| Popis: |
Characterising morphological changes in mountain areas is of fundamental importance for scienceand engineering. Intense floods usually involve massive sediment transport, which may significantlyalter basin and river characteristics. Sediment erosion and deposition control the dynamicsof morphological structures such as alternate bars and meanders. By using unmanned aerial vehicles(UAV), it has been possible to obtain high-precision bed elevation data at the sediment scale.Our project aims to develop a consistent and optimised methodology for monitoring morphologicalchanges in an Alpine watershed using an UAV. Since 2017, we have been monitoring the Plat de laLé area drained by the River Navisence (Zinal, canton Valais, Switzerland). In mountainous regions,poor accessibility and light conditions make it difficult to set control points on the ground. We firstanalysed the relevance and influence of certain ground control points (GCP) on the the accuracy ofthe digital elevation model (DEM) obtained from the UAV’s images. Errors in the GCP localisationwere much larger than the DEM resolution. Not only did the GCP number and flight height affectthese errors, as expected, but their positions and orientations also played a part. We then carriedout an additional monitoring campaign to understand the influence of these parameters on the DEMaccuracy. This campaign was ran on two areas: a steep-slope area with irregular topography anda low-slope area that comprises the river channel and its floodplain. We built DEMs for each areaconsidering different GCP numbers (in the 3–18 range with 14 additional checkpoints) and flightheights (in the 40–140-m range). The present study provides guidelines, including an optimal combinationof parameters that significantly reduce errors in the DEM, and a methodology that can beused for monitoring Alpine watersheds on a regular basis. |
