Stereophotogrammetry of archive data and topographic approaches to debris-flow torrent measurements : calculation of channel-sediment states and a partial sediment budget for Manival torrent (Isère, France)
Résumé
This paper focuses on a topographic methodology to characterize the amount of sediment stored in channels and the use of historical photographs for aerial survey by stereophotogrammetry, as part of wider research on debris-flow magnitude prediction. The topographic methodology uses equidistant four-point cross-sections along the long profile of the channel. Each cross-section is representative of a 50-m reach of the channel. To calculate the volume of each reach, the difference is calculated between a reference level and the topographic surface. The reference level is the lowest level where the debris flow can erode, and in the current method this level is estimated from fixed points along the long profile of the channel. The accuracy of the method has been estimated by comparing results of a detailed topographic survey, with a standard deviation corresponding to about 6 per cent of the total calculated sediment volume. This topographic methodology has been used on aerail photographs by photogrammetry. This tool was applied to photographs taken on 12 past dates. The scales of the archive photographs used range from 1:3000 to 1:30 000, but results are consistent and permit us to calculate sediment states of the channel for different past dates with an uncertainty of about 6 per cent of the total volume. The application of the technique to the Manival debris-flow torrent has permitted us to propose some partial sediment budget and erosion-rate estimates.