Decline In Sediment Supply Research Articles

Morphodynamics of a meandering channel over decadal timescales in response to hydrological variations

AbstractThis article addresses a major theme in fluvial geomorphology on which Professor Ken Gregory contributed much pioneering research, that of quantitative relations of morphological change to discharge variations. It examines the morphodynamics and processes of adjustment of river channels to flow characteristics on event to decadal timescales, using field and remote sensing data collected over a 40‐year period for a very active meandering reach of the River Bollin, northwest England. The free meandering and rapid rates of changes provide insight into response timescales and processes. Morphological variations and rates of change are analysed in relation to peak flow parameters. The results show that the sinuosity has continuously increased since a resetting of the planform by multiple cutoffs in 2001, this autogenic trend underlying the effects of flow events. Width of the channel has varied by 50% between map dates, corresponding to discharge characteristics of each 6‐year period and integrating the impacts of individual events. A trend of decrease in intensity of erosion and deposition and of narrowing for the period 2001–2019 is apparent and may be an indication of the recovery and adjustment time to the major morphological changes in 2001. Other possible influences include decline in sediment supply but no obvious external cause is identified. A feedback effect of lower process rates producing less sediment from banks which decreases rates of channel movement may be occurring. A major contributor to the process rate decline and narrowing could be an increase in riparian vegetation cover. Complex sequences of change between peak flow events emerge, unrelated to discharge magnitude or numbers of peak events, and a process of basal bank sedimentation followed by upper bank deposition is evident. Channel capacity varies by up to 30% year to year, which has major implications for variability of flood risk and floodplain inundation.

Morphodynamic changes in the Yangtze Estuary under the impact of the Three Gorges Dam, estuarine engineering interventions and climate-induced sea level rise

The estuarine turbidity maximum (ETM) in the Yangtze Estuary Delta (YED) is muddy by definition and lacks bottom undulations. However, since 2013, a remarkable change has occurred in the YED. Recent images detected by a multibeam echosounder system, SeaBat 7125, for the first time have confirmed widespread regions of subaqueous dunes in the Yangtze ETM channel. This abnormal change is the result of morphodynamic transformation from the combination of an abrupt decline in sediment supply resulting from the construction of the Three Gorges Dam (TGD) and hydrodynamic changes caused by sea level rise. The latter includes anthropogenic-induced sea level rises (from land subsidence and coastal engineering) of 7–37 cm and a climate-induced sea level rise of 8 cm during the past four decades. Obvious evidence of hydrodynamic changes includes tidal amplification, i.e., a 10–28 cm rise in the tidal range, 42–65 cm rise in the lowest tidal level in the dry season, 45–67 cm rise in the highest tidal level in the flood season and 10–30% increase in the amplitude of the major tidal component. These findings will likely have global implications in formulating strategies to combat the superimposed effects of human interventions and climate change on upstream river and downstream coastal developments.

Marine sediment sustains the accretion of a mixed fluvial-tidal delta

There has been a decline in fluvial sediment inputs to the world's deltas but it is still unclear to which extent upstream changes in sediment supply directly relate to variations in wetlands extent. We address this gap by investigating the contribution of marine and fluvial sediment sources to tidal flats in Changjiang Delta. Surprisingly, field measurements show that tidal flats accreted despite a > 50% decline in fluvial suspended sediment discharge due to the Three Gorges Dam. Results show that at a decadal time scale only a minimal portion of fluvial sediments contribute to sediment deposition on tidal flats while the adjacent estuary and ocean zone contributed over 90% of the deposit. We conclude that marine sediments give a substantial contribution to the maintenance of coastal wetlands and that there can be a long-time lag between human interventions and the delta's response to the decline in sediment supply.

Long-term sediment decline causes ongoing shrinkage of the Mekong megadelta, Vietnam

Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was < +1 km2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels.

Lithofacies Superimposition in a Shallow Basin - Interplay of Tectonics and Sedimentation: Evidences from Kolhan Basin, Eastern India

The 2.2-2.1 Ga pear shaped Kolhan basin show the development of a time transgressive group in a half-graben setting developed during the fragmentation of the Rodinia supercontinent. The overall style of sedimentation reflect a switchover from low-sinuous avulsed channels developed within a braided-fluvial-ephemeral streams to a lacustrine fan-delta complex during the later part of the sedimentation history. The fan-delta facies indicate sediment dispersal by hyperconcentrated flows in the form of sheetfloods and channelized flows. These different-scale cycles are interpreted as the sedimentary response to pulses of deformation of the basin margin at variable frequencies, related to the contemporary thrusts (ca. 20 km away from the basin). The episodes of tectonism downwarped the basin margin sediments and made the basin shift periodically toward the margin, and created progressive lithological changes in the sedimentary succession. The immediate effects of a tectonic pulse included lake transgression and accentuation of the structural hinge of the basin margin, causing a decline of sediment supply from the source rock. As the basin margin was subsequently reduced by denudation, the fans prograded and fan deltas were formed in normal conditions of graben subsidence. The sediment geometries and the climate exerted a major control on the processes of sediment transfer. Our results show that the fluvio-lacustrine strata show on lap-pinch out relationship at the centre of the basin but only on lap relationship along the lateral edges. This transition can be best explained by the fault growth models.

