Hydroacoustic Measurements Research Articles

Sounding out the river: Seismic and hydroacoustic monitoring of bedload transport

AbstractSeismological observations provide a non‐invasive and continuous means for indirectly measuring fluvial bedload transport. A significant challenge remains in independently characterising the seismic signature of bedload transport from other sources such as turbulence. We present a unique dataset from an alluvial Scottish river, combining seismic data and hydroacoustic measurements, to analyse bedload transport during three high‐flow events occurring within the same year. By studying three successive events, we assess the consistency of bedload transport thresholds in response to changing flow conditions and explore the presence of hysteresis in seismic data versus water level as an indicator of coarse bedload transport. Through the use of hydroacoustic data to independently characterise bedload transport, our findings reveal that bedload transport occurred during all three events but that the threshold for entrainment varied. These entrainment thresholds were influenced by antecedent events, with a drop of 15%–20% of the threshold flow depth following the largest of the three events. In agreement with recent studies, we also found that hysteresis in the seismic versus water level data is not sufficient for identifying and analysing bedload transport: Distinct hysteresis was only observed during the largest of the three events despite all events experiencing bedload transport as observed through the independent hydroacoustic data. Our work shows the value in combining independent datasets for long‐term monitoring of bedload transport to understand the evolution in the thresholds of bedload motion, providing crucial information for effective river and land‐use management in a changing climate with potentially impacted high‐flow events.

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

AbstractThis study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (<19 nM and <70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region.

Experimental investigation of partial and cloud cavitation control on a hydrofoil using bio-inspired riblets

We experimentally investigated a passive cavitation control approach to control partial and cloud cavitation on a National Advisory Committee for Aeronautics 0015 hydrofoil. For this approach, we implemented two different kinds of mesoscale bio-inspired riblets, known as scalloped riblets and sawtooth riblets, on the suction side of the hydrofoil near its leading edge. We studied the dynamics of partial and cloud cavitation on the hydrofoil with and without two kinds of riblets using a high-speed camera. In addition, we performed hydro-acoustic measurements to analyze the effects of this passive control on the cavitation induced noise in the wake of the hydrofoils. We considered flows at Reynolds numbers 0.5 × 106 and 0.6 × 106 with the hydrofoils at angles of attack of 8° and 10°. Results revealed that the cloud cavitation shedding on the hydrofoil suction surface was mitigated significantly due to the reduction in the re-entrant jet momentum. Also, the influence of tip vortex cavitation was reduced on the modified hydrofoils. Furthermore, the large-scale cavitation induced noise for the unmodified hydrofoil was located mostly at lower frequency in the range of 30 Hz, which was similar to the cavity shedding frequency. The small-scale vortex-induced noise concentrated at relative higher frequency between 100 and 120 Hz. The noise reduction in this study was achieved by scalloped and sawtooth riblets at low frequency domain and for some cases at higher frequency range by manipulating of the cavitation oscillation mechanism and elimination of a large-scale cavitation structure. This study provided new insight into controlling the destructive effects of cavitation using bio-inspired riblets, which should be relevant for various engineering applications.

Data-driven characterization of viscoelastic materials using time-harmonic hydroacoustic measurements

Any numerical procedure in mechanics requires choosing an appropriate model for the constitutive law of the material under consideration. The most common assumptions regarding linear wave propagation in a viscoelastic material are the standard linear solid model, (generalized) Maxwell, Kelvin-Voigt models or the most recent fractional derivative models. Usually, once the frequency-dependent constitutive law is fixed, the intrinsic parameters of the mathematical model are estimated to fit the available experimental data with the mechanical response of that model. This modelling methodology potentially suffers from the epistemic uncertainty of an inadequate a priori model selection. However, in this work, the mathematical modelling of linear viscoelastic materials and the choice of their frequency-dependent constitutive laws is performed based only on the available experimental measurements without imposing any functional frequency dependence. This data-driven approach requires the numerical solution of an inverse problem for each frequency. The acoustic response of a viscoelastic material due to the time-harmonic excitations has been calculated numerically. In these numerical simulations, the non-planar directivity pattern of the transducer has been taken into account. Experimental measurements of insertion loss and fractional power dissipation in underwater acoustics have been used to illustrate the data-driven methodology that avoids selecting a parametric viscoelastic model.

