3 s 1. The resulting peaks over threshold (POT) series extends from 1815 to 2000. In analysing an augmented POT series, it makes sense to set the threshold as the lowest historical flood, unless that flood level is below an instrumental event which is not an AM value. When this occurs, the threshold should be given a value just above the largest POT event not included in the AM series. In this case, the 19th January 1974 flood (1360 m3 s 1) is not part of the AM series but is larger than the smallest historical flood; the threshold accordingly is set at 1361 m3 s 1. Since the original AM series has been censored at a high threshold and augmented with reconstructed historical flows, the Generalised Pareto Distribution (GPD) using L-moment parameters is now used to fit the data. The resulting flood flows with 50- to 500-year return periods are reported in Table 4. Comparison of the estimated values for the 100- and 200-year floods based on the GPD (augmented data) with Fsingle site_ and Fpooled_ AM methods reveals only minimal differences, all three values for the 200-year flood lying within 25 m3 s 1 of each other.

115

The Scottish Executive (2004) has specified the 200-year flood as the reference flood for second generation flood risk maps. FSingle site_ analysis based on the post 1952 Ballathie record can only yield reliable flood risk estimates up to the 100-year flood (approximately twice the record length). FPooled_ analyses generate much longer synthetic records but the inclusion of specific sites within the Fpool_ is a matter of judgement and can be questioned. However, the addition of historical data and re-analysis of a censored data set using the GPD enables the 200-year flood to be calculated without resort to pooling. In this case, the Fsingle site_, Fpooled_ and Faugmented historical_ estimates of the 200-year flood all converge within the range 2250 – 2300 m3 s 1. Thus the independent support provided by the Faugmented historical_ analysis significantly increases the confidence attached to the estimates based on Flood Estimation Handbook (Institute of Hydrology, 1999) procedures.

5. Discussion The first aim of this paper is to determine whether proxy flood records can be successfully extracted from sediment archives in floodplain palaeochannels and reliably dated. The second aim is to determine how far the resulting proxy record agrees with an independently obtained documentary/ epigraphic record. The degree to which both of these aims have been achieved is now assessed. Cores from the FBloody Inches_ on the lower River Tay just upstream of Perth record many discrete coarser sandy facies which, on being dated by 210Pb, correspond with historical flood stages inscribed into the westernmost pier of Smeaton’s Bridge in Perth. The correspondence between the dates of the flood facies in the FBloody Inches_ core and the epigraphic record on Smeaton’s Bridge is most precise between 1814 and 1868 when all major floods can be identified in the core. This was a period when medium- and fine-sized sands optimally captured flood signatures and when sedimentation rates within the palaeochannel exceeded 0.5 mm year 1. Although flood facies can be identified throughout the core up to 1989– 93, retrieval of individual floods within the sediment stack becomes increasingly difficult as silts replace sands, the proportion of organics increases and the sedimentation rate declines. Nevertheless, in the uppermost section of the core (dated by 137 Cs and coincident with the instrumental record on the Tay), clusters of floods (1949 –51 and 1989– 93) correspond to discrete packets of minerogenic sediment embedded within organic sediments. In summary, it was proved possible to compile a list of major floods from a sediment stack in a palaeochannel by granulometric analysis. The radiometric dates for the inferred floods in the sediment stack have been independently verified by reference to documentary and epigraphic sources in Perth. The environmental history of the Tay catchment is sufficiently well known that it too can provide a check on

