Evaluating the Landslide Stability and Vegetation Recovery: Case Studies in the Tsengwen Reservoir Watershed in Taiwan

The sediment yield from numerous landslides triggered in Taiwan’s mountainous regions by 2009 Typhoon Morakot have had substantial long-term impacts on the evolution of rivers. This study evaluated the long-term evolution of landslides induced by 2001 Typhoon Nari and 2009 Typhoon Morakot in the Tsengwen Reservoir Watershed by using multiannual landslide inventories and rainfall records for the 2001–2017 period. The landslide activity, vegetation recovery time, and the landslide spatiotemporal hotspot analyses were used in the study. Severe landslides most commonly occurred on 35–45° slopes at elevations of 1400–2000 m located within 500 m of the rivers. The average vegetation recovery time was 2.29 years, and landslides with vegetation recovery times exceeding 10 years were most frequently retrogressive landslide, riverbank landslides in sinuous reaches, and the core area of large landslides. The annual landslide area decline ratios after 2009 Typhoon Morakot in Southern Taiwan was 4.75% to 7.45%, and the time of landslide recovery in the Tsengwen reservoir watershed was predicted to be 28.48 years. Oscillating hotspots and coldspots occupied 95.8% of spatiotemporal patterns in the watershed area. The results indicate that landslides moved from hillslopes to rivers in the 2001–2017 period because the enormous amount of sediment deposited in rivers resulted in the change of river geomorphology and the riverbank landslides.

Invasion of Siberian Elm (Ulmus Pumila) Along the South Platte River: The Roles of Seed Source, Human Influence, and River Geomorphology

Riparian ecosystems in the western USA have been invaded by non-native woody species deliberately introduced for stream bank stabilization, agricultural windbreaks, and urban shade. Recent work suggests that the non-native tree Ulmus pumila (Siberian elm) is capable of significant spread in western riparian ecosystems, that range infilling is still incomplete, and that the invasion is dispersal-limited. We analyzed the interacting roles of propagule pressure from upland U. pumila, human influence based on road and farmstead density, and river geomorphology in promoting U. pumila invasion into riparian corridors along the South Platte River, Colorado, USA. U. pumila stem density increased with increasing channel and floodplain restriction and increasing human influence from both urban and rural development. Model selection indicated that local upland U. pumila seed sources were relatively unimportant to riparian U. pumila stem density, suggesting that upland propagule pressure is currently contributing less than human influences to U. pumila spread along the South Platte River. In particular, higher road density was the most important predictor for the proportional abundance of smaller U. pumila individuals (DBH < 5-cm and 5-15-cm), suggesting that human influence in densely populated areas has been the primary driver of recent U. pumila population expansion. U. pumila stem density was only weakly associated with abundance of other common riparian tree species. Land managers and other entities concerned with non-native tree invasion into important riparian habitat may be able to reduce U. pumila spread most effectively by focusing U. pumila control efforts where human influences are greatest.

Empirical analysis of effects of dike systems on channel morphology and flowlines

A phased study of the dike fields within the Vicksburg and Memphis Districts of the US Army Corps of Engineers was conducted to document the channel morphology trends since dike construction on the Lower Mississippi River (LMR). This included the development of the hydrographic survey database and methodology utilized to identify changes in channel geometry in response to dike construction. A subsequent report will provide further refinements to the approach and results of the comprehensive assessment. Recent Mississippi River Geomorphology and Potamology program efforts have employed the database developed by Mr. Steve Cobb to assess the geomorphic changes in 21 dike systems along the LMR. Previous studies using this database have indicated that the dike fields have not caused a loss of channel capacity. Furthermore, these efforts suggested that the trends in the dike fields are closely related to the long-term geomorphic trends along the LMR. Previous efforts using the Cobb database provided considerable insight into the dike effects on the LMR, but they were limited spatially and temporally. In this study, a database and protocols were developed to allow for a more robust assessment of dike field impacts and to extend the spatial and temporal extents of the analysis.

Does river restoration result in improved environmental heterogeneity?

Rivers can shape diverse landscapes, determine the spatial connectivity of river and terrestrial life, and provide a variety of resources and services. Rivers are often over-bound due to the need for flood control and irrigation. Rivers affected by human disturbance often require restoration to improve the ecosystem services they provide. Environmental heterogeneity is generally considered to be the non-uniform variation of environmental elements in space and/or time. The relationship between variability in physical characteristics of restored rivers and biological communities in the river environment is a highly complex feedback, and studying and summarising changes in environmental heterogeneity following river restoration can help refine methodologies for monitoring river restoration outcomes. This study highlights the variability in river geomorphology and river ecology, and demonstrates the feasibility and necessity of incorporating environmental heterogeneity indicators into river restoration outcome evaluation systems at three levels: hydrological, geomorphological and ecological.

A natural approach to river engineering practice. A case study of the Ljiljanska river

In the past 30 years the efforts to protect river beds and banks have increased significantly. The selection and design of proper structural solution means finding a solution in accordance with construction principles, river geomorphology, avoiding channel aggradation, bed scour, bank erosion, resulting structure failure and significant harm to the stream and nearby property. On the other hand, the structure should be environmentally-friendly. Hydraulic structures generally have a strong impact on the environment, so providing ?the right solution? presents a real challenge to engineers.