SOUTHWEST ENGLAND A CHRONOLOGY OF FLASH FLOODING Introduction There is no agreed definition of a flash flood. For the purposes of this chronology and in line with the requirements of the SINATRA project to investigate ‘flooding from intense rainfall’, the following definition (Archer and Fowler in press) is used: A flood resulting from intense short duration rainfall, usually convective, that exceeds drainage capacity in urban areas or infiltration capacity in rural areas and hence can flood land and property far from rivers. River response is rapid with possible near vertical wave fronts in river channels; river users and floodplain residents may be endangered by rapid rates of rise in river level which may be enhanced by failure of upstream structures or antecedent saturation of the catchment. Flash floods may cause serious erosion of hillsides and river channels and may carry heavy loads of floating debris and boulders which may be deposited in berms and terraces. Hence the chronology includes flooding from surface water (pluvial flooding) and from rivers. An attempt has been made to identify any event during which such a flash flood did or might have occurred. In the early records, descriptions may only include information on thunder or severe hail but during which flooding may also have occurred. Such events have been retained in the chronology to provide a basis for a further search on that date which might confirm a flash flood occurrence. On the basis of this definition not all river floods are included irrespective of their magnitude and indeed, whilst most annual maximum river floods occur in winter, most flash floods occur in summer. However, since the boundary between a flash flood and a ‘normal’ flood has not yet been clearly defined, some events are included which may not qualify as ‘flash floods’ but are retained to help establish a future boundary between flash and normal events. The flood chronicle is presented with a summary text description of each event in a WORD document in a three-column table. The first column gives the date of the event with the source(s) of the information (including the newspaper date). The second column gives associated meteorological information mainly from British Rainfall and Climatological Observers Link but occasionally from other sources. Short period rainfall amounts are especially noted, but daily rainfall totals are also listed where these might have included short bursts of high intensity. The third column contains descriptive information, notably locations which have been flooded and the depth and extent of flooding. Accompanying the WORD document is a summary EXCEL file with one line per event. For each event associated meteorological information is provided, including the occurrence of thunder, lightning or hail (and the size and severity of effects), and whether short period or daily rainfall information is available. Impacts of the event are indicated in terms of drowning and bridge failure and a code to indicate the overall regional extent and severity of flooding to property. Flooding is characterised as river or pluvial (or both) and as urban or rural (or both). The location of the flooding and the severity at individual locations is shown by a code for urban centres such as Exeter (1E to 3E) and Truro (1T to 3T). The location of the flood within main river catchments is shown. An indication is given as to the occurrence of rapid short-period rate of rise and whether geomorphological effects have been associated. Finally a summary note characterises the flood. Sources of Information An initial source for this study has been the BHS Chronology of British Hydrological Events (CBHE) which in turn derived much of its information from British Rainfall. Frank Law and others have added information from a variety of newspaper and other published accounts. Newspaper Sources The Lexis British Newspaper Archive has digitised large continuous runs of newspapers which have been accessed on the web. A search facility identifies all occurrences of given words or combinations of words, by newspaper, by county, by region and by date. The phrase ‘flash flood’ is a 20th century description, so alternative words were required to ensure that all potential flash flood events were identified, without producing an excessive number of irrelevant events. The words ‘flood’ and thunder’ were adopted. The search facility includes within these words variants such as ‘flooding’ and ‘thundering’. Up to 50% of the occurrences identified were of descriptions of ‘floods of tears’ or ‘thundering speeches by politicians’ but is likely to have produced a comprehensive set of flash flood events. All relevant information was extracted from the identified descriptions, such as location, associated meteorological conditions, the occurrence of hail, magnitude and extent of flooding, depth in buildings, fatalities from drowning or lightning, animal losses, bridges and buildings damaged by flood or lightning, severe channel erosion and descriptions of rapid rate