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The quantification of the erodibility of cohesive sediments is fundamental for an advanced understanding of estuarine sediment transport processes. In this study, the surface erosion threshold tc for cohesive sediments collected from two sites in the area of the Port of Hamburg in the River Elbe is investigated in laboratory experiments. An improved closed microcosm system (C-GEMS) is used for the erosion experiments, which allows the accumulation of suspended sediment concentration (SSC) over an experimental run. A total of 34 erosion experiments has been conducted with homogenized samples and bulk densities between 1050 kg/m³ and 1250 kg/m³. The covered range of bulk densities is seen to represent the values commonly exhibited by freshly deposited cohesive sediments. Two approaches to derive tc based on the erosion rate (e-method) and the SSC (SSC-method) were elaborated and compared. For both approaches, only one parameter has to be set in order to facilitate transferability to other devices. The results show a better performance of the SSC-method in terms of lower uncertainties, especially at the upper application limits of the utilized C-GEMS. The application of the SSCmethod yields values for tc between 0.037 N/m² and 0.305 N/m², continuously increasing with bulk density. Repetition tests proved the repeatability of the experimental procedure and utilized methods to derive tc. The derived data for tc is used to fit two mathematical models: i) a highly empirical model relating tc to dry bulk density and ii) a recently proposed model relating tc to the physical properties of the sediment-mixture. While the derived parameters for the first model vary widely for the two sampling sites, the fit-parameter for the lattermodel is virtually independent of the investigated site, suggesting the superiority of this approach. [ABSTRACT FROM AUTHOR]

The quantification of the erodibility of cohesive sediments is fundamental for an advanced understanding of estuarine sediment transport processes. In this study, the surface erosion threshold τc for cohesive sediments collected from two sites in the area of the Port of Hamburg in the River Elbe is investigated in laboratory experiments. An improved closed microcosm system (C-GEMS) is used for the erosion experiments, which allows the accumulation of suspended sediment concentration (SSC) over an experimental run. A total of 34 erosion experiments has been conducted with homogenized samples and bulk densities between 1050 kg/m³ and 1250 kg/m³. The covered range of bulk densities is seen to represent the values commonly exhibited by freshly deposited cohesive sediments. Two approaches to derive τc based on the erosion rate (ε-method) and the SSC (SSC-method) were elaborated and compared. For both approaches, only one parameter has to be set in order to facilitate transferability to other devices. The results show a better performance of the SSC-method in terms of lower uncertainties, especially at the upper application limits of the utilized C-GEMS. The application of the SSC method yields values for τc between 0.037 N/m² and 0.305 N/m², continuously increasing with bulk density. Repetition tests proved the repeatability of the experimental procedure and utilized methods to derive τc. The derived data for τc is used to fit two mathematical models: i) a highly empirical model relating τc to dry bulk density and ii) a recently proposed model relating τc to the physical properties of the sediment-mixture. While the derived parameters for the first model vary widely for the two sampling sites, the fit-parameter for the latter model is virtually independent of the investigated site, suggesting the superiority of this approach.

The erosion threshold, beyond which bed sediments start to move, is a key parameter describing sediment transport processes. For silt‐dominated mixtures, in which the grain size is between sand and clay, existing experimental studies exhibit contradictory observations. That is, the erosion was either sand‐like or clay‐like, suggesting transitional erosion behavior. To explore the underlying mechanism of the transitional erosion behavior of silt‐sized sediment, we revisited the topic of the erosion threshold of sand‐silt mixtures by carrying out a series of erosion experiments for different bed compositions. The results suggest that there exists a critical silt content of approximately 35%, separating two domains. Below this critical value, the critical bed shear stress follows the Shields criterion, whereas above this value, the erosion threshold of a mixed bed increases abruptly and remains relatively constant with a further increase in silt content. By combining with existing data, we found that the proposed critical silt content acts as a tipping point, beyond which the mixed bed shifts from a sand‐dominated to a silt‐dominated domain. For the silt‐dominated domain, a stable silt skeleton can be formed by attraction forces that resist erosion. However, the attraction forces are too weak to form a stable silt skeleton when the silt content is too small. Based on this finding, a modified critical bed shear stress formula is proposed for silt‐dominated mixtures, which results in a better agreement with experimental data (an averaged bias of 10%), performing better than existing formulas (larger than 30%). Plain Language Summary: When exposed to a certain flow velocity, sediment particles are dislodged from the seabed, namely, the erosion threshold. Attributed to sizes, shapes, minerals, etc., sediments of different grain sizes behave differently, resulting in various erosion behaviors. Sand particles (62.5–2,000 μm) are not sticky and erode particle by particle, whereas clay particles (<4 μm) are sticky and collectively erode as chunks. Silt (4–62.5 μm), of which the grain size is between sand and clay, exhibits either sand‐like or clay‐like behavior, suggesting transitional erosion behavior. Coastal sediments are usually mixtures of clay, silt, and fine sand. Different bed compositions result in different erosion behaviors, leading to a variety of bed forms, morphological patterns, etc. Therefore, a thorough understanding of the erosion behavior of sediment mixtures is of great significance to the topic of coastal sediment transport, which further benefits coastal geomorphology, ecology, etc. This study indicates that, for sand‐silt mixtures, a critical silt contents exists, beyond which the bed mixtures shift from a sand‐like erosion behavior to a clay‐like erosion behavior. The widely adopted parameterized expression of the erosion threshold (namely, the Shields curve) is modified to mimic the transitional erosion behavior of sand‐silt mixtures by considering the effects of silt content. Key Points: Silt content is an important factor controlling the erosion of sand‐silt mixtures in regards to noncohesive or cohesive propertiesThere exists a critical silt content of 35%, beyond which a stable silt skeleton is formed that increases the erosion thresholdA modified critical bed shear stress is proposed to mimic the transitional erosion behavior of sand‐silt mixtures [ABSTRACT FROM AUTHOR]

Cohesive sediment is ubiquitous in aquatic systems, which often forms fractal aggregates due to cohesive and adhesive forces between particles and is generally eroded as aggregates at low bed shear stresses. The erosion of aggregates plays a significant role in cohesive sediment dynamics. However, the effects of fractal bed aggregation on the erosion threshold of sediment have not been well understood. The incipient motion condition of cohesive sediment is investigated, in which particle aggregation is taken into account by employing the fractal theory and the van der Waals force between particles. A formula for the critical shear stress for surface erosion of cohesive sediments composed of fractal aggregates is developed based on the balance analysis of momentums acting on an aggregate in the bed surface. The developed formula has been successfully applied to different kinds of cohesive sediment. The fractal dimension is found as a function of the solid volume fraction and the diameter of primary particles. The contribution rate of the effective weight of aggregate to the erosion threshold of cohesive sediment is quantified.

