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Monitoring sediment transport in the coastal zone of Tallinn Bay
Abstract
Continuous near bottom measurements of wave characteristics, bottom velocity and turbidity were performed using an acoustic Doppler velocimeter (Sontek ADV Ocean Hydra) integrated with turbidity meter (YSI OBS-3+) and a pressure wave gauge (PTR Group). An experimental autonomous submersible camera system (by the Center for Biorobotics) was used to monitor the motion of particles in the bottom boundary layer (BBL). The ADV measured flows consisted of wind induced currents, wave induced orbital motions and turbulence. Maximum of wind induced currents reached up to 10-15 cm/s, while the maximum near bed orbital motions peaked over 40 cm/s. From the comparison of ADV, turbidity and wave characteristics it followed that turbidity was clearly depending on the wave energy. It means only long and quite high waves generating bottom orbital velocities (calculated from the wave gauge data and/or measured using ADV) over 20 cm/s were able to resuspend bottom sediment and induce some increase in turbidity values - 5-12 NTU in stormy days. Sediment fluxes were estimated using measuring data, sediment characteristics and BBL model. It followed from the modeling results that the height of all BBL varied from 1 cm to 10 cm, and the height of the bottom-most layer (skin friction layer) varied from 0.01 cm to 1 cm. Running the model has shown also that the currents have a significant impact to the sediment resuspension only if their speed reaches to 15-20 cm/s occurring very seldom in Estonian sea areas. Absolute majority of measurements showed an error below 5% using the wave skin friction shear velocity instead of the total skin friction velocity, i.e. excluding the existence of a skin friction sublayer. Captured video clips were split into frames and pairs of images were analyzed using particle image velocimetry software by MatPIV toolbox for MATLAB. The velocity and vorticity vector fields of bottom boundary layer were visualized.
... Another study showed that c was a function of bed shear stress . Some findings support that the total transport of sediment in coastal zones mostly depends on the wave mode (Buschmann et al., 2012;Pomeroy et al., 2018). Sediment suspension and hydrodynamics should both be considered together from the perspective of ecosystem recovery and restoration in coastal areas (Duarte et al., 2015;Fatahi Nafchi et al., 2021;Yu et al., 2020). ...
... Many studies in recent years have focused on the short-term or long-term monitoring of c but without continuous data (Adamo et al., 2015;Buschmann et al., 2012;Robert et al., 2016). The traditional method for monitoring c is to collect and analyze water samples. ...
Suspended sediment concentration (c) has been considered a critical environmental factor in reef habitats; however, the values and variations of c are not evident in a unique reef mainly created by crustose coralline algal concretions compared to abundant studies in coral reefs. The results of real-time and long-term monitoring of waves and c over the intertidal algal reef are reported because of the construction of an offshore industrial harbor near the reef. The real-time monitoring systems were based on techniques, including optical backscatter sensors (OBSs) for measuring c, pressure sensors for measuring waves, data loggers, and wireless networks for data transmission. The instruments sampled every hour and ran continuously and automatically for years. The OBS measurement was compared and validated with biweekly water sampling. A good correlation between the results of the two methods was observed. Nevertheless, more calibrations of OBSs in different seasons reduced the variance between the two methods over a year-long timescale. The year-long data showed a remarkable seasonal variation in c. The average c was approximately 140 mg/l during the winter season, while it was only approximately 70 mg/l during the summer season. The observed c was higher than that in other coral reef environments; the elevated and highly variable c, ranging from approximately 0 to 500 mg/l, may be one factor that creates the unique algae reef environment. The year-long measurement of waves and c showed that the variation in c was mainly due to the variation in waves in different seasons and was well correlated with the wave-induced bed shear stress. The real-time and long-term data measured by the system will aid in better understanding and providing useful environmental data for accessing future environmental changes and protecting reef habitats.
... The magnitude of suspended sediment concentration is directly proportional to the bed materials, sizes and the resuspension of bed sediment induced by waves and currents 7 . The sediment transport using currents, turbulence and wave orbital motion were measured using the Acoustic Doppler Velocimeter (ADV) 8 . Sedimentation rate in coral reefs is a key factor for controlling the reef development and distribution. ...
The Kavaratti Island of Lakshadweep archipelago exhibits high coastal erosion and shoreline changes due to the anthropogenic stresses on the island morphology. This study focuses on the shoreline analysis using Digital Shoreline Analysis System model and nearshore sediment dynamics using an integrated modeling system with high resolution bathymetry of ~15 m. The sediment load and bed level change were estimated using the sediment transport model through the feedback mechanism of flow and wave forces. The model simulations were performed during winter, pre-monsoon, monsoon and post-monsoon seasons for the year 2015 and calibrated using the field observation datasets. Shoreline change studies indicate the coastal erosion of about 83.43 % and accretion of about 16.55 % along the island coast. The study signifies the erosion around the fishing jetty and accretion at the western (Katchery) jetty due to wave induced currents and tidal influx. The scouring effect due to the presence of tetrapods along the island coast is observed between 0.02 and 0.18 m. Introduction The coastal processes are driven by strong interaction of nearshore waves, currents, tides and local wind field such as land and sea breeze. This process majorly leads to accretion and erosion features along the coast by changing the shoreline morphology. However, spatial variation of erosion/accretion rates depends on natural and manmade activities. Hydrodynamic conditions tend to change the nearshore dynamics and morphology of the coast 1-3. Dynamic variation occurs in the lagoon due to the continuous striking of ocean waves and wave breaking over the reef crest 4. Wave breaking causes the longshore drift, sedimentation and sediment suspension over the coral reefs 5,6. The magnitude of suspended sediment concentration is directly proportional to the bed materials, sizes and the resuspension of bed sediment induced by waves and currents 7. The sediment transport using currents, turbulence and wave orbital motion were measured using the Acoustic Doppler Velocimeter (ADV) 8. Sedimentation rate in coral reefs is a key factor for controlling the reef development and distribution. The sediment transport and the water circulation within the lagoon predict the shoreline changes. Thus, it is
Satellite sensors provide a valuable tool in understanding the seasonal variability of ocean color properties. The vertical diffuse attenuation coefficient (Kd) was evaluated for the waters around Puerto Rico and the US Virgin Islands. The MODIS K490 of Band 3 Level82 daily images were processed with a resolution of 1 kilometer for the year 2008. The images were projected to WGS884 geographic coordinate system and then compiled using the Layer Stacking routine to include all images within each month. A total of 184 were processed and monthly averages were developed using the Sum Data Bands routine for the study area to minimize cloud cover and seasonal averages were created from the monthly averages. Additional MODIS K490 Level82 monthly and seasonal average products (euphotic depth, Lee et al., 2005), and (euphotic depth, Morel et al., 2007) were downloaded from the NASA's Ocean Color web site with a resolution of 4 kilometers. The values for the dry season (December8April) ranged from 0.206 (1/m) to 0.666 (1/m), with a mean of 0.482 (1/m) and for the rainy season (May8November) the values ranged from 0.516 (1/m) to 0.860 (1/m) with a mean of 0.698 (1/m). The monthly averages were higher for December (0.206 1/m) and lower for June (0.860 1/m). There is seasonal variability in the K490 values in the waters around Puerto Rico and the USVI the MODIS K490 product at 1 km provides the necessary resolution to evaluate the coastal land influence in the values of K490 at this geographic extent. Further development may include comparison of K490 additional products (Morel and Lee Kd algorithms) values at comparable spatial resolution to evaluate the overestimation of values derived from remote sensors. The data obtained from this project provides information on the seasonal variability of this parameter and the possible effects in the marine ecosystems.
The potential impact of high-speed vessel wakes on the longshore drift of semi-sheltered, medium-energy beaches is evaluated based on recent studies in almost tideless Tallinn Bay, the Baltic Sea. Energy flux and wave propagation direction of vessel wakes is estimated based on high-resolution water surface profiling in summer 2008. The wind-wave time series in 1981–2008 is modelled on the basis of a simplified scheme for a long-term wave hindcast with the use of a triple-nested version of the WAM model. Longshore drift, created by wind waves and by vessel wakes, is estimated by the energy flux model, also known as the Coastal Engineering Research Centre (CERC) model. Vessel wakes cause longshore drift that in 2007–2008, as compared to the drift produced by wind waves on the SW coast of the Aegna Island at the entrance to Tallinn Bay, had a magnitude about 75% less and the opposite direction.
The longshore transport along a sandy beach resulting from the combination of a longshore current and waves is computed. This computational procedure is based on the principle that because of the waves the bed shear of the current is increased. The bed load can be computed with any bed load formula (the formula of Frijlink which is commonly used at the Delft Hydraulics Laboratory is used in the paper), taking into account the previously mentioned effects of the waves. The transport in suspension is computed according to the procedure suggested by Einsgein. The influence of the assumed thickness of the layer in which the bed load is considered to take place and which determines the concentration of suspended material at the bed is presented. The results of measurements in a laboratory basin and along the coast of Queensland (Australia), are presented.
In this chapter the essential principles are being put into practice, such as in the study of colloids, microwave properties of superconductors, hydrodynamic properties of suspensions, porous media, ultrasonics etc. Where predicted results don’t accord with observed physics the reader new to the field will learn to deal with the question of whether the fault lies with the failure of EMT or is the result of inadequate understanding of the physical mechanisms and he or she can use the examples in this chapter as templates for the application of EMT to their own specified interest areas. The seasoned worker can also use these sections as references and platforms for a deeper analysis and understanding of EMT. The final section deals with non-linear composites for which many outstanding and challenging problems remain unsolved.
In a recent result of Gérard-Varet and Dormy (2010) [4], they established ill-posedness for the Cauchy problem of the linearized Prandtl equation around non-monotic special solution which is independent of x and satisfies the heat equation. In Guo and Nguyen (2010) [5] and Gérard-Varet and Nguyen (2010) [6], some nonlinear ill-posedness were established with this counterexample. Then it is natural to consider the problem that does this linear ill-posedness happen whenever the non-degenerate critical points appear. In this paper, we concern the linearized Prandtl equation around general stationary solutions with non-degenerate critical points depending on x which could be considered as the time-periodic solutions and show some ill-posedness.
Dependence of near bottom currents and turbidity on wind and wave parameters is analyzed. Measurements campaigns with an acoustic Doppler velocymeter (ADV Field/Hydra, SonTek/YSI) were curried out in two bays in north western Estonia - the first one on Naissaar Shallow in Tallinn Bay (22.12.2009-12.01.2010, water depth 9 m, 37 cm from the bottom) and the second one in Keibu Bay (03.06.2010-26.06.2010, water depth 7m, 27 cm). Near bottom velocities were recorded with frequencies 2 Hz (currents) and 0.2 Hz (wave induced orbital motion). Additionally the water turbidity at the same level as flow measurements was performed using an integrated turbidity meter OBS 3+ (YSI). Wave parameters were recorded using a pressure wave gauge (PTR Group, Tallinn). The ADV measured flows consist of wind induced currents, wave induced orbital motions and turbulence. Maximum of wind induced currents reached meanly 10-15 cm/s at both measurement locations, while the maximum near bed orbital motions peaked over 40 cm/s. Measurements showed that the near bottom velocities in Keibu Bay were in correlation with wind speed, but turbidity values showed a significant increase only in some special weather conditions. From the comparison of ADV, turbidity meter and wave gauge characteristics it followed that turbidity was clearly depending on the wave energy. It means only quite long and high waves inducing bottom orbital velocities (calculated from the wave gauge data) over 20 cm/s were able to resuspend bottom sediments.
1] The diffuse attenuation coefficient for downwelling irradiance (K d) is an important parameter for ocean studies. For the vast ocean the only feasible means to get fine-scale measurements of K d is by ocean color remote sensing. At present, values of K d from remote sensing are estimated using empirical algorithms. Such an approach is insufficient to provide an understanding regarding the variation of K d and contains large uncertainties in the derived values. In this study a semianalytical model for K d is developed based on the radiative transfer equation, with values of the model parameters derived from Hydrolight simulations using the averaged particle phase function. The model is further tested with data simulated using significantly different particle phase functions, and the modeled K d are found matching Hydrolight K d very well (12% maximum error). Such a model provides an improved interpretation about the variation of K d and a basis to more accurately determine K d (especially using data from remote sensing).
Wake waves produced by fast ferries bring about significant changesin the optical parameters of sea water in the c. 1 m thick near-bottomlayer of the coastal areas of Tallinn Bay. The greatest of thesechanges occur at relatively small depths, but the duration ofthe influence increases with increasing depth. Rough quantitativeestimates suggest that the overall influence of fast ferry trafficin Tallinn Bay may result in an annual loss of the order of severalhundred litres of fine sediments from each metre of the coastline.
Artifical structures and shoreline of Estonian SSR. – Artifical Structures and Shorelines
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