Welsh Journals

Rill patterns derived from air photographs of theGrwyneFechan catchment, Black Mountains M. D. STAGG Lecturer, Department of Landscape Architecture, Gloucestershire College of Art and Design, Cheltenham The potential value of air photographs in hydrological research is considerable, especially in upland regions where differences in land- forms, soils, drainage and vegetation are often numerous and well defined on the photographs, but it has only been realised to a limited degree (Barrett and Curtis, 1976) probably because of cost, the time consuming nature of analysis and the need for continuous checking of results (Gardiner, 1975). Air photographs can be best used for areas where vegetation cover is less dense and analysis is expected to yield additional information to mapwork (Gardiner, 1975) and most economically if existing stocks of photographs can be purchased. Monochrome air photographs of the Grwyne Fechan catchment (SO 240226) have been used to delineate the drainage network to include intermittent and ephemeral channel courses which are not recorded on the Provisional Edition 1 25,000 series map. Thus a network analysis more closely related to runoff and erosion processes, as demonstrated by Gregory and Walling (1968), Jones (1971), Stagg (1973) and Weyman (1970), was established. Analysis The drainage network was delineated from a Provisional Edition 1 25,000 series map, compiled from 1 10,560 sheets last fully revised 1891-1903, with other partial systematic revision 1948. Contour crenulations were used to determine channel extremities, thus including channel courses not always designated by blue lines, but possibly carrying runoff at frequent intervals. Stream ordering was by the Strahler method (Gardiner, 1975) resulting in the main channel being classified as a fourth order. Monochrome air photo- graphs at 1 20,000 scale (source: Department of the Environment; survey 1961) were used to add channels not shown on the map- mainly rills and gullies on valley side slopes. Many of the difficulties of air photograph interpretation (Gregory and Walling, 1973, Gardiner, 1975) were not encountered due to the low vegetation cover and distinct features of the catchment. Cloud cover on some photographs was overcome by using overlapping photographs.