Settling Of Suspended Sediment Research Articles

Understanding the factors associated with long-term reconstructed turbidity in Lake Diefenbaker from Landsat-imagery

Turbidity affects a variety of aquatic ecosystem processes. Turbidity events are dominated by suspended sediment in many systems. High levels of suspended sediment in lakes can occur during periods of high inflows from turbid tributaries or suspension of sediment from lake beds. This study reconstructed historic turbidity levels using Landsat-imagery on Lake Diefenbaker (LD), a large river-reservoir constructed in the late 1960's on the naturally turbid South Saskatchewan River (SSR). We examined the factors that were associated with it. Reconstructed turbidity levels, from Landsat-images, were similar to actual turbidity. The SSR flow and wind speed explained 64%, 54% and 69% of the variability in estimated turbidity levels at the riverine zone, the transition zone and the entire reservoir, respectively. The decrease in estimated turbidity from June to October and down the length of the reservoir is likely associated with the decline in the SSR flow and the settling of suspended sediments. The relationship between estimated turbidity and wind speed may be associated with the re-suspension of bottom sediment at the upper reach of LD. Wind speed and direction were related to estimated turbidity at the lacustrine zone (r2adj = 0.19, P < 0.05), which may be attributed to the persistence of sediments. We observed high turbidity in 2002 that exceeded other estimates of turbidity. Since 2002 was preceded by a prolonged drought, the high estimate turbidity may be related to an increase in sediment loads from the SSR flow and an increase in shoreline erosion from a rise in LD's water level. Hence, extreme events (drought and flooding) are associated with high turbidity in LD. As the Canadian Prairies continues to undergo climate change, lakes located in this region are predicted to experience more frequent extreme events. These extreme events will cause further deterioration of water quality.

Tale of Two Storms: Impact of Extreme Rain Events on the Biogeochemistry of Lake Superior.

Climate change is expected to profoundly affect the Great Lakes region of North America. An increase in intensity and frequency of rain events is anticipated to deliver more runoff and to increase riverine inputs to Lake Superior's ecosystem. The effects of these changes on key biogeochemical parameters were analyzed by coupling satellite data, water column sensor profiles, and discrete surface-water sampling after two "500-year" flood events in the Lake Superior basin. This study provides both a spatial and a temporal sense of how plumes interacted within the ecosystem. We also determined the significant differences in water quality parameters for plume versus non-plume waters. These two plumes were important for delivery of nutrients, with variable transport of sediments and colored dissolved organic matter (CDOM). Data from the 2012 storm event showed a significant input of total nitrogen (TN), total phosphorous (TP) and CDOM to the system. In the 2016 storm event, carbon cycling parameters (acidity, total inorganic carbon (TIC), and dissolved organic carbon (DOC), and ammonia levels were elevated within the plume. In neither storm event was there a significant difference in chlorophyll a between plume and non-plume waters during our sampling cruises. These two plume events were similar in amount of precipitation, but their effect on the biogeochemistry of Lake Superior varied due to differences in the watersheds where the rain fell. The studied plume events were dynamic, changing with currents, winds and the settling of suspended sediments.

Field studies of estuarine turbidity under different freshwater flow conditions, Kaipara River, New Zealand

We present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the Kaipara River (New Zealand) under both low and high freshwater inputs and a neap tidal cycle. During the first day, we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence. During the second day, longitudinal surveys were conducted over a distance of 6 km centred on the upstream station. The data reveal a turbidity maximum in the form of a high-concentration ‘plug’ of suspended mud that was advected downstream on the ebbing tide past the upper (HB) measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide. The data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low-flow conditions. On the third day of measurements we repeated the experiments of the first day but later in the year, for a much higher freshwater flow. This interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended-sediment signals, including resuspension of local bed sediment, advection by the tidal current, settling of suspended sediment over a long timescale compared to the advection timescale, advection of longitudinal gradients in suspended sediment, and suppression of vertical mixing by density stratification of the water column. The level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed-point sensors.

