Remotely sensed mid-channel bar dynamics in downstream of the Three Gorges Dam, China

The downstream reach of the Three Gorges Dam (TGD) along the Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). MCBs dynamics are crucial to the river’s hydrological processes and local ecological function. However, a systematic understanding of such dynamics and their linkage to TGD remains largely unknown. Using Landsat-image-extracted MCBs and several spatial-temporal analysis methods, this study presents a comprehensive understanding of MCB dynamics in terms of number, area, and shape, over downstream of TGD during the period 1985−2018. On average, a total of 140 MCBs were detected and grouped into four types representing small ( 2 km2), middle (2 km2 − 7 km2), large (7 km2 − 33 km2) and extra-large size (>33 km2) MCBs, respectively. MCBs number decreased after TGD closure but most of these happened in the lower reach. The area of total MCBs experienced an increasing trend (2.77 km2/yr, p-value 0.01) over the last three decades. The extra-large MCBs gained the largest area increasing rate than the other sizes of MCBs. Small MCBs tended to become relatively round, whereas the others became elongate in shape after TGD operation. Impacts of TGD operation generally diminished in the longitudinal direction from TGD to Hankou and from TGD to Jiujiang for shape and area dynamics, respectively. The quantified longitudinal and temporal dynamics of MCBs across the entire Yangtze River downstream of TGD provides a crucial monitoring basis for continuous investigation of the changing mechanisms affecting the morphology of the Yangtze River system

Response of the Downstream Braided Channel to Zhikong Reservoir on Lhasa River

Lhasa River basin is situated in the southern part of the Qinghai-Tibet Plateau, which is the most important region of economic and social development in Tibet. In order to efficiently utilize water resources in the basin and ease the shortage of regional electric power supply, Zhikong Reservoir was built in the upstream reach of the Lhasa River in 2006. Impoundment of this reservoir evidently affected the morphology and stability of the downstream braided channel below the dam. Yet, little is known about the complex responses of the downstream braided channel to the Zhikong Dam. Landsat images in the 2000–2016 period, together with daily discharges and field observations in the 2017–2018 period, were used to investigate the morphological response of the braided channel to the Zhikong Dam. The downstream Lhasa River below the Zhikong Dam was divided into four reaches (i.e., RS1, RS2, RS3 and RS4) based on the confluence of three downstream tributaries. Results showed that the number and area of central bars in the braided reach closest to Zhikong Dam (RS1) were increased because of main channel incision and water level drop. This increasing trend attenuated along the downstream channel of this reach. Braiding number index of multithread channels in RS1 obviously increased by 3 in one section and reduced by 2 in two sections, while changed in all sections randomly with no pronounced trend along the RS2 to RS3 and RS4 reaches. The average bar area in two focus reaches, RS1_B1 and RS2_B2, 6.0 km and 36.8 km far away to the Zhikong Dam, respectively, followed opposite trends with the former increasing and the later reducing. Furthermore, the mean dissection, landscape dissection and fragmentation shape indices in RS1, showed an increasing trend from 2001 to 2016, indicating the shape of irregular central bars varied greatly because clean water release of Zhikong Dam eroded the downstream braided channel