Sedimentary Dynamics along the West Coast of Bohai Bay, China, during the Twentieth Century

Wang, F.; Wang, H.; Zong, Y.; Andersen, T.J.; Pei, Y.D.; Tian, L.Z.; Li, J.F., and Shang, Z.W., 2014. Sedimentary dynamics along the west coast of Bohai Bay, China, during the twentieth century.To investigate the most recent changes in sedimentation along the west coast of Bohai Bay, China, we collected twelve 1–2 m short cores of undisturbed sediment from tidal flats off the city of Tianjin, using an Eijkelkamp corer. Based on the excess or unsupported 210Pb and 137Cs activities measured from the sediment cores, the chronology of the sediment sequences was established, from which the sedimentary history of the tidal flats was reconstructed. Results inform spatial and temporal variations in sedimentation rate along the Tianjin coast, from which sedimentary dynamics can be deduced. Two important time boundaries are identified. As the Luanhe River had migrated northwards since 1915, sediment supply to the northern section of the coast had reduced. Around 1958, several rivers were dammed. Since 1985, the need for dredging the Tianjin Port has decreased. These actions have resulted in a decline of sediment supply from these rivers, including Haihe, to both the northern and southern sectors of the coast. Once the supply of sediment from rivers was reduced, tidal processes acted to modify sediment characteristics, and an upward-fining pattern is recorded in the majority of sediment cores in this study. This study highlights the usefulness of applying both 210Pb and 137Cs dating methods. These dating methods, together with detailed sedimentary analysis, can provide valuable sedimentary evidence of coastal change and natural and human causes of change.

Intra-tidal sedimentary processes associated with combined wave–current action on an exposed, erosional mudflat, southeastern Yangtze River Delta, China

Abstract Numerous deltaic coasts in the world are eroding in response to a decline in sediment supply as a consequence of river damming. Near-bed sediment dynamics are key mechanisms of erosion and deposition. To understand the sediment dynamics of an exposed, erosional mudflat on the delta front of the Yangtze River, China, we measured wave parameters, near-bed current profiles (for 30 layers located up to 50 cm above the seabed), suspended sediment concentration (SSC) profiles (at 6, 15, 35, and 75 cm above the seabed), bed-level changes, and sediment properties, at a low flat site. We found that bed shear stresses induced by waves can be important to sediment dynamics on the mudflat, even in periods of offshore winds. SSC-measurements close to the seabed revealed the presence of a highly dynamic fluid mud layer. At the high tides, SSC at the near-bed 6-cm layer increased to > 8 kg/m3, as a result of sediment deposition from the overlying water column. During the ebb tides, however, an increase in hydrodynamic variations resulted in resuspension, and the near-bed SSC was reduced to

An Evaluation of Digital Elevation Models to Short-Term Monitoring of a High Energy Barrier Island, Northeast Brazil

The morphological short-term evolution of Ponta do Tubarao Island (PTI) was investigated through high accurate surveys based on post-processed kinematic (PPK) relative positioning on Global Navigation Satellite Systems (GNSS). PTI is part of a barrier island system on a high energy northeast Brazilian coastal environment and also an area of high environmental sensitivity. Surveys were carried out quarterly over a two years period from May 2010 to May 2012. This paper assesses statically the performance of digital elevation models (DEM) derived from different interpolation methods to represent morphologic features and to quantify volumetric changes and TIN models shown the best results to that purposes. The MDE allowed quantifying surfaces and volumes in detail as well as identifying the most vulnerable segments of the PTI to erosion and/or accumulation of sediments and relate the alterations to climate conditions. The coastal setting and geometry of PTI protects a significant mangrove ecosystem and some oil and gas facilities installed in the vicinities from damaging effects of strong oceanwaves and currents. Thus, the maintenance of PTI is extremely required but the prediction of its longevity is uncertain because results indicate an irregularity of sedimentary balance and a substantial decline in sediment supply to this coastal area

Effect of Human Activities on Overall Trend of Sedimentation in the Lower Yellow River, China

