Fine sediment transport and storage in a gravel bed river, a pilot study in the Geul River, the Netherlands
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This pilot study investigates the storage of fine sediments in the river bed of the Geul River, the Netherlands. The sediment infiltration into the gravel bed is measured at four locations in the Geul River using two different methods: a gravimetric method and a metal concentration-based method. Both methods concerned the placement of sediment traps in the gravel bed, consisting of cylindrical mesh cages with a diameter of 15 cm and a height of 10 cm. In the first method, the cage was filled with clean gravel larger than 12.5 mm (the size of the mesh openings) collected from the local river bed (with a mean gravel size of 19 mm). After two to sixty-five days, the sediment traps were removed. In the second method, the sediment traps were filled with clean gravel and 700 grams of fine sand with low metal concentrations. During the sampling period, this fine sand was contaminated by deposition of metal-contaminated fine sediment from the Geul River. After four to eight days, the sediment traps were removed. For both methods the trap is placed in a bag. The bag was pulled to the bottom of the trap when the trap was placed and was pulled up when the traps were removed to retain the fine sediment. The fine sediment was washed from the sediment traps and subsequently dried and weighed. For the second method, the zinc concentrations of the fine sand and the fine sediment collected from the sediment traps were measured using a handheld XRF analyser. The sediment flux was then calculated from the differences between the zinc concentrations in the sediment samples and the fine sand. The amount of gravel-stored fine sediment was measured with a resuspension cylinder. It was found that the mean and variation of the fine sediment deposition rates increased with stream discharge during the sampling period. Changes in the trapped fine sediment weight were related to changes in discharge. The average fine sediment flux was determined by fitting a mass balance to the measurements. Based on this flux-discharge relation and resuspension measurements, average sediment residence times were calculated. A travel time model for suspended and bed load sediment based on the probability of sediment having a certain residence time was made. It was found that it takes bed load on average 13 days to travel through a river reach of 20 km and suspended sediment 126 days.