Variations in Channel Centerline Migration Rate and Intensity of a Braided Reach in the Lower Yellow River

The Yellow River (YR) covers three climatic zones including arid region, semi-arid region and temperate monsoon region, with frequent appearance of flow intermittence in the Lower Yellow River (LYR) before 1999. Channel migration occurs frequently in braided rivers, which is a major focus of study in geomorphology and river dynamics. The braided reach in the LYR is featured by a complexly spatio-temporal variation in channel migration parameters owing to the varying condition of flow and sediment. It is crucial to investigate the migration characteristics of channel centerline for the sake of fully understanding channel evolution. A detailed calculation procedure is proposed to quantify migration rates and intensities of channel centerline at section- and reach-scales, using the measurements of remote sensing images and cross-sectional topography. Migration rates and intensities of channel centerline at section- and reach-scales from 1986 to 2016 were calculated, with the characteristics and key factors to control the migration intensity of channel centerline being identified quantitatively. Calculated results indicate that: (i) the mean probability of centerline migrating toward the left side was approximately equal to the probability of rightward migration from a long-term sequence; (ii) the mean reach-scale migration rate of channel centerline was reduced from 410 m/yr in 1986–1999 to 185 m/yr in 1999–2016, with a reduction of 55% owing to the Xiaolangdi Reservoir operation in 1999, and the mean reach-scale migration intensity of channel centerline was decreased from 0.28 to 0.16 m/(yr·m), with a reduction of 43%; (iii) the incoming flow-sediment regime was a dominant factor affecting the degree of channel migration, although the channel boundary conditions could influence the intensity of channel migration; and (iv) the reach-scale migration intensity of channel centerline can be written as a power function of the previous two-year average incoming sediment coefficient or fluvial erosion intensity, and the reach-scale migration intensities of channel centerline calculated using the proposed relations are generally in close agreement with the measurements over the period of 30 years.

Changes in sediment budget and morphology in the floodplains of the braided reach of the lower Yellow River

Floodplains provide valuable social and ecological environment functions, and understanding the rates and patterns of floodplain sedimentation/erosion is critical for floodplain management and rehabilitation. The sediment entering the lower Yellow River has been dramatically reduced, and the geomorphology has changed greatly during the operation of the Xiaolangdi (XLD) reservoir since 2000. Utilizing sediment resources is the key to managing the downstream river, and the floodplains not only play the roles of flood mitigation, detention and de-sanding but also provide land to support local residents and economic development; however, the floodplain currently faces competition between land development and protection. This research presents a detailed investigation of changes in the sediment budget and morphology of the braided reach between Huayuankou (HYK) and Gaocun (GC) during 2000-2017 using digital elevation models (DEMs) and the historical bathymetry of the braided reach. During the implementation of the water-sediment regulation scheme (WSRS), the long-term low-concentration flow released from the XLD reservoir leads to a fully scoured long channel, further improving the bank-full discharge and reducing the risk of floods on floodplains. However, the floodplains have gradually changed from sedimentation to erosion due to the continual construction of farm dykes and control works, land use changes and other forms of land disturbance, including water and soil conservation measures and climate change. The cumulative eroded volume was approximately 11.47×108 m3 along the HYK - GC reach between 2000 and 2017, of which 3.08×108 m3 originated from the floodplains, with an average annual erosion rate of 1.3 cm/yr. To develop the economy and guide floodplain construction, we propose a new method of environmental management to reconstruct the floodplain domain into different zones for immigration resettlement areas, agricultural areas and resource development and utilization areas, with the methods of river dredging and floodplain filling.

Spatiotemporal response of an Alpine braided river reach to snow melt and flood events

<p>Alpine environments are responding to accelerated climate warming through the release and mobilization of large amounts of unconsolidated sediment. Sediment fluxes delivered to Alpine streams may be buffered, filtered and/or modulated as they pass through braided river reaches, which play a key role in the downstream transfer and dynamics of bed material. The functioning of these braided reaches is however still poorly understood, particularly during high magnitude events whose effects are very difficult to monitor but play an ever more prominent role in river system evolution.</p><p>In this study, we investigate the transfer of bedload material and river bed morphological change in a braided reach of the Séveraisse River (France), over the course of the melt season and two large flood events with an estimated return period of 5 and 50 years. To quantify braided reach dynamics, a multi-physical approach is employed that combines both temporally and spatially resolved techniques. We use bank-side geophones and locally derived parameters that describe seismic wave propagation in the subsurface to accurately quantify bedload transport and gain a unique insight in its temporal dynamics, particularly during the flood events. River bed elevation changes are determined from intermittent UAV-based LiDAR and photogrammetric acquisition. These are complemented with hourly (daytime) time-lapse images that register planform changes during the flood events.</p><p>Our results show strongly contrasting morphodynamic behavior with different flow conditions. During ‘normal’ bedload transport conditions driven by annual snow-melt, channel aggradation occurs leading to progressively lower bedload export from the reach for a given discharge. During the flood with a 5 year return period, which occurred at the end of the melt season, the braided riverbed morphology is rearranged and net sediment export took place. Most interestingly, in the autumn an extreme flood event led to the development of a single channel, meandering planform with significant outer bend erosion on alternating banks. Although this morphological change may be only temporary, i.e. a braided configuration may be expected to be gradually re-instated, it has important implications on the general functioning and morphological evolution of the reach and the downstream transfer of sediment.</p>