The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attent...The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attention for low-head dam removal is increasing as one of alternatives for river restoration. Thus, this study intends to investigate the impacts of low-head dam removal on river geomorphology and riparian vegetation with developing a quantitative method to predict the changes of river morphology as well as invasion, growth, expansion and destruction of riparian vegetation after a low-head dam removal. To verify the numerical simulation model, the low-head dam removal case in Gongreung River was employed with investigation of low-head dam removal responses on river geomorphology and riparian vegetation. Following the low-head dam removal, the results of monitoring and numerical simulation indicated that new sand bars has formed as well as increasing the extent of existing sand bars in the upstream of the low-head dam. The sand bars have been colonized in a year after the low-head dam removal by grass type plants. After a decade to several decades, the riparian vegetation in sand bars often developed to tree type plants in several low-head dam removal cases. As other cases, Gongreung River also showed the growth of tree type plants in 5 years after the removal.展开更多
Riparian vegetation is known to affect the flood flow and the riparian environment, and it is important for river engineers to know the vegetation dynamics in river. Despite a number of researches have been performed ...Riparian vegetation is known to affect the flood flow and the riparian environment, and it is important for river engineers to know the vegetation dynamics in river. Despite a number of researches have been performed for vegetation dynamics, the mechanism of initial vegetation recruitment is still not clear. In this study, two field surveys by using Unmanned Aerial Vehicle (UAV) remote sensing, UAV aerial photographs analysis and a numerical simulation were conducted to detect the location of initial vegetation recruitment and explore its relationship with hydrology and river morphology. The initial vegetation recruitment was identified by comparing vegetation distributions before and after vegetation germination. Most of the initial vegetation recruitment locations were found either along dunes on sandbar or along the wet/dry rotational area of shore bank. Results of the aerial photographs analysis showed that the initial vegetation recruitment occurred at the downstream side of crest of dunes. As for the vegetation recruitment zone located along shore bank, the relative elevation of different recruitment zones is almost same, and the mean value of the relative elevation is around 0.33 m. With the comparison of the contour map of river morphology and the initial vegetation recruitment zone, the shape of initial vegetation recruitment zone is positively consistent with the shape of river morphology distribution. The results of numerical analysis show that the initial vegetation recruitment zone located along shore bank experienced intermediate flood pulse. The inundation frequencies during seed dispersal and germination period for the internal and external boundaries of vegetation recruitment zone are 23.87% and 6.2%, respectively.展开更多
The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict th...The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.展开更多
文摘The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attention for low-head dam removal is increasing as one of alternatives for river restoration. Thus, this study intends to investigate the impacts of low-head dam removal on river geomorphology and riparian vegetation with developing a quantitative method to predict the changes of river morphology as well as invasion, growth, expansion and destruction of riparian vegetation after a low-head dam removal. To verify the numerical simulation model, the low-head dam removal case in Gongreung River was employed with investigation of low-head dam removal responses on river geomorphology and riparian vegetation. Following the low-head dam removal, the results of monitoring and numerical simulation indicated that new sand bars has formed as well as increasing the extent of existing sand bars in the upstream of the low-head dam. The sand bars have been colonized in a year after the low-head dam removal by grass type plants. After a decade to several decades, the riparian vegetation in sand bars often developed to tree type plants in several low-head dam removal cases. As other cases, Gongreung River also showed the growth of tree type plants in 5 years after the removal.
文摘Riparian vegetation is known to affect the flood flow and the riparian environment, and it is important for river engineers to know the vegetation dynamics in river. Despite a number of researches have been performed for vegetation dynamics, the mechanism of initial vegetation recruitment is still not clear. In this study, two field surveys by using Unmanned Aerial Vehicle (UAV) remote sensing, UAV aerial photographs analysis and a numerical simulation were conducted to detect the location of initial vegetation recruitment and explore its relationship with hydrology and river morphology. The initial vegetation recruitment was identified by comparing vegetation distributions before and after vegetation germination. Most of the initial vegetation recruitment locations were found either along dunes on sandbar or along the wet/dry rotational area of shore bank. Results of the aerial photographs analysis showed that the initial vegetation recruitment occurred at the downstream side of crest of dunes. As for the vegetation recruitment zone located along shore bank, the relative elevation of different recruitment zones is almost same, and the mean value of the relative elevation is around 0.33 m. With the comparison of the contour map of river morphology and the initial vegetation recruitment zone, the shape of initial vegetation recruitment zone is positively consistent with the shape of river morphology distribution. The results of numerical analysis show that the initial vegetation recruitment zone located along shore bank experienced intermediate flood pulse. The inundation frequencies during seed dispersal and germination period for the internal and external boundaries of vegetation recruitment zone are 23.87% and 6.2%, respectively.
文摘The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.
