Sediment replenishment to downstream reaches below dams: implementation perspectives

Abstract

Sediment replenishment below dams is an increasingly common practice in Japan to compensate sediment deficits downstream and improve habitat quality and ecological functions. We reviewed implementation methods in the context of sediment placement, and compared the design and implementation activities undertaken in the Nunome River of Japan and Trinity River of California. Nunome River was supplied with a maximum of 500 m3 of mixed sand-gravel to the stream channel from a single site through a high-flow stockpile method during a relatively small (80 m3 /s) and short (less than 4 hours) peak flow. Trinity River was supplied through an in-channel stockpile, high-flow stockpile and high-flow direct injection in combination with mechanical rehabilitation with the aim of re-creating gravel bar features through fluvial processes. More (max 51,000 m3) and greater (gravel to boulder) sediment has been added from more than four sites along the downstream channels during a longer (max 5 days) and higher (up to 311 m3 /s) peak flow regime that are designated depending on the water year types which are determined by systematic analyses in a given year. This comparative study provides the present programs in Japan with some recommendations that will inform proper methods corresponding to river-specific high-flow and sediment regimes.