Hydrodynamics and sediment transport in an estuary with an abrupt depth step

ABSTRACTThe hydrodynamic patterns resulting from the combination of tide flow, riverine discharge and bathymetry and their effects on water circulation and sediment transport are studied to assess the impact of geomorphic changes in the Quequén Grande River estuary (Buenos Aires, Argentina). A depth step located at about 2 km from the sea, created by dredging to provide favourable navigational conditions in the harbour, induces different flow patterns in the deeper and shallow estuarine parts, creating an attractive system to study. The main purpose of this paper is to report key physical features of this water system and the results of detailed longitudinal and transverse measurements of velocity and backscatter intensity. The data reveal the selective settling of suspended sediments in the harbour and a net loss of sediments to the sea. The effects of local severe storms that cause an intense tidal intrusion and mixing as well as a stronger ebb flow and seiches are also described.

Late winter coccolithophore bloom off central Portugal in response to river discharge and upwelling

Coccolithophore communities collected during late winter (9–19 March of 2010) over the central Portuguese margin showed a major change in species abundance and composition within a few days' time, closely related to the highly transient meteorological and oceanographic conditions.Particularly favourable conditions for coccolithophore growth resulted from late winter continental runoff combined with northerly winds prevailing over the shelf, under clear sky conditions. A nutrient-rich Buoyant Plume (BP) resulting from intense river water runoff prior to and during the start of the cruise, was observed to spread out over the denser winter mixed layer water beneath, and extend equatorwards and offshore under influence of Ekman superficial dynamics. Stabilization of buoyancy, settling of suspended sediment from the BP and the prevailing clear sky conditions in the transition to the 2nd leg of the cruise resulted in optimum conditions for coccolithophores to develop, at the expense of nutrient availability in the superficial sunlit layer. Within a few days, coccolithophore cell densities and associated phytoplankton biomass more than tripled, reaching maximum values of 145,000cells/l and ~13µg/l Chl-a, respectively.Often considered as a uniform functional group of calcifying phytoplankton thriving in low-turbulence, low-nutrients and high-light environments, results presented in this study clearly show that coccolithophore life strategies are much more diverse than expected. The increase of cell densities was mainly due to the bloom of Emiliania huxleyi and Gephyrocapsa oceanica in the coastal region west off Cape Carvoeiro, together with other opportunistic phytoplankton genera (Chaetoceros s.l., Thalassiosira s.l and Skeletonema s.l.). This confirms their role as early succession r-selected taxa, capable of rapid growth within nutrient-rich environments. On the contrary, Syracosphaera spp. and Ophiaster spp. displayed the characteristics of K-selected species, being consistently more abundant in more oceanic and less eutrophic waters, away from the influence of the BP, during both low- and high-productive periods.A general description of coccolithophore communities as well as the environmental conditions during this period is presented in this study (i.e. hydrography and nutrient availability). Multivariate analysis was used to investigate the impact of short-term environmental changes on the productivity and ecology of this group. In view of the observed dominance of coccolithophores off Portugal during winter, this transitional period appears particularly favourable for coccolithophores to develop. The results highlight the importance of taking short-term hydrographic and meteorological variability into account when interpreting the ecological preferences of coccolithophores from coastal-neritic-oceanic transitional settings.

Removal of total suspended solids and turbidity within experimental vegetated channel: optimization through response surface methodology

Vegetation plays an important role in transport and settling of suspended sediment in natural waterways. In this study, effectiveness of emergent vegetation in an open channel for the removal of Total Suspended Solids (TSS) and turbidity is empirically investigated. To this end, response surface methodology (RSM) was applied to optimize three variables of vegetated channel including flow velocity at the entrance, density of vegetation, and the length of vegetation zone. Analysis of Variance (ANOVA) was employed to verify the precision of the mathematical models and their relative parameters. Optimum values generated by RSM were compared with experimental results of percentage TSS and turbidity removal and good agreement was observed.