Erosion of cohesive sediments is a ubiquitous phenomenon in estuarine and intertidal environments. Several methods have been proposed to determine the surface erosion threshold (τc0), which are still debatable because of the numerous and uncertain definitions. Based on erosion microcosm experiments, we have compared three different methods using (1) eroded mass (EM), (2) erosion rate (ER), and (3) suspended sediment concentration (SSC), and suggested a suitable method for revealing the variation of erodibility in intertidal sediments. Erosion experiments using a microcosm system were carried out in the Muuido tidal flat, west coast of South Korea. The mean values of τc0 for three methods were: 0.20 ± 0.08 Pa (EM); 0.18 ± 0.07 Pa (ER); and (3) 0.17 ± 0.09 Pa (SSC). The SSC method yielded the lowest τc0, due to the outflow of suspended sediment from the erosion chamber of the microcosm. This was because SSC gradually decreased with time after depleting the erodible sediment at a given bed shear stress (τb). Therefore, the regression between SSC and applied τb might skew an x-intercept, resulting in the underestimation (or “not-determined”) of τc0. The EM method yielded robust and accurate (within the range of τb step at which erosion begins) results. The EM method represents how the erodible depth thickens as τb increases and therefore seems better suited than the SSC and ER methods for representing depth-limited erosion of cohesive sediments. Furthermore, this study identified the spatiotemporal variations of τc0 by EM method in an intertidal flat. The τc0 in mud flat was about two times higher than that in mixed flat. Compared to the end of tidal emersion, the sediment was 10–40% more erodible at the beginning stage.

The erosion threshold of sand-mud mixtures is investigated by analyzing the momentum balance of a sand particle or a mud parcel in the mixture bed surface, and a formula for the critical shear stress of sand-mud mixtures is developed, which also applies for pure sand and mud. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture. The developed formula reproduces well the variation of the critical shear stress of sand-mud mixtures over mud content and can predict the critical shear stress of both sand-mud mixtures and pure mud in the process of consolidation. The developed formula promises to be convenient for application by relating the critical shear stress to mud content and the dry bulk density of sediment.

• The bentonite gel generated by hydration is proved to have a fractal structure by X-ray small-angle scattering test. • The erosion threshold of bentonite buffer in the high-level radioactive waste repository is better predicted by fractal theory. • Cumulative erosion mass is assessed based on the fractal structure characteristic of the gel eroded out. Bentonite exhibits good self-sealing performance and low permeability after hydration, and therefore is selected as a buffer material for high-level radioactive waste repositories, isolating the canisters containing nuclear waste from the external environment. The bentonite cushion directly contacts the surrounding rock and generates gels at the contact interface with fracture water. Surface erosion occurs once the shear stress exerted by the flowing water exceeds the erosion threshold of the bentonite gel, resulting in gel particle detachment from the soil–water interface and transport through fissure. The mass loss of the buffer by erosion will weaken its functionality as designed and directly endanger the long-term safety of the repository. The erosion behavior of bentonite is related to the structure of the gel formed by bentonite hydration. In this paper, the X-ray small-angle scattering experiments were used to prove that the gel structure has fractal characteristics and to obtain the mass fractal dimension. Based on the fractal structure of the bentonite gel, the erosion threshold that determines whether erosion will occur was quantified as a function of the solid volume fraction of the gel by taking the inter-particle attractive bond as the primary source of resistance. Furtherly, the cumulative erosion mass model that evaluates the extent of erosion was constructed to reveal the relationship between the cumulative erosion mass and cumulative water flow after the erosion has achieved stability. The proposed relations are verified by the experimental results from published literature and are of great significance for judging whether erosion will occur and evaluating the degree of erosion. [ABSTRACT FROM AUTHOR]

Intertidal zones by definition are exposed to air at low tide, and the exposure duration can be weeks (e.g. during neap tides) depending on water level and bed elevation. Here we investigated the effect of varying exposure duration (6 h to 10 days) on intertidal mudflat erosion (measured using the EROMES device), where the effects of water content and biofilm biomass (using chlorophyll‐a content as a proxy, Chl‐a μg g−1) were taken into account. Sediments were collected between spring and summer (in October 2018, January 2019 and February 2019) from an intertidal site in the Firth of Thames, New Zealand. Longer exposure duration resulted in more stable sediments [higher erosion threshold (Ƭcr, N m−2) and lower erosion rate (ER, g m−2 s−1)]. After 10 days, exposure increased Ƭcr by 1.7 to 4.4 times and decreased ER by 11.6 to 21.5 times compared with 6 h of exposure. Chl‐a and water content changed with exposure duration and were significantly correlated with changes in Ƭcr and ER. The stability of sediments after two re‐submersion periods following exposure was also examined and showed that the stabilizing effect of exposure persisted even though water content had increased to non‐exposure levels. Re‐submersion was associated with an increase in Chl‐a content, which likely counteracted the destabilizing influence of increased water content. A site‐specific model, which included the interplay between evaporation and biofilm biomass, was developed to predict water content as a function of exposure duration. The modelled water content (WMod.) explained 98% of the observed variation in water content (WObs.). These results highlight how the exposure period can cause subtle changes to erosion regimes of sediments. An understanding of these effects (e.g. in sediment transport modelling) is critical to predicting the resilience of intertidal zones into the future, when sea‐level rise is believed to exacerbate erosion in low‐lying areas. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Sediment resuspension may play a major role in sediment-water exchange of nutrients, matter and energy in coastal areas where waves and currents dominate sediment transport. Biogeochemical sediment properties regulate sediment erodibility, but there is only limited knowledge of how temporal variability in environmental variables is reflected in the resuspension potential, especially for subtidal habitats. Further, the significance of resuspension on nutrient fluxes in coastal environments has remained unclear as contradicting results have been reported. Here we quantified the temporal variation in resuspension potential metrics (erosion threshold (τc; N m−2) and erosion constant (me; g N−1 s−1)) and associated nutrient fluxes from three sites in the Hanko archipelago (Finland) using a core-based erosion device (EROMES). The sites were sampled bi-monthly from April to December. We also quantified the temporal variation in biogeochemical sediment properties at each site. The τc exhibited the clearest temporal pattern in muddy sediment, where the coefficient of variation (= 67) was two to three times higher than the mixed (= 29) and sandy (= 16) sediments. Dry bulk density was the best predictor for sediment erodibility at all sites explaining 26–46% of the temporal variation in τc despite its limited variability at sandier sites. In addition, temporal variations in the macrofaunal community were important predictors of muddy sediment erodibility and therefore community dynamics need to be considered in sediment transport studies. All sites were potential nutrient sources, yet the overall role of sediment resuspension on nutrient release from the sediments was small. [ABSTRACT FROM AUTHOR]

Abstract Erosion of cohesive soils is represented typically as an erosion threshold or erosion rate. Predicting the erosion threshold of the cohesive soils requires taking into account cohesion, which results from interactions between clay particles. Beyond the usual dependence on grain size, a significant correlation between erosion threshold and porosity or density measurements is confirmed for the cohesive soils. Correlations of the erosion threshold to the solid volume fraction and effective density of cohesive soils are proposed based on the fractal model for cohesive aggregates. Cohesion forces, the long-range van der Waals interaction between two particles is taken as the resource of the erosion threshold. The model verification is conducted by comparison with experiments of the erosion thresholds published in literature. The results show the proposed model for the erosion threshold offers good agreement with experimental data. Highlights • A fractal model for the aggregates of cohesive sediment is proposed. • Erosion threshold is calculated from the long-range van der Waals interaction. • The predicted erosion threshold is in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Bentonite has been considered as a candidate buffer material for the disposal of high-level radioactive waste (HLW) because of its low permeability, high sorption capacity, self-sealing characteristics and durability in a natural environment. Bentonite erosion caused by groundwater flow may take place at the interface of the compacted bentonite and fractured granite. Surface erosion of bentonite flocs is represented typically as an erosion threshold. Predicting the erosion threshold of bentonite flocs requires taking into account cohesion, which results from interactions between clay particles. Beyond the usual dependence on grain size, a significant correlation between erosion threshold and porosity measurements is confirmed for bentonite flocs. A fractal model for erosion threshold of bentonite flocs is proposed. Cohesion forces, the long-range van der Waals interaction between two clay particles are taken as the resource of the erosion threshold. The model verification is conducted by the comparison with experiments published in the literature. The results show that the proposed model for erosion threshold is in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The critical shear stress for erosion of sand and mud mixtures is theoretically investigated and expressed as a function of the critical shear stresses of pure sand and mud, mud content, and sand diameter. The proposed formula accounts for different structures and behaviours of the mixtures with low and high mud contents. The critical shear stress of pure mud is related to the solid/void volume ratio, and this relation is extended to the mud component in the mixture to consider the effect of compaction. It is found that the mud dry density, not the mixture dry density, is a direct factor affecting the mixture erosion. The developed formula has been calibrated and tested using four sets of experimental data collected from the literature. It reproduces the variations of the critical shear stress and mud dry density well. [ABSTRACT FROM AUTHOR]