116

A. Werritty et al. / Catena 66 (2006) 107 – 119

the robustness of the inferred proxy flood record. The precise history of palaeochannel infill is determined by catchmentwide and local controls which change over time. Inverting this relationship, one can use the land use history of a catchment to check on how accurately a core has been dated and whether this is congruent with the environmental history. The pattern of steady upward fining punctuated by discrete sand-rich flood facies in core T5 agrees remarkably well with the land use history for the Tay river basin. Extensive embankments were not in place until the 1830s and this corresponds exactly with a rapid decrease in the > 250-Am sand fractions. The final separation of the FBloody Inches_ from the main channel by 1864 also coincides with the next major phase of fining around 1870. By 1905, the proportion of sand-sized fractions deposited in the FBloody Inches_ had stabilised at around 80% reflecting the effect of river engineering from Pitlochry to Dunkeld which, during the nineteenth century, had transformed the middle Tay from a wide, unstable braided channel to a narrower, sinuous more stable channel confined by rock revetment and flood embankments (Gilvear and Winterbottom, 1998). During most of the twentieth century, the proportion of sand routinely reaching the FBloody Inches_ remained broadly stable. But the reduction in sand-sized fractions in the 1930s and 1940s and again after 1970 corresponds to the installation and development of hydroelectric schemes on Lochs Tummel, Rannoch and Garry (Payne, 1988). It is clear that changes in the granulometry of core T5 are consistent with what is known about the environmental history of the Tay catchment from c. 1800 onwards. This provides a second, albeit weaker, confirmation of the 210Pb dates reported for the core. A third aim of this paper is to explore the added value of including historic floods to improve estimates of flood risk. The River Tay at Perth is exceptional in terms of the quality of available information on historic floods which has enabled a detailed flood chronology to be compiled back to 1210. Following the construction of Smeaton’s Bridge in 1771, the quality of this record was enhanced by the practice of inscribing the stages of the highest floods on the westernmost pier. Since the river bed at this historic crossing has remained relatively stable both vertically and laterally since the late eighteenth century, present-day stage-discharge ratings can be used to generate estimates of historic peak flows. This has enabled the compilation of flood peak flows back to 1814: the date of an ice-jam flood which registered 7.0 m at Smeaton’s Bridge. When combined with the instrumental record (from 1952) this site yields the longest continuous high quality flood record in Scotland. Such a record is inevitably censored with only the very highest floods being recorded. This implies that an improved estimate of flood risk should be based a ‘‘peaks over threshold’’ (POT) series rather than the annual maxima series. Using a relatively high threshold of 1361 m3 s 1 and the Generalised Pareto Distribution to model the resulting POT series, estimates of the 50-, 100- and 200-year floods

are 2250, 2050 and 1875 m3 s 1, respectively. For this site the results conform to ‘‘single site’’ and ‘‘pooled’’ analyses using standard Flood Estimation Handbook (Institute of Hydrology, 1999) software. Thus, the inclusion of historical floods in the POT series has provided a valuable independent corroboration of results using standard methods. This, however, is not generally true elsewhere in the UK. For example, Macdonald’s (2004) incorporation of historical records into POT series for the River Trent at Nottingham generates markedly different values for the 100- and 200year floods to those generated by Flood Estimation Handbook methods. It is hoped that the results reported in this paper will add to the recent renewed interest in modelling POT series using GDP and Bayesian statistics (Parent and Bernier, 2003). Compiling high-quality proxy flood records is also valuable in identifying ‘‘flood-rich’’ and ‘‘flood-poor’’ periods as evidence of non-stationary flood series. These in turn provide potential analogues for the increased flood risks predicted for the 2050s and 2080s based on UKCIP02 climate change scenarios (Werritty et al., 2002). In this paper, the Perth chronology captures a flood record which extends through the later part of the Little Ice Age and includes several potential ‘‘flood-rich’’ periods (see Macklin et al., 1992 for similar episodes in northern England). The Perth chronology also captures floods triggered by storms and frontal systems originating in the Atlantic and generating significant orographic precipitation over the Western Grampians. The timing and location of these storms is governed by the quasi-cyclical behaviour of the North Atlantic Oscillation which is closely aligned with the incidence of ‘‘flood-rich’’ and ‘‘flood-poor’’ periods across Northern Britain (Werritty and Foster, 1998). The conceptual model outlined at the beginning of this paper claimed that proxy flood records can be retrieved from sediment stacks in palaeochannels, providing certain conditions are met. This claim has been substantiated at the FBloody Inches_ on the lower River Tay. The next stage is to test the model at other sites where independent documentary or epigraphic evidence is also available. Such sites should ideally have a cross-section with minimal vertical or horizontal change and a robust historical record of floods. The local flood hydraulics controlling the ingress of flood waters into the nearby palaeochannel should also be broadly stable. Preliminary research has already identified a number of sites on the Rivers Teith and Spey where these conditions appear to be met. The requirement of a steady sediment supply and minimal land use change appears to be less important. Thus, the introduction of hydroelectric schemes and upstream reservoirs has not prevented the recovery of sediment signatures from floods which post date the schemes. During very rare floods, such as that on the River Tay in 1993, most Scottish hydroelectric schemes spill inundating the floodplain downstream with sediment-rich water. Since the flood storage in these schemes is small in proportion to the volume of the largest floods, a modest

