Close range digital photogrammetric analysis of experimental drainage basin evolution

Despite the difficulties of establishing formal hydraulic and geometric similarity, small-scale models of drainage basins have often been used to investigate the evolution and dynamics of larger-scale landforms. Historically, this analysis has been restricted to planform basin characteristics and only in the last decade has the topographic similarity of experimental landscapes been explored through explicitly three-dimensional parameters such as the distributions of cumulative drainage area, area-slope and catchment elevation. The current emphasis on three-dimensional morphometry reflects a growing awareness of the descriptive paucity of planform data and the need for more robust analysis of spatial scaling relationships. This paradigm shift has been significantly facilitated by technological developments in topographic survey and digital elevation modelling (DEM) which now present the opportunity to acquire and analyse high-resolution, distributed elevation data. Few studies have, however, attempted to use topographic modelling to provide information on the changing pattern and rate of sediment transport though an evolving landscape directly by using multitemporal DEM differencing techniques. This paper reports a laboratory study in which digital photogrammetry was employed to derive high-resolution DEMs of a simulated landscape in declining equilibrium at 15 minute frequency through a 240 minute simulation. Detailed evaluation of the DEMs revealed a vertical precision of 1·2 mm and threshold level of change detection between surfaces of ±3 mm at the 95 per cent confidence level. This quality assurance set the limits for determining the volumetric change between surfaces, which was used to recover the sediment budget through the experiment and to examine local - and basin-scale rates of sediment transport. A comparison of directly observed and morphometric estimates of sediment yield at the basin outlet was used to quantify the closure of the sediment budget over the simulation, and revealed an encouragingly small 6·2 per cent error. The application of this dynamic morphological approach has the potential to offer new insights into the controls on landform development, as demonstrated here by an analysis of the changing pattern of the basin sediment delivery ratio during network growth