Quantifying Suspended Sediment using Acoustic Doppler Current Profiler in Tidung Island Seawaters

ABSTRACTClimate change will impact the water and sediment conveyance into periglacial reservoirs. It is therefore important to understand and forecast future reservoir sedimentation processes with regard to climate change. In the present project, particle size distribution (PSD) and suspended sediment concentrations (SSC) were measured in three reservoirs in the Swiss Alps whose catchment areas are covered by glaciers by at least 40%. The threefold combination of water sample analysis, laser in-situ scattering and transmissometry (LISST) and acoustic Doppler current profiler (ADCP) was applied and the results were compared to each other. The combination of the three measuring techniques was proven suitable for assessing PSD and SSC in periglacial reservoirs. Water sample analysis and LISST records showed that most of the suspended sediments in the reservoir are in the range of clay and silt. SSC was relatively low in the order of 100 mg/l. An increase in both PSD (e.g. median diameter d50) and SSC with increasing reservoir depth could be observed in deep reservoirs. Flow velocities and Signal-to-Noise ratios (SNR) were measured with ADCP. SNR values allowed to study the mixing of inflowing river water and the evolution and decay of turbidity currents. There is evidence of dominant homopycnal flows, whereas stratified flow was restricted to the regions close to the inflow. Flocculation, influence of mica, and organic content could be neglected. This paper presents detailed information about PSD and SSC gained with water sample analysis and LISST measurements. Furthermore, flow field measurements and tracking of mixing by means of ADCP will be illustrated. Finally, application experiences and limitations will be discussed.

The application of Acoustic Doppler Current Profiler (ADCP) can be used not only for measuring ocean currents, but also for quantifying suspended sediment concentrations (SSC) from acoustic backscatter strength based on sonar principle. Suspended sediment has long been recognized as the largest sources of sea contaminant and must be considered as one of the important parameters in water quality of seawater. This research was to determine SSC from measured acoustic backscattered intensity of static and mobile ADCP. In this study, vertically mounted 400 kHz and 750 kHz static ADCP were deployed in Lembeh Strait, North Sulawesi. A mobile ADCP 307.2 kHz was also mounted on the boat and moved to the predefined cross-section, accordingly. The linear regression analysis of echo intensity measured by ADCP and by direct measurement methods showed that ADCP is a reliable method to measure SSC with correlation coefficient (r) 0.92. Higher SSC was observed in low water compared to that in high water and near port area compared to those in observed areas. All of this analysis showed that the combination of static and mobile ADCP methods produces reasonably good spatial and temporal data of SSC.

Estimating the distribution of suspended sediment concentration in submerged vegetation flow based on gravitational theory

Two-dimensional depth-averaged model simulation of suspended sediment concentration distribution in a groyne field

Dynamic release process of pollutants during suspended sediment transport in aquatic system

This thesis presents the results of new and original research on suspended sediment (SS) and nutrient concentrations and loads on the Rivers Bandon and Owenabue in the south of Ireland. It addresses the knowledge gap in SS and nutrient concentration data and in particular SS load/yield estimation methods. A comprehensive database of SS and nutrient concentrations has been established. An investigation into the relationship between water discharge and sediment and nutrient transport is presented and new original relationships between turbidity and suspended sediment concentration (SSC) and nutrients are developed. Turbidity-based suspended sediment loads (SSL) are estimated from the continuous turbidity record using the developed relationships with annual loads of 9.7 tonnes per square kilometre per year (t km‘‘ year’') and 18 t km'^ year ' estimated for the Rivers Bandon and Owenabue, respectively. Rating curves are developed to assess their accuracy compared to the turbidity-based SSL for the first time in an Irish context. Generally, rising stage and winter rating curves explain the variation in sediment concentration well (r‘ > 0.4), but winter and falling stage curves are not a good fit. Rating curves are shown to provide acceptable estimates of SSL at the large (annual) scale on both rivers, without the need for stage or seasonal rating curve separation. The variation in SSC between and within storm events is found to be large and over 73% of the total annual load is transported when flows are above the 10% exceedance flow rate on both rivers, indicating that stonn-based events are critical to load delivery. The sediment associated nutrient transport in the two rivers is also examined, with brief surges in particulate forms of nitrogen and phosphorus observed at the onset of the events analysed. These are accompanied by decreases in the dissolved forms of the monitored nutrients. Turbidity is used as a surrogate for phosphorus concentrations, as it correlates well with most of phosphorus species concentrations. Nitrogen does not correlate well with turbidity or SSC, and loads are estimated using an averaging estimator (interpolation). The particulate fraction of nutrient transport is found to be 25% and 35% of the respective total nitrogen and total phosphorus yields on the River Bandon, and 41% and 52% on the River Owenabue. The total portion of SSL consisting of particulate nutrients is estimated to be in excess of 10% on both the River Bandon and the River Owenabue.

Present study shows suspended sediment dynamics in the meltwater of Chhota Shigri glacier, Himachal Pradesh, India for different melt seasons during the period 2011-2014. Maximum suspended sediment concentration in the meltwater was found during the month of July 2011, 2012 and 2014 constituting to 55.2%, 48.3% and 46.9%, respectively. Whereas in 2013, maximum suspended sediment concentration was observed in August accounting for 46.1% of the total. On the other hand, maximum suspended sediment load was monitored in the month of July 2011, 2012 and 2014 constituting 59.5%, 63% and 55.7% of the total, respectively. Whereas in 2013, maximum suspended sediment load was observed in the month of August accounting for 49.8% of the total suspended sediment load. Annual distribution of suspended sediment concentration (SSC) and suspended sediment load (SSL) in the Chhota Shigri glacier shows higher value of SSC and SSL during the study period 2012 and 2013, which may be due to the presence of high glacial runoff and negative mass balance of the studied area during these time periods. Marked diurnal variation has been observed in the SSC of meltwater. Strong correlation was observed between SSC and SSL with discharge. On the other hand, SSC and SSL also showed strong exponential correlation with air temperature of the studied area. Sediment yield from the catchment of Chhota Shigri glacier is high during the peak melt season (July and August) and low during the late melt season (September and October). The average value of erosion rate for Chhota Shigri glacier basin during the study period 2011-2014 was calculated to be 1.1 mm/yr, which is lower than the average erosion rate of other Himalayan glaciers such as Rakiot, Chorabari and Gangotri glaciers, which may be caused by its geological setting containing high erosion resistant rocks such as granite, granite gneiss and porphyritic granite.

In Finland, the monitoring of river water quality is crucial for measuring nutrient loads and assessing ecosystem condition; however, the dynamics of suspended sediment are poorly characterized due to a lack of temporal information from routine sampling. This prevents the further development of digital water systems that demand continuous or near-real-time indications of sediment transport. This study proposes a framework that transforms acoustic Doppler current profiler (ADCP) information into quantitative, data-driven knowledge on suspended sediment concentration across multiple survey campaigns in some Finnish boreal rivers. It leverages a Bayesian-optimized support vector regression to predict suspended sediment concentration by integrating multiple combinations of hydrological parameters, acoustic backscatter features, and a robust backscatter magnitude (RBM). The method is applied to ADCP surveys from four Finnish boreal rivers, and model performance is assessed using leave-one-campaign-out cross-validation to evaluate out-of-survey predictive performance under new hydrological and sedimentary conditions. Moreover, probabilistic modeling is utilized in order to evaluate the level of uncertainty associated with all predictions. Despite the results of log-transformed suspended sediment concentration demonstrating consistent and comparable performance across all the rivers, substantial differences in the accuracy of model predictions and uncertainty were identified within various study sites. Prediction intervals exhibited strong calibration, characterized by narrow uncertainty in more stable systems and much greater uncertainty in rivers influenced by episodic, event-driven sediment movement. Feature-scenario studies indicated that the main factors influencing suspended sediment concentration estimates varied among rivers; in some systems, hydrological variables mostly explained suspended sediment concentration variability, but in others, acoustic structure provided the key predictive insights. Furthermore, in sediment-rich rivers, the incorporation of RBM was essential for more accurate suspended sediment concentration predictions. For the purpose of providing a scalable foundation for near-real-time digital monitoring of suspended sediment and nutrient transport in river networks, the examined framework proposed valuable links between limited acoustic surveys with continuous hydrological data and provides physically meaningful uncertainty estimates.

In this study, we investigate how changes in the vertical distribution of suspended sediment affect continuous suspended sediment flux measurements at a location in the San Francisco Estuary. Current methods for measuring continuous suspended sediment flux estimates relate continuous estimates of suspended-sediment concentration (SSC) measured at-a-point (SSCpt) to discrete cross-section measurements of depth-averaged, velocity-weighted SSC (SSCxs). Regressions that compute SSCxs from continuous estimates of SSCpt require that the slope between SSCpt and SSCxs, controlled by the vertical distribution of SSC, is fixed. However, in tidal systems with suspended cohesive sediment, factors that control the vertical SSC profile—vertical turbulent mixing and downward settling of suspended sediment mediated by flocculation of cohesive sediment—constantly vary through each tide and may exhibit systematic differences between flood and ebb tides (tidal asymmetries in water velocity or particle size). We account for changes in the vertical SSC profile on estimates of SSCxs using time series of the Rouse number of the Rouse-Vanoni-Ippen equation combined with optical turbidity measurements, a surrogate for SSCpt, to predict SSCxs from 2009 to 2011 and 2013. Time series of the Rouse number were estimated by fitting the Rouse-Vanoni-Ippen equation to SSC estimated from optical-turbidity measurements taken at two elevations in the water column. When accounting for changes in the vertical SSC profile, changes in not only the magnitude but also the direction of cumulative sediment-flux measurements were observed. For example, at a mid-depth sensor, sediment flux estimates changed from − 319 kt (± 65 kt, negative indicating net seaward transport) to 482 kt (± 140 kt, positive indicating net landward transport) for 2009–2011 and from − 388 kt (± 140 kt) to 1869 kt (± 406 kt) for 2013–2016. At the study location, estimation of SSCxs solely from SSCpt resulted in sediment flux values that were underestimates on flood tides and overestimates on ebb tides. This asymmetry is driven by covariance between water velocity and particle settling velocity (Ws) with larger Ws on flood compared to ebb tides. Results of this study indicate that suspended-sediment-flux measurements estimated from point estimates of SSC may be biased if systematic changes in the vertical distribution of SSC are unaccounted for.

An improved method for evaluating the seasonal variability of total suspended sediment flux field in the Yellow and East China Seas

The German Wadden Sea (southern North Sea) sediments are composed of both cohesive and non-cohesive deposits. The spatial distribution patterns are mainly driven by wind-induced waves and tidal currents. Transport intensity and duration depend on the hydrodynamic conditions, which vary over time. In this paper, the transport of suspended sediment was investigated on seasonal, tidal and hourly time scales in the back-barrier system of Spiekeroog Island. Long- and short-term data of fair weather periods and two storm events were investigated based on stationary and mobile measurements of currents and waves by Acoustic Doppler Current Profiler (ADCP), in situ particle size and suspended sediment concentration (SSC) measurements by laser in situ scattering and transmissometry (LISST) as well as wind records. The ADCP backscatter intensities were calibrated by means of LISST volume concentration data in order to quantify longer term SSCs and fluxes in the back-barrier system. Values up to 120 mg l−1 were recorded, but concentrations more commonly were below 60 mg l−1. The long-term results confirm former observations of a balanced budget during low-energy (fair weather) conditions in the study area. In general, SSCs were higher during spring tides than during neap tides. The data also clearly show the remobilisation of sediment by tidal current entrainment. The records include two severe storm events, “Britta” (1st November 2006) and “Kyrill” (18th January 2007). The data reveal very complex temporal flow and transport patterns. During both storm events, the export of material was mainly controlled by the interaction of wind, waves and tidal phase. The typical ebb-dominance occurring during fair-weather conditions was temporarily neutralised and even reversed to a flood-dominated situation. During “Kyrill”, the wind and high-waves setup in conjunction with the tidal phase was even able to compress the duration of two successive ebb cycles by over 70%. Although SSCs increased during both storms and higher turbulence lifted particle clouds upwards, an export of suspended matter towards the North Sea was only observed under the conditions taking place during “Britta”. Such fluxes, however, are currently still difficult to quantify because the backscatter intensity during high energy events includes a substantial amount of noise produced by the high turbulence, especially near the water surface.

Numerical experiments were carried out to evaluate the physical effects of upwelling/downwelling on the suspended sediment concentration (SSC) on continental shelves. The results show that (1) because the upwelling or downwelling velocity tends to be of the same order of magnitude as the settling velocity of suspended sediment, various SSC distribution patterns are possible, depending on the relative importance of the velocities; (2) the presence of upwelling can enhance the SSC; and (3) in an upwelling/downwelling system, maximum concentrations may be situated in the middle of the water column, if the settling velocity is larger than the upwelling velocity. Hence, the physical processes associated with upwelling/downwelling are relevant to the SSC distribution observed over the mud patch near Cheju Island, East China Sea. Furthermore, a qualitative analysis indicates that the presence of upwelling/downwelling in the overlying water column increases the accumulation rate at the seabed.

Suspended sediment dynamics play an important role in controlling nearshore and estuarine geomorphology and the associated ecological environments. Modeling the transport of suspended sediment is a complicated and challenging research topic. The goal of this study is to improve the accuracy of modeling the suspended sediment concentrations (SSCs) with newly developed techniques. Based on a three-dimensional suspended cohesive sediment transport model, the transport of suspended sediment and SSCs are simulated by assimilating SSCs retrieved from the Geostationary Ocean Color Imager (GOCI) with the adjoint data assimilation in the Hangzhou Bay, a typical strong tidal estuary along the coast of the East China Sea. To improve the effect of the data assimilation, the penalty function method, in which the reasonable constraints of the estimated model parameters are added to the cost function as penalty terms, will be introduced for the first time into the adjoint data assimilation in the SSCs modeling. In twin experiments, the prescribed spatially varying settling velocity is estimated by assimilating the synthetic SSC observations, and the results show that the penalty function method can further improve the effect of data assimilation and parameter estimation, regardless of synthetic SSC observations being contaminated by random artificial errors. In practical experiments, the spatially varying settling velocity is firstly estimated by assimilating the actual GOCI-retrieved SSCs. The results demonstrate that the simulated results can be improved by the adjoint data assimilation, and the penalty function method can additionally reduce the mean absolute error (MAE) between the independent check observations and the corresponding simulated SSCs from 1.44 × 10−1 kg/m3 to 1.30 × 10−1 kg/m3. To pursue greater simulation accuracy, the spatially varying settling velocity, resuspension rate, critical shear stress and initial conditions are simultaneously estimated by assimilating the actual GOCI-retrieved SSCs to simulate the SSCs in the Hangzhou Bay. When the adjoint data assimilation and the penalty function method are simultaneously used, the MAE between the independent check observations and the corresponding simulated SSCs is just 9.90 × 10−2 kg/m3, which is substantially less than that when only the settling velocity is estimated. The MAE is also considerably less than that when the four model parameters are estimated to be without using the penalty function method. This study indicates that the adjoint data assimilation can effectively improve the SSC simulation accuracy, and the penalty function method can limit the variation range of the estimated model parameters to further improve the effect of data assimilation and parameter estimation.

The use of sidelooking acoustic Doppler velocity meters (ADVMs) to estimate fluvial suspended-sediment concentrations (SSC) by the U.S. Geological Survey has become more operational in recent years; however, direct transfer of these techniques to downlooking acoustic Doppler current profilers (ADCPs) currently is not feasible. The use of ADCPs to estimate SSC has been investigated by other researchers, but the requirements and limitations of an operational method that could be successfully applied at many locations are not well defined. In order to evaluate the efficacy of using ADCPs of multiple frequencies to estimate SSC, a dataset was collected on the Missouri River at St. Charles, Missouri, in July 2016. The Missouri River dataset is being compared to similar datasets to determine: 1) what factors influence successful use of downlooking ADCPs to estimate SSC, 2) what are the minimum datasets required to develop SSC calibrations, and 3) what modifications are needed to existing methods, instruments, and software tools.

Modelling diagnosis of heavy metal (copper) transport in an estuary