A. Werritty et al. / Catena 66 (2006) 107 – 119

attenuation in the downstream peak flow results with minimal impact on flood risk estimates. Should further testing of the model prove successful, one could then reconstruct proxy flood records from sediment stacks in palaeochannels at many other sites where documentary and epigraphic sources are poor. Taken in conjunction with other palaeohydrological investigations, this would significantly add to our understanding of changes in flood frequency across upland Britain during the later part of the Little Ice Age. It could also help develop historical analogues for the ‘‘warmer and wetter’’ Britain predicted by the 2080s by climate scientists.

6. Conclusions This paper has demonstrated that proxy flood records extending over 200 years can be successfully recovered from sediment stacks in palaeochannels adjacent to major rivers. The hypothesis that each local coarsening in the stack represents a flood signature which can be individually dated by 210Pb or 137Cs has also been confirmed. The optimal conditions for the recovery of flood series from palaeochannel infills include rates of sedimentation of at least 0.5 mm year 1 and flood flows capable of transporting fine- to medium-sized sand in suspension across the floodplain followed by deposition. Ideally there should be a steady sediment supply and known land use change upstream throughout the record and stable local hydraulic controls which determine the quantity and frequency with which flood waters reach the palaeochannel. These conditions can be relaxed providing changes in the local controls (usually due to the construction of flood embankments) are well documented. A mandatory requirement is that the channel is not subject to incision or aggradation. Reconstructing historic floods in order to augment short instrumental records can significantly improve estimates of flood risk. This is especially important where instrumental records are short, the stationarity of flood series can be questioned and their natural variability is high. By setting the 200-year flood as the design level for flood risk mapping and planning controls, the Scottish Executive (2004) poses a major challenge for consultants. The inclusion of historical floods in augmented POT flood series and their modeling by the Generalised Pareto Distribution provides a framework for meeting that challenge.

Acknowledgements JLP acknowledges a University of Dundee PhD studentship and NM a NERC Industrial Case studentship (NER/S/ C/2000/03289). The Beluthie and Atholl estates allowed JLP and LJMcE access to their archives and muniments for the retrieval of information on the history of flooding and embankment construction on the Lower Tay. LJMcE and

