Variation In Sediment Deposition Research Articles

Sedimentary geochemistry mediated by a specific hydrological regime in the water level fluctuation zone of the Three Gorges Reservoir, China.

The water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) acts as an important sink for inflowing suspended sediment loads over the inundation periods following regular dam operations. This study depicts the sedimentary geochemical dynamics along a sedimentary profile based on the determined chronology and explores its links to the specific hydrological regime created by dam flow regulation and riverine seasonal suspended sediment dynamics. A compact 345-cm-long sediment core was extracted near the base water level (145.3m) from the WLFZ of the TGR and sectioned at 5-cm intervals. Extracted sediment subsamples were analyzed for grain size composition, organic matter (OM), total nitrogen (TN), and geochemical elements (Na, K, Ca, Mg, Pb, Zn, Ni, Co, Mn, Cr, Fe, and Cu). The sediment core chronology was determined using 137Cs elemental analysis. Sedimentary geochemistry and grain size properties of extracted sediment core exhibited greater variations during initial submergence years till the first complete impoundment of the TGR (2006-2010). Afterward (2011-2013), although upstream inflowing suspended sediments and reservoir water level were comparable, sediment deposition and concentrations of sedimentary geochemical constituents showed considerably fewer variations. Seasonal variations in sediment deposition and geochemical composition were also observed during the rainy (October-April) and dry (May-September) seasons, in addition to annual variations. Grain size, OM, and other sediment geochemical constituents all had significant correlations with each other and with sediment core depth. The concentrations of geochemical elements in various sediment stratigraphic layers exhibited staggering associations with each other and were dependent on each other in several ways. The arrangement of geochemical elements in various stratigraphic layers of the extracted core illustrated amalgamation with inputs from upstream seasonal suspended sediment dynamics and reservoir water levels. During shortened submergence periods and higher input sediment loads, geochemical elements demonstrated impulsive distributions. Alternatively, during longer submergence periods, elemental distributions were relatively uniform attributed to higher settling time to deposit according to grain size and geochemical affinities. Higher suspended sediment loads in association with seasonal floods also resulted in rough sediment deposition patterns, imparting variations in the distributions of geochemical elements. Interim mediations in geochemical element concentrations are associated with seasonal distal flash floods and local terrace bank collapses, which generate significant amounts of distal sediment loads that are quickly deposited and are not sorted hydrodynamically. Overall, although a specific mechanism was devised to circumvent the siltation process, a considerable amount of sediment is trapped at pre-dam sites. In addition, siltation caused nutrients and geochemical elements' enrichment.

Spatiotemporal Patterns of Mineral and Organic Matter Deposition Across Two San Francisco Bay-Delta Tidal Marshes

Sediment deposition in tidal wetlands is a critical process that determines whether vertical growth will keep pace with sea-level rise. However, more information is needed on how sediment deposition varies spatially and temporally across wetlands, including the effects of elevation, tidal inundation, vegetation, and weather. We investigated variation in sediment deposition due to season, distance from channel, channel size, and vegetation composition at low and high salinity tidal marshes in the San Francisco Bay-Delta Estuary, California using sediment traps deployed monthly between December 2015 and November 2016. Over the course of the year, sediment deposition ranged widely (1.4 – 174.0 kg m−2 yr−1) and averaged 19.5 ± 3.5 kg m−2 yr−1. Deposition increased with flooding duration, decreased with increasing distance from channels, and was highest during the spring and early summer. Higher wind speeds during the spring may have driven re-suspension from mudflats, promoting deposition. Ratios of organic-to-mineral deposition were twice as high at the fresher site and were correlated with differences in vegetation composition between sites. Our results suggest that seasonality, distance from sediment source, salinity regime, and channel size are important sources of spatiotemporal variation in deposition. These results are relevant to accretion sampling and wetland restoration design.