Cohesive sediments are gradually consolidated after settling in dam reservoirs, settling ponds and channels. For hydraulic flushing of the sediments it’s necessary that the erosion threshold velocity and shear stress of these types of sediments to be known. In this study, by using the annular flume in Shahrekord University, this subject was investigated. Fine sediments used in the experiments, were collected from the reservoir bed of Pirbaloot earthfill dam located in north Karun basin, in Chaharmahal and Bakhtiari province. By conducting physicochemical tests the sediments properties were obtained. Five experiments to determine the erosion threshold of cohesive sediments with consolidation of five weeks, three weeks, two weeks, one week and three days was performed and velocity and shear stress of erosion threshold of sediments was measured by the ADV velocimeter. Results showed that erosion threshold of these sediments have a logarithmic relationship with the sediment consolidation time. The amount of erosion threshold shear stress of sediments for surface sediments were obtained in a range of 0.12 to 0.14 N/m2 for sediment with difference age of self-weight consolidation. These values for subsurface sediments were obtained at 0.44 and 0.69 N/m2, respectively.

Soft-bottom bioturbators are ecosystem engineers in the sense that they can have considerable effects on sediment erodibility and resuspension. The common cockle Cerastoderma edule is a bioturbating filter feeder that is widespread along the European Atlantic coastline. Its presence and activity can decrease sediment erosion thresholds in cohesive sediments but little is known about its effect on non-cohesive sediments. Using controlled annular flume experiments, we investigated the relative effects of different cockle densities on sediment resuspension in cohesive vs. non-cohesive sediments by assessing the following: (i) the mud and sand burrowing behavior of cockles, (ii) critical erosion thresholds, (iii) the mass of eroded sediment and (iv) erosion rates. Our results show that cockles were more active in non-cohesive sediment compared with cohesive sediment. Despite their lower activity, the presence of cockles in cohesive sediment increased sediment erodibility by reducing the critical erosion threshold ( U crit ) and increasing both the mass of eroded sediment and erosion rate. In contrast, cockles had no effect on erodibility in non-cohesive sediment, especially on the eroded sediment mass and erosion rate. The mass eroded was not significantly different between cohesive and non-cohesive sediments when cockles were present. Our experiments show that the increased erodibility of cohesive sediment due to the bioturbation by cockles is density dependent: higher cockle density results in stronger effects on erodibility. Moreover, this increase in cohesive sediment erosion due to cockle bioturbation was positively correlated with current velocity. In contrast, the erosion of non-cohesive sediment only depended on the current stress and was unaffected by cockle density. Considering the high abundance of C. edule , its widespread distribution and its extensive activities, the results of this study could be widely applicable to intertidal mud flats around the world. [ABSTRACT FROM AUTHOR]

Shi, B.; Wang, Y.P.; Yang, Y.; Li, M.; Li, P.; Ni, W., and Gao, J., 2015. Determination of critical shear stresses for erosion and deposition based on in situ measurements of currents and waves over an intertidal mudflat. Accurate determination of the critical shear stress associated with the erosion and deposition of sediments is an important component of numerical models used to predict and quantify sediment behavior and transport across intertidal flats. In this study, water depth, wave parameters, near-bed turbulent velocity, suspended sediment concentration (SSC), and intratidal bed-level changes were measured to determine the erosion ( τce) and deposition ( τcd) thresholds of sediments on an intertidal mudflat at Jiangsu, China. Based on integrated field measurements of bed-level changes and hydrodynamics, the bed shear stresses of currents ( τc), waves ( τw), and combined current-wave action ( τcw) were calculated, and the critical shear stress required for erosion ( τce = 0.14 N/m2) and deposition ( τcd = 0.08 N/m2) of these sediments was determined. Both values are in agreement with an estimate of τce (0.13 N/m2) that was based on water content and τcd values calculated by previous works, indicating that the value of τce estimated for these sediments is controlled primarily by the water content of the sediments. During field measurements, deposition occurred ( τcw < τcd) when current action exceeded wave energy ( τc > τw) during calm weather, whereas erosion occurred ( τcw > τce) when wave action increased dramatically during rough weather. Our field data showed that high current velocities lead to low τc, possibly because high SSC reduced the drag coefficient, which is variable during a tide, and further caused low τc under high current velocities. Additionally, bedforms characteristic of intertidal mudflat ( e.g., gullies, small creeks, ripples, or saltmarsh) has a significant influence on the drag coefficient of the bed. These observations suggest that the bed level responds strongly to changing hydrodynamic conditions, and also that waves are of great importance to erosion. This study demonstrates that the in situ determination of the parameters that control erosion and deposition is a useful approach to obtaining values of τce and τcd, which provide the basis for a mechanistic understanding of the morphological evolution and development of predictive sediment transport and erodibility models. [ABSTRACT FROM AUTHOR]

Mangrove expansion has been observed in numerous harbours and estuaries of the North Island of New Zealand. Recently, the removal of above-ground mangrove vegetation has been initiated to halt this expansion. A key objective of mangrove clearing is to return a site to a sandier substrate, however limited monitoring has been undertaken to determine if that objective is reached, and also over what timeframes. A sub-estuary of the Whangamata Harbour was partially cleared of mangroves in 2005. This study revisited the site four and six years after the removal activity to assess the physical condition of the site, and also to explore the erodibility of surface sediments. An EROMES, a small semi-portable instrument, was used to collect sediment cores and subsequently measure the erosion threshold and erosion rate of sediments collected in mangrove habitat, cleared habitat, and unvegetated/undisturbed sandflats. Grain size analysis shows a flushing of silt and clay in the cleared location, which mostly occurred between 3 and 4 years post-clearance, whereas surface sediments appeared to maintain ~15% mud between 4 and 6 years. The coarsening of sediment was limited to the top 5 mm, creating a sand cap that armours the underlying mud-dominated sediments. The sand cap was coupled with a higher erosion threshold, suggesting this cleared site has become more resistant to typical hydrodynamic conditions. This has implications for the on-going ‘recovery’ of these sites which now require stronger tidal currents or wave velocities to entrain and redistribute surface sediments. [ABSTRACT FROM AUTHOR]

