Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field,flume and modelling experiments

The objectives of the present study were twofold: (1) to identify spatial sedimentation and erosion patterns developing within patches of epibenthic structures (i.e. physical structures that protrude from the sediments, originating either from animals or plants) as a consequence of biophysical interactions; and (2) to assess the relevance of hydrodynamic flume studies for the long-term sediment dynamics in the field. We addressed these objectives by using patches of well-defined artificial structures (bamboo canes) for which we could easily monitor the long-term sediment dynamics in the field, measure the hydrodynamic effects in detail in the flume, and simulate the field and flume set-up with a commercially available hydrodynamic model. Two-year monitoring in the field showed that sedimentation was much larger in the high-density patches than the low-density ones. Within the high-density patches, comparable spatial patterns emerged at different field sites: erosion at the front and the side of the patches, and sedimentation more down-stream within the patches. The low-density patches showed no such patterns, and were generally characterised by some small-scale erosion directly around individual bamboo canes. Sedimentation and erosion in the field was well explained by the patterns in bed shear stress that were derived from our flume measurements. The 3D hydrodynamic modelling facilitated up-scaling of the flume results to the field, but failed to simulate accurately the effects at the leading edge. We conclude that: (A) field observations on sedimentation revealed interesting spatial patterns, but could not elucidate underlying processes; (B) detailed hydrodynamic measurements in a flume can elucidate these underlying processes, provided that appropriate scaling is being used; (C) flume studies are by definition not able to capture all spatial scales that are relevant for estuarine landscape formation and will always cause some flow artefacts; (D) hydrodynamic modelling offers a valuable tool to upscale flume observations, even though present models are not yet capable of fully reproducing all detailed spatial patterns; and (E) spatial heterogeneity is very important when looking at small-scale patches. There is a need for more spatially explicit and scale-dependent knowledge on bio-physical interactions.     

Field experiments on remediation of coastal sediments using granulated coal ash

Field experiments were carried out to evaluate the effect of Granulated Coal Ash (GCA) on remediation of coastal sediments in terms of removing phosphates and hydrogen sulfide. Phosphate concentrations in the sediment were kept below 0.2 mg/l after the application of GCA, whereas those in the control sites increased up to 1.0 mg/l. The concentration of hydrogen sulfide in the sediment was maintained at almost zero in the experimental sites (GCA application sites) for over one year, whereas it ranged 0.1–2.4 mg S L⁻¹ in control sites. Meanwhile, individual number of benthos increased in the experimental sites by several orders of magnitude compared to the control sites. The major process involved in hydrogen sulfide removal by GCA was thought to be the increase in pH, which suppresses hydrogen sulfide formation. From our findings, we concluded that GCA is an effective material for remediating organically enriched coastal sediment.     

Relationship between bed shear stress and suspended sediment concentration:annular flume experiments

In this study,annular flume experiments were carried out,using the sediment samples collected from the lower part of the inter-tidal zone at Xiaoyangkou,Jiangsu coast,China.The Ariathurai-Partheniades equation was used to determine the bed shear stress,by evaluating variations in the suspended sediment concentration within the water column.The derived relation between the bed shear stress and suspended sediment concentration shows that,at various stages of seabed erosion, suspended sediment concentration increases rapidly when the flow velocity is increased,but the pattern of change in the bed shear stress does not follow suit.At low concentrations,bed shear stress initially increases markedly with increasing flow velocity.However,when the concentration reaches an apparently critical level around 0.55 kg m"3,the rate of change in the bed shear stress abruptly slows down,or becomes almost constant,in response to further increases in the flow velocity.Results of experiments indicate that,from a critical level onward,suspended sediment concentration has a strong influence on the bed shear stress.     