Sediment dynamics of mountain streams as a function of variation in sediment deposition

In Japan, the management of sediment in dam reservoirs has become challenging owing to heavy rain. Unexpected increase of sediment volume in dam reservoirs can increase flood risk owing to a rise in the level of the riverbed. It is therefore necessary to conduct sediment management to clarify the area of sediment production to control the amount of sediment transported and the timing of its movement. Sediment discharge from tributaries is often calculated based on catchment area and the deposited sediment volume already present in the dam reservoir. However, our preliminary surveys have implied that it is also necessary to consider regional geology in the evaluation of sediment transport. Moreover, variations in sediment deposition should be evaluated for each site where the geology changes.
 In this study, we observe in detail changes in the riverbed in two tributaries with different regional geologies. Changes in the shape of the riverbed were analyzed from unmanned aerial vehicle observation data, following which the effects of erosion and sedimentation on the riverbed were quantitatively evaluated. Finally, sediment discharge was calculated by a one-dimensional numerical model of sediment transport. These results indicate that the tributary that is mostly composed of mudstone produced considerably more sediment than the tributary composed mainly of sandstone. This suggests that an understanding of regional geology is necessary in the evaluation of sediment dynamics and should be considered in addition to the catchment area.

Variation in the sediment deposition behind check‐dams under different soil erosion conditions on the Loess Plateau, China

AbstractTo maintain a reasonable sediment regulation system in the middle reaches of the Yellow River, it is critical to determine the variation in sediment deposition behind check‐dams for different soil erosion conditions. Sediment samples were collected by using a drilling machine in the Fangta watershed of the loess hilly–gully region and the Manhonggou watershed of the weathered sandstone hilly–gully (pisha) region. On the basis of the check‐dam capacity curves, the soil bulk densities and the couplet thickness in these two small watersheds, the sediment yields were deduced at the watershed scale. The annual average sediment deposition rate in the Manhonggou watershed (702.0 mm/(km2·a)) from 1976 to 2009 was much higher than that in the Fangta watershed (171.6 mm/(km2·a)) from 1975 to 2013. The soil particle size distributions in these two small watersheds were generally centred on the silt and sand fractions, which were 42.4% and 50.7% in the Fangta watershed and 60.6% and 32.9% in the Manhonggou watershed, respectively. The annual sediment deposition yield exhibited a decreasing trend; the transition years were 1991 in the Fangta watershed and 1996 in the Manhonggou watershed (P < 0.05). In contrast, the annual average sediment deposition yield was much higher in the Manhonggou watershed (14011.1 t/(km2·a)) than in the Fangta watershed (3149.6 t/(km2·a)). In addition, the rainfalls that induced sediment deposition at the check‐dams were greater than 30 mm in the Fangta watershed and 20 mm in the Manhonggou watershed. The rainfall was not the main reason for the difference in the sediment yield between the two small watersheds. The conversion of farmland to forestland or grassland was the main reason for the decrease in the soil erosion in the Fangta watershed, while the weathered sandstone and bare land were the main factors driving the high sediment yield in the Manhonggou watershed. Knowledge of the sediment deposition process of check‐dams and the variation in the catchment sediment yield under different soil erosion conditions can serve as a basis for the implementation of improved soil erosion and sediment control strategies, particularly in semi‐arid hilly–gully regions. Copyright © 2018 John Wiley & Sons, Ltd.

Particle deposition, resuspension and phosphorus accumulation in small constructed wetlands

To improve understanding of phosphorus (P) retention processes in small constructed wetlands (CWs), we analysed variations in sediment deposition and accumulation in four CWs on clay soils in east-central Sweden. Sediment deposition (in traps) generally exceeded the total suspended solids (TSS) load suggesting that resuspension and wetland base erosion were important. This was confirmed by quantification of particle accumulation (on plates) (1–23 kg m−2 year−1), which amounted to only 13–23% of trap deposition. Spatial mean P concentrations in accumulated sediment on plates (0.09–0.15%) were generally similar to temporal mean P concentrations of particles in water (0.11–0.15%). Deposition/accumulation was minor in one wetland with high hydraulic load (400 m year−1), suggesting that such small wetlands are not efficient as particle sinks. Economic support for CWs are given, but design and landscape position are here demonstrated to be important for effective P retention.