Much research has been devoted to the development of numerical models of river incision. In settings where bedrock channel erosion prevails, numerous studies have used field data to calibrate the widely acknowledged stream power model of incision and to discuss the impact of variables that do not appear explicitly in the model's simplest form. However, most studies have been conducted in areas of active tectonics, displaying a clear geomorphic response to the tectonic signal. Here, we analyze the traces left in the drainage network 0.7 My after the Ardennes region (western Europe) underwent a moderate 100-150 m uplift. We identify a set of knickpoints that have traveled far upstream in the Ourthe catchment, following this tectonic perturbation. Using a misfit function based on time residuals, our best fit of the stream power model parameters yields m = 0.75 and K = 4.63 × 10-8 m-0.5y-1. Linear regression of the model time residuals against quantitative expressions of bedrock resistance to erosion shows that this variable does not correlate significantly with the residuals. By contrast, proxies for position in the drainage system prove to be able to explain 76% of the residual variance. High time residuals correlate with knickpoint position in small tributaries located in the downstream part of the Ourthe catchment, where some threshold was reached very early in the catchment's incision history. Removing the knickpoints stopped at such thresholds from the data set, we calculate an improved m = 0.68 and derive a scaling exponent of channel width against drainage area of 0.32, consistent with the average value compiled by Lague for steady state incising bedrock rivers. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The incipient motion and erosion behavior of bottom muds in coastal areas have wide applications in coastal engineering, water environment management, and also the protection of marine benthic communities. Due to the strong inter-particle cohesion of fine sediments and the diversity in its physical properties, the mechanism of the coastal mud erosion is not well understood and the determination of the threshold for the incipient motion remains a challenge. In order to investigate the influence of the mud density on the incipient motion of fully disturbed coastal mud, experiments were carried out using a 22-m-long laboratory flume and mud samples from Huangmaohai Estuary, South China Sea. Muds with densities ranging from 1100 to 1550 kg/m were tested under unidirectional open channel flows, which yielded threshold velocities ranging from 0.11 to 1.67 m/s, corresponding to bed shear stresses ranging from 0.029 to 4.191 N/m. Based on the experimental results, an empirical formula for the threshold velocity at the incipient motion of coastal muds with different densities is presented. The computed results together with the proposed formula were also compared with other theories. Besides, four different erosion patterns of coastal muds at incipient motion were identified according to experimental observations, namely erosion in the pattern of fluid muds, strips, pieces, and blocks. The physical mechanisms for different erosion patterns were also analyzed and interpreted. The presented experimental achievements well enrich our knowledge of coastal mud behavior and lead to a deeper understanding of the mechanisms of the incipient motion of bottom muds. [ABSTRACT FROM AUTHOR]

Abstract: The disturbance or removal of vegetation by wildfire influences channel incision following intense rainfall events. Here we empirically examine relationships between the severity of vegetation disturbance and geomorphic controls on threshold conditions that lead to channel incision. We conducted post-fire field mapping and digital spatial analyses across 97 recently formed channel heads in the Rocky Mountains of Montana and Idaho, USA, to identify the relationship between remotely-sensed fire severity and vegetation disturbance and the source area and gradient conditions required for channel initiation. We found that the relationship between the size of source areas and source-area steepness was described by an inverse power function, consistent with established theory, across the range of fire severity, but that the magnitude of the slope–area relationship was significantly correlated with increasing fire severity. Further, at higher levels of fire severity, source areas above channel heads had lower slopes and somewhat larger areas. The findings suggest that the onset of channel incision defined by location of channel heads is controlled by fire severity and that the threshold for channel initiation decreases as vegetation disturbance increases. We also found that, in a subset of catchments for which LiDAR data were available, total curvature explained channel head location across the range of fire severity, with a small but significant contribution from source area steepness. Steepness remains more important at lower fire severity, however, and total curvature dominates where fire severity is most extreme. This suggests that forces of convergent flow are not fully expressed until a significant proportion of vegetation has been consumed such that flow resistance is minimized. Our findings, and the use of a continuous fire severity metric, contribute an ecohydrological and biogeomorphical template for studies of post-fire geomorphic responses and landscape evolution. [Copyright &y& Elsevier]

Abstract: The impact of three dominating benthic invertebrates on sediment stability and erosion conditions of cohesive sediments in the Mondego Estuary, Portugal, was examined in laboratory annular flume experiments. The purpose was to test how the life habits and body size of the three involved species (Hydrobia ulvae, Nereis diversicolor and Scrobicularia plana) in terms of density or biomass influence sediment erosion. All three species decreased the free-stream erosion threshold (u c) and increased erosion rate (E), since their feeding activities diminished the surface stabilizing effect of extracellular polymeric substances (EPS) produced by microphytobenthos. S. plana had the highest and H. ulvae the lowest impact when related to density (factor of 29 for u c and factor of 19 for E), while H. ulvae was more important than S. plana when related to biomass (factor of 4 for u c and factor of 6 for E). N. diversicolor had intermediate density-specific (4–6 times higher than H. ulvae) and lowest biomass-specific (2–3 times lower than S. plana) effects on erosion. It appears that faunal erosion impacts preferably should be reported in biomass units for comparative purposes because individual behavioural effects of a small-bodied species like H. ulvae functionally can be relatively more important than those of a 100 times heavier S. plana individual. This is clearly evidenced from the strongly diminished response in suspended Chlorophyll-a content in the presence of the former than the latter species, which is caused by an efficient microphytobenthos grazing by H. ulvae. It is also important to emphasize that the total faunal impact on erosion threshold in a certain area is dictated by combination of contributions from individual species. The total outcome is unpredictable and controlled by synergistic and antagonistic species-specific effects, species interactions as well as environmental and sediment conditions. [Copyright &y& Elsevier]

Purpose: Lowland chalk streams in the UK are experiencing increased deposition of fine sediment due to changes in land-use practices, channel modifications, and groundwater abstraction. The excessive fine sediment deposits have been linked to benthic habitat degradation, the obstruction of surface-groundwater flow, and the storage of contaminants, such as nutrients and pesticides. While research has been conducted on the provenance, transport, deposition, and storage of fine sediment in chalk streams, none has expressly investigated the erosion of fine sediment deposits. Materials and methods: A year-long field survey was conducted in two reaches of the Frome-Piddle catchment (Dorset, UK) to quantify spatial and temporal variations in the erosion thresholds of surficial fine sediment deposits. Erosion thresholds were measured at randomly located points within areas of sediment accumulation using a cohesive strength meter (CSM). The threshold measurements were paired with sediment cores for analysis of the physical, chemical, and biological properties of the sediment. Spatial and temporal patterns in the erosion thresholds of fine sediment were analyzed using nonparametric statistical tests and visualized with GIS. The sediment properties underlying the variations in erosion thresholds were examined through correlation and linear regression analyses. Results and discussion: Erosion thresholds varied significantly over space and time within the stream reaches. Erosion thresholds were greater for fine sediment deposits found in the center of the channel than in the margins. Thresholds were highest in September 2008 and declined substantially to a minimum in May 2009, with a small peak in March 2009, indicating an annual cycle in erosion thresholds. Effective particle size was identified statistically as the most important sediment property influencing erosion thresholds and was probably underlying much of the spatial variation within the reaches. None of the measured sediment properties adequately characterized the temporal variation in erosion thresholds, however, the results suggest that biological sediment properties and water geochemistry (i.e., cation content) may play a role. Conclusions: By identifying significant spatial and temporal variations in erosion thresholds, this study provides valuable information on the stability of fine sediment deposits, and sediment-bound contaminants, in lowland river systems. This is a crucial step in assessing their local environmental impacts and developing models of fine sediment transport for the effective management of catchment sediment budgets and water resources. [ABSTRACT FROM AUTHOR]