117

NM acknowledge the assistance of staff in Perth Public Library in compiling a history of flooding in Perth. Granulometric analysis on the cores was undertaken by Craig Phillips in the Environmental Diagnostics Laboratory in Dundee. Jim Ford drew Fig. 1. References Appleby, P.G., 2001. Chronostratigraphic techniques in recent sediments. In: Kluwer, P.G., Last, W.M., Smol, J.P. (Eds.), Tracking Environmental Change Using Lake Sediments: Volume 1. Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic, Dordecht, pp. 171 – 203. Baker, V.R., 2003. Palaeofloods and extended discharge records. In: Gregory, K.J., Benito, G. (Eds.), Palaeohydrology: Understanding Global Change. Wiley, Chicester, pp. 307 – 323. Ballantyne, C.K., Whittington, G., 1999. Late Holocene alluvial fan formation and floodplain incision, Central Grampian Highlands, Scotland. Journal of Quaternary Science 14, 651 – 671. Bayliss, A.C., Reed, D.W., 2001. The Use of Historical Data in Flood Frequency Estimation. Centre for Ecology and Hydrology, (NERC), Wallingford, UK. Benito, G., 2003. Palaeoflood hydrology in Europe. In: Thorndycraft, V.R., Benito, G., Barriendos, M., Llasat, M.C. (Eds.), Palaeofloods, Historical Data and Climatic Variability: Applications in Flood Risk Assessment, Proceedings of the PHEFRA International Workshop, Barcelona, 16 – 19th October 2002. Centro de Ciencias Medioambientales, Madrid, pp. 19 – 24. Benito, G., Thorndycraft, V.R. (Eds.), Systematic, Palaeoflood and Historical Data for the Improvement of Flood Risk Estimation: Methodological Guidelines. Centro de Ciencias Medioambientales, Madrid. Benson, M.A., 1950. Use of historical information in flood frequency analysis. Transactions of the American Geophysical Union 3, 419 – 424. Black, A.R., Anderson, J.L., 1994. The Great Tay flood of January 1993. Hydrological Data UK, 1993 Yearbook. Institute of Hydrology, Wallingford, pp. 25 – 34. Black, A.R., Burns, J.C., 2002. Re-assessing the flood risk in Scotland. The Science of the Total Environment 294, 169 – 184. Britton, C.E. (1937) A meteorological chronology to AD 1450, Geophysical Memoirs, 20, (Volume VIII) Meteorological Office, Air Ministry, HMSO, London. Cant, J., 1806. Memorabilia of the City of Perth. William Morison, Perth. Centre for Ecology and Hydrology, 2003. Hydrometric Register and Statistics 1996 – 2000. Hydrological Data UK Centre for Ecology and Hydrology, Wallingford. Coates, H., 1916. Floods and droughts of the Tay valley. Proceedings of the Perthshire Society of Natural Science 7, 103 – 126. Cowan, S., 1904. The Ancient Capital of Scotland: The History of Perth from the Invasion of Agricola to Passing of the Reform Bill, second ed. London (2 Vols.). Gilvear, D.J., Black, A.R., 1999. Flood-induced embankment failures on the River Tay: implications of climatically induced hydrological change in Scotland. Hydrological Sciences Journal 44, 345 – 362. Gilvear, D.J., Winterbottom, S.J., 1992. Channel change and flood events since 1783 on the regulated River Tay, Scotland: implications for flood hazard management. Regulated Rivers: Research and Management 7, 247 – 260. Gilvear, D.J., Winterbottom, S.J., 1994. Mechanisms of flood embankment failure during large flood events: River Tay, Scotland. Quarterly Journal of Engineering Geology 27, 319 – 332. Gilvear, D.J., Winterbottom, S.J., 1998. Changes in channel morphology, floodplain land use and flood damage on the Rivers Tay and Tummel over the last 250 years: implications for floodplain management. In: Bailey, R.G., Jose´, P.V., Sherwood (Eds.), United Kingdom Floodplains. Otley, Westbury, pp. 93 – 116.

