Impact assessment of sand-trap flushing

Sand traps or 'desanders' are a common component of the water intake for many small hydroelectric power plants. These traps remove a large part of the silt, sand and gravel as well as organic debris carried by the water. This prevents sedimentary deposits from building up in the penstock before that water reaches a turbine for power generation. This accumulated material is regularely flushed downstream from the sand trap.
A study into the effects of sandtrap flushings on the benthic invertebrate communities was conducted in an alpine gravel stream above and below the hydropower water intake and sedimentation basin. The intensity and frequency of sand-trap flushings resulted in a colmatation of the streambed sediments with densly packed silt and sand among larger stones. Compared to the upstream section, species diversity and population densities were disinctly lower in those river sections characterized by pore-spaces clogged by silt and sand particles. In addition, streambed surface sections were scoured by the released sedimentary deposits removing surface-dwelling organisms. Similarly to this study, more recent investigations also concluded that the benthic invertebrate and fish community is severely impaired by sand-flushing operations1. A change in the sand-trap flushing regime, in particular, a marked reduction in flushing intensity would minimize sedimentation and clogging of interstices, improving benthic habitats and consequently species diversity.