Showing papers on "Bank erosion published in 1972"

Patterns of Present Sub-Aerial Erosion and Landforms in Mid-Wales

Abstract: Ten sampling units, with an area of the order of 0o4 km2, were selected for the study of fluvial landforms in the upper Wye valley in mid-Wales during 1963-67. Within each sampling unit, two sampling plots (a stream reach and its tributary slopes) and twenty sampling sites were selected. The sampling was designed to give data at three scales, namely (a) for river basins, (b) for slope and stream reaches, and (c) for individual sites. Various field measurements were chosen to indicate the rates at which sediment is transported at each of the three scales. Soil creep, rock-debris creep, sheet erosion, bank erosion, suspended sediment, bed load and scour and fill were measured in the field. The data for individual sites showed that the movement of sediment could be understood in terms of presentday site parameters. Most of the stream reach and tributary slope data also suggested that the landforms and present rates of supply and removal of sediment were mutually adjusted. The river basin data, although showing that most of the secondand third-order basins in the area were in a state of quasi-equilibrium with respect to sediment supply and removal, provided several examples of severe disequilibrium over the 4-year period of study. It is suggested that, in general, meso-scale and micro-scale fluvial landforms in the mid-Wales area are explicable in terms of time-independent models. Macro-scale fluvial land-forms, on the other hand, may never be satisfactorily evaluated without investigations of considerably longer duration. THREE broad schools of thought can be recognized with respect to the interpretation of fluvial landforms. The first comprises those who maximize the importance of the longcontinued operation of erosional processes, such as vertical down-cutting by streams, without explicit reference to the multivariate nature of erosional processes over space and through time. The work of R. J. Rice (1954, I957a and i957b) and E. H. Brown (1960) exemplifies this mode of interpretation in the context of mid-Wales, and directs attention to a particular assemblage of fluvial landforms, consisting essentially of meso-scale and macro-scale elements. These elements include the orientation of the river trace, the longitudinal profile of the river, and any remnants of terraces and planation surfaces that have regional significance. The second approach is illustrated by the work of J. T. Hack (1957, 1960, 1965 and 1966). For different physiographic environments, he directs attention to the great variety of erosional processes operating on an assemblage of landforms over a short period of time and the associated tendency towards 'a steady state of balance' as long as the energy state of that landscape remains constant. The meso-scale and micro-scale elements of the landscape are emphasized, such as the nature of the river bed and bank material, the nature of the regolith, the local gradient of the stream, elements of the relief and the kind of bedrock. The third position, held explicitly by R. J. Chorley (I962), S. A. Schumm and R. W. Lichty (1965) and A. D. Howard ( 965), acknowledges the existence of a tendency towards equilibrium and the importance of the time factor 'although it is an impossibly restricted view ... to imagine a universal approach to landform study being based only upon considerations of historical development' (Chorley, I962, p. 6). Such a 47 This content downloaded from 207.46.13.72 on Sat, 06 Aug 2016 06:22:24 UTC All use subject to http://about.jstor.org/terms viewpoint considers the micro-scale, meso-scale and macro-scale landform elements. The fluvial landforms of mid-Wales have been examined extensively from the viewpoint of the first school of thought; other modes of interpretation have been largely ignored. Two exceptions to this are the work of J. Challinor (I930) who invoked a concept very similar to that propounded by Hack, and D. Q. Bowen (1967) who suggested that Hack's hypothesis might be relevant in an interpretation of the upland plains (planation surfaces) of Wales, but neither writer developed any detailed discussion. It is therefore proposed to consider some of the ways in which a study of present erosion processes in mid-Wales can add to our understanding of its fluvial landforms. DESCRIPTION OF THE STUDY AREA Research was conducted in the upper drainage basin of the River Wye, an area of approximately 370 km2 (Fig. i). The region consists of Ordovician and Silurian sedimentary strata, dominated by shales and greywackes, with local development of conglomerates (O. T. Jones, I909; K. A. Davies, I926 and 1933; W. D. V. Jones, I945). Intricate folding but a relative dearth of faulting is characteristic of the area. The Elan river drains approximately half of the area (I95 km2) and joins the Wye as a right-bank tributary a little over 3 km south of Rhayader in Radnorshire. The confluence of the two streams is the lower limit of the drainage basin studied. The gross morphometries of the two halves of the basin, namely, the basin of the Wye above its confluence with the Elan and the basin of the Elan, display major contrasts which can be summarized by their respective hypsometric curves (A. N. Strahler, I952). The Elan basin, with a hypsometric integral of 56-2 per cent, has a high average altitude (445 m O.D. with a range from 185 to 635 m) compared with that of the Wye. The Wye basin has a hypsometric integral of 32-6 per cent and has an average altitude of 363 m, with a range from i85 to 745 m. The Elan has a large area of relatively undissected upland with mean annual precipitation in excess of I500 mm; the Wye includes a restricted area of greater absolute and available relief with a mean annual precipitation in excess of 2500 mm. The variety of gross slope form in the area has been well described by A. Young (I963). From the point of view of the sediment produced by present erosion processes, a slope sequence (consisting of a convexity, a maximum slope segment and a concavity) of smaller dimensions than those discussed by Young is highly significant. This slope sequence has a maximum slope segment averaging about 60 cm in length and occurs widely in the upper Wye basin. It is the site of a slope-failure form, termed a 'sheep hollow' by T. M. Thomas (1964) on the assumption that it is normally initiated by sheep. From these unvegetated scars sediment is produced at a high rate owing to the combined effects of sheet erosion and biogenic activity. Although it is clear that the growth rate of sheep hollows is accelerated by the presence of sheep, their initiation at places where the rate of increase of slope is greater than on adjacent parts of the slope suggests a natural instability of the slope independent of biogenic activity. The vegetation cover over the whole area exceeds 99 per cent. Molinia caerulea and Nardus stricta characterize the poorly drained interfluves of the Elan: Festuca ovina and Agrostis tenuis are dominant on the well-drained steep slopes; and there is significant peat development on the broader and higher interfluves, covering 2-3 per cent of the area and reaching recorded depths of i8o cm. The eastern extension of the basin, drained by the Afon Marteg, and the small area in the north-western corner of the basin, drained by the 48 H. O. SLAYMAKER This content downloaded from 207.46.13.72 on Sat, 06 Aug 2016 06:22:24 UTC All use subject to http://about.jstor.org/terms Present erosion in midWales 49 Source of River Severn