Simplification bias: lessons from laboratory and field experiments on flow through aquatic vegetation

We present a critical analysis of experimental findings on vegetation–flow–sediment interactions obtained through both laboratory and field experiments on tidal and coastal environments. It is well established that aquatic vegetation provides a wide range of ecosystem services (e.g. protecting coastal communities from extreme events, reducing riverbank and coastal erosion, housing diverse ecosystems), and the effort to better understand such services has led to multiple approaches to reproduce the relevant physical processes through detailed laboratory experiments. State-of-the-art measurement techniques allow researchers to measure velocity fields and sediment transport with high spatial and temporal resolution under well-controlled flow conditions, yielding predictions for hydrodynamic and sediment transport scenarios that depend on simplified or bulk vegetation parameters. However, recent field studies have shown that some simplifications on the experimental setup (e.g. the use of rigid elements, a single diameter, a single element height, regular or staggered layout) can bias the outcome of the study, by either hiding or amplifying some of the relevant physical processes found in natural conditions. We discuss some observed cases of bias, including general practices that can lead to compromises associated with simplified assumptions. The analysis presented will identify potential pathways to move forward with laboratory and field measurements, which could better inform predictors to produce more robust, universal and accurate predictions on flow–vegetation–sediment interactions. © 2020 John Wiley & Sons, Ltd.     

Aminostratigraphy of surface and subsurface Quaternary sediments,North Carolina coastal plain,USA

The Quaternary stratigraphy and geochronology of the Albemarle Embayment of the North Carolina (NC) Coastal Plain is examined using amino acid racemization (AAR) in marine mollusks, in combination with geophysical, lithologic, and biostratigraphic analysis of 28 rotasonic cores drilled between 2002 and 2006. The Albemarle Embayment is bounded by structural highs to the north and south, and Quaternary strata thin westward toward the Suffolk paleoshoreline, frequently referred to as the Suffolk Scarp. The Quaternary section is up to ～90 m thick, consists of a variety of estuarine, shelf, back-barrier, and lagoonal deposits, and has been influenced by multiple sea-level cycles. The temporal resolution of the amino acid racemization method is tested statistically and with the stratigraphic control provided by this geologic framework, and it is then applied to the correlation and age estimation of subsurface units throughout the region. Over 500 specimens (primarily Mercenaria and Mulinia) from the subsurface section have been analyzed using either gas chromatographic (GC) or reverse-phase liquid chromatographic (RPLC) techniques. The subsurface stratigraphic data are compared with AAR results from numerous natural or excavated exposures from the surrounding region, as well as results from NC beach collections, to develop a comprehensive aminostratigraphic database for the entire Quaternary record within the NC coastal system. Age mixing, recognized in the beach collections, is also seen in subsurface sections, usually where major seismic reflections or core lithology indicate the presence of stratigraphic discontinuities. Kinetic models for racemization are tested within the regional stratigraphic framework, using either radiocarbon or U-series calibrations or comparison with regional biostratigraphy. Three major Pleistocene aminozones [AZ2, AZ3, and AZ4] are found throughout the region, all being found in superposition in several cores. Each can be subdivided, yielding a total of at least eight stratigraphically and statistically distinct aminozones. Kinetic modeling, supplemented with local calibration, indicates that these aminozones represent depositional events ranging from ～80 ka to nearly 2 Ma. Three prominent seismic reflections are interpreted to represent the base of the early, middle, and late Pleistocene, respectively, roughly 2 Ma, 800 ka, and 130 ka. The large number of samples and the available stratigraphic control provide new insights into the capabilities and limitations of aminostratigraphic methods in assessing relative and numerical ages of Atlantic Coastal Plain Quaternary deposits.     

Perfluoroalkyl and polyfluoroalkyl substances in sediments from South Bohai coastal watersheds,China

This study investigated the concentrations and distribution of Perfluoroalkyl and polyfluoroalkyl substances (PFAS) in sediments of 12 rivers from South Bohai coastal watersheds. The highest concentrations of ΣPFAS (31.920 ng g⁻¹ dw) and PFOA (29.021 ng g⁻¹ dw) were found in sediments from the Xiaoqing River, which was indicative of local point sources in this region. As for other rivers, concentrations of ΣPFAS ranged from 0.218 to 1.583 ng g⁻¹ dw were found in the coastal sediments and from 0.167 to 1.953 ng g⁻¹ dw in the riverine sediments. Predominant PFAS from coastal and riverine areas were PFOA and PFBS, with percentages of 30% and 35%, respectively. Partitioning analysis showed the concentrations of PFNA, PFDA and PFHxS were significantly correlated with organic carbon. The results of a preliminary environmental hazard assessment showed that PFOS posed the highest hazard in the Mi River, while PFOA posed a relative higher hazard in the Xiaoqing River.     