Accuracy Assessment of the Positioning of a Swarm of Underwater Vehicles in Relation to Four Surface Vehicles Using the TDOA Method

This paper presents the results of research on the accuracy assessment of the positioning of a swarm of underwater vehicles based on hydroacoustic measurements made with respect to four surface vehicles under the time difference of arrival (TDOA) method. The assessment consisted of the estimation of accuracy parameters for determining the position of an underwater vehicle in relation to surface vehicles forming a so-called moving geometrical measurement structure (MGMS) in the following shapes: square, rectilinear, triangular, and three-pointed. This demonstrated that MGMS makes it possible to estimate the relative position of underwater vehicles in a swarm with an accuracy of 2.1 m (RMS) over an area of approx. 1000 m2 and approx. 3.0 m (RMS) over an area of approx. 1600 m2. The most favourable MGMS shapes include three-pointedwhile maximising the size of the positioning area, where the positioning accuracy should not exceed 3.0 m (RMS)—and rectilinear—while maximising the size of the positioning area, where the positioning accuracy should not exceed 10.0 m (RMS).

Design of a hydro sound intensity probe for quantification and localization of acoustic sources — Applied to a hubless marine rim drive

The reduction of hydro noise emission by ship propulsion systems is of importance since background noise levels in the oceans have increased by 21 dB in the past decades. Thus, the acoustical improvement of ship propulsion systems is a necessary and challenging task. It involves not only the ship propeller but also the components of the entire drive train like diesel or electrical engines, electrical inverters as well as gearboxes. Acoustical measurement systems for the quantification and localization of noise sources are essential for any kind of optimization and are state of the art in the airborne noise field. This is not the case in the hydro acoustic field and measurements are mostly limited to the recording of the hydro sound pressure using single hydrophones. The objective of this paper is the presentation of a new three dimensional hydro sound intensity probe capable of acoustical sound power quantification and noise source localization. The probe can be used for the determination of the hydro sound pressure, the 3D sound intensity vector and the sound power. Furthermore, the probe can be utilized for hydro sound source localization purposes using beamforming methods based on virtual acoustical arrays. The developed probe was used for hydro acoustic investigations of a full scale marine rim drive during a measurement campaign in the river Warnow in Rostock, Germany. The results indicate the different hydro sound generating mechanism of the rim drive.

Rapid in situ assessment of high-resolution spatial and temporal distribution of cyanobacterial blooms using fishery echosounder

Cyanobacterial blooms are increasing in frequency, magnitude, and duration globally because of enhanced eutrophication and climate change. Thus, comprehensive investigation and systematic monitoring of the spatial and temporal distribution of cyanobacteria in aquatic environments are urgently needed to better understand bloom development and complex interactions within a dynamic environment. Various methods have been used to investigate the distribution of cyanobacteria, however, none of them can provide high-resolution data for the three-dimensional spatial structure of the bloom and its dynamics in real time. In the present study, we investigated the applicability of a high-frequency (200 kHz) fishery echosounder, a type widely used in fisheries acoustics, to detect and estimate the cyanobacterial genus Microcystis bloom distribution and biomass in a shallow lake (Sulejów Reservoir, Poland). Verification of the usefulness of in situ acoustic quantification of bloom-forming cyanobacteria was based on a comparison of acoustic estimates of cyanobacterial biomass with the ground truth—that is, fluorometric measurements and chlorophyll a concentrations. We compared the acoustic estimates with other methods for continuous measurements along 10 predetermined parallel transects and point samples at 14 stations situated on the transects. In vertical hydroacoustic measurements at night, we observed that cyanobacterial biomass was highest in the uppermost layer and diminished continuously with depth. For both horizontal and vertical continuous measurements, we found significant positive correlations between acoustic and fluorometric estimates of cyanobacterial biomass. The traditional point samples measurements, however, did not agree equally well with the acoustic estimates, especially for vertical beam. We argue that the point measurements have more stochastic character and less adequately describe dynamic changes in the cyanobacteria distribution than continuous acoustic estimates. More studies are required to explore the cyanobacteria distribution patterns under different biological, physical, and meteorological conditions.

