Role Change of In-Channel Vegetation with Regard to Sediment Retention at the Decadal Scale

Abstract

Human modification to rivers may induce considerable barriers to sediment movement, whether directly and immediately or otherwise. For example, dams may prevent sediment from moving through the system once constructed. In-channel vegetation is a form of sediment trap, which could be an indirect consequence of human interference with river, and has received considerable attention. However, sediments retained in various landforms may be remobilized by one cause or another. Better understanding of sediment remobilization mechanisms and related time scales are needed for better sediment management. The present study is motivated by the fact that the long-term behavior of sediment trapped by in-channel vegetation is still poorly understood. It focuses on the temporally changing role of in-channel vegetation with its growth in regard to sediment retention in the lower Tenryu River, Japan. It reveals that although in-channel vegetation can trap sediment during growing stage, it could become a source of sediment supply at the decadal time scale, when the vegetation was well established. The mechanism behind is the formation and destruction of forested overhanging bank. The process can be characterized by the time period scale ratio of riparian forest growth to lateral channel migration. The findings can help identify the residence time of sediment stores and subsequently aid in planning to increase fluvial sediment delivery from river to the coast.

Keywords: riparian vegetation, storage-transport, bank erosion, time scale, Lower Tenryu River