Author(s): Takashi Asaeda, Kelum Sanjaya
Linked Author(s): Takashi Asaeda
Keywords: Gravel sediment, steep rivers, vegetation recovery, riparian vegetation, nutrient and moisture
Abstract: Steep slope segments of rivers that originally covered with gravels, transport gravelly sediments and fill up the river channel. But, a significant vegetation encroachment can be seen on these riparian areas in modern days and the reason is not yet clear. Therefore, we hypothesized that a reduction in movable gravelly sediments may have caused the difference. With that, the objective of the study is to assess the effect of soil erosion and deposition processes on riparian vegetation colonization. The recovery process of vegetation after sediment deposition or erosion is investigated in six rivers by aerial photographs. Field investigations have been carried out at depositional and erosional locations. In addition, the vegetation colonization is modeled with the Dynamic Riparian Vegetation Model (DRIPVEM) for further understanding the intact processes. The survey conducted with aerial photos elucidated that the vegetation colonization is significantly delayed at sites where gravel is deposited in comparison to sediment eroded sites. The sandy rivers has a faster recovery of vegetation compared to gravelly rivers. Moreover, the tree recovery begin slowly by recruitment from seeds at deposited sites of gravelly sediments, whereas the new shoots sprouted in the next spring from the collapsed live trees, achieving a rapid recovery in the succeeding years at eroded sites. The sediment nutrient and moisture are significantly higher at the eroded sites in comparison to gravel deposited sites. The simulated results are also in agreement with the observations in the present study. The results suggest that the gravelly sediment deposition creates a resistance for the colonization of riparian vegetation. Because, the gravel sediments are deposited after washing during floods and free from fine sediments, thus gravels lack moisture and nutrients. The reduction of gravel sediment seems to be the major reason for the rapid increase of riparian vegetation Japanese rivers
Year: 2017