The Yellow River has been intensively affected by human activities, particularly in the past 50 years, including soil-water conservation in the upper and middle drainage basin, flood protection in the lower reaches, and flow regulation and water diversion in the whole drainage basin. All these changes may impact sedimentation process of the lower Yellow River in different ways. Assessing these impacts comprehensively is important for more effective environmental management of the drainage basin. Based on the data of annual river flow, sediment load, and channel sedimentation in the lower Yellow River between 1950 and 1997, the purpose of this paper is to analyze the overall trend of channel sedimentation rate at a time scale of 50 years, and its formative cause. It was found in this study that erosion control measures and water diversion have counteractive impacts on sedimentation rate in the lower Yellow River. Although both annual river flow and sediment decreased, there was no change in channel sedimentation rate. A regression analysis indicated that the sedimentation in the lower Yellow River decreased with the sediment input to the lower Yellow River but increased with the river flow input. In the past 30-40 years, the basin-wide practice of erosion and sediment control measures resulted in a decline in sediment supply to the Yellow River; at the same time, the human development of water resources that required river flow regulation and water diversion caused great reduction in river flow. The former may reduce the sedimentation in the lower Yellow River, but the reduction of river flow increased the sedimentation. When their effects counterbalanced each other, the overall trend of channel sedimentation in the lower Yellow River remained unchanged. This fact may help us to better understand the positive and negative effects of human activities in the Yellow River basin and to pay more attention to the negative effect of the development of water resources. The results of this study demonstrate that, if the overuse of river water cannot be controlled, the reduction of channel sedimentation in the lower Yellow River cannot be realized through the practice of erosion and sediment control measures.

Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next half-century

In order to examine the delta response to decline in sediment supply from the Yangtze River during the recent decades, bathymetric maps of the Yangtze subaqueous delta surveyed from 1958 to 1997 were analyzed together with the river sediment dataset of the same period. The net accretion rate over the study area decreased from 38 mm/yr in 1958–1978 to 8 mm/yr in 1978–1997, while the river sediment discharge decreased from 466×10 6 to 394×10 6 t/yr and the suspended sediment concentration decreased from 0.543 to 0.448 kg/m 3 during the same periods, respectively. The outer subaqueous delta is more sensitive to decline in river sediment load in contrast with the inner subaqueous delta. From 1958–1978 to 1978–1997, the net accretion rate decreased from 51 to 2 mm/yr in the outer subaqueous delta and from 25 to 14 mm/yr in the inner subaqueous delta. The accretion rate decreased more rapidly than the river sediment discharge because the amount of sediment transported out of the delta has not decreased. A concept of the critical threshold of river sediment discharge that separates delta progradation from recession is proposed. Although this critical value was found to change temporally and spatially, it was instrumental in predicting the future delta response. Under the impacts of human activities, particularly, the Three Gorges Dam Project, the river sediment load would be reduced below the critical value during the next five decades. This would result in a general delta recession replacing delta progradation; the recession would occur first in the outer subaqueous delta.

Facies analysis and tectonic significance of lacustrine fan-deltaic successions in the Pliocene–Pleistocene Mugello Basin, Central Italy

The Mugello Basin is an intermontane asymmetric basin, trending WNW–ESE and filled with Pliocene–Pleistocene alluvial and lacustrine deposits. The study focuses on the sedimentary succession deposited at the basin's northern margin and uses facies analysis to reconstruct the margin's depositional and deformation history. The controversial concepts of “fan delta” and “hyperconcentrated flow,” adopted in this study, are firstly discussed. The early Pleistocene succession consists of two higher order, coarsening-upward successions (time span ∼10 5 years), separated by an angular unconformity and referred to as the lower fan delta (LFD) and upper fan delta (UFD) systems. They show an alternation of alluvial fan, fan delta front, lacustrine, and floodplain facies associations. The alluvial facies indicate sediment dispersal by flashy, hyperconcentrated flows in the form of sheetfloods and moderately channelized flows. Deposits of the former predominate in the LFD system and deposits of the latter in the UFD system, and this change is attributed to the greater volumes of water yielded by enlarged fan catchments. The fan deltas are of Gilbert and shoal water types, indicating variable water depths at the lake margin. Each of the successions consists of fining- and coarsening-upward cyclothems of medium (tens of meters) and small scale (meters). These different-scale cycles are interpreted as the sedimentary response to pulses of a compressional deformation of the basin margin at variable frequencies, related to the contemporary Northern Apennine active thrusts (ca. 50 km away from the basin). The high-frequency flood events are attributed to episodes of heavy rainfall. The episodes of compression downwarped the basin margin alluvium, made the central lake shift periodically toward the margin, and created progressive unconformities in the sedimentary succession. The immediate effects of a compressional pulse included lake transgression and accentuation of the structural hinge of the basin margin, causing a decline of sediment supply from the catchments. As the hinge relief was subsequently reduced by denudation, the alluvial fans prograded and fan deltas were formed in normal conditions of graben subsidence. The time lag determined the amount of fine-grained sediment accumulated at the lake margin, and thus the water depth and fan delta type.