A 3-D coastal tidal rectification process: observations, theory and experiments

In homogeneous rotating fluid, when there is an oscillating forcing in the interior fluid with a period long enough for an Ekman layer to develop, there is an interaction between the oscillatory Ekman layer and the vertical wall, since the latter imposes an alternating adjustment flow confined near the wall. As a result, this coastal rectification process leads to a Lagrangian transport along the coast. The Ekman number, the Rossby number and the temporal Rossby number of the forcing flow are the governing parameters of that mechanism which can be described by a simplified analytical model taking into account both the vertical time-dependent structure of the current and the presence of the wall. The model shows that the residual (rectified) current flowing with the coast to its right results from the strong nonlinear interaction between along- and cross-shore tidal currents leading to asymmetrical momentum exchanges between the Ekman bottom layer and the coastal boundary layer. The model provides simple scaling laws for the maximum intensity and width of the residual current. The latter is significantly larger than the friction (Stokes) lateral boundary layer of the forcing flow. A comprehensive set of experiments is performed in the 13 m diameter rotating tank by oscillating an 8 m×2 m horizontal plate and vertical wall in a homogeneous fluid at rest in solid-body rotation and measuring the two horizontal components of the current at several locations and depths above the central part of the plate. The predicted and experimentally measured maximum intensity and width of the residual current are in very good agreement, within the range of validity of the model, i.e. when the Ekman number is sufficiently small. However experiments also show that the residual current still occurs when the Ekman layer thickness is of the same order as the fluid depth, but it is then confined to a narrower band along the vertical wall. The flow structure found experimentally is also correctly described by a numerical model developed by Zhang et al. (1994). Current measurements in the Eastern part of the English Channel near the French coast reveal a significant coastal residual current flowing Northward and the coastal rectification process described here may account for part of it.     

Elemental composition and mineralogical characteristics of coastal marine sediments of Tutuila, American Samoa

Surface sediment samples were collected from 5 pristine coastal areas and 1 potentially contaminated coastal site on Tutuila, the main island of American Samoa, an isolated island group in the South Pacific Ocean. Samples were analysed for total element analysis (15 elements) and mineralogy. The results indicated no evidence of trace element contamination at any site, including Pago Pago Harbour. Inter-site variations could be explained assuming the sediments consisted predominantly of coralline sand and rubble with varying quantities of basaltic materials derived from local catchments.     

Fingerprinting upland sediment sources: particle size‐specific magnetic linkages between soils,lake sediments and suspended sediments

Accelerated erosion of fine‐grained sediment is an environmental problem of international dimensions. Erosion control strategies and targeting of mitigation measures require robust and quantitative identification of sediment sources. Here, we use magnetic ‘fingerprinting’ to characterize soils, and examine their affinity with and contribution to suspended sediments transported within two subcatchments feeding Bassenthwaite Lake, northwest England. A high‐resolution soil magnetic susceptibility survey was made using a field susceptometer (ZH Instruments, SM400 probe). Combining the spatial and vertical (down‐profile) soil magnetic data, a subset of soil profiles was selected for detailed, laboratory‐based magnetic remanence analyses. The magnetic properties of the catchment soils are highly particle size‐dependent. Magnetic analyses were performed on the 31–63 µm fraction, for particle size‐specific comparison both with the suspended sediments and lake sediments. Fuzzy cluster analysis groups the soil magnetic data into six clusters, apparently reflecting variations in parent material and horizon type, with three magnetically hard soils as unclassified outliers. Examination of the cluster affinity of the soils, suspended sediments and lake sediments indicates that topsoils of the upper Newlands Valley and subsoils around Keskadale Beck are a major source of the Newlands Beck suspended load, and the recent (post‐nineteenth century) sediments in the deep lake basin. Older lake sediments show strong affinity with a small number of the Derwent suspended sediments and one of the Glenderamackin soils. A large number of Derwent suspended sediments show no affinity with any of the soils or lake sediments, instead forming a coherent, discrete and statistically unclassified group, possibly resulting from mixing between the magnetically hard subsoils of the medium to high‐altitude Glenderamackin and Troutbeck areas and softer, lower altitude Glenderamackin soils. The lack of any affinity of these suspended sediments with the lake sediments may indicate deposition along the Derwent flood plain and/or in the shallow delta of Lake Bassenthwaite. Particle size‐specific magnetic fingerprinting is thus shown to be both highly discriminatory and quantitatively robust even within the homogeneous geological units of this catchment area. Such a methodological approach has important implications for small–large scale catchment management where sources of sediment arising from areas with uniform geology have been difficult to determine using other approaches, such as geochemical or radionuclide analyses. Copyright © 2009 John Wiley & Sons, Ltd.     