Chapter 7: Wetlands

Contains fulltext : 205862.pdf (publisher's version ) (Open Access

SEDIMENT TRANSPORT AND CHANNEL MORPHOLOGY OF A NATURAL AND A LEVEED ALLUVIAL RIVER

Alluvial rivers are shaped by interactions of flow and sediment transport. Their lower reaches to the world’s oceans are highly dynamic, often presenting engineering and management challenges. This thesis research aimed to investigate channel dynamics and sediment transport in a natural river and a highly engineered river in South Louisiana, in order to gain much-needed science information for helping develop sustainable practices in river engineering, sediment management, and coastal restoration and protection. Especially, the thesis research examined (1) riverbed deformation from bank to bank in the final 500-km reach of the Mississippi River, (2) bed material transport at the Mississippi-Atchafalaya River diversion, and (3) long-term and short-term flood effects on the morphological changes of the Amite-Comite River confluence. The research employed morphological, hydrodynamic, and geospatial modeling and analysis. The research found that from 1992 to 2013 the lowermost Mississippi River channel trapped 337 × 106 m3 sediment, equal to about 70% of riverine sand input from the upstream channel. The finding rejects the initial hypothesis that the highly engineered Mississippi River acts as a conduit for sediment transport. Sediment deposition mainly occurred in the immediate channel downstream of the Mississippi-Atchafalaya River diversion and the reach between RK 386 and RK 163, reflecting flow reduction and backwater influences. The bed material transport assessment revealed that in the recent decade the engineering-controlled Mississippi-Atchafalaya River diversion showed a slight disproportional transport of bed material loads. On average 24% of the Mississippi River was diverted into the Atchafalaya, but only 22% of bed material loads moved into the diversion outflow channel (i.e. 47 MT out of 215 MT). The confluence of Amite and Comite River continuously migrated about 55 m downstream between 2002 and 2017. Sediment deposition on the main channel side of the confluence mouth bar is the major driver for the confluence migration. Regression analysis shows that the increase rate of the vegetated area of the bar is highly related to the days of moderate floods. Short-term Laser scanning measurements reveal that a single flood with the intensity close to a moderate flood could double the projected surface area of the mouth bar and increased its volume by 68%. Overall, the thesis research shows the complexity of sediment transport in the lower reach of a large alluvial river, in that distinctive bed deformation can occur in different reaches because of flow deduction and backwater effects. Our study is the first try of estimating bed material load at a largely controlled bifurcation based on a simple, well-established bed material transport model. The study also highlights the importance of episodic floods on the evolution and migration of a river confluence

Using Remote Sensing Techniques to Improve Hydrological Predictions in a Rapidly Changing World

Remotely sensed geophysical datasets are being produced at increasingly fast rates to monitor various aspects of the Earth system in a rapidly changing world. The efficient and innovative use of these datasets to understand hydrological processes in various climatic and vegetation regimes under anthropogenic impacts has become an important challenge, but with a wide range of research opportunities. The ten contributions in this Special Issue have addressed the following four research topics: (1) Evapotranspiration estimation; (2) rainfall monitoring and prediction; (3) flood simulations and predictions; and (4) monitoring of ecohydrological processes using remote sensing techniques. Moreover, the authors have provided broader discussions on how to capitalize on state-of-the-art remote sensing techniques to improve hydrological model simulations and predictions, to enhance their skills in reproducing processes for the fast-changing world

Exploring sediment dynamics in coastal bays by numerical modelling and remote sensing

