Sensitivity of morphometric estimates of sediment transport in large gravel-bed rivers

The estimation of fluvial sediment transport rate from measurements of morphological change has received growing recent interest in the last five years. The revival of the 'morphological method' reflects continuing concern over traditional methods of rate determination but also the availability of new survey methods capable of high-precision, high-resolution topographic monitoring. In particular, remote sensing of river channels through aerial digital photogrammetry is a potentially attractive alternative to labour intensive ground surveys. However, while photogrammetry presents the opportunity to acquire survey data over large areas, data precision and accuracy, particularly in the vertical dimension is lower than traditional ground survey methods. This paper presents results of recent research in which DEMs have been developed for a reach of a large braided gravel bed river in Scotland using both digital photogrammetry and high resolution RTK GPS ground surveys. For both approaches a statistical level of detection of change is assessed by intercomparing surfaces with independent check points. The sensitivity of the annual channel sediment budget to this level of detection is presented. Preliminary results suggest that as much as 60% of channel deposition and 30% of erosion may be obscured by the lower level of precision associated with photogrammetric monitoring.