Particle Image Velocimetry Analysis for the Assessment of Bedload
Sampling Using Helley-Smith
Abstract
Excess sediment is one of the main agents of water pollution, affects
the aquatic ecosystem and causes siltation problems in reservoirs and
rivers. Bottom sediments are coarse and easily deposited, being the
major responsible for morphological alterations in rivers. Therefore, is
important to estimate the amount of sediment transported in other to
establish mitigation strategies and recovery of affected areas. There
are several methodologies for estimating bedload transport: equations,
numerical modeling, direct physical sampling, and indirect method
measuring dune tracking. Most studies measure bedload transport using
the equipment Helley-Smith, generally taken as reference value for other
methods. However, Helley-Smith causes changes in the flow field, often
resulting in overestimates in sand bed rivers. Those overestimates were
not yet described and quantified in detail. This article tries to solve
this gap by observing the Helley-Smith sampler at the time of sampling.
For this, we used a video camera recording processed by Particle Image
Velocimetry (PIV) technique to qualitatively evaluate changes that the
intrusive sampler causes in the displacement of bottom sediments in a
sand bed river. When the sampler was properly allocated to the bottom,
PIV resulting velocity maps of bedload particle transport identified the
acceleration of the bottom particles as they approached the mouth of the
sampler. We also noticed areas of convergence and vorticity of the
sediments towards the sampler’s mouth, which may overestimate sampling.
When the sampler’s mouth was disconnected from the bed due to the
morphology of the bottom, we find regions of divergence in the
displacement of the sediments creating unrealistic results for bedload
transport analysis. In addition, the presence of organic matter can be
underestimated, which causes difficulties in analyzing the quality of
the collections because it is not possible to guarantee an equity in the
positioning of the equipment at the bottom of the channel. We thus
recommend complementation Helley-Smith measurements with surrogate
methods, for example such as using ADCPs or optical methods, as the one
described here.