МОДЕЛИРОВАНИЕ ПРОЦЕССОВ ИЗМЕРЕНИЯ ПАРАМЕТРОВ И ВОССТАНОВЛЕНИЯ ГИДРОЛОКАЦИОННОГО ОТОБРАЖЕНИЯ

An algorithm for restoring the bottom topography profile from sonar display for various elements of a complex underwater topography is proposed. A sonar profile representation in the form of a second-order polynomial equation is given. Analytical expressions for restoring various forms of reflecting the surface are obtained considering hydroacoustic measurement results. Applying the described restoration algorithm is analysed on the example of a complex profile of the bottom topography. The article shows that when certain values of echo distances are reached, relief distortions occur with a violation of the profile geometric similarity.

Assessment of Risks Induced by Countermining Unexploded Large-Charge Historical Ordnance in a Shallow Water Environment—Part II: Modeling of Seismo-Acoustic Wave Propagation

The goal of this work presented in a two-companion paper is to pave the way for reliably assessing the risks of damage to buildings on the shore, induced by the detonation of large-charge historical ordnance (i.e., countermining) in variable shallow water environments. Here, we focus on the impact of the marine environment, more specifically the unconsolidated sedimentary layer, on detonation-induced seismo-acoustic wave propagation. We rely on a multidisciplinary cross-study including real data obtained within the framework of a countermining campaign, and numerical simulations of the seismo-acoustic propagation using a spectral-element method. We first develop a strategy relying on physical insights into the different kind of waves that can propagate in a coastal environment, to provide clues for a computational cost reduction. The geological surveys and the hydroacoustic measurements provide input data for the 3-D axisymmetric modeling of wave propagation. The numerical simulations, obtained for one specific source–receiver path with a variable sedimentary facies, are compared with the real seismic data induced by the detonation of a charge either on the seabed, or in the water column, and recorded on the coast. Numerical analysis sheds light on the strong interaction between surface waves and the sedimentary facies. The short-scale and deep sedimentary basins favor a local wave-amplitude amplification and a frequency shift toward the low-frequency domain. However, the seismo-acoustic waves are globally attenuated during their propagation because of intrinsic attenuation and geometrical spreading, which generally prevents any large damage to nearby buildings on the shore.

Remote Sensing Monitoring of Anthropogenic Changes in the Desenka River Channel (Kyiv, Ukraine)

The article is devoted to developing an universal methodological apparatus of ecological monitoring and practical assessment of the state of hydroecosystems to determine the nature of the anthropogenic impact. The authors analyzed the transformation of the Desenka River channel (Kyiv, Ukraine) in the 1965 – 2021 years. The primary attention is paid to changes in the coastline of Kyiv to determine the nature of the anthropogenic impact on the study area. The authors improved the technology of monitoring the dynamics of the water regime of the riverbed by constructing bathymetric maps based on the results of hydroacoustic measurements and the space imagery interpretation.The complex use of methods for selecting and processing information was applied through the use of GIS technologies (thematic classification of remote sensing results in the conditions of data exchange of ground-based verifications with independent features of objects). The results are presented in a way that is easy to interpret. It was found that the main reason for the change in area is sand mining. The bathymetric survey allowed to specify the maximum depth of the reservoir, which is 16.8 m. It was determined that the relief of the bottom is typical for a quarry. The river's depth in its central part increases from west to east in proportion to the increase in the width of the reservoir. The study found that sand was mined in the same place, washing away huge underwater quarries and forming numerous silt alluviums. According to the results of the analysis of changes in areas presented in this study, the authors proved that uncontrolled sand mining has a negative impact on the biotic stability of landscapes and causes irreparable damage to the environment. The effectiveness of remote sensing methods for determining the ecological status of hydroecosystems is proved. The presented studies indicate the need to stabilize the ecological balance of the river ecosystem, take appropriate measures to increase the productivity of hydrolandscapes, improve the environment and ensure the environmental safety of the Desenka River and coastal areas.