Coastal bays and salt marshes are buffer zones located at the interface between land and ocean, and provide ecologically and commercially important services worldwide. Unfortunately, their location makes them vulnerable and sensitive to sea-level rise (SLR), reduced sediment loads and anthropogenic modifications of the shoreline. Sediment budget and sediment availability are direct metrics for evaluating the resilience of salt marshes and coastal bays to various stressors (e.g. SLR). Salt marshes requires adequate sediment inputs to maintain their elevation with respect to sea level. Understanding sediment trajectories, sediment fluxes and sediment trapping capacities in different geomorphic unit facilitates efficient restorations and coastal management. In this research I used remote sensing, field observations and numerical modelling in the Plum Island Sound in Massachusetts, USA, to explore mechanisms controlling sediment dynamics and their feedbacks with SLR. The analysis of remote-sensed suspended sediment concentrations (SSC) reveals that a 5-year record (2013-2018) is sufficient to capture a representative range of meteorological and tidal conditions required to determine the main drivers of SSC dynamics in hydrodynamically-complex and small-scale coastal bays. The interplay between river and tidal flows dominated SSC dynamics in this estuary, whereas wind-driven resuspension had a more moderate effect. The SSC was higher during spring because of increased river discharge due to snowmelt. Tidal asymmetry also enhanced sediment resuspension during flood tides, possibly favoring deposition on marsh platforms. Together, water level, water-level rate of change, river discharge and wind speed were able to explain > 60% of the variability in the main channel SSC, thereby facilitating future prediction of SSC from these readily available variables. To determine the fate of cohesive sediments and spatial variations of trapping capacity in the system, a high-resolution (20 m) numerical model coupled to a vegetation module was developed. The results highlight the importance of the timing between sediment inputs and tidal phase and show that sediment discharged from tidal rivers deposit within the rivers themselves or in adjacent marshes. Most sediment is deposited in shallow tidal flats and channels and is unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platform. Trapping capacity of sediment in different intertidal subdomains decreases logarithmically with the ratio between advection length and the typical length of channels and tidal flats. Moreover, sediment deposition on the marsh decreases exponentially with distance from the channels and marsh edge. This decay rate is a function of settling velocity and the maximum value of water depth and velocity on the marsh platform. Bed sediment compositions were generated to further explore feedbacks between SLR, sediment dynamics and morphological changes. The results show SLR increases tidal prism and inundation depth, facilitating sediment deposition on the marsh platform. At the same time, SLR enhances ebb-dominated currents and increases sediment resuspension, reducing the sediment-trapping capacity of tidal flats and bays, leading to a negative sediment budget for the entire system. This bimodal distribution of sediment budget trajectories will have a profound impact on the morphology of coastal bays, increasing the difference in elevation between salt marshes and tidal flats and potentially affecting intertidal ecosystems. The results also clearly indicate that landforms lower with respect to the tidal frame are more affected by SLR than salt marshes. Therefore, Salt marshes, shallow bays, tidal flats, and barrier islands are inherently and physically connected systems, and evaluating the effect of SLR on salt marshes should involve all these units

Modelling of River Flows, Sediment and Contaminants Transport

This book presents five articles that are also part of a Special Issue titled: Modelling of River flows, Sediment and Contaminants Transport published in the Water Journal under the section: Water Erosion and Sediment Transport. It covers a wide range of topics, such as predicting the impacts of wildfires on sediment transport and water quality in a mountainous region and estimating the sediment erosion due to release of ice-jams in cold region rivers

Morphodynamics of active meandering rivers reviewed in a hierarchy of spatial and temporal scales

Meandering planforms are common on rivers but active, rapidly changing meandering channels are particularly instructive in indicating meander processes and dynamics and can extend our understanding of fluvial behaviour more generally. Questions arise in relation to the relative effects of flow events, phases and sequences of conditions, on the extent of autogenesis in changes, and the spatial propagation of change. In this review, the direct field and real-world evidence of the morphodynamics is examined through a hierarchy of spatial and temporal scales. Within bends, the channel interaction with bars is a major component of the morphodynamics. High variability in rates and patterns between events and years is evident, but a systematic sequence of mid-channel bar development emerges. Relations to discharge parameters are complex. At bend scale, clear autogenic sequences of bend evolution from simple loops to compound forms then cut-offs are apparent, in spite of short timescale episodicity and variability, and modulation by bank inhomogeneities. Two major morphodynamic issues are discussed, that of migration rate-curvature relations and of push-pull of bends. Cut-offs are important in active bend morphodynamics and an immediate phase of widening and multiple bars within the cut-off zone commonly occurs but then the channel stabilises. Conditions for clustering of cut-offs are discussed. Analysis at reach scale of multiple bends tends to produce much more systematic morphodynamics relations but may be obscuring the spatial and temporal variability. Evidence is equivocal on the extent and rates of spatial propagation of changes, some indicating change tends to be localised but other showing systematic interaction between bends. Much adjustment is by local feedbacks. The high rates and variability of the morphodynamics of active meandering rivers have implications and challenges for management. A strategy of allowing for the mobility is advocated. That requires understanding of the morphodynamics