Total sediment transport in the lower reaches of river Þjórsá : results from the year 2003

Abstract

This report introduces results from a comprehensive sediment sampling program in the lower reaches of river Þjórsá that was initiated in 2001. This year, suspended sediment samples were taken in five sampling campaigns from cableways at Krókur and Urriðafoss and from beneath the old bridge on Highway 1. In four of these campaigns bedload samples were also obtained at Krókur. The main objectives of the program is to get more suspended sediment samples from the lower reaches of Þjórsá, evaluate the older suspended samples taken at this site by taking comparison samples, and obtain some estimation of bedload transport in lower Þjórsá. The total concentration and grain size of suspended samples varies greatly between campaigns, but tend to increase with discharge, especiallly the bridge samples. Those and the Krókur samples appear to underestimate the concentration of total sediment and the coasest size fraction (>0.2mm) compared to samples taken at the Urriðafoss cableway. Bedload transport was highest at the 50 to 80 m stations and total transport was from 1.7 to 16.5 kg/ (values adjusted by extra sampling). The bedload material was mainly medium and coarse grained samd with pebbles smaller tha -5.0 ö.