Abstract: Slope incision and subsequent development of rills, gullies and channels are responsible for significant soil losses and are often irreversible with very high restoration costs particularly in semiarid environments. The location of potential areas of erosion where these processes occur is vital to land management and conservation. The study of the relationship between the local slope (S) and the drainage area (A) through the geomorphological relationship S =αA b, combined with hydrologic simulation models, has proved to be appropriate for the identification and characterisation of potential areas of incision, especially when it takes into account the spatial distribution of soil properties and the evolution of hydrological processes. However, physical properties of soil, digital elevation models (DEMs) and flow algorithms used may affect the results. This study employs a distributed, physically-based hydrological model to evaluate the infiltration–runoff relationships and their influence on selecting critical area from three DEMs with different resolutions. The results show a significant scale effect on flow distribution and the location of threshold points on slopes. The results obtained from a 30-m DEM significantly differ from those obtained from 10 and 5-m DEMs because the former was unable to capture the spatial variability of geomorphic processes. The selected set of critical points shows high S–A correlations for different values of critical shear stress. The physical model confirmed the dominance of surface runoff in the study site and was validated from field identification of erosion risk areas, although for incision areas <2ha, an appreciable error remains in relation to the calculation algorithm used for the drainage network and DEM resolution. [Copyright &y& Elsevier]

Abstract: The stability of cohesive sediments of a saltmarsh in Southern England was measured in the field and the laboratory using a Cohesive Strength Meter (CSM) and a shear vane apparatus. Cores and sediment samples were collected from two tidal creek banks, covered by Atriplex portulacoides (Sea Purslane) and Juncus maritimus (Sea Rush), respectively. The objectives of the study were to examine the variation of sediment stability throughout banks with cantilevers present and investigate the influence of roots and downcore consolidation on bank stability. Data on erosion threshold and shear strength were interpreted with reference to bank depth, sediment properties and biological influences. The higher average erosion threshold was from the Sea Purslane bank whilst the Sea Rush bank showed higher average vane shear strength. The vertical variation in core sediment stability was mainly affected by roots and downcore consolidation with depth. The data obtained from the bank faces revealed that vertical variations in both erosion threshold and vane shear strength were affected primarily by roots and algae. A quantitative estimate of the relative contributions of roots and downcore consolidation to bank sediment stability was undertaken using the bank stability data and sediment density data. This showed that roots contributed more to the Sea Purslane bank stability than downcore consolidation, whilst downcore consolidation has more pronounced effects on the Sea Rush bank stability. [Copyright &y& Elsevier]

Abstract: Cohesive sediment is an important component of aquatic environments, which must be monitored and managed for environmental, engineering, and human health reasons. While the hydrodynamic aspects of sediment erosion and transport are well understood, the erodibility of cohesive sediment has proved more difficult to address and predict. Erodibility is the propensity for the sediment to be eroded, and is represented typically as an erosion threshold or erosion rate. It is an attribute of the sediment itself, and is dependent on the sediment properties that dictate the resistive forces in the sediment, such as gravity, friction, cohesion, and adhesion. This paper reviews recent findings from a range of disciplines to create a comprehensive picture of the physical, geochemical and biological properties that influence the erodibility of cohesive sediment. By identifying these key sediment properties, we provide the background for a discussion on how changes in and interactions between the properties generate significant spatial and temporal variations in erodibility. We discuss the development of a predictive model of erodibility, and emphasize the need for more comparable field and laboratory data. [Copyright &y& Elsevier]

This article describes the relationship between 10 selected properties of the sediments (chlorophyll a and b, colloidal and total carbohydrate, water concentration, sediment type, organic matter, erosion threshold and erosion rate) and meio- and macrofauna within and among three different habitats in an urbanized intertidal mudflat/mangrove forest in Tambourine Bay, Sydney Harbour, Australia. Many of the biogeochemical variables were significantly different among habitats, often grading from mudflat to mangrove canopy. In contrast to previous studies, patterns of distribution of macrofauna among habitats were weak. For the meiofauna, only copepods showed any significant difference among habitats, with the greatest numbers in the open mudflat habitat and least under the mangrove canopy. There was a gradient in fauna among the habitats; overall macrofauna abundances were greatest under the mangrove canopy and least on the mudflat, while meiofauna abundance was greatest in the pneumatophore habitat and least under the canopy. Correlations between fauna and properties of sediment were generally weak. When the habitats were analysed separately, some correlations were strengthened but relationships were inconsistent. Thus, while some taxa vary significantly among habitats there was not a strong relationship between biogeochemical properties and either macro- or meiofauna. This suggests that localised factors other than the measured properties of the sediments are driving patterns in fauna at these small scales, which requires further investigation to be unravelled. [ABSTRACT FROM AUTHOR]

Previous studies suggest that current-driven plant transport in shallow lagoons and estuaries is associated with increased turbidity. Our hypothesis is therefore that macroalgae erode surface sediment while drifting as bedload. This ballistic effect of moving plants on surface sediment was tested in a series of controlled annular flume experiments, where simultaneous measurements of macrophytes transport and turbidity were conducted at increasing current velocities. Sediment erosion always started earlier in experiments with plants than in control experiments without plants. Turbidity increased immediately when plants started to move at current velocities of 2–4 cm s−1. From a background concentration of 7–10 mg SPM l−1, turbidity increased to 30–50 mg SPM l−1 for Ceramium sp., Ulva lactuca and Chaetomorpha linum, while the more rigid Gracilaria sp., caused much higher turbidities (50–180 mg SPM l−1). Such plant induced sediment erosion at low current velocity can explain the observed appearance of turbid waters in estuaries and lagoons in the absence of strong wind and wave action. Based on 3-D hydrodynamic modelling, it was determined that plant driven erosion occurs during most of the growth season in a shallow eutrophic estuary (Odense Fjord, Denmark). [ABSTRACT FROM AUTHOR]

Abstract: For studies on sediment transport processes experimental data on the erosion behaviour of sediments are necessary. Because of significant differences in experimental setups and subsequently in resulting values comparisons and, where possible, correlations between methods are required. This study presents measurements with two different erosion devices (straight flume and microcosm erosion chamber), which were used for the determination of critical shear stress velocities for sandy submarine sediments and sieved sediment fractions. An approach is presented to convert measured current velocities into shear stress velocities via roughness length values and drag coefficients under hydrodynamically smooth and transitional turbulent flow conditions. The results from both devices show a good agreement and the measured erosion threshold values fit to established correlations between critical shear stress velocity and grain size. In the grain size range below 200 µm results for naturally composed sediments are influenced by effects caused by the silt- and clay fraction. Sieved sediment fractions in this grain size range tend to show lowered erosion thresholds in relation to the Shields'' curve. [Copyright &y& Elsevier]