The influence of Pacific salmon decay products on near‐field streambed sediment and organic matter dynamics: a flume simulation

Pacific salmon are biogeomorphic agents shown to induce positive feedbacks on their natal watersheds. However, the literature documenting their ecological effects on in‐stream natal environments is more divisive. The disturbance salmon create during redd construction has the potential to reduce stream productivity. The pulse of salmon organic matter (SOM) and marine derived nutrients (MDNs) released during carcass decay has been reported as either stimulating in‐stream productivity or having no local effect. To evaluate the ecological costs and benefits of salmon spawning events, MDN delivery and storage processes need to be identified and quantified. A simulation was conducted in three flow‐through flumes (2 m × 2 m × 30 m) over a 33‐day period (consisting of 15 baseline, four MDN exposure, and 14 post‐exposure days) to assess near‐field sediment and organic matter dynamics during active and post‐spawn simulations. The objective of the study was to measure changes in the amounts and particle sizes of suspended and gravel‐stored fine sediment, in order to elucidate the process and significance of SOM recruitment to the gravel bed via sedimentation. Gravel beds in all flumes were enriched with SOM following treatments but the response was highest in the active spawn simulation. The more effective delivery in the active spawn simulation was attributed to its higher inorganic sediment concentration, which is known to enhance floc formation. Although the active spawn simulation delivered more SOM to the gravel bed, the post‐spawn phase may be equally important to natural streams because its decay phase is longer than the active spawn and consequently can provide SOM to the streambed as long as carcasses remain in‐stream. The delivery, and potential retention, of SOM to spawning streambeds and the intergravel environment may be particularly important for interior streams, which experience low flow conditions during the spawning phase and accordingly have the potential for hyporheic nutrient recruitment and storage. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of variable‐density groundwater flow on nitrate flux to coastal waters

A two‐dimensional variable‐density groundwater flow and transport model was developed to provide a conceptual understanding of past and future conditions of nitrate (NO₃) transport and estimate groundwater nitrate flux to the Gulf of Mexico. Simulation results show that contaminant discharge to the coast decreases as the extent of saltwater intrusion increases. Other natural and/or artificial surface waters such as navigation channels may serve as major sinks for contaminant loading and act to alter expected transport pathways discharging contaminants to other areas. Concentrations of NO₃ in the saturated zone were estimated to range between 30 and 160 mg?L^?1 as NO₃. Relatively high hydraulic vertical gradients and mixing likely play a significant role in the transport processes, enhancing dilution and contaminant migration to depth. Residence times of NO₃ in the deeper aquifers vary from 100 (locally) to about 300 years through the investigated aquifer system. NO₃ mass fluxes from the shallow aquifers (0 to 5.7 × 10⁴ mg?m^?2?day^?1) were primarily directed towards the navigation channel, which intersects and captures a portion of the shallow groundwater flow/discharge. Direct NO₃ discharge to the sea (i.e. Gulf of Mexico) from the shallow aquifer was very low (0 to 9.0 × 10¹ mg · m^?2?day^?1) compared with discharge from the deeper aquifer system (0 to 8.2 × 10³ mg?m^?2?day^?1). Both model‐calibrated and radiocarbon tracer‐determined contaminant flux estimates reveal similar discharge trends, validating the use of the model for density‐dependent flow conditions. The modelling approach shows promise to evaluate contaminant and nutrient loading for similar coastal regions worldwide. Copyright © 2015 John Wiley & Sons, Ltd.     