Transport and deposition of fine sediment in open channels with different aspect ratios

ABSTRACTThis paper investigates by means of several large eddy simulations how the channel aspect ratio affects the transport and settling of suspended sediments. The numerical method is successfully validated using data of a physical experiment of fine sediment net deposition in an open channel flow. The channel aspect ratio, A, is known to be the determining factor for the development, strength and distribution of the turbulence‐driven secondary flow, and it is demonstrated that A influences the primary flow, turbulence quantities and the transport and fate of fine sediments. The secondary flow locally supports or hinders the falling of fine sediment particles in a turbulent flow, which results in a non‐uniform deposition of fine sediments over the cross‐section. While the channel aspect ratio has a large influence on the distribution of suspended sediments within the cross‐section, its effect on the cross‐sectional averaged deposition is negligibly small. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Morpho-sedimentary characteristics of the volcaniclastic apron around Stromboli volcano (Italy)

Submarine portions of Stromboli volcano account for about 98% of the whole extent of the volcanic edifice and are mostly covered by volcaniclastic sediments that made up a modern volcaniclastic apron. The architecture and main sedimentary processes acting within the apron are depicted through the integration of geophysical evidence (multibeam, long-range side scan sonar and seismic data) and seafloor sampling. The apron shows, on the whole, a large variability both across and along slope of morphologies and deposits related to mass-wasting and reworking processes, passing into areas covered by hemipelagic sedimentation. A large spectrum of erosive-depositional features was recognized on the surface of the apron. On shallower areas (i.e. submerged shelves, shore platforms and depositional terraces), submarine features related to wave action and sea-level fluctuations are present; they mainly act as temporary storing and reworking areas for volcaniclastic material derived from subaerial portions. Unconfined cohesionless inertial flows commonly dominate the upper slope, where gradients are very high (up to 35°) and grain-to-grain and grain-to-bed collisions are favoured within the flows. These flows tend to evolve into turbidity currents on the lower slope, in response to a marked decrease of slope gradients below ∼ 8°, generating various erosive-depositional features, such as erosive furrows, troughs, channel-levee features and coarse-grained sediment waves. The most active area of the apron lies offshore Sciara del Fuoco (on the NW flank), where a large amount of coarse-grained volcaniclastic material, derived by the persistent Strombolian activity, is collected and deposited by means of high- and low-density turbidity currents on the lower slope down to 2600 m bsl, more than 20 km far from the island. Conversely, other areas are mostly dominated by deposition via settling of suspended sediments, related both to hemipelagic sedimentation and to the dispersion of fine tephras by the prevailing wind and marine currents. Such processes interfinger and overlie in space and time, giving rise to a very complex morpho-sedimentary facies distribution. However, the structural bilateral symmetry of Stromboli volcano allows defining distinct morpho-sedimentary zones, where one (or more) type of processes dominates, and related different stratal architecture of the apron. The unbuttressed portions of the island (NW and SE) are characterized by the emplacement of wide and thick debris avalanche deposits (volume in the order of 1–2 km 3 for each event) related to large-scale sector collapses, representing the most important and efficient mass-wasting processes making up the apron. Such deposits are embedded within different volcaniclastic sequences and can be eroded or exhumed by successive gravity flows. The result is a highly complex (both spatially and vertically) succession of facies, where it is often difficult to depict an arranged series of processes. Conversely, on buttressed flanks (SW and NE) a more ordered evolution of processes and deposits making up the apron is observed, giving rise to a less complex apron architecture.