Microalgal biofilms are associated with considerable variability in the properties of natural sediments, yet little effort has been made to isolate micro-scale spatial and temporal changes in sediment properties caused by the growth of a biofilm. Understanding the changes associated with biofilm growth and quantifying the time scales over which these changes occur is important for developing suitable experimental designs and for understanding how biofilms mediate sediment properties and processes. The development of a microphytobenthic biofilm and associated changes in the sediment was investigated over 45 days in the laboratory. The biogeochemical properties of the sediment: bulk density, water content, chlorophyll a concentration and colloidal carbohydrate concentration were measured on a sub-millimetre scale in the top 2 mm. The erosion threshold was measured with a Cohesive Strength Meter (CSM). Biofilm development was rapid, with changes in the properties occurring after 1 day and a visible film forming after just 3 days. The largest changes in sediment properties tended to occur in the surface 200 μm through time, with some variables also showing a differing response with depth. There were significant changes in water content, chlorophyll a concentration, colloidal carbohydrate concentration and erosion threshold in the surface 2 mm, with a general trend to increase with time. Bulk density was highly variable and did not show a consistent pattern of change with time. Erosion threshold was positively correlated with water content, chlorophyll a and colloidal carbohydrate in the surface 200 μm and these were also positively correlated with each other. Low Temperature Scanning Electron Microscopy (LTSEM) images revealed changes in the surface sediment structure and the formation of a thick multi-layer biofilm. The rapidity of biofilm growth and development and the associated changes to the sediment should be considered when designing experiments that investigate biofilms and properties of sediments and/or that involve biocide treatments or disturbance to the sediment. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that benthic diatoms and bacteria are able to affect the stability of cohesive sediments. Their production of new extracellular polymeric substances (EPS) increases the erosion threshold and decreases the erosion rate. To predict this build up of biostability in cohesive sediments, it is therefore vital to understand the EPS production rates for both diatoms and bacteria under different ecological conditions. The present study examined the production of EPS as function of light intensity and linked this to biostabilisation. Microbenthos was sampled from a Danish marine embayment at 5-m depth. A 10-day comparison of EPS production under light and under dark conditions showed that the bacterial EPS production hardly changed during the experiment, while the algal EPS production was significantly high already from day 1 and reached a maximum production on day 10. Erosion threshold of natural cohesive sediments was determined in annular flumes after variable consolidation periods under similar light/dark conditions. The evolutions of erosion threshold and EPS concentration correlated well under light conditions, while the development in dark conditions showed no significant correlation, indicating that diatoms are more efficient in sediment stabilisation than bacteria. Further it seems, that the studied biological-sedimentary system needs 2–3days of acclimatisation before the production of EPS can be well correlated with the increase of sediment stability. [Copyright &y& Elsevier]

Abstract: Vertical profiles of the critical erosion threshold (τ crit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John''s Ford. The τ crit of surface sediment increased from 0.04Pa in the upper, brackish estuary to 0.09Pa in the lower estuary. In the upper estuary τ crit only increased slightly with depth whereas in the marine estuary τ crit increased rapidly from 0.09Pa at the surface to 0.25Pa at 15cm below the sediment surface. The results showed that the relationship between τ crit and bulk density (ρ b) obtained previously for surface sediment was also applicable to sediments from depths of 10–15cm and probably deeper. Profiles of ρ b were measured to depths of 70cm using a corer. In the lower (marine) estuary ρ b increased with depth in the sediment from 1580kgm−3 at the surface to 1720kgm−3 at 70cm. In the upper estuary ρ b values were lower at 1170–1200kgm−3 and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes. [Copyright &y& Elsevier]

Abstract: Coastal erosion is an immense economic and social problem that has been receiving increased attention in recent years. A number of devices have been developed to determine the sediment stability in coastal areas: laboratory and field flumes; a range of different erosion devices; shear vanes and fall cone penetrometers. The cohesive strength meter (CSM) erosion device was developed to determine in situ the temporal and spatial variations in the erosion threshold of muddy intertidal sediments. Technological developments have enabled considerable improvements to be made to the original design over the last 15 years. This paper describes modifications to the CSM system that extend the range of eroding pressures the device can generate, to enable measurements to be made on very stable and consolidated sediments such as saltmarshes. A recalibration of the modified device found inconsistencies in the calibration for CSM devices presently in use, therefore a completely new calibration method is presented. This calibrates the CSM jet pressure to the pressure on the surface sediment (henceforth termed the “stagnation pressure”). The stagnation pressure when erosion is detected is assumed to be a relative (but not absolute) measure of the erosion threshold. The application of the device using the new calibration under laboratory conditions on muddy sediment is also presented. The following calibration equations were generated for the individual CSM models, where y=stagnation pressure at the sediment surface (Nm−2) and x=jet exit pressure (kPa): Mark IV (high pressure): y=22.652x; Mark IV (prototype): y=8.528x; and Mark III: y=15.844x. [Copyright &y& Elsevier]

Abstract: Five erosion devices were compared using five intertidal estuarine sites covering a range of sediment stability from newly settled mud to very cohesive mud at the margins of a saltmarsh. The erosion devices use different methods of fluid shearing from horizontal currents/bed shear stresses to vertical water jets, and have different ‘footprint’ areas. The devices included: (1) the annular flumes (AFs—diameter 64cm; footprint area 0.17m2) of the Plymouth Marine Laboratory (PML); (2) PML''s mini-annular flume (MAF—diameter 19cm; area 0.026m2); (3) the annular mini-flume (AMF—diameter 30.5cm; area 0.032m2) of the National Oceanography Centre Southampton (NOC); (4) NOC''s Cohesive Strength Meter (CSM—diameter 3cm; area 0.0007m2); (5) NOC''s EROMES (ER—diameter 10cm; area 0.0079m2). The quantification of threshold shear stress for bed erosion (τ e) and sediment erosion rate was complemented by the measurement of physical, chemical and biological properties of the sediment (grain size, bulk density, water content, organic content, chlorophyll a, carbohydrates, macrofauna). The results demonstrated a significant correlation (r 2=0.98) between the PML AF (laboratory measurement of undisturbed cored sediment) and PML MAF (in situ) for measurement of erosion thresholds for bed sediment. However, there were no significant correlations between AFs, the CSM and EROMES. There were no consistent correlations with physical or biological sediment properties due to the spatially unrelated sites and the marked differences in benthic assemblages. The sources of differences and the lack of correlations between erosion devices were due to several factors, including operational procedures (e.g., sediment resuspension during filling with water), definition of erosion threshold, the nature of the force applied to the bed, and method of calibration. In contrast to the CSM and EROMES, both types of AFs were able to record significant differences in the erodability of soft sediments from four sites. This indicates that the CSM and EROMES may not be very effective at measuring the differences in erosion thresholds of soft estuarine sediments. [Copyright &y& Elsevier]

Abstract: Critical erosion thresholds of intertidal sediments have been measured throughout the Tamar Estuary, SW England, between March 2004 and August 2005. Erosion measurements were made with a miniature, portable annular flume, which could be deployed in situ or on large cores transferred to a research vessel or to the laboratory. Parallel measurements were made of sediment bulk properties (silt content, water content, wet bulk density, chlorophyll and carbohydrate). The data set included sediments with bulk densities ranging from 1555kgm−3 in the lower, marine estuary where silt and water contents were low (45% and 43%, respectively) to 1138kgm−3 in the upper estuary, at the upper limit of the estuarine turbidity maximum, where silt and water contents were higher (75% and 84%, respectively). For this data set, spanning the region from almost fully marine to the summer, low river flow, limit of the turbidity maximum, critical erosion thresholds ranged from 0.245 to 0.025Pa and were highly correlated with sediment wet bulk density. There were no significant correlations with chlorophyll or carbohydrate. This is interpreted, at the whole-estuary scale, as reflecting an underlying physical control of sediment properties. The control is exerted through the combined influence of tidal pumping of fine material up-estuary to form the turbidity maximum, resulting in a reduction in sand content with distance up-estuary, and the generation of increasingly wet, low-density sediments in the upper estuary through repeated tidal mobilisation and deposition. It is speculated that adverse environmental conditions of low and fluctuating salinity, combined with regular tidal sediment disturbance in the upper estuary, prevent the benthic stabilisers (mainly algae) from colonising the sediment to the same extent as they do in the lower, marine estuary. The implication is that the distribution of benthic species in macro-tidal estuaries is determined by a combination of physical influences that (a) control the nature of the sediment along the estuary and, (b) result in a hostile, fluctuating salinity in the low-salinity region associated with variable river flows. Conversely, because of the less variable conditions in the marine end of the estuary, elements of the biota are able to colonise the mud flats where they can modify sediment properties. In these physically dominated, macro-tidal estuaries, seasonal (biological) effects on sediment stability contribute to the variability of the erosion–bulk density relationship. [Copyright &y& Elsevier]