118

A. Werritty et al. / Catena 66 (2006) 107 – 119

Gregory, K.J. (Ed.), Fluvial geomorphology of Great Britain, Geological Review Conservation Series, Joint Nature Conservancy Council. Chapman and Hall, London. He, Q., Walling, D.E., 1996. Use of fallout Pb-210 measurements to investigate longer-term rates and patterns of overbank sediment deposition on the floodplains of lowland rivers. Earth Surface Processes and Landforms 21 (2), 141 – 154. Hirsch, R.M., 1987. Probability plotting position formulas for flood records with historical information. Journal of Hydrology 96, 185 – 199. Hulme, M., Jenkins, G.J., Lu, X., Turnpenny, J.R., Mitchell, T.D., Jones, R.G., Lowe, J., Murphy, J.M., Hassell, D., Boorman, P., McDonald, R., Hill, S., 2002. Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. Hutton, G., 1995. Old Perth. Richard Stentlake, Ayrshire. Institute of Hydrology,, 1999. Flood Estimation Handbook. Institute of Hydrology, Wallingford. (5 volumes). Jetter, H.W., 2000. Determining the ages of recent sediments using measurements of trace radioactivity. Terra et Aqua 78, 21 – 28. Kidson, R.L., Richards, K.S., 2005. Flood frequency analysis: assumptions and alternatives. Progress in Physical Geography 29, 392 – 410. Kiem, A.J., Franks, S.F., Kuczera, G., 2003. Multi-decadal variability of flood risk. Geophysical Research Letters 30 (2) ((art no.1035, Jan 17, 2003)). Knox, J.C., 1999. Long-term episodic changes in magnitudes and frequencies of floods in the upper Mississippi Valley. In: Brown, A.G., Quine, T.A. (Eds.), Fluvial Processes and Environmental Change. Wiley, Chichester, pp. 255 – 281. Macdonald, N., 2004. The application of historical flood information in reassessing flood frequency in Britain, Unpublished PhD thesis, University of Dundee. Macklin, M.G., Rumsby, B.T., Heap, T., 1992. Flood alleviation and entrenchment: Holocene valley floor development and transformation in the British uplands. Geological Society of America Bulletin 104, 631 – 643. Marshall, T.H., 1849. The History of Perth. Fisher, Perth. McEwen, L.J., 1993. Issues in flood hazard management: a case-study of the River Tay floods, in Scottish Geographical Studies. In: Dawson A.H., Jones, H.R., Small, A., Soulsby, J.A. (Eds.), Departments of Geography, Universities of Dundee and St. Andrews, 109-125. McEwen, L.J., 1997. Derry Burn. In: Gregory, K.J. (Ed.), Fluvial Geomorphology of Great Britain, Geological Review Conservation Series, Joint Nature Conservancy Council. Chapman and Hall, London, pp. 52 – 53. McEwen, L.J., in press. Flood seasonality and generating conditions in the Tay catchment, Scotland from 1200 to present. Area. Nicholas, A.P., Walling, D.E., 1997. Modeling flood hydraulics and overbank deposition on floodplains. Earth Surface Processes and Landforms 17, 687 – 697. Nicholas, A.P., Walling, D.E., 1997. Modeling flood hydraulics and overbank deposition on floodplains. Earth Surface Processes and Landforms 17, 687 – 697. Oldfield, F., Appleby, P.G., 1984. A combined radiometric and mineral magnetic approach to recent geochronology in lakes affected by catchment disturbance and sediment re-distribution. Chemical Geology 44, 67 – 83. Ove Arup et al., 1994. River Tay Catchment Study Commissioned by Tayside Regional Council Water Services Department. Final Report. Paine, J., Rowan, J.S., Werritty, A., 2002. Reconstructing historic floods using sediments from embanked floodplains: a case study of the River Tay in Scotland. The Structure, Function and Implications of Fluvial Sedimentary Systems, International Association of Hydrological Sciences Publication, vol. 276, pp. 211 – 218. Parent, E., Bernier, J., 2003. Bayesian POT modeling for historical data. Journal of Hydrology 274, 95 – 108. Payne, P.L., 1988. The Hydro. Aberdeen University Press, Aberdeen.