Water quality of a stream recently fenced‐off from deer

Water quality was monitored on a tributary of the Dow Stream on the AgResearch Invermay deer farm near Mosgiel. The aim was to see if fencing‐off an area of the stream channel with a known contaminant source (a wallow) and riparian planting improved water quality as measured by the two‐weekly concentrations and annual loads of nitrogen (NH4 +‐N, NO3 ‐‐N), phosphorus (P; dissolved reactive P, participate P and total P) species, suspended sediment (SS) and the faecal indicator bacteria, Escherichia coli (E. coli). Measurements were taken 2 years before and after fencing‐off and riparian planting. Analysis of the data indicated that the mean concentrations of N03 ‐‐N, NH4 +‐N, SS and TP improved after fencing‐off and planting compared to those beforehand. As a result, loads of these constituents decreased by 78% to 98%. Mean concentrations of E. coli and DRP showed no significant difference with fencing‐off and planting. Mitigation effects were attributed to (i) the settling of SS and associated NH4 +‐N and PP in an area formerly used by deer for wallowing and (ii) the absence of direct excretal input by deer. Hence, fencing‐off and riparian planting is recommended for areas where known sources of contaminant loss occur (e.g., wallows or degraded areas of a stream channel). However, this must be balanced with the cost of fencing and potential loss of land from production and the suitability of the stream, or area, for fencing. For instance, the potential mitigation performance of fencing‐off and planting is likely to be poorer in areas without large contaminant sources or already good water quality.

Significance of fine-grained sediment lofting from melt-water generated turbidity currents for the timing of glaciomarine sediment transport into the deep sea

Sediment lofting from turbidity currents is a process that occurs in density currents generated from fresh-water discharges into the sea and other gravity currents that can produce reversed buoyancy. Settling of suspended sediment from the top or deposition from the bottom of the flows may lower the density below that of ambient seawater causing the currents to lift up from their substrate either in part or as a whole through buoyancy reversal, as is well known from experiments. Fine-grained sediment lofting on a large scale during times of Heinrich events has been inferred for ice-proximal regions of the northwest Labrador Sea adjacent to the margin of the Late Pleistocene Laurentide Ice Sheet (LIS). The evidence comes from a distinct depositional facies of stacked graded mud layers containing ice-rafted debris (IRD). The significance of the lofted fine-grained depositional facies arises from its genetic link to large sand and gravel transporting turbidity currents that were generated by melt-water discharges from the Hudson Strait outlet of the LIS and are the likely source of the lofted sediment. The observed lofted depositional facies is exclusively found in Heinrich layers. Through this stratigraphic relationship the lofted facies ties the main pulses of sediment supply in the Labrador Basin to Heinrich events. Heinrich events thus appear to be the times of the main depositional activity in the Labrador Sea, a hitherto unknown aspect of these phenomenal events besides their known role as intense iceberg discharges.

Spatio-temporal dynamics of a Zostera noltii dominated community over a period of fluctuating salinity in a shallow lagoon, Southern France

The consequences of a sudden decrease in water salinity on a macrophyte community dominated by the marine species Zostera noltii were investigated in a shallow coastal lagoon (Southern France). The expected change in the community assemblage was a competitive displacement of Z. noltii by Potamogeton pectinatus and Ruppia cirrhosa. However, the results of an eight-year survey did not validate this hypothesis. Zostera noltii remained dominant after three years at low salinity (ca. 5) and then suddenly disappeared within one year. The decline of Z. noltii was not associated with an increase of other species. The maximum depth colonized by all species decreased, suggesting that light availability became a limiting factor for the aquatic vegetation. The high concentration in suspended sediments that was measured when salinity was low may result from the synergetic effects of low salinity that slowed down the settling of suspended sediments and the disappearance of Z. noltii meadow that trapped sediments and reduced wave energy. The progressive recovery of the Z. noltii meadow following the increase in water salinity was associated with an increase in depth limit of the other species. However, the two other angiosperms remained present at low frequency of occurrence and the two charophytes ( Lamprothamnium papulosum and Chara galioides) declined over the study period. A geo-referenced database was constructed to investigate the spatial variation in Zostera noltii cover in relation to depth, slope, wave exposure and sediment characteristics using logistic regressions. During the first years of re-colonization, when Z. noltii was restricted to the shallow borders of the lagoon with the highest slope values, the best explanatory variables were depth and slope. In the following years, depth and wave exposure were the most important factors determining the spatial distribution of Z. noltii.

