As streams and rivers flow, they carry sediments of various sizes, from fine sand to large boulders. Knowing the quantity of such transported sediments is vital for river engineering and natural hazard assessment. For example, in the case of flooding, knowing how much sediment a river can carry can help prevent disasters.
In a new study, Schneider et al. studied sediment transport at steep and gentle channel slopes in a glacier-fed Swiss mountain stream called Riedbach. Near the glacier source, the Riedbach has a comparatively gentle slope of 3%–6%. Over a 1-kilometer stretch, the Riedbach channel becomes very steep, with a slope of 30%–40%. Because it is driven by glacial melt, the stream fluctuates daily and seasonally, with its peak flow during late afternoons in the summer.
The scientists used two methods to measure sediment transport rates in the 1-kilometer study stretch. In the low-gradient part of the Riedbach, they installed bed load traps, long, trailing nets with aluminum frame openings that collect gravel and cobbles as they roll along the streambed. The trapped sediment was emptied and weighed every hour.
Downstream of the steep reach, the scientists set up a Swiss plate geophone system, consisting of metal plates with vibration sensors lining the streambed. As gravel and cobbles hit these plates, they produce vibrations that are picked up by the sensors. The signals from the sensors were tallied each minute to measure the volume of sediment carried by the stream.
Although the Riedbach streambed gradient increases tenfold over the 1-kilometer stretch, the scientists found that the flow velocity and sediment transport rate were nearly the same in the steep reach and the gentle one. The flow velocity in the steep reach is limited by the drag of the flow against large boulders lining the bed of the channel. The increased flow resistance in the steep channel reduces the flow energy that is available to transport sediment.
The authors used the sediment transport observations to validate flow velocity and sediment transport equations that were created for low-gradient and moderately steep streams. They found that these equations could accurately predict flow velocities and sediment transport rates at very steep slopes if the additional drag from boulders was taken into account.
These equations work best during high flows, which are especially relevant for forecasting flood hazards. The results may help environmental researchers predict sediment transport during floods in mountain regions. (Water Resources Research, doi:10.1002/2016WR019308, 2016)
—Alexandra Branscombe, Freelance Writer
© 2017. The authors. CC BY-NC-ND 3.0