Author(s): Tetsuro Tsujimoto; Atsuko Teramoto; Hiromichi Matsui
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Keywords: Riffle-pool structure; Sand bar; Alternate bar; Double-row bar; Flood; Daily landscape; River ecosystem
Abstract: When we discuss the river ecosystem, the importance of riffle-pool structure is demonstrated because it supports habitats for various organisms. Riffle-pool structure is observed in low-water stage when sand bars are formed during flood. The discharge of flood determines the dimension and the shape of sand bars, while the riffle-pool structure appears at the low-water discharge flowing over the morphology developed during flood. In this paper, equilibrium geometry of sand bar for large discharge corresponding to flood and the hydraulic properties of flow over that morphology at the lower discharge are investigated by laboratory experiment and numerical simulation. Though there are several types of sand bars, alternate bars are focused on because the riffle-pool structure is most dominantly observed for such regimes. However, there are different types of riffle-pool: (1) The low-water stream crosses the bar-front line and the clear riffle appears there; and (2) the low-water stream flows along the bar-front line, and riffle-pool structure is not always clear. Under the condition that the alternate bars develop from an initially flattened bed by flood discharge, Type (1) appears. On the other hand, under the condition that double-row bars appear at first and then it changes into alternate bars during flood, Type (2) appears.
Year: 2005