Fine-grained sediment lofting from meltwater-generated turbidity currents during Heinrich events

Turbidity currents generated from sediment-carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater. From experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal. This ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows. Evidence for large-scale lofting of suspended sediment from the top of giant sand- and gravel-carrying turbidity currents in the Labrador Sea comes from two independent lines of observations: (1) The first is a distinct sedimentary facies consisting of stacked, centimeter-thick graded mud layers that contain grains of ice-rafted debris (IRD) supported by the mud. Deposition of these unusual layers requires a gradedlayer‐forming process that is slow enough to allow the incorporation of IRD; this is not possible with normal mud-carrying turbidity currents. (2) The second observation is the presence of a huge abyssal sand and gravel plain in the central Labrador Sea that received its sediment from bed-load‐rich meltwater discharges from the Hudson Strait outlet of the Pleistocene Laurentide Ice Sheet. These discharges turned into turbidity currents that released rising columns of freshwater that carried fine-grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater. Deposition from these slow turbid interflows would allow the incorporation of IRD in the accumulating graded mud deposits. The IRD-spiked graded mud facies is restricted to Heinrich layers within 300 km radius of the Hudson Strait ice-stream terminus, tying the sand-carrying turbidity currents via fine-grained sediment lofting to Heinrich events. Estimated total discharge volumes of individual currents are on the order of 10 3 km 3 , supporting the notion that Heinrich ice-rafting events were times of maximum meltwater generation.

Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests

Root systems in mangrove swamps have captured the attention of scientists for decades. Among the postulated roles of root structures include a contribution to the geomorphological stability of mangrove soils through sediment trapping and binding. In this study, we used feldspar marker horizons and sediment pins to investigate the influence of three different functional root types—prop roots in Rhizophora spp., root knees in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia alba—on vertical accretion and elevation change in three mangrove forests in the Federated States of Micronesia. Prop roots facilitated vertical accretion (11.0 mm year −1) more than pneumatophores or bare soil controls (mean, 8.3 mm year −1). Sediment elevation, on the other hand, increased at an average rate of only 1.3 mm year 1 across all root types, with rate differences by root type, ranging from −0.2 to 3.4 mm year −1, being detected within river basins. This investigation demonstrates that prop roots can assist in the settling of suspended sediments from estuarine waters, yet prop root structures are not as successful as pneumatophores in maintaining sediment elevation over 2.5 years. As root densities increase over time, an increase in turbulence-induced erosion and in shallow subsidence as organic peat layers form is expected in Micronesian mangrove forests.

Potential long-term ecological impacts caused by disturbance of contaminated sediments:a case study.

Several submerged barges were recently removed from the Passaic River, New Jersey, USA, in two areas (areas 1 and 2) where contaminated sediments are known to exist. During removal of the single barge in area 1, elevated turbidity levels and chemical parameters were measured. Greater increases were measured in area 2, where several barges were removed. In both areas, water column concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and several metals exceeded one or more water quality criteria; turbidity levels in area 2 also exceeded regulatory criteria. Potential chemical bioaccumulation from the water column into residential aquatic receptors was estimated using standard models and assumptions. The modeled results predicted that steady-state tissue concentrations of bioaccumulative chemicals would not occur as a result of the brief increase in water column concentrations that occurred during barge removal but that metals and PCDD/Fs could bioaccumulate to levels that exceed regulatory ecological criteria during long-term sediment disturbance activities. In addition, based on some simplistic assumptions regarding settling of suspended sediments, we estimate that chemical bioaccumulation from surface sediments into the food web could result in substantial increases in PCDD/F body burdens in the benthic forage fish, mummichog. Our findings are consistent with the limited number of field studies that have measured increased body burdens of bioaccumulative chemicals following dredging. We suggest that, prior to consideration of extensive dredging as a remedial alternative for any river system, the potential significant and long-term impacts on the food web must be evaluated.