of rise in level. Except where events were of very unusual magnitude, winter floods produced by long duration rainfall or snowmelt events were excluded. Only information which could be used to define the magnitude or severity of a historical flood in relation to a recent gauged flood has been included. The following is a list of the newspapers that have been accessed on the British Newspaper Archive, with the dates currently available. It is noted that although some papers continue publication to the present, for copyright reasons the current archive extends only to 1900 or to 1950. Hence records are most comprehensive for the period from 1850 to 1900, but sufficient newspapers are archived up to 1950 to provide comprehensive data up to that date. Newspaper Place of Publication Available dates Cornishman Penzance 1878-1950 Royal Cornwall Gazette Truro 1811-1900 Western Times Exeter 1827-1950 Exeter and Plymouth Gazette Exeter 1827-1950 Western Morning News Plymouth 1921-1950 North Devon Journal Barnstable 1824-1950 Exeter Flying Post Exeter 1800-1900 The period after 1950 required visits to Devon (Exeter) and Cornwall (Redruth) county record offices. Two main sets of information were inspected: 1. For periods extending from the late 1970s to 2005, the record offices took cuttings from local newspapers on the subject of storms and floods. All relevant material was extracted from these cuttings. (Photographs were taken and are stored in a separate folder). These records are often more comprehensive than those obtained from the British Newspaper Archive since extracts were taken from a large number of local as well as regional newspers including: Camborne and Redruth Packet, Camelford and Delabole Post, Express and Echo, Falmouth Packet, West Briton, Helston Packet, Cornish Times, Cornish Guardian, St Ives Times and Echo and Post and Weekly News. 2. For the period from 1950 to 1980, unavailable from the cuttings, microfilm of limited newspaper sources was inspected, West Briton for Cornwall and Express and Echo for Devon. Dates previously identified from meteorological data when heavy rainfall had occurred were inspected. Not all these events yielded information on flooding but are retained in the WORD archive in the event that other local sources will identify a flood on those dates; they are excluded from the EXCEL archive. It is probable that some local but possibly severe local events are omitted. Information on the period after 2000 was primarily obtained by internet searches. Newspaper accounts again provided the main source of information but BBC and other news reports added information and Environment Agency reports provided lists of floods for specific locations. Early information was obtained from the monthly Gentlemans Magazine held as hardcopy in the British Library at St Pancras London. Editions from 1731 to 1800 were inspected. Each annual volume contains an index which provided a ready means of identifying ‘floods’ and ‘storms’. The flood record is far from comprehensive but it provided a few additions to the information already held. Occasionally, historic newspapers collated useful information on flood events at earlier dates. For example, Exeter Flying Post for 17 January 1866 gives a list of flood levels from 1800 to 1866 on the River Exe at Exeter but detailed location not given. With the exception of events in 1829 (which were not at a very high level) these high river levels occurred in winter. Thunderstorms and flash floods rarely generate extreme flood peaks on major catchments due to the limited area which they cover and possibly also due to the fact that in the summer at least some of the rainfall can be absorbed in dry soils. Some major events, such as that in 1800 were the result of snowmelt. The note states that these are ‘some’ of the highest floods in the present century. It is not clear, for example, whether all floods above 6 feet have been included. The Exeter and Plymouth Gazette Daily Telegrams for 20 Jan 1881 gives a list of severe weather events from the 16th century which includes some thunderstorms mainly up to 1830. Miscellaneous sources Jones et al (1984) provided a source of early information on floods (and droughts) with the focus on the River Exe catchment and covering the period from 1588 to 1676. Information in turn has been taken from a variety of sources including chronicles, almanacs, parish registers and diaries. Reference has also been made to other published historical accounts including Bonacina (1946), Morris (1949), Oliver (1969) (with reference to Borlase diaries) and Whetter (1991). A paper by Brierley (1964) provides details of the extreme event in 1960 and previous occurrences of flooding on the River Exe. Associated meteorological information The publication British Rainfall 1863 – 1968 (Symons British Rainfall from 1863 to 1900) was further inspected to identify potential flood dates and to ascertain the meteorological conditions associated with flooding – thaw, thaw with rain, frontal rainfall, convectional storms etc.