| Decreased sediment transport and channel degradation have been a typical issue recently for environment and ecosystems of rivers in Japan. Although sediment is transported during floods, sediment shortage and bedrock exposure are evidently increasing in many Japanese rivers due to a decreased ability of channels to retain sediment. Bedrock exposure can result in significant reduction in aquatic species diversity and ecosystem function of rivers, because many benthic species rely on gravel-beds with much interstices for their habitats and as reproduction sites. Thus, in the present status of channel sediment, effective methods to increase sediment retention in the channel are required in river management practices. We have investigated processes of bedrock exposure below a dam in gravel-bed mountain river (Kinu River), Tochigi, Japan. Amount of sediment in the channel in our field site decreased due to reduced supply of sediment from upstream by dams in the main stem and tributaries, and also channel excavations in the past decades. Although a large amount of sediment still passes through a major dam during floods, coarser materials (>10 cm in diameter) are likely trapped at immediately upstream of the dam, which was implied by differences in grain size distribution between upper and lower reaches of the dam and by relationships between transport distance of bed materials with different sizes during floods. At lower reaches of the dam, a shortage of 10-40 cm materials is considered to enhance mobility of finer materials (<10 cm) and large boulders (>50 cm), which results in export of whole surface sediment and increase exposure of bedrocks. Once bedrock is exposed, a limited amount of transported sediment can be retained in the channel reach. To recover gravel-beds at bedrock exposed reaches, structures that enhance channel roughness and sediment retention are required. Boulders have been installed in some rivers to enhance channel roughness. However, such methods cannot be applied in mountain rivers with steep slopes, which inhibit the transport of power vehicles. We examined the usefulness of rectangle geotextile bags (2.5 cm mesh, 1m×2m×0.5m), which were filled with gravels, for recovering gravel-beds without using power vehicles in mountain rivers. Although we created and located a limited number of bags in a bedrock-exposed reach, the bags successfully retained sediment around them after a flood of moderate magnitude. The nets of the bags were mostly intact despite the abrasion by transported materials, even though some bags were moved downstream for a few-meters. Benthic invertebrates and fish appeared to increase in abundance after the retention of sediment. Limitations and possibilities of using geotextile bags on river restoration works were discussed. |