Subglacial and subaqueous processes near a glacier grounding line: sedimentological evidence from a former ice‐dammed lake, Achnasheen Scotland

Subglacial and subaqueous sediments deposited near the margin of a Late‐glacial ice‐dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre‐existing glaciolacustrine deposits, and the subaqueous sediments consist of ice‐proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment‐gravity flows on prograding outwash fans. Beyond the ice‐marginal environment, deposition was from turbidity currents, ice‐rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.

Comments on the paper by D. Thirkell and R. Keatch

We present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the Kaipara River (New Zealand) under both low and high freshwater inputs and a neap tidal cycle. During the first day, we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence. During the second day, longitudinal surveys were conducted over a distance of 6 km centred on the upstream station. The data reveal a turbidity maximum in the form of a high-concentration ‘plug’ of suspended mud that was advected downstream on the ebbing tide past the upper (HB) measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide. The data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low-flow conditions. On the third day of measurements we repeated the experiments of the first day but later in the year, for a much higher freshwater flow. This interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended-sediment signals, including resuspension of local bed sediment, advection by the tidal current, settling of suspended sediment over a long timescale compared to the advection timescale, advection of longitudinal gradients in suspended sediment, and suppression of vertical mixing by density stratification of the water column. The level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed-point sensors.

Unsteady one‐dimensional settling of suspended sediment

A one‐dimensional, unsteady numerical model for the prediction of the vertical distribution of suspended sediment concentration and rate of suspended sediment deposition in water stirred uniformly has been developed. The model is derived from the convective‐diffusive transport equation and uses a fully implicit, exponential finite difference scheme. The model is unconditionally stable regardless of vertical diffusivities and particle fall velocities. Such a model has not previously been given in the sedimentation literature and is of interest for theory and applications. A generic study of vertical suspended sediment profiles and trap efficiencies has been made. The results, presented in dimensionless form, cover Peclet numbers Vh/D from zero to infinity. V is the particle fall velocity, h is the water depth, and D is the mean vertical exchange coefficient. In studies of suspended sediment behavior the parameters V and D are often unknown or not well determined. The generic results generated show what kind of variations in suspended sediment concentrations and trap efficiencies can be expected with different values of V and D. Sediment deposition is shown to occur through downward frontal movement when Vh/D is large (≥ 20) and through columnar precipitation when Vh/D is small (≤ 0.2). Residence time is shown theoretically to be far more important for estimates of sediment deposition rate than turbulence. This is in agreement with empirical findings for reservoir trap efficiency.

Diffusion and Settling of Suspended Sediment at River Mouths: a Computer Simulation Model: ABSTRACT

A Fortran IV computer program has been written for simulating the diffusion and settling of suspended sediment at river mouths. The rate of sediment accumulation at any point in front of the channel mouth is governed by water and sediment discharge, sediment grain-size distribution, sediment density, the porosity of the resulting sediment, width and depth of the river channel, and the geometry of the basin. A plane jet model is used for determining the velocity field and the rates of sediment diffusion. By adjusting the input parameters, a variety of deposits may be created. The shape and foreset slope of the delta fan are found to be closely controlled by grain size and discharge. By allowing the model to respond dynamically to the buildup of sediment at the chan el mouth, a distributary mouth bar and submerged levees can be formed. Delta-simulation experiments are monitored by printing (1) maps showing the rates of sedimentation for each grain size at every point in a digital accounting grid and (2) facies maps using alphabetic symbols. Maps and stratigraphic cross sections are drawn with a digital plotter. Computer simulation is an important method for sedimentology, and should be used in combination with hydraulic models and direct observations of natural phenomena. End_of_Article - Last_Page 2162------------