On the Persistence of Landscape Features Formed by a Large Flood

Abstract

An assessment is made of the persistence of landscape features formed by a flood with a large recurrence interval. The landscape of part of Exmoor is studied at two dates: following the passage of the 1952 flood and the present day. Evidence suggests that channel deepening resulting from the flood will survive the mean recurrence interval of the flood event, while channel-side erosional scars will not do so: downstream boulder fields deposited at the time of the flood will tend to become a general flood-plain feature rather than a specific flood-deposition feature. More generally, it is suggested that the sequence and inter-arrival times between such events, rather than their mean frequency alone, are more significant in their control on landscape form and development than has hitherto been acknowledged. THIS paper is concerned with the relative importance of large infrequent geomorphic events and smaller more frequent geomorphic events in the landscape. It centres in particular on the degree of persistence of fluvial and hillslope features associated with the large flood which occurred on Exmoor, south-west England, in August 1952. It has been recognized that the effect of geomorphic processes depends on their frequency as well as their magnitude (Wolman and Miller, 1960), and that specific landscape features may be related to the event whose size and frequency cause it to accomplish most total work over a period. For example, the idea that aspects of channel form are related to a dominant discharge has long been in existence (see, for example, Inglis (194i)), and this discharge is today frequently associated with bankfull discharge. Some landscape features however, must, by their size or their nature, be the work of unusually large events: in these categories must be considered the movement of very large grades of debris, for example, and the erosion of high valley side bluffs. Little is, of course, known about the whole frequency distribution of sizes of geomorphic event. Predictive models exist in both hydrological and climatological literature which suggest direct extrapolation from the recurrence interval of small measured events to the recurrence interval of large events. A number of large flood events have been investigated shortly after their time of occurrence. Representative of many articles on such work are those of Woolley (1946), Jahns (1947), Wolman and Eiler (1958), Tricart (1961), McPherson and Rannie (1969), Hanwell and Newson (1970) and Gupta and Fox (1974). Many of these papers deal with the meteorological causes of high rainfalls, with geomorphological or geological features as observed during or shortly after the passage of the flood, and with causes of high run-off, in some cases from the point of view of some measure of flood control or prediction. In this paper attention is focused on the degree of persistence of the landscape features produced by a flood. Existing information on the durability of features formed in association with large floods is limited and found mostly as theoretical or speculative comment (e.g. Gilbert, 1890; Woolley, 1946) or in description of 243 This content downloaded from 207.46.13.111 on Sat, 21 May 2016 06:12:50 UTC All use subject to http://about.jstor.org/terms 244 MALCOLM G.ANDERSON AND ANN CALVER Area Valley of Rockof:an oH Constriction ...e...ic..B und rie (eanGRIT a H n1 303 LYNTON agwothyBEDS LOCATION 6 Watr WterNORTH EXMOOR 00 HANGMAN AR gArea of Cannon Hill Valley referred to in Figure 3 MORTE 0--SLATES Principal Valley /Constrictions 300.-Geological Boundaries (after Hamling, 1910) C__ t-300Height in Metres 0 2 4 6 Kmns r n0 1 2 3 4 Mis ,i , ...... 0 ,