Impact of sediment cohesion by root systems on river channel formation.

Tomonori Nagata, Yasuharu Watanabe, Jungo Funaki

Tuesday 30 june 2015

12:48 - 12:51h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling, Poster pitches

Parallel session: Poster pitches: 5A. Sediment - Erosion

Throughout Japan, riparian woods have been expanding in recent years. The Satsunai River used to have a braided channel with numerous sandbars covered with gravel. However, over the past few decades, willow forest has gradually invaded the channel and, the bed morphology has been changing from double-row bars to single-row alternate bars. The development of alternating sand bars can make the flow regime monotonous and bring considerable change to aquatic habitats. As causal factors in such channel migration, various possibilities have been proposed: flood control by a dam, reduction in river width and the like. However, the dominant factor has not been fully investigated yet. Previous studies addressed intrusion by vegetation mainly as a factor in flow resistance. However, the cohesion afforded by the underground portion of the vegetation’s root system is also regarded as playing an important role in river channel formation by resisting sediment transport. The present study addresses such resistance, which varies with the vegetation growth, as a factor that determines the amount of sediment transport, and a movable-bed hydraulic model experiment was conducted in which short fibers were used to model the root system. We investigated the reduction in sediment transport afforded by the addition of short fibers to silica sand at various mix ratios. The measurements determined that sand particle movement can be significantly inhibited by the addition of a fibrous substance to the sediment. Based on experimental results, numerical experiments using plane two-dimensional analysis of riverbed evolution were conducted in which short fibers were used at a mix ratio that reproduced the growth of willows, and the relation between tree age and the percent reduction in sediment transport was modeled. ¬The analysis revealed that areas which are frequently inundated during floods are prone to disturbance while the willows are still saplings; however, after the height differential between the sandbar and the main channel has increased to a certain extent, vegetation on the sandbar is less prone to disturbance, which promotes the growth of the vegetation and gradually increases that height differential, all of which together promote the development of alternating sand bars over time.