Peacock, D., 1849. Perth: Its Annals and Its Archives. Thomas Richardson, Perth. Perthshire Courier, 1814. Dundee. Robson, A.J., 2002. Evidence for trends in UK flooding. Philosophical Transactions of the Royal Society of London A 360, 1327 – 1343. Rowan, J.S., Higgitt, D.L., Walling, D.E., 1993. Chernobyl-derived radiocaesium into the sediments of Lake Vyrnwy, Mid-Wales. In: McManus, J., Duck, R.W. (Eds.), Geomorphology and Sedimentology of Lakes and Reservoirs. Wiley, Chichester, pp. 55 – 72. Rowan, J.S., Black, S., Schell, C., 1999. Floodplain evolution and sediment provenance reconstructed from channel fill sequences: the Upper Clyde basin, Scotland. In: Brown, A.G., Quine, T.A. (Eds.), Fluvial Processes and Environmental Change. Wiley, Chichester, pp. 223 – 240. Rowan, J.S., Price, L.E., Fawcett, C.P., Young, P.C., 2001. Reconstructing historic sedimentation rates using data-based mechanistic modeling. Physics and Chemistry of the Earth 26, 77 – 82. Rumsby, B.T., Macklin, M.G., 1994. Channel and floodplain response to recent abrupt climate change: the Tyne basin, Northern England. Earth Surface Processes and Landforms 19, 499 – 515. Scottish Executive,, 2004. Scottish Planning Policy SPP7: Planning and Flooding. Scottish Executive, Edinburgh. Sheffer, N., Ensel, Y., Benito, G., 2003. Palaeofloods in southern France: the Arde`che River. In: Thorndycraft, V.R., Benito, R., Barriendos, G., Llasat, M.C. (Eds.), Palaeofloods, Historical Data and Climatic Variability: Applications in Flood Risk Assessment, Proceedings of the PHEFRA International Workshop, Barcelona, 16 – 19th October 2002. Centro de Ciencias Medioambientales, Madrid, pp. 25 – 31. Smith, K., 1995. Precipitation over Scotland, 1757 – 1992: some aspects of temporal variability. International Journal of Climatology 15, 543 – 556. Smout, T.C., 2000. Nature Contested: Environmental History in Scotland and Northern England since 1600. Edinburgh University Press, Edinburgh. Starkel, L., 2002. Change in the frequency of extreme events as the indicator of climate change in the Holocene, (in fluvial systems). Quaternary International 91, 25 – 32. The New Statistical Account of Scotland, 1845. Volume X (Perth). William Blackwood, Edinburgh and London. Sinclair, Sir John (Ed.), The Statistical Account of Scotland: Volume XI (South and East Perthshire). EP Publishing, Wakefield. (reprinted in 1976). Stedinger, J.R., Cohn, T.A., 1986. The value of historical and paleoflood information in flood frequency analysis. Water Resources Research 23, 715 – 727. Wang, Q.J., 1990. Unbiased estimation of probability weighted moments and partial probability moments from systematic and historical information and their application to estimating the GEV distribution. Journal of Hydrology 120, 115 – 124. Walling, D.E., He, Q., 1997. Use of fallout 137Cs in investigation of overbank sediment deposition on river floodplains. Catena 29, 263 – 282. Werritty, A., Chatterton, J., 2004. Future flooding Scotland. Office of Science and Technology, Department of Trade and Industry, London. Werritty, A., Foster, M., 1998. Climatic variability and recent changes in rainfall and river flows in Scotland. In: Lemmela, R., Helenius, N. (Eds.), Proceedings of the Second International Conference on Climate and Water, vol. 3. Espoo, Finland, pp. 1110 – 1119 (August). Werritty, A., Leys, K.F., 2001. The sensitivity of Scottish rivers and upland valley floors to recent environmental change. Catena 42, 2 – 4. Werritty, A., Black, A.R., Duck, R.W., Finlinson, W., Thurston, N., Shackley, S., Crichton, D., 2002. Climate change: flooding occurrences review. Report to the Central Research Unit Scottish Executive, Edinburgh. Werritty, A., Paine, J.L., Rowan, J.S., 2003. Developing proxy flood records from sediment stacks in palaeochannels: the FBloody Inches_ on the River Tay, Scotland. In: Thorndycraft, V.R., Benito, G., Barriendos, M., Llasat, C. (Eds.), Palaeofloods,

A. Werritty et al. / Catena 66 (2006) 107 – 119 Historical Data and Climatic Variability: Applications in Flood Risk Assessment, Proceedings of the International PHEFRA Workshop, Barcelona, Spain, 16th – 19th October 2002. Centro de Ciencias Medioambientales, Madrid, pp. 125 – 130. Werritty, A., Hoey, T.B., Black, A.R., 2005. The geomorphology and management of a dynamic unstable gravelbed river: the Feshie/Spey

119

confluence, Scotland. In: Garcia, C., Batalla, R. (Eds.), Catchment Dynamics and River Processes. Elsevier, pp. 213 – 214. Winter, L.T., Foster, I.D.L., Charlesworth, S.M., Lees, J.A., 2001. Floodplain lakes as sinks for sediment-associated contaminants – a new source of proxy hydrological data? Science of the Total Environment 266, 187 – 194.