Rates, timing, and mechanisms of rainfall interception loss in a coastal redwood forest

Rainfall, throughfall, and stemflow were monitored at 5-min intervals for 3 years in a 120-year-old forest dominated by redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii) at the Caspar Creek Experimental Watersheds, located in northwest California, USA. About 2.5% of annual rainfall reaches the ground as stemflow at the site, while 22.4% is stored on foliage and stems and evaporates before reaching the ground. Comparison of the timing of rainfall and throughfall indicates that about 46% of the interception loss occurs through post-storm evaporation from foliage and 54% is either evaporated during the storm or enters long-term storage in bark. Until bark storage capacity is saturated, the proportion of rainfall diverted to bark storage would be relatively constant across the range of rainfall intensities encountered, reflecting primarily the proportional incidence of rainfall on surfaces contributing to bark storage. In any case, loss rates remain high—over 15%—even during the highest-intensity storms monitored. Clearcut logging in the area would increase effective annual rainfall by 20–30% due to reduction of interception loss, and most of the increase would occur during large storms, thus potentially influencing peakflows and hillslope pore-pressures during geomorphically significant events.     

Explosive magma-water interactions: Thermodynamics,explosion mechanisms,and field studies

Physical analysis of explosive, magma-water interaction is complicated by several important controls: (1) the initial geometry and location of the contact between magma and water; (2) the process by which thermal energy is transferred from the magma to the water; (3) the degree to and manner by which the magma and water become intermingled prior to eruption; (4) the thermodynamic equation of state for mixtures of magma fragments and water; (5) the dynamic metastability of superheated water; and (6) the propagation of shock waves through the system. All of these controls can be analyzed while addressing aspects of tephra emplacement from the eruptive column by fallout, surge, and flow processes. An ideal thermodynamic treatment, in which the magma and external water are allowed to come to thermal equilibrium before explosive expansion, shows that the maximum system pressure and entropy are determined by the mass ratio of water and magma interacting. Explosive (thermodynamic) efficiency, measured by the ratio of maximum work potential to thermal energy of the magma, depends upon heat transfer from the pyroclasts to the vapor during the expansion stage. The adiabatic case, in which steam immediately separates from the tephra during ejection, produces lower efficiencies than does the isothermal case, in which heat is continually transferred from tephra to steam as it expands. Mechanisms by which thermal equilibrium between water and magma can be obtained require intimate mixing of the two. Interface instabilities of the Landau and Taylor type have been documented by experiments to cause fine-scale mixing prior to vapor explosion. In these cases, water is heated rapidly to a metastable state of superheat where vapor explosion occurs by spontaneous nucleation when a temperature limit is exceeded. Mixing may also be promoted by shock wave propagation. If the shock is of sufficient strength to break the magma into small pieces, thermal equilibrium and vapor production in its wake may drive the shock as a thermal detonation. Because these mechanisms of magma fragmentation allow calculation of grain size, vapor temperature and pressure, and pressure rise times, detailed emplacement models can be developed by critical field and laboratory analysis of the resulting tephra deposits. Deposits left by dense flows of tephra and wet steam as opposed to those left by dilute flows of dry steam and tephra show contrasts in median grain size, dispersal area, grain shape, grain surface chemistry, and bed form.     

Present state and historical changes of trace metal pollution in Kaoping coastal sediments, southwestern Taiwan

Surface and gravity-cored sediments were collected from the Kaoping coastal area off southwestern Taiwan to determine particle size, organic carbon, trace metal concentration and enrichment factor (EF), 210Pb dating, and 206Pb/207Pb ratio for determining present and historical metal pollution. Surface distributions of trace metals ranged from 0.02 to 0.13 mg/kg for Cd (EF: 0.16-1.65), from 35 to 189 mg/kg for Cr (EF: 0.85-2.92), from 25 to 64 mg/kg for Ni (EF: 0.53-2.38), from 10 to 32 mg/kg for Pb (EF: 1.20-4.94), and from 29 to 129 mg/kg for Zn (EF: 1.18-3.50). Trace metal concentrations correlate closely with distributions of mud (<63 microm) and organic carbon which accumulate largely around river mouths and within the Kaoping Canyon. With the exception of Cd, Cr and Ni in certain areas with rather coarse sediments, metals were generally elevated above the baseline levels over the studied area. Metals were also relatively enriched in areas with high contents of mud and organic carbon. Sedimentation rates derived from the excess 210Pb data in core sediments sampled from the canyon illustrate metal pollution beginning around 1970 that is corresponding to the booming time of economic growth in Taiwan. Meanwhile, the status of Pb pollution in core sediments is verified by an inversed correlation between 206Pb/207Pb ratios and Pb concentrations. Anthropogenic Pb and other metals in the studied area were apparently derived primarily from the Kaoping River and accumulated around the river mouth and within the Kaoping Canyon. The Kaoping Canyon appears to act as a major sink for river borne trace metals.     

