Fine Suspended Sediment Research Articles

Environmental Impact of River Bank Erosion on Stream Morphology: A Study of the Mini Piti Stream, Obio-Akpor, Nigeria

This study was designed to assess the impact of river bank erosion on the environment and the channel morphology for the Mini-Piti stream in Obio-Akpor local government area of Rivers State, Nigeria. This study determined the depth variations from width measurements due to bank erosion activities and the impact of indiscriminate waste dumps on the river channel. Physical water quality parameters of temperature, turbidity, transparency and TSS were determined using standard methods. The stream was divided into three stations; upstream 2.5km, midstream 2.5km, and downstream with intervals of 1km measured to differentiate each station as 5 samples were obtained per station at intervals of 0.5km apart. The flow velocity and discharge were estimated as the average depth recorded upstream being DS 0.9m, WS 1.4m, Midstream DS 1.3m, WS 1.7m, and Downstream = DS 1.5m WS 1.7m. The maximum width of 11m and minimum of 0.1m were recorded though natives confirmed that the stream is closing up unless maintenance dredging to expand the depth and width, the loss of this channel cannot exceed 20 years. The field observation and the impacts of precipitation and storm water from the built area of midstream were very intensive whose action increased the water level above its channel capacity and eroded the bank. From t-test analysis, there were significant seasonal variations in the depth for the measured widths for the upstream T (8) =2.1, p= 0.036 and the midstream, T (8) =2.36, p=0.023. Similarly using the one-ANOVA, a significant variation in the depths of five locations during the dry season downstream F (4, 60) = 2.79, p= .03 was recorded. The Tukey’s HSD post hoc test showed the locations have no significantly high depths for the studied widths sizes (p<0.01 and p<0.05). A one-way ANOVA revealed a significant variation in the depths of five locations during the wet season upstream F (4, 60) = 5.966, p= .000. The Tukey’s HSD post hoc test showed the locations have significantly high depths for the studied widths sizes (p<0.01 and p<0.05) at locations L2:L4 at Q=5.53 (p=0.00216, L2:L5 at Q= 4.72 (p=0.1214), L3:L4 at Q=4.90 (p=0.00841) and L3:L5 at Q=4.10 (p=0.0403). The studied physical parameters were all above WHO permissible standards and there were no significant variations between dry and wet seasons (38% turbidity midstream, TSS-33%, 31% and 30.6% variation upstream, midstream and downstream respectively) as well as spatial changes. The collapsed sediments and waste dumped at the bank of the stream were transported downstream due to reduced velocity and coarser sediments trapped at the stream bed causing shallow depth while the suspended fine sediment on the water column reduced the clarity of the stream resulting in adverse flooding over 20 years now. The research revealed that bank erosion has significant impact on water quality due to poor transparency and geometry of stream by the alterations of the channel depth and width, flow rate, scouring and deposition of sediments in the stream. The need for regulatory agencies to control anthropogenic inputs and enhance bank erosion prevention programmes cannot be over-emphasized. The monitoring of indiscriminate urbanization programmes to limit excess scouring activities by National Inland Waterways Authority (NIWA) is very essential to reduce excessive sand mining.

Assessment of bed clogging in surface water reservoirs under different hydrologic conditions

Physical clogging of porous media plays an important role in controlling their hydraulic conductivity. This study investigates how uniformly graded medium-to-coarse sand in a reservoir’s bed performs in response to the inflow of turbid water with fine suspended sediments. Different particle sizes, heads, and concentrations were simulated to cover a range of hydrologic conditions. A 1D sand column was used with three bed material sizes and three suspended particle sizes. Seven particle size ratios were tested under a constant head. Four heads and three suspended sediment concentrations are also evaluated. It was concluded that the decline in the relative hydraulic conductivity of the bed material is directly proportional to the ratio of sediments-to-bed particle sizes. Increasing the concentration of suspended sediments accelerated the rate of clogging, while bigger heads increased the depth of clogging. The study highlights how physical clogging affects the water balance in surface reservoirs and threatens the safety of dams.