Cyclic 100-ka (glacial-interglacial) migration of subseafloor redox zonation on the Peruvian shelf

The coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood. We examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial-interglacial cycles in a sedimentary section drilled on the Peruvian shelf (Ocean Drilling Program Leg 201, Site 1229). Multiple and distinct layers of diagenetic barite and dolomite, i.e., minerals that typically form at the sulfate-methane transition (SMT), occur at much shallower burial depth than the present SMT around 30 meters below seafloor. These shallow layers co-occur with peaks of (13)C-depleted archaeol, a molecular fossil of anaerobic methane-oxidizing Archaea. Present-day, non-steady state distributions of dissolved sulfate also suggest that the SMT is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the SMT. Reaction-transport modeling substantiates our hypothesis that shallow SMTs occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages. Long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales, e.g., glacial-interglacial periods. However, our study demonstrates how dynamically sediment biogeochemistry of the Peru Margin has responded to glacial-interglacial change and how these changes are now preserved in the geological record. Such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling.

Linking floodplain hydraulics and sedimentation patterns along a restored river channel: River Odense, Denmark

Re-establishment of riparian freshwater wetlands often involves re-meandering of former channelized river sections by narrowing the channel width and depth, which creates frequently inundated floodplains. Inundations are important for floodplains in terms of erosion/accretion of sediment and for deposition of nutrients and organic matter. The links between floodplain hydraulics and deposition of sediment, organic matter and phosphorus were investigated along a 6km re-meandered channel section of River Odense, Denmark. A 2D dynamic river and floodplain model was set up for the investigated floodplain area. The flow model was validated against in situ measurements of flow velocities and depths during a one-day field campaign in January 2012. Deposition of sediment was measured on 30 artificial grass mats deployed on the floodplain during winter 2011/2012. Total deposition of sediment, organic matter and total phosphorus was estimated by Kriging and amounted to 4.72kgm−2, 0.65kgm−2 and 11.4gm−2, respectively. The predicted duration of inundations, distances to inflow, flow velocities, and information on microtopography were compared with sedimentation patterns on the floodplain. Three distinct flow regimes were predicted by the hydraulic model and were classified as small, medium and large inundation events according to spatial extent and inundation depth. The model predicts spatially and temporally changing zones of confluence on the floodplain due to variations in inundation depth. The position of these zones correlates with zones of high sediment deposition. Simple regression models described only up to 76% of the variation in sediment deposition. We conclude that use of dynamic spatial distributed 2D models is necessary if the links between floodplain hydraulics and deposition of sediment, organic matter and phosphorus are to be accurately described.

Fluvial architecture of Belgian river systems in contrasting environments: implications for reconstructing the sedimentation history

AbstractAccurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Amblève catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Amblève catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking.

Variability of Mediterranean coralligenous assemblages subject to local variation in sediment deposition

This study tested whether the development of coralligenous assemblages on horizontal and vertical surfaces differed between localities of high and low sediment deposition. The development and structure of these assemblages varied in predictable ways according to the level of sediment deposition. These differences were largely independent of the orientation of substratum. Turfs were more extensive in areas of high sediment deposition while erect and encrusting algae were most extensive in areas of low sediment deposition. Encrusting invertebrates characterised vertical surfaces and were most extensive in areas of high sediment deposition. These results are consistent with studies from other temperate regions of the globe, suggesting that effects of sedimentation on temperate coasts are substantial and widespread.