Distribution, enrichment and accumulation of heavy metals in coastal sediments of Alang-Sosiya ship scrapping yard, India

Since its inception in 1982, the Alang-Sosiya yard has become the largest ship scrapping works in the world. Several hundreds of ships arrive every year. The degree of heavy metal contamination has been studied in bulk and fine sediments from the intertidal zone of this ship scrapping yard, two stations, one on either side at 5 km distance and one reference station 60 km distance near Mahuva, towards the south. The samples have been subjected to a total digestion technique and analysed for elements: Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, Zn and Al, and %TOC. The absolute metal concentrations reflected variations in BF and FF sediment samples with organic matter content. Enrichment factors (EF) and geoaccumulation indices (Igeo) have been calculated and the relative contamination levels are assessed at these sites. At Alang-Sosiya, the enrichment of heavy metals has been observed to be relatively high.     

Analytic solutions for interface flow in combined confined and semi-confined,coastal aquifers

Analytic solutions are presented for steady interface flow in aquifers consisting of a confined and a semi-confined section. The total discharge is constant in the confined section and is directed towards the semi-confined section, which is bounded on top by a leaky layer that separates the aquifer from the sea. All solutions are based on the Dupuit approximation. The interface position is fully determined by two dimensionless parameters. The first parameter is the product of the uniform gradient towards the sea and the dimensionless leakage factor divided by the dimensionless density difference. The second dimensionless parameter is the length of the semi-confined section divided by the leakage factor. Four types of flow are distinguished. For cases I and II the interface does not reach the end of the semi-confined section, while it does for cases III and IV. For cases I and III the interface extends into the confined section, while for cases II and IV it remains entirely in the semi-confined section. Analytic solutions are presented for the position of the interface for all four cases. Diagrams are presented to determine the type of flow and the position of the interface toe based on the two dimensionless parameters. The pre-development position of the interface along the Georgia coast is computed as an illustration of the practical application of the presented formulas. Another practical application is the estimation of how far a numerical model of seawater intrusion should be extended into the sea for accurate simulations.     

Natural and anthropogenic controls on sediment rating curves in northern California coastal watersheds

The watersheds along the north coast of California span a wide range of geologic settings, tectonic uplift rates, and historic timber harvest activity. Known trends in how each of these factors influence erosion rates provides an opportunity to examine their relative importance. We analyzed 71 watersheds within nine larger river basins, investigated the factors influencing suspended sediment rating curves (SRCs), investigated how SRCs varied among our study watersheds, and used Random Forest modeling (RFM) to determine which environmental characteristics and land management metrics influence SRC shapes, vertical offsets, and slopes. While SRCs typically take the form of a power function, they also can exhibit threshold or peak relationships. First, we found both power and threshold relationships for the SRCs within our study watersheds. Second, the SRC offsets and slopes systematically varied with regional tectonic uplift. Third, SRC offsets increased in several watersheds following intensive timber harvest events and SRC slopes decreased due to a greater relative increase in suspended sediment concentration at lower flows than higher flows. Our RFM correctly classified 96% of the SRC shapes using two near-channel metrics; near-channel precipitation-sensitive deep-seated landslide susceptibility and near-channel soil erodibility. Our RFM models also showed that timber harvest activity and near-channel local relief can explain 40% of the variability in SRC offsets, whereas tectonic uplift rates, millennial-scale erosion rates, and precipitation patterns explain 40% of the variability in SRC slopes.