Composition and Flux of Dissolved and Particulate Carbon and Nitrogen in the Lower Tocantins River

AbstractThe Tocantins River contributes ∼5% of the total flux of water to the Amazon River plume in the Atlantic Ocean. Here, we evaluate monthly variability in the composition and abundance of carbon, nitrogen, and suspended sediment in the lower reaches of the Tocantins River from 2014 to 2016. Dissolved organic carbon concentrations generally increased during periods of high discharge and are ∼1.5 times lower than average concentrations at the mouth of the Amazon River. Dissolved inorganic carbon similarly increased during periods of high discharge. Total dissolved nitrogen and individual nitrogen species followed a similar temporal pattern, increasing during high water. predominated the dissolved inorganic nitrogen pool, followed by , and , characteristic of environments with a relatively low anthropogenic impact. Dissolved fractions represented 92% of the total carbon exported and 78% of the total nitrogen exported. The suspended particulate sediment flux was 2.72 × 106 t yr−1, with fine suspended sediment dominating (71.3%). Concentrations of carbon relative to nitrogen indicate a primarily terrigenous source of organic matter and CO2 derived from in situ respiration of this material during the rainy season and a primarily algal/bacterial source of organic matter during the dry season. Considering past estimates of dissolved carbon and nitrogen fluxes from the Amazon River to the Atlantic Ocean, the Tocantins River contributes 3% and 3.7% to total fluxes to the Amazon River plume region, respectively. While this contribution is relatively small, it may be influenced by future changes to the basin's land use and hydrology.

Sedimentation of cohesive sediments at the subtidal flat affected by wind wave in high turbidity estuary

Sedimentation is an important mechanism to mitigate the shrinking of tidal flat and to restore its ecological function by means of sand or mud nourishment. To explore the sedimentation of cohesive sediments, a seabed tripod observation system was deployed at the subtidal region of the Hengsha Shoal adjacent to the turbidity maximum zone of the Yangtze Estuary for 11 days. The results showed that the fine sediment with the median grain size around 8 mm occupied the whole water column. The seabed was in relative equilibrium state with the fluctuation of bed level smaller than 16 mm during the moderate wind condition while the seabed experienced a rapid erosion of 38mm and a successive intensive accretion of 68mm during the process of wind wave and swell. The bottom hydrodynamic at 0.3mab during the bed accretion was stronger than that during the bed erosion. The deposition process of cohesive sediments can be better described by the simultaneous deposition paradigm than that by the exclusive deposition paradigm according to the direct data-model comparison of the bed level changes, especially during the impact of fluid mud. Three possible reasons for the better performance of the simultaneous deposition paradigm were proposed. The first possibility is that the fine suspended sediments do maintain a continuous contact with the sediment bed since the direct bed level changes during our observation period has been well reproduced by the simultaneous deposition paradigm. The second possibility is the SSC-induced turbulence damping which facilitates the fine sediment settling in the form of cohesive sediment flocs, indicating the settling of sediments can’t be judged by the critical shear stress for deposition just based on the single particle grain size. The last possibility is the fluid mud-induced overestimated bed shear stress by using turbulent velocity fluctuation above the fluid mud-water interface, which produces excess sediment erosion waiting to be compensated by the simultaneous deposition paradigm. For practical modeling purposes, modeling under the simultaneous deposition paradigm can give satisfactory results for the sedimentation of cohesive sediment especially during the impact of wave or swell.

Numerical modeling of microplastic interaction with fine sediment under estuarine conditions

Microplastic (MP) pollution is an important challenge for human life which has consequently affected the natural system of other organisms. Mismanagement and also careless handling of plastics in daily life has led to an accelerating contamination of air, water and soil compartments with MP. Under estuarine conditions, interactions with suspended particulate matter (SPM) like fine sediment in the water column play an important role on the fate of MP. Further studies to better understand the corresponding transport and accumulation mechanisms are required. This paper aims at providing a new modeling approach improving the MP settling velocity formulation based on higher suspended fine sediment concentrations, as i.e. existent in estuarine turbidity zones (ETZ). The capability of the suggested approach is examined through the modeling of released MP transport in water and their interactions with fine sediment (cohesive sediment/fluid mud). The model results suggest higher concentrations of MP in ETZ, both in the water column as well as the bed sediment, which is also supported by measurements. The key process in the modeling approach is the integration of small MP particles into estuarine fine sediment aggregates. This is realized by means of a threshold sediment concentration, above which the effective MP settling velocity increasingly approaches that of the sediment aggregates. The model results are in good agreement with measured MP mass concentrations. Moreover, the model results also show that lighter small MP particles can easier escape the ETZ towards the open sea.