An evaluation of mobile mud dynamics in the Mississippi River deltaic region

Large rivers are the primary interface between terrestrial and ocean environments. A relatively small number of rivers account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean. Sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are considered to be the main repositories of organic carbon in marine sediments. Particulate material in these environments are typically deposited and resuspended several times before permanent accumulation or transport off the shelf. This sediment cycling is an important component influencing biogeochemical processes that occur in coastal environments. During two cruises in April and October 2000 on the shelf adjacent to the Mississippi River, water and sediment samples were collected for analysis of suspended solids and particle reactive radionuclides ( 210Pb, 137Cs, 7Be and 234Th) to evaluate the transport and fate of terrestrial and marine material. A comparison of the distribution of these tracers provides insight about the pathways and residence times of particulate materials on the shelf. Inventories of these short-lived radiotracers showed variations of more than two orders of magnitude, indicating dramatic variations in sediment deposition between sampling events. Short-lived radiotracers indicate that river-borne materials are transported less than ∼30 km from the river mouth before initial deposition. However, seasonal variations in 7Be and 137Cs indicate significant remobilization of sediment and potential export of sediment out of the study area during the high energy (e.g., wind/wave) winter months. In addition, depth profiles of 7Be and excess 234Th indicate sediment deposition rates between 0.8 and 3.9 cm month −1 (0.4–2.1 g cm −2 month −1) at two shelf locations (near river and open shelf environments). These rates are much greater than those observed on decadal time scales (1.3–2.0 cm year −1 or 0.6–1.5 g cm −2 year −1) via 210Pb at the same sites. This further substantiates active sediment reworking and potential export off the shelf as has been observed in other river-dominated ocean margins.

Seasonal Patterns of Sediment Supply to Coastal Foredune of Seungbong Island, Korea

The seasonal patterns of sediment supply were investigated during the period of June 1999 to June 2000 on a coastal foredune of Seungbong Island, Korea. Sediment supply was determined from measurements of geomorphic changes in the foredune and beach along six lines. Most sands were deposited on the dunefoot and foredune area during the winter and spring, from November to April. The largest amount of sands was deposited along the lines 5 and 6 near the sea-dike in the southern tip of the dune area. In general, the sand on the beach was gradually eroded in spring, summer and fall but deposited in winter. Total sediment accumulation over the study period was for the foredune and for the beach. The volume of the foredune increased in the winter and spring, whereas the volume of beach increased in the winter. Variation in sediment deposition appears to be controlled primarily by variations in the seasonal wind regime.

The Ganges and Brahmaputra rivers in Bangladesh: basin denudation and sedimentation

Every year the Ganges and Brahmaputra rivers in Bangladesh transport 316 and 721 million tonnes of sediment, respectively. These high loads of suspended sediment reflect the very high rate of denudation in their drainage basins. The average mechanical denudation rate for the Ganges and Brahmaputra basins together is 365 mm 103 yr−1. However, the rate is higher in the Brahmaputra Basin than that in the Ganges Basin. Several factors, including mean trunk channel gradient, relief ratio, runoff, basin lithology and recurring earthquakes are responsible for these high denudation rates. Of the total suspended sediment load (i.e. 1037 million tonnes) transported by these rivers, only 525 million tonnes (c. 51% of the total load) are delivered to the coastal area of Bangladesh and the remaining 512 million tonnes are deposited within the lower basin, offsetting the subsidence. Of the deposited load, about 289 million tonnes (about 28% of the total load) are deposited on the floodplains of these rivers. The remaining 223 million tonnes (about 21% of the total load) are deposited within the river channels, resulting in aggradation of the channel bed at an average rate of about 3·9 cm yr−1. Although the Brahmaputra transports a higher sediment load than the Ganges, the channel bed aggradation rate is much higher for the Ganges. This study also documents a wide range of interannual, seasonal and daily variation in suspended sediment transport and water discharge. Interannual variation in sediment deposition within the basin is also suggested. Copyright © 1999 John Wiley & Sons, Ltd.