A new approach for the estimation of extreme roughness in torrents by hydraulics and photogrammetry

In this paper we present a new approach to estimate extreme roughness in torrents based on the transformation of an irregular bed geometry of a torrent reach into a geometrically well determined "idealized" channel. An important parameter is the surface of the water, which was determined by a photogrammetric method using stereo photos on the Analytical Plotter WILD Aviolyt AC1. Two torrent reaches were photographed with two synchronized Rollei 6006 metric cameras, which were installed above the torrent. The mean velocity and the discharge were simultaneously measured by the salt-dilution method. With this approach, the calculated Strickler coefficients are considerably lower than the values in the often used coefficient tables.