Abstract: This study was carried out to describe the difference in erodibility and aggregation in a tidal basin including both subtidal and intertidal study sites and to use these results to explain the shifting erosion/deposition cycles at the sites. Erosion thresholds, erosion rates and settling velocities of the eroded material were measured at a mudflat transect and at sediment cores taken from a nearby tidal channel during surveys made in May 2000 and March 2002. Surface samples were analysed for grain-size, chl. a content, faecal pellet content, dry bulk density and organic content. Additionally, surface samples were taken at eight occasions in the period January 2002 to May 2003 from shallow tidal channels in the area. These samples were analysed for mud content and showed that major shifts in sediment distribution occurred in the period. The erodibility of the mudflat was generally high due to pelletization by the mudsnail Hydrobia ulvae but close to the salt marsh much lower erodibility was found, probably due to stabilisation by microphytobenthos. In contrast, the erodibility of the channel bed seemed to be very little influenced by biological activity and the relatively low erodibility found here was caused by physical characteristics of the sediment. The sediment eroded from the mudflat was generally strongly pelletized and showed high settling velocities whereas less aggregation and lower settling velocities were found for the channel bed sediments. Temporal variations of the mudflat stability and hydrodynamics resulted in temporal variations of deposition and erosion and the changing stability at the mudflat is likely to be one of the main reasons for a general transport of fine-grained sediment from the mudflat to the channel in the cold seasons and vice versa during the rest of the year. [Copyright &y& Elsevier]

ABSTRACT In situ measurements of lakebed sediment erodibility were made on three sites in Hamilton Harbour, Lake Ontario, using the benthic flume Sea Carousel. Three methods of estimating the surface erosion threshold (τc (0)) from a Carousel time series were evaluated: the first method fits measures of bed strength to eroded depth (the failure envelope) and evaluates threshold as the surface intercept; the second method regresses mean erosion rate (E m ) with bed shear stress and solves for the floc erosion rate (E f ) to derive the threshold for E m = E f = 1 × 10-5 kg m-2 s-1 ; the third method extrapolates a regression of suspended sediment concentration (S ) and fluid transmitted bed shear stress (τ0 ) to ambient concentrations. The first field site was undisturbed (C) and acted as a control; the second (W) was disturbed through ploughing and water injection as part of lakebed treatment, whereas the third site (OIP) was disturbed and injected with an oxidant used for remediation of contaminated sediment. The main objectives of this study were: (1) to evaluate the three different methods of deriving erosion threshold; (2) to compare the physical behaviour of lacustrine sediments with their marine estuarine counterparts; and (3) to examine the effects of ploughing and chemical treatment of contaminated sediment on bed stability. Five deployments of Sea Carousel were carried out at the control site. Mean erosion thresholds for the three methods were: τc (0) = 0·5 (±0·06), 0·27 (±0·01) and 0·34 (±0·03) Pa respectively. Method 1 overpredicted bed strength as it was insensitive to effects in the surface 1–2 mm, and the fit of the failure envelope was also highly subjective. Method 2 exhibited a wide scatter in the data (low correlation coefficients), and definition of the baseline... [ABSTRACT FROM AUTHOR]

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a short-term laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10 000 and 50 000 ind m−2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10 000 to 50 000 ind m−2. The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions. [Copyright &y& Elsevier]

A portable erosion flume has been developed that is capable of estimating erosion threshold and erosion rate relationships for fine-grained specimens over the depth of a typical sample tube. This newly-designed apparatus is a recirculating flume capable of generating steady currents over the exposed section of the sample. In this paper, the erosion properties of two marine sediments have been determined and show a significant systematic variation with depth at centimetre scale that would have implications for the potential need for scour protection engineering. The tests showed that the critical erosion onset velocity doubled over the upper 200 mm of each sample, and the erosion rate fell by an order of magnitude. The increased erosion resistance with depth is consistent with the general trend of erodibility reducing with decreasing moisture content. Ignoring this depth effect when selecting design values of the erosion properties could lead to erroneous predictions of scour rate and extent around subsea structures, and unnecessary scour protection engineering costs. [ABSTRACT FROM AUTHOR]

The erosion of marine sediments, although difficult to predict, can lead to important implications in offshore engineering, sedimentology and coastal management. Continued research is, therefore, warranted to compile high-quality erosion data from which to develop models to better predict the erosion resistance of different types of marine sediments. In this paper, dimensional analysis is performed to express the threshold shear stress as a function of a selection of soil properties that are commonly linked to the erosion process of sediments. To identify the dominant dimensionless group, an experimental investigation on the erosion threshold was carried out using fine-grained sediments that were systematically prepared to ensure variations in (i) particle size distribution (i.e. fines content), (ii) bulk density, and (iii) hydraulic permeability. The samples included silica, carbonate and marine sediments, each of which are expected to have limited or no clay-mineral content. The measurements were analysed and compared with existing literature and predictive models. It was found that marine sediment samples with limited fines content showed good agreement with the empirical Shields curve, irrespective of particle size distribution, bulk density and permeability. In contrast, for finer marine sediment it was found that variations in these soil properties modify the threshold shear stress away from the Shields curve. Across each of these parameters only permeability appeared to independently correlate with the observed range of threshold measurements. Motivated by this finding, a model is introduced to predict the threshold shear stress as a function of permeability and the reference erosion rate that is used to define when the threshold is reached. The resulting expression is shown to quantitatively explain the experimental data and is found to also agree with existing data from the literature for quartz sediments with a wide range in fines content. An apparent advantage of the new model is that it is consistent with existing studies that identify variations in threshold shear stress due to changes in bulk soil parameters – including fines content and bulk density – since each of these parameters also affect permeability. • The effect of soil properties on the erosion behaviour of marine sediments. • Unique relationship between permeability and erosion threshold discovered. • Novel empirical relationship proposed to predict threshold shear stress for fine-grained sediments. • New model predicting threshold conditions agrees well with existing data from literature. [ABSTRACT FROM AUTHOR]

