Sohei Kobayashi

Effects of sediment bypass tunnels on grain size distribution and benthic habitats in regulated rivers

ABSTRACT Four dams in Japan and Switzerland with Sediment Bypass Tunnels (SBT) as a measure against reservoir sedimentation were monitored to analyse the effects of sediment supply on the downstream environment based on up- to downstream differences in geomorphological and biological characteristics. SBT operation times ranged from 93 years at Pfaffensprung and 17 at Asahi to only three years at Solis and no operation at Koshibu. Sediment grain size distribution was monitored, and microhabitats and invertebrates were analysed in terms of richness and composition. Results showed that grain sizes were coarser down- than upstream at dams with newly established SBTs, while they were similar or finer for dams with long SBT operation. Analysis of biotic data revealed that microhabitat and invertebrate richness was low directly below the dam but increased further downstream the longer the SBT operation. Sedentary species dominated at locations where bed conditions were stable, e.g. directly downstream of the dam at Koshibu. Recovery of downstream environment with increasing SBT operation time was disclosed by the Bray–Curtis similarity index, which evaluated an overlap between up- and downstream reaches for both microhabitat composition and invertebrate communities. With increasing operation time, both indices increased, revealing the positive effects of long-term SBT operation.

Riffle topography and water flow support high invertebrate biomass in a gravel-bed river

Abstract.  We compared biomass and community structure of macroinvertebrates among 3 flow zones (deep, rapid, flat) of riffles at 3 sites in a gravel-bed river. We evaluated bed stability in these zones with a 2-dimensional hydrodynamic simulation over a range of discharge levels. Deep zones had higher flow velocity and coarser bed materials than other zones. Rapid zones were shallower with higher flow velocity than flat zones. The probability of bed movement was greatest in rapid zones and was lowest in deep zones based on bed shear stress and the size of bed materials. Total macroinvertebrate biomass was dominated by filterer insects and was highest in deep zones and lowest in rapid zones across the sites. This trend was most conspicuous for taxa that build retreats on stones, such as net-spinning caddisflies, which have a sessile life form and prefer stable environments. The trend was less apparent for taxa that move freely on the bed, such as baetid and heptageniid mayflies. The macroinvertebrate community differed between the middle and peripheral areas at deep zones. Peripheral areas were dominated more by taxa that stay under stones. The channel bed topography in deep zones of riffles is likely to support high macroinvertebrate biomass by providing greater bed stability and higher water flow, the combination of which is relatively uncommon in gravel-bed rivers.

Heavy flooding generating a precipitous decrease of fine particles along a riverbed: the case of a downstream reach of the futase dam

Dams and their reservoirs are operated for multiple purposes, including flood protection.1 However, all dams trap sediment and inevitably lead to downstream physical and ecological changes.1 In the downstream reaches, flows are more stable and sediment transport is more limited than in those upstream.2 This can cause several problems in downstream reaches including sediment deficiency and riverbed degradation.3 To compensate the sediment deficit downstream and reduce ecological damage, sediment replenishment is sometimes performed.3 Specifically, a certain amount of sediment is excavated from an upstream reservoir, transported to downstream channels, and then flushed via channel flooding.4 However, the total volume of sediment transported downstream is influenced not only by the annual volume of replenished sediment, but also by the flood discharge. The combination of these factors complicates prediction of sediment flow patterns. Analysis of a range of flood discharge rates and sediment flow data over many years at a single site is required in order to better predict these sediment flow patterns. However, existing data of this type spans only relatively short time periods (a few years at most). In this study, we examine the impacts of flood discharge on fine material on a riverbed by investigating the effects of sediment replenishment in the Futase Dam in Saitama Prefecture, Japan.