Turbidity maxima in estuarine networks: Dependence on fluvial sediment input and local deepening/narrowing with an exploratory model

An estuarine turbidity maximum (ETM) results from various subtidal sediment transport mechanisms related to, e.g., river, tides, and density gradients, which have been extensively analysed in single-channel estuaries. However, ETMs have also been found in estuaries composed of multiple interconnected tidal channels, where the water and suspended fine sediments are exchanged at the junctions with possible occurrence of sediment overspill. The overall aim of this study is to understand the processes that determine the ETM dynamics in such channel networks. Specifically, focusing on the ETMs formation due to sediment transport by river flow and density-driven flow, the dependence of ETM locations in an idealised three-channel network on fluvial sediment input and the local deepening and narrowing of a seaward channel is investigated. It is found that the ETM dynamics in channels of a network is coupled, and hence, changes in one channel affect the ETM pattern in all channels. Sensitivity results show that, keeping river discharge fixed, a larger fluvial sediment input leads to the upstream shift of ETMs and an increase in the overall sediment concentration. Both deepening or narrowing of a seaward channel may influence the ETMs in the entire network. Furthermore, the effect of either deepening or narrowing of a seaward channel on the ETM locations in the network depends on the system geometry and the dominant hydrodynamic conditions. Therefore, the response of the ETM location to local geometric changes is explained by analysing the dominant sediment transport mechanisms. In addition to the convergence of sediment transport mechanisms in single-estuarine channels, ETM dynamics in networks is found to be strongly affected by net exchange of sediment between the branches of a network. We find that considering the sensitivity of net sediment transport to geometric changes is needed to understand the changing ETM dynamics observed in a real estuarine network.

Sustainable Development of Coastal Areas: Port Expansion with Small Impacts on Estuarine Hydrodynamics and Sediment Transport Pattern

With the expansion of global trade and the growing traffic of increasingly larger ships to meet this demand, the need to expand port infrastructure appears as the main alternative. In this way, dredging operations for the maintenance and deepening of navigation channels, as well as the expansion of evolution basins and berthing wharf areas become fundamental, generating large amounts of material removed from the bottom. Aiming at port expansion based on a sustainable development and minimization of the environmental impacts generated by these operations, it is necessary to seek alternatives for the destination of large volumes of dredged material. A sustainable alternative is to dispose these volumes on land in order to expand coastal areas (such as fattening beaches) and mooring pier areas. The present work presents a numerical modeling case study using the TELEMAC-3D model coupled to the suspended sediment module SEDI-3D. As an alternative, an existing island (Ilha do Terrapleno) will receive material dredged from the access channel to the Port of Rio Grande (southern Brazil) and will have its mooring area expanded. The study evaluated the impact that this change in the island’s configuration would generate on the hydrodynamics and on the deposition patterns of fine suspended sediments in its surroundings and in the adjacent navigation channel. Results indicated that in the navigation channel adjacent to Ilha do Terrapleno, the new configuration promoted changes in the hydrodynamics with a decrease in the mean current velocity around the island and throughout the water column, mainly in the north and central portions of the channel. This difference decreased towards the southern portion of the channel. This change in local hydrodynamics promoted small changes in the suspended sediment deposition patterns and in the bottom evolution. Thus, the sustainable expansion of the Port of Rio Grande operational capacity considering the creation of 3600 m of berthing wharf areas and minimum environmental impact proved viable when considering the Ilha do Terrapleno proposed configuration. Furthermore, this idea offers the possibility to dispose in land 722,910 m3 of dredged material, a sustainable alternative to the Port of Rio Grande development, and an inspiration for the sustainable development of other ports worldwide.