The sediment dynamics and the long-term evolution of intertidal environments depend on the erosion threshold, which is often increased by microbial biofilms. Intertidal biostabilization has been well studied in temperate and subtropical environments, but little is known for subarctic conditions with strong seasonal changes in vegetation cover and sea ice presence in winter because of the difficulty in studying the sediments under ice cover. The present project investigated the role of biofilms and their seasonal as well as spatial variations for sediment biostabilization in a subarctic intertidal area near Rimouski (Quebec, Canada). Grain size distribution reflected a seaward increasing trend in sand content and mean grain size. The poor sorting of sediments results from the contribution of sediment transport by ice rafting in addition to the hydrodynamic transport processes in this subarctic intertidal area. A Cohesive Strength Meter (CSM) was used to measure the erosion threshold (τ crit) across the entire intertidal area. Chlorophyll a (Chl a) and extra polymeric substances (EPS) were measured in the top 3 mm. The high marsh had a far higher τ crit than the low marsh, but there was much less variation between low marsh, mudflat and sandflat. τ crit was significantly correlated with Chl a , elevation, and organic matter content. EPS was not a good indicator for τ crit , except for the low marsh/high marsh difference. The results showed a seasonal cycle in intertidal bed erodibility. Lower intertidal areas had highest τ crit in autumn during the storm season, followed by lowest τ crit in winter under sea-ice cover, and no major difference between summer and spring. This study highlights the characteristics of cold climate intertidal areas, where low temperatures in autumn and land-fast ice cover during winter severely affect biofilms and intertidal vegetation, and ultimately their capacity to shield sediments from erosion. • Sediments were more stable in ice free periods than in winter. • Significant changes in erosion threshold were recorded, from highest values in autumn to lowest in winter. • Biofilms were affected by frequent inundations, poor drainage, and winter ice cover. • Biofilm stabilization was less effective in subarctic winter conditions compared with temperate environments. [ABSTRACT FROM AUTHOR]

• Flow-biofilm-sediment interactions in relation to biostabilization are reviewed. • This is supported by a joint workshop, testing feasibility of an integrated approach. • Development in optical tools and molecular approaches increased biofilm understanding. • Mechanical understanding of biostabilization have not been well understood. • Challenges include realism, scalability and methodological limitations. Biofilm activities and their interactions with physical, chemical and biological processes are of great importance for a variety of ecosystem functions, impacting hydrogeomorphology, water quality and aquatic ecosystem health. Effective management of water bodies requires advancing our understanding of how flow influences biofilm-bound sediment and ecosystem processes and vice-versa. However, research on this triangle of flow-biofilm-sediment is still at its infancy. In this Review, we summarize the current state of the art and methodological approaches in the flow-biofilm-sediment research with an emphasis on biostabilization and fine sediment dynamics mainly in the benthic zone of lotic and lentic environments. Example studies of this three-way interaction across a range of spatial scales from cell (nm – µm) to patch scale (mm – dm) are highlighted in view of the urgent need for interdisciplinary approaches. As a contribution to the review, we combine a literature survey with results of a pilot experiment that was conducted in the framework of a joint workshop to explore the feasibility of asking interdisciplinary questions. Further, within this workshop various observation and measuring approaches were tested and the quality of the achieved results was evaluated individually and in combination. Accordingly, the paper concludes by highlighting the following research challenges to be considered within the forthcoming years in the triangle of flow-biofilm-sediment: i) Establish a collaborative work among hydraulic and sedimentation engineers as well as ecologists to study mutual goals with appropriate methods. Perform realistic experimental studies to test hypotheses on flow-biofilm-sediment interactions as well as structural and mechanical characteristics of the bed. ii) Consider spatially varying characteristics of flow at the sediment-water interface. Utilize combinations of microsensors and non-intrusive optical methods, such as particle image velocimetry and laser scanner to elucidate the mechanism behind biofilm growth as well as mass and momentum flux exchanges between biofilm and water. Use molecular approaches (DNA, pigments, staining, microscopy) for sophisticated community analyses. Link varying flow regimes to microbial communities (and processes) and fine sediment properties to explore the role of key microbial players and functions in enhancing sediment stability (biostabilization). iii) Link laboratory-scale observations to larger scales relevant for management of water bodies. Conduct field experiments to better understand the complex effects of variable flow and sediment regimes on biostabilization. Employ scalable and informative observation techniques (e.g., hyperspectral imaging, particle tracking) that can support predictions on the functional aspects, such as metabolic activity, bed stability, nutrient fluxes under variable regimes of flow-biofilm-sediment. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Accelerating sea-level rise (SLR) threatens salt marshes globally, and thus understanding how these ecosystems respond to this stress is vital to increasing their resiliency. In order to maintain their surface elevation as sea level rises, salt marshes must add below ground biomass and accrete sediment. In many cases, biota can significantly affect marsh accretionary and erosional processes by stabilizing or helping to mobilize sediment, which in turn can profoundly affect the morphological evolution of the marsh. However, these effects are poorly understood. Bioturbation by dense populations of the marsh crab Sesarma reticulatum has been found to facilitate expansion of tidal creeks and creek bank erosion in multiple areas. The influence of S. reticulatum bioturbation on sediment erodibility has not yet been quantified, due to difficulties involved in measuring the processes in a field setting. In this study, we used a laboratory flume to examine the effects of burrowing by common marsh crab species S. reticulatum and Uca pugnax on sediment surface roughness and erodibility. Measurements and observations of surface elevation, flow velocities, and sediment movement indicate that burrowing and feeding by S. reticulatum , and to a lesser extent, Uca pugnax, increase surface roughness and decrease the threshold velocities and shear stresses required for sediment erosion. Erosion associated with observed burrowing is of a similar magnitude to mean annual marsh surface accumulation, and, consequently, this mechanism for sediment loss in heavily burrowed areas has the potential to produce large-scale morphological changes. Ecological engineering, Coastal setting, Salt marsh, Geomorphological aspects, Soil erosion. USA, Texas, Galveston. • We determine the impact of Sesarma reticulatum and Uca pugnax on erosion thresholds. • S. reticulatum increase surface roughness, more so than other crab species. • Crab burrowing decreases velocity and shear stress thresholds needed for erosion. • Extensive burrowing by S. reticulatum facilitates headward creek erosion and marsh landscape evolution. [ABSTRACT FROM AUTHOR]

The prediction of the erosion of mudflats is hampered by inaccurate estimates of the erodibility distribution of the sediment bed. To investigate how erodibility varies in space and what the vertical distribution over the sediment depth is, comprehensive observations of the sediment properties, hydrodynamics and bed-level changes were conducted on an intertidal flat in the Western Scheldt Estuary, the Netherlands. The erosion potential on a mudflat is determined by the critical shear stress for erosion (τ e), erosion rate coefficient (M) and local hydrodynamic conditions. A clear difference in hydrodynamic forcing was observed, leading to significant bed level variations at the low water line, where erosion often occurs during very shallow water condition, and a nearly constant bed level at the upper part. The erosion parameters τ e and M could be determined over a sediment bed of 12 cm at the low water line. The erosion coefficient M can be considered constant with depth, although there is a large spreading. A clear vertical variation of τ e was found: τ e increased significantly downward from 0.10 Pa at the sediment surface to 1.13 Pa at 12 cm below the surface. Additionally, there was a strong indication that the presence of diatoms enhanced τ e in the upper 2 mm of sediment by five times of the abiotic τ e (from 0.09 Pa to 0.46 Pa). These findings lead to the following improvement for predicting morphological changes of tidal mudflats: (1) very shallow conditions should be better simulated, (2) the vertical distribution of τ e should be considered. Otherwise, erosion rates can be overestimated, especially during extreme events, because exposure of the deeper well-consolidated layer likely occurs; and (3) an appropriate description of the effect of diatoms should be considered as part of the bottom boundary condition. • The erodibility of the bed at the elevation just below the low tide is more variable. • Bed level and bed shear stress measurements enable assessing the vertical τ e profile. • Diatoms increase τ e of surficial sediment by 5 times of the abiotic value. • Erosion coefficient M is vertically constant in the erosion model E = M (τ cw − τ e). [ABSTRACT FROM AUTHOR]