Sentinel-1 Satellite Radar Images: A New Source of Information for Study of River Channel Dynamics on the Lower Vistula River, Poland

The amount of sediments transported by a river is difficult to estimate, while this parameter could influence channel geometry. It is possible to derive the bedload transport rate per unit width of the river channel by measuring the migration distance of bedform profiles over time and thickness of bedload layer in motion. Other possible methods include instrumental measurements using bedload traps and empirical formulas. It is possible to use remote-sensing techniques to measure the dynamics of bedform movements and geometries. Landsat images and aerial photographs have been used for this. A new source of remote-sensing information is radar satellite images. Sentinel-1 images have a temporal resolution of 2–3 days and spatial resolution of 25 m at middle latitudes, which make them usable on large rivers. The research area is the 814–820 km reach of the Lower Vistula River, where seven alternate sandbars were selected. The bank lines of the sandbars were delineated on Sentinel-1 images sensed during two low-flow periods of 4 August–26 September 2018 and 1 July–31 August 2019, when discharges at low flow were similar. From water stage observations at gauges, water elevations were assigned to every bank line of the alternate sandbars. The following morphometric parameters were calculated: alternate sandbar centers, volumes and longitudinal profile. Average daily movement of the sandbars in the period 4 August 2018–1 July 2019 was calculated as 0.97 m·day−1. A similar alternate sandbar movement velocity was obtained from a study of Sentinel-2 optical satellite images and hydro-acoustic measurements on the Lower Vistula River. Having depth of bedload in motion and alternate sandbar shift velocities, it was possible to calculate the rate of bedload transport according to the Exner approach formula. Rate of bedload transport was estimated as qb = 0.027 kg·s−1·m−1. This study shows a novel use of Sentinel-1 images to study the 3D geometry and movement rate of sandbars.

Hydroacoustics for density and biomass estimations in aquaculture ponds

The use of hydroacoustics is currently being studied and developed as a promising non-intrusive methodology to monitor and manage fish stocks in aquaculture farms. The main objective of this study was to develop an acoustic method for the estimation of fish density and biomass in inland aquaculture farms and test the accuracy and precision of the estimates with real data provided by the company. The study was conducted in sea bass (Dicentrarchus labrax) production ponds located in Seville (Southern Spain). A Simrad EK60 echosounder with two split-beam circular transducers operating simultaneously at 200 kHz was used for hydroacoustic surveys. Two different hydroacoustic designs were considered: central trajectories and zigzag trajectories. The accuracy and precision of the estimates were examined in order to select the best sampling design. Due to a non-homogeneous fish distribution in the pond caused by the avoidance behaviour, as a response to the sampling disturbance presented by fish, acoustic density and biomass were corrected by applying sampling theory according to the probability of fish detection. When density and biomass were corrected, the estimates became highly accurate and precise with respect to real data, which confirms that the proposed method is adequate. Similarly, acoustic estimates of fish weight were highly in agreement with real data, due to the use of specific equations developed “in situ” for the study. Although no significant differences were recorded in the density and biomass estimates with regard to the trajectory used (central vs. zigzag), it was observed that the most accurate agreement and precision were always obtained in central trajectories. Therefore, central design is proposed as the most appropriate design for hydroacoustic measurements in inland ponds. The results obtained in this study provide estimates of density and biomass that accurately match the real data, supporting the use of hydroacoustics as a potentially valid tool to manage inland aquaculture farms.

A Very Large Spawning Aggregation of a Deep-Sea Eel: Magnitude and Status

Multiple lines of evidence substantiate the existence of a very large aggregation of the basketwork eel, Diastobranchus capensis, on the small (3 km2) Patience Seamount off southeast Australia. The aggregation appears to be present year-round, but largest in the austral autumn when composed of spawning eels. Twenty eels caught in April 2015 (14 female, 6 male) were all in advanced stages of spawning condition. The eel’s abundance in the aggregation was very high as measured at seamount, local and regional scales. Hydroacoustic measurement of the spawning aggregation’s dimