Modelling sea surface currents in the eastern coast of Bawean Island, East Java

Bawean Island is located in the middle of the Java Sea, approximately 80 km north of Gresik Regency, East Java Province. The coastal area of Bawean Island is famous for its potential as a marine tourism area because it has a well-preserved coral reef ecosystem. The potential for tourism development on this island requires the support of environmental suitability. The dynamics of ocean currents as an important parameter for small island development is important to be analyzed. This study aims to determine the characteristics of currents in the eastern coast of Bawean Island through the hydro-oceanographic model. The data used in this modelling was hourly wind and tide data from the period of 2020-2021. The results showed that the velocity of surface current speed in the study area was weak (<0,5 m/s). There was a significant difference of current direction during the west monsoon season and the first transitional season. Validation of model simulation and ADCP measurements produce MAE values 0,014 and 0,035 as well as MAPE values 12,75% and 27,48%.

Accurate Open Channel Flowrate Estimation Using 2D RANS Modelization and ADCP Measurements

Boat-mounted Acoustic Doppler Current Profilers (ADCP) are commonly used to measure the streamwise velocity distribution and discharge in rivers and open channels. Generally, the method used to integrate the measurements is the velocity-area method, which consists of a discrete integration of flow velocity over the whole cross-section. The discrete integration is accomplished independently in the vertical and transversal direction without assessing the hydraulic coherence between both dimensions. To address these limitations, a new alternative method for estimating the discharge and its associated uncertainty is here proposed. The new approach uses a validated 2D RANS hydraulic model to numerically compute the streamwise velocity distribution. The hydraulic model is fitted using state estimation (SE) techniques to accurately reproduce the measurement field and hydraulic behaviour of the free-surface stream. The performance of the hydraulic model has been validated with measurements on two different trapezoidal cross-sections in a real channel, even with asymmetric velocity distribution. The proposed method allows extrapolation of measurement information to other points where there are no measurements with a solid and consistent hydraulic basis. The 2D-hydraulic velocity model (2D-HVM) approach discharge values have been proven more accurate than the ones obtained using velocity-area method, thank to the enhanced use of the measurements in addition to the hydraulic behaviour represented by the 2D RANS model.

A close look at the middle Adriatic upwelling: schematized ROMS model simulations

&lt;p&gt;Strong upwelling driven by the NNW winds was detected off the eastern middle Adriatic coast in May 2017. High resolution CTD data revealed thermocline doming by about 20 m at approximately 20 km from the coast. Main characteristics of the upwelling event are reproduced in the realistic ROMS model simulation. Adriatic scale ROMS model having 2.5 km horizontal resolution, forced by the air-sea fluxes calculated using surface fields from operational weather forecast model ALADIN-HR (Tudor et al., 2013; Termonia et al., 2018), river discharges, tides and water mass exchange through the Strait of Otranto, reproduces cold water dome and two-layer offshore flow in accordance with CTD and shipborne ADCP measurements. Significant improvement in the upwelling simulations is obtained using increased drag coefficient. The location of upwelling is correctly modelled, although with somewhat lower upper layer temperatures if compared with measurements. Moreover, the surface cyclonic circulation indicated by ADCP measurements along the cross-Adriatic transect is also evident in the model results. In order to improve understanding of the upwelling mechanism, several schematized numerical experiments are conducted. Wind fields from dynamical adaptation (Zagar and Rakovec, 1999; Ivatek-Sahdan and Tudor, 2004) of ALADIN-HR8 (8 km horizontal grid spacing) wind forecast to 2 km grid, are decomposed by the Natural Helmholtz-Hodge Decomposition (HHD) into divergence-free (incompressible), rotation-free (irrotational), and harmonic (translational) component (Bhatia et al., 2014). The components thus obtained and their combinations are used for calculation of the wind stress instead of the total wind field. Simulations with decomposed wind stress are conducted in the Adriatic domains with both flat bottom and realistic topography. Schematized simulations reveal that the positive rotational wind component is responsible for the rising of thermocline through Ekman pumping and it is more pronounced in the flat bottom basin. In the simulations with divergent wind component, the thermocline doming disappears and only coastal upwelling is reproduced. Additional idealised simulations with homogeneous NW wind stress are performed assuming both two-layer and uniform initial density field.&lt;/p&gt;

From inflow to interflow, through plunging and lofting: uncovering the dominant flow processes of a sediment-rich negatively buoyant river inflow into a stratified lake

&lt;p&gt;Lake and reservoir water quality is impacted greatly by the input of momentum, heat, oxygen, sediment, nutrients and contaminants delivered to them by riverine inflows. When such an inflow is negatively buoyant, it will plunge upon contact with the receiving ambient water and form a gravity-driven current near the bed (density current). If such a current is sediment-laden, its bulk density can be higher than that of the surrounding ambient water, even if its carrying fluid has a density lower than that of the surrounding ambient water. After sufficient sediment particles have settled however, the buoyancy of the current can reverse and lead to the plume rising up from the bed, a process referred to as lofting. In a stratified environment, the river plume may then find its way into a layer of neutral buoyancy to form an intermediate current (interflow). A deeper understanding of the wide range of hydrodynamic processes related to the transitions from open-channel inflow to underflow (plunging) and from underflow to interflow (lofting) is crucial in predicting the fate of all components introduced into the lake or reservoir by the inflow.&lt;/p&gt;&lt;p&gt;Field measurements of the plunging inflow of the negatively buoyant Rh&amp;#244;ne River into Lake Geneva (Switzerland/France) are presented. A combination of a vessel-mounted ADCP and remote sensing cameras was used to capture the three-dimensional flow field of the plunging and lofting transition zones over a wide range of spatial and temporal scales.&lt;/p&gt;&lt;p&gt;In the plunge zone, the ADCP measurements show that the inflowing river water undergoes a lateral (perpendicular to its downstream direction) slumping movement, caused by its density surplus compared to the ambient lake water and the resulting baroclinic vorticity production. This effect is also visible in the remote sensing images in the form of a distinct plume of sediment-rich water with a triangular shape leading away from the river mouth in the downstream direction towards a sharp tip. A wide range of vortical structures, which most likely impact the amount of mixing taking place, is also visible at the surface in the plunging zone.&lt;/p&gt;&lt;p&gt;In the lofting zone, the ADCP measurements show that the underflow undergoes a lofting movement at its edges. This is most likely caused by a higher sedimentation rate due to the lower velocities at the underflow edges and leads to a part of the underflow peeling off and forming an interflow, while the higher velocity core of the underflow continues following the bed. Here, the baroclinic vorticity production works in the opposite direction as that in the plunge zone. Further downstream, as more particles have settled and the surrounding ambient water has become denser, the remaining underflow also undergoes a lofting motion. The remnants of these lofting processes show in the remote sensing images as intermittent &amp;#8216;boils&amp;#8217; of sediment rich water reaching the surface and traces of surface layer leakage.&lt;/p&gt;

Deriving the full turbulent stress tensor from paired ADCP measurements: application to under-ice convection

&lt;p&gt;The Reynolds stress tensor (RST) is the key characteristic of turbulence describing the paths of turbulent kinetic energy transfer and its anisotropy. Despite recent technical advances in application of multi-beam acoustic Doppler current profilers (ADCPs) to in situ acquiring of the RST components, derivation of the full Reynolds tensor from raw flow measurements remains a challenging problem. We present a method for derivation of the full set of turbulent stresses, based on combined use of two ADCPs with two beams from adjacent devices crossing at some point. &amp;#160;In the proposed framework, two 3-beam ADCPs with vertically aligned axes constitute the minimum configuration sufficient to derive 6 equations for all 6 RST components.&amp;#160;&lt;br&gt;The method was applied to studying turbulence in a convectively mixed layer in ice-covered Lake Kilpisj&amp;#228;rvi. The calculated dynamics of all six stress components revealed diurnal periodicity along with the variations with the periods of a few hours. The pulsations intensities (diagonal components of RST) remained positive except short outliers; less than 5% of cases did not meet the so-called realizability requirements (positive definiteness of the stress matrix). The off-diagonal stresses demonstrated sign-changing dynamics, mirroring the inter-component energy transfer.&lt;br&gt;The ratio of pulsation intensities along vertical and horizontal axes varied in the range from 0.02 to 0.25. The r.m.s. values of horizontal and vertical pulsations reached diurnal maximums of 4 and 1 mm/s correspondingly, the latter being close to 1/3 of the convective velocity w*, in accordance with the previous studies on free convection.&amp;#160;&lt;br&gt;The new approach provides an immediate insight into the internal structure of the turbulent boundary mixing, especially relevant to anisotropic non-stationary flows, like buoyancy-driven convection. The preliminary results on under-ice convection elucidate strong anisotropy of the convective flow &amp;#8212; a key to understanding the heat and mass transport in ice-covered waters.&lt;/p&gt;

Removing Wave Bias from Velocity Measurements for Tracer Transport: The Harmonic Analysis Approach

Estimates of turbulence properties with Acoustic Doppler Current Profiler (ADCP) measurements can be muddled by the influence of wave orbital velocities. Previous methods—Variance Fit, Vertical Adaptive Filtering (VAF), and Cospectra Fit (CF)—have tried to eliminate wave-induced contamination. However, those methods may not perform well in relatively energetic surface gravity wave or internal wave conditions. The Harmonic Analysis (HA) method proposed here uses power spectral density to identify waves and least squares fits to reconstruct the identified wave signals in current velocity measurements. Then, those reconstructed wave signals are eliminated from the original measurements. Datasets from the northeastern Gulf of Mexico and Cape Canaveral, Florida, are used to test this approach and compare it with the VAF method. Reynolds stress estimates from the HA method agree with the VAF method in the lower half of the water column because wave energy decays with depth. The HA method performs better than the VAF method near the surface during pulses of increased surface gravity wave energy.