Field testing a High-Frequency acoustic attenuation system for measuring fine suspended sediments and algal movements

Acoustic measurements of suspended sediment have the potential to allow remote, autonomous monitoring of sediment movements at much higher temporal resolution than traditional manual sampling methods. Although suspended sands present a challenging measurement problem due to their logarithmic distribution with depth, fine clay sediments are distributed evenly throughout a stream cross section, making them amenable to point measurements. In order to improve measurement capabilities for fine sediments in stream channels, The National Center for Physical Acoustics at The University of Mississippi has developed a remote, autonomous acoustic system to monitor fine sediments transported in streams. The system was tested on the Middle Rio Grande near San Acacia, New Mexico, and in Goodwin Creek in Panola County, Mississippi. The acoustic instruments were compared to sediment concentrations from physical samples in both deployments. Diurnal patterns were found in the acoustic signals from the Middle Rio Grande, and a follow up experiment at The University of Mississippi Biological Field Station was used to investigate the potential effects of algal biomass on acoustic attenuation measurements. The results showed diurnal patterns in attenuation were associated with patterns in light, temperature, and dissolved oxygen. These results combined with information from the literature indicate diel movement of algal colonies in the water column of some water bodies may interfere with high-frequency acoustic measurements in natural environments and that acoustic methods have the potential to allow ecological researchers to evaluate mass algal movements in the field. Results from Goodwin Creek demonstrate that the acoustic system is able to provide measurements of sediment concentration with high temporal resolution that track well with expected sediment transport patterns in response to discharge hydrographs.

Wildfire impacts on the persistent suspended sediment dynamics of the Ventura River, California

RegionVentura River watershed, California, USA FocusAfter wildfire, small mountainous watersheds can produce dramatic increases in fluvial sediment transport, but these changes are often evaluated under the assumption of stationary pre-fire sediment dynamics. The objectives of this study were to investigate temporal dependence in the fine (silt and clay) suspended sediment dynamics of the Ventura River before and after 48.5 % of the 194 km2 undammed portion of the watershed burned in the 2017 Thomas Fire. New hydrological insightsFine sediment concentrations in the Ventura River decreased between 1971 and 2008. In 2018 and 2019, fine suspended sediment concentrations were 14.2 and 5.1 times higher than predicted based on the entire monitoring record, but 32.4 and 9.5 times higher than the most recent period of persistently low concentrations. The Thomas Fire resulted in a 200x increase in fine sediment flux in 2018 relative to pre-fire conditions. However, fine suspended sediment flux during both post-fire years, 2018: 0.073 (0.050-0.105) MT and 2019: 0.393 (0.284-0.447) MT, was comparable to the long term average 0.177 (0.140-0.230) MT because of relatively little post-fire precipitation. These findings highlight the importance of considering preexisting time-dependent behavior when characterizing the fire impacts on suspended sediment dynamics, and the critical role that storm event magnitudes play in determining the sediment yield of small mountainous watersheds after wildfire.

Stream Suspended Mud as an Indicator of Post-Mining Landform Stability in Tropical Northern Australia

Mining can cause environmental disturbances and thus mined lands must be managed properly to avoid detrimental impacts in the future. They should be rehabilitated in such a way that post mining landforms behave similarly as the surrounding stable undisturbed areas. A challenge for government regulators and mine operators is setting closure criteria for assessment of the stability of the elevated post-mining landforms. Stability of a landform is often measured by the number and incision depth of gullies. This can assess mass stability and bulk movement of coarse material. However, there is a need for a more sensitive approach to assess catchment disturbances using the concept of waves of fine suspended sediment and thus determine the dynamics of recovery of a post mining landform. A more environmentally meaningful approach would be to assess the fine suspended sediment (FSS, silt + clay (0.45 µm < diameter < 63 µm)) leaving the system and entering downstream waterways. We propose assessing stability through relationships between rainfall event loads of FSS and event discharge (Q) in receiving streams. This study used an innovative approach where, instead of using instantaneous FSS concentration, it used total FSS load in waves of sediment driven through the system by rainfall runoff events. High resolution stream monitoring data from 2004 to 2015 in Gulungul and Magela Creeks, Northern Territory, Australia, were used to develop a relationship between sediment wave and event discharge, ∑FSS α f(Q). These creeks are adjacent to and receive runoff from Ranger Mine. In 2008, a 10 ha elevated waste rock landform was constructed and instrumented in the Gulungul Creek catchment. The earthworks required to build the landform created a considerable disturbance in the catchment, making a large volume of disturbed soil and substrate material available for erosion. Between 2008 and 2010, in the first two wet seasons immediately after construction, the downstream monitoring site on Gulungul Creek showed elevated FSS wave loads relative to discharge, compared with the upstream site. From 2010 onwards, the FSS loads relative to Q were no longer elevated. This was due to the establishment of vegetation on the site and loose fine sediment being trapped by vegetation. Large scale disturbance associated with mining and rehabilitation of elevated landforms causes elevated FSS loads in receiving streams. The predicted FSS loads for the stream as per the relationships between FSS and event discharge may not show a 1:1 relation with the observed loads for respective gauging stations. When downstream monitoring shows that FSS wave loads relative to rainfall runoff event discharge reduce back to pre-construction catchment levels, it will indicate that the landform is approaching equilibrium. This approach to assess landform stability will increase the sensitivity of assessing post-mining landform recovery and assist rehabilitation engineers to heal the land and benefit owners of the land to whom it is bestowed after rehabilitation.

Do legacy effects of deposited fine sediment influence the ecological response of drifting invertebrates to a fine sediment pulse?

The deposition of excess fine sediment and clogging of benthic substrates is recognised as a global threat to ecosystem functioning and community dynamics. Legacy effects of previous sedimentation create a habitat template on which subsequent ecological responses occur, and therefore, may have a long-lasting influence on community structure. Our experimental study examined the effects of streambed colmation (representing a legacy effect of fine sediment deposition) and a suspended fine sediment pulse on macroinvertebrate drift and community dynamics. We used 12 outdoor stream mesocosms that were split into two sections of 6.2 m in length (24 mesocosm sections in total). Each mesocosm section contained a coarse bed substrate with clear bed interstices or a fine bed substrate representing a colmated streambed. After 69 days, a fine sediment pulse with three differing fine sediment treatments was applied to the stream mesocosms. Added fine sediment influenced macroinvertebrate movements by lowering benthic density and taxonomic richness and increasing drift density, taxonomic richness, and altering drift assemblages. Our study found the highest dose of sediment addition (an estimated suspended sediment concentration of 1112 mg l−1) caused significant differences in benthic and drift community metrics and drift assemblages compared with the control treatment (30 l of water, no added sediment). Our results indicate a rapid response in drifting macroinvertebrates after stressor application, where ecological impairment varies with the concentration of suspended sediment. Contrary to expectations, bed substrate characteristics had no effect on macroinvertebrate behavioural responses to the fine sediment pulse.

Observations on Floc Settling Velocities in the Tamar Estuary, United Kingdom

Measurements in the mesotidal Tamar estuary (UK) reported previously indicate a dependence of the floc settling velocity on the shear rate and the suspended fine sediment concentration or volume fraction. Typical time-independent analytic formulas for the floc settling velocity tend to follow the mean trend but fail to provide an explanation for the characteristic data spread. Moreover, they assume floc diameter to be single-valued such as the mean or the median. Given this constraint, an examination of tide-induced trends in the Tamar settling velocities is attempted by simple time-dependent modeling of aggregation, i.e., the dynamics of floc growth and breakup. The effect of aggregation on the settling velocity and diameter is simulated over a representative one-half tidal cycle. Starting at low water (LW) slack at the onset of the sediment erosional phase during the first quarter tide, as the shear rate and volume fraction increase, floc growth is shown to increase the settling velocity that peaks at a shear rate in the range of 15−30 s−1. At higher shear rates, as floc breakup supersedes the effect of sediment concentration in promoting growth, the settling velocity gradually decreases until the shear rate reaches its maximum value on the order of 102 s−1 at the strength of flow. During the following depositional phase in the second quarter tide, as the shear rate decreases the settling velocity increases continually until high water (HW) slack when it achieves its overall maximum value as the shear rate approaches zero. Thus, the loci of settling velocity versus shear rate differ between the quarter tides and result in shear rate versus settling velocity hysteresis. Moreover, it is shown that in the Tamar, tidal variation prevents the settling velocity and diameter from always achieving the equilibrium assumed in analytic formulas. Thus, aggregation modeling serves as a useful guide for resolving temporal trends in floc properties.

Multiple framings of uncertainty shape adoption of reference states during reform of water policy

There is growing recognition that multiple forms of uncertainty influence policy reform, but we need case studies illustrating how uncertainties influence reform, and strategies used to manage uncertainties in practice. We present a case study of how science was generated and incorporated into national water policy. We estimated turbidity (suspended fine sediment) reference states throughout the river network of New Zealand. A key decision-problem was: how region-specific should reference states be? We developed classifications of the national river network that grouped rivers with similar turbidity regimes into classes at four spatial resolutions. Choosing a classification was complicated by ontological uncertainties. Scientific actors framed these ontological uncertainties in different ways, introducing framing uncertainties (‘ambiguities’) into the decision-making process. Well-established statistical techniques were employed to objectively identify the most parsimonious classification, but those techniques had little impact on decision-making. Reframing ontological uncertainties in terms of risks to management objectives was required to reconcile divergent frames, and showed that the classification with the highest resolution was the most credible option. Nonetheless, local governments highlighted that the most credible classification was not the most relevant or legitimate one in light of implementation constraints. The final decision was made against a backdrop of ambiguities, was partially subjective, and balanced trade-offs among credibility, relevance and legitimacy. Looking ahead, ambiguities could be reduced by framing decision problems in terms of risks to management objectives. A barrier to doing this is the short duration of policy windows, so translational science must take place when such windows are shut.

Exploring Macroinvertebrates Ecological Preferences and Trait-Based Indicators of Suspended Fine Sediment Effects in the Tsitsa River and Its Tributaries, Eastern Cape, South Africa

The taxonomy-based response pattern of macroinvertebrates to sediment stress is well established, with tolerant taxa increasing in impacted conditions, while sensitive taxa decrease along a deteriorating water quality gradient. However, the distribution patterns of traits in response to environmental stress gradient, including suspended sediments, remain unclear, particularly in Africa, where trait-based studies are under-explored. We examined the distribution patterns of macroinvertebrate traits along a suspended sediment stress gradient and identified tolerant and sensitive traits for suspended sediment stress. We sampled macroinvertebrates and environmental variables seasonally in winter, spring, summer and autumn of 2016 to 2018 in eight selected sites in the Tsitsa River and its tributaries. We selected 12 traits and ecological preferences, resolved them into 47 trait attributes, and analysed them using the RLQ and fourth-corner analyses. Our results revealed that macroinvertebrate traits and ecological preferences were differentially influenced by fine suspended sediments in the Tsitsa River and its tributaries. Traits such as a preference for CPOM, collector-filtering, and a high sensitivity to oxygen depletion, were deemed sensitive to suspended sediments stress, exhibiting positive associations with the control sites, and negatively associated with any of the environmental parameters (sediment grain sizes, turbidity, TSS and EC). Tolerant indicator traits included a high tolerance of oxygen depletion, skating and a preference for FPOM. The fourth-corner analysis results indicated that suspended fine sediment grain sizes, (including coarse sand, fine silt and clay) were the most important variables influencing macroinvertebrate trait distribution patterns during the dry season, while gravel, mud and medium sand were more important during the wet season. Overall, our study provided critical insights towards trait-based responses of macroinvertebrates communities to suspended sediment stress, key information that could stimulate the development of macroinvertebrate trait-based biomonitoring tools for the assessment of suspended sediment stress in the Afrotropical region.

Intensive landscape-scale remediation improves water quality of an alluvial gully located in a Great Barrier Reef catchment

Abstract. Gully erosion can be a major disruptor to global fluvial sediment budgets. Gully erosion in the catchments of the Great Barrier Reef is attributed to ∼40 % of fine suspended sediment pollution to the freshwater and marine ecosystems downstream. Mitigating this source of erosion will have a lasting positive impact on the water quality of connected rivers and the receiving marine environment. Here we conduct a preliminary evaluation of the ability of intensive landscape-scale gully remediation to reduce suspended sediment and associated nutrient export from a catchment draining to the Great Barrier Reef. The gully remediation method was a first attempt, in the region, at investing a high level of financial (total cost of remediation AUD ∼90 000) and logistical effort (e.g. intensive earthworks, including the establishment of an on-site quarry) to develop long-lasting erosion mitigation measures (i.e. regraded, compacted, and battered gully walls, rock armouring of banks and channel, and installation of rock check dams). A novel suspended sediment monitoring network, comprised of a suite of new and established automated monitoring methods capable of operating in remote environments, was used to evaluate the water quality of a remediated gully, a control gully, and their respective catchments. The recently developed pumped active suspended sediment (PASS) sampler optimised to sample ephemeral water flows was deployed in gully outlets and catchment runoff flow paths. This study demonstrates how the combination of low- and high-cost water quality monitoring techniques can be deployed in a configuration that ensures sample collection redundancy and complementary data collection between methods. Monitoring was conducted during two consecutive wet seasons and, thus, can only provide preliminary information. Monitoring over longer timescales (i.e. 5–10 years) will need to be carried out in order to validate the findings discussed herein. Samples collected from the remediated gully had significantly lower suspended sediment concentrations compared to the control gully, providing preliminary evidence that the remediation works were successful in stabilising erosion within the gully. Dissolved and particulate nutrient concentrations were also significantly lower in the remediated gully samples, consistent with the decreased suspended sediment concentrations. The novel combination of suspended sediment measurements from both the gully channels and overland flows in the surrounding gully catchments suggests that sediment and nutrients at the remediated site are likely sourced from erosion processes occurring within the catchment of the gully (at relatively low concentrations). In contrast, the primary source of suspended sediment and associated nutrients at the control gully was erosion from within the gully itself. This study demonstrates the potential of landscape-scale remediation as an effective mitigation action for reducing suspended sediment and nutrient export from alluvial gullies. It also provides a useful case study for the monitoring effort required to appropriately assess the effectiveness of this type of erosion control.

Estimation of policy-relevant reference conditions throughout national river networks

A method for objectively estimating reference states for suspended fine sediment (turbidity) is presented. To be fit for water policy development and implementation the method had to satisfy four requirements: (1) the method must not be dependent on data from minimally-disturbed reference sites; (2) the method must facilitate characterization of reference states throughout heterogeneous river networks, given patchy data; (3) the classification of reference states must be relevant and legitimate to end-users; (4) the method should provide several classifications of reference states at different spatial resolutions allowing selection of the resolution yielding the most parsimonious classification of reference states throughout the network. Implementing the method involves two stages: (1) Development of a river classification based on sediment supply and retention regimes (defining ‘turbidity classes’) at multiple spatial resolutions. (2) At each resolution, for each turbidity class, estimation of a reference state based on relationships between turbidity and anthropogenic stressors, then objective selection of the resolution yielding the most parsimonious classification of reference states throughout the network. Implementing the method requires a river network GIS and turbidity data within classes, preferably from monitoring sites spanning the domains of the anthropogenic stressor variables used for model-based estimation of reference states.•A method is presented for estimating reference states for suspended fine sediment (turbidity) throughout spatially heterogeneous river networks.•Development of the method was guided by the requirements of policy analysts during reform of water policy in New Zealand.•The method presented was used to develop fine sediment regulatory thresholds of national water policy.

Experimental study on the mechanisms of flow and sediment transport in a vegetated channel

Laboratory experiments were conducted in a flume containing three types of artificial flexible submerged and emergent vegetation. Detailed measurements of velocity and sediment concentration in the channel were taken. The results showed that submerged and emergent vegetation generates a much greater resistance to flow and significantly alters the vertical distributions of velocity, especially in vegetated and downstream regions. In comparison with the non-vegetated case, the turbulence kinetic energy and Reynolds stresses in the vegetated and downstream regions were much higher, indicating strong flow turbulence and momentum exchange in these areas. The high turbulence also resulted in a nearly constant fine suspended sediment concentration in the water column for all cases, while the increased resistance caused the coarser suspended sediment concentration to decrease. In addition, sediment retention by small-height vegetation was found to be insignificant while, for a canopy of large height, significant sediment deposition was found at the upstream region of the vegetated region.

Impacts of Suspended Clay Particle Deposition on Sand‐Bed Morphodynamics

AbstractFine particles (0.1–100 microns) are ubiquitous within the water column. Observations on the interactions between suspended fine particles and sediment beds remain limited, reducing our ability to understand the interactions and feedbacks between fine particles, morphodynamics, and hyporheic flow. We performed laboratory experiments to explore changes in bedform morphodynamics and hyporheic flow following the progressive addition of kaolinite clay to the water column above a mobile sand bed. We characterized these interactions by taking high‐frequency time series measurements of bed topography and freestream clay concentration combined with solute injections and bed sediment cores to characterize subsurface properties. Deposition of initially suspended clay resulted in a decrease of bedform height, celerity, and sediment flux by 14%, 22%, and 29% when 1000 g was accumulated within the bed (equal to clay/sand mass ratio of 0.4% in the bed). The hyporheic exchange flux decreased by almost a factor of 2 for all clay additions, regardless of the amount of clay eventually deposited in the bed. Post experiment sediment cores showed clay accumulation within and below the mobile layer of the bedforms, with the peak concentration occurring at the most frequent bedform scour depth. These results demonstrate the tight coupling between bed sediment morphodynamics, fine particle (clay) deposition, and hyporheic exchange. Suspended and bed load transport rates are diminished by the transfer of suspended load to the sediment via hyporheic exchange. This coupling should be considered when estimating sediment transport rates.

Suspended sediment monitoring in alluvial gullies: A laboratory and field evaluation of available measurement techniques

AbstractGully erosion is a significant source of fine suspended sediment (<63 μm) and associated nutrient pollution to freshwater and marine waterways. Researchers, government agencies, and monitoring groups are currently using monitoring methods designed for streams and rivers (e.g., autosamplers, rising stage samplers, and turbidity loggers) to evaluate suspended sediment in gullies. This is potentially problematic because gullies have several hydrological features and monitoring operational challenges that differ to those of continually flowing streams and rivers (e.g., short and intense flows, high suspended sediment concentrations, and rapid scouring and aggradation). Here we present a laboratory and field‐based assessment of the performance of common suspended sediment monitoring techniques applied to gullies. We also evaluate a recently‐described method; the pumped active suspended sediment (PASS) sampler, which has been modified for monitoring suspended sediment in gully systems. Discrete autosampling provided data at high temporal resolution, however, it had poor collection efficiency (25 ± 10%) of coarser sediment particles (i.e., sand). Rising stage sampling, while robust and cost‐effective, suffered from large amounts of condensation under field conditions (25–35% of sampler volume), due to harsh climatic conditions creating large diurnal temperature differences at the field site, thereby diluting sample concentrations and introducing additional measurement uncertainty. The turbidity logger exhibited a highly variable response when calibrated at each site with physically collected suspended sediment samples (R2 = 0.17–0.83), highlighting that this approach should be used with caution. The modified PASS sampler proved to be a reliable and representative measurement method for gully sediment water quality, however, the time‐integrated nature of the method limits its temporal resolution compared to the other monitoring methods. We recommend monitoring suspended sediment in alluvial gully systems using a combination of complementary techniques (e.g., PASS and RS samplers) to account for the limitations associated with individual methods.

Rainfall control on Amazon sediment flux: synthesis from 20 years of monitoring

The biodiversity and productivity of the Amazon floodplain depend on nutrients and organic matter transported with suspended sediments. Nevertheless, there are still fundamental unknowns about how hydrological and rainfall variability influence sediment flux in the Amazon River. To address this gap, we analyzed 3069 sediment samples collected every 10 days during 1995–2014 at five gauging stations located in the main rivers. We have two distinct fractions of suspended sediments, fine (clay and silt) and coarse (sand), which followed contrasting seasonal and long-term patterns. By taking these dynamics into account, it was estimated, for first time, in the Amazon plain, that the suspended sediment flux separately measured approximately 60% fine and 40% coarse sediment. We find that the fine suspended sediments flux is linked to rainfall and higher coarse suspended sediment flux is related with discharge. Additionally this work presents the time lag between rainfall and discharge, which is related to the upstream area of the gauging. This result is an important contribution to knowledge of biological and geomorphological issues in Amazon basin.