Morfometrisk analyse av step-pools i Vekveselva, Trollheimen: Betydning av gradient for step-pool morfologi

Abstract

This thesis focuses on the step-pool morphology in a steep alpine channel, Vekveselva in Trollheimen near Oppdal. Step-pool sequences in the river have been measured by Leica GNSS/GPS (RTK). The rod has been used to measure the thalweg for every one meter in the field, resulting in a long profile of 756 meters. Data from fieldwork have been used, to investigate the relationship between bed gradient and step-pool geometry, including the relationships between step height, residual pool depth, step drop height, pool length and step length. Key stones are observed most places in the channel and might have great importance on hydraulics and the morphological structure. Step height and residual depth are highly controlled by the grain size of the step and the random location of key stones across the channel. There is no correlation between residual depth and stream gradient. The effects of recent flood events might be the reason why pools don’t get deeper with increasing stream gradient because of submergence and newly established and reorganized steps. The pools get deeper with increasing step heights and vice versa. The thesis also discusses how difficult it can be to identify step-pools from a stream longitudinal profile and why it is important to use a standardized classification scheme. Standardized algorithms overcome the subjectivity associated with surveying a long profile.