Monitoring and modelling morphological change in a braided gravel-bed river using high resolution GPS-based survey

Abstract

Recent research in fluvial geomorphology has emphasized the direct monitoring of channel topography as a useful tool to better understand the interrelationship between river form and process, and in particular to estimate bedload transport rates and reach-scale sediment budgets. Until recently, however, this morphometric approach has been limited to comparison of repeatedly surveyed cross-sections with the associated problems of low frequency sampling and an emphasis on cross-stream as opposed to downstream morphological dynamics. Recent advances in analytical photogrammetry have, however, permitted the acquisition of data at sufficient resolutions to represent macroform (101 m) and microform (10° m) channel geometry, in fully three dimensions. While inherently spatially distributed, the photogrammetric approach to land-surveying is, however, restricted to observable areas of the channel and is thus of limited use in the subaqueous zone, especially if flows are turbid or highly coloured. This paper presents an alternative approach to the study of three-dimensional morphological dynamics of a divided reach of the gravelly River Feshie, Scotland, in which topographic survey of both exposed and submerged areas of the reach was undertaken using the Global Positioning System (GPS). Survey results from two field programmes in 1998 and 1999 are presented in which a Geotronics Geotracer 2000, real-time kinematic GPS receiving unit was used to survey a 200 × 80 m reach. Rapid radial survey methods were employed permitting the collection of 2000 + survey points per day, giving a mean density over the 13 ha reach of 1·1 points per square metre. A detailed assessment of survey errors and DEM quality is presented and a limit of detection of c. 10 cm proposed as an upper threshold for the identification of significant change between DEMs. Preliminary interpretation of channel dynamics in 1998–99 reveals relatively little change to the overall macroscale reach structure. DEM differencing techniques, however, highlight important local changes including the progradation of submerged bar fronts, bar head aggradation and scour along major channel thalwegs and the erosion of fines from sloughs. Copyright © 2000 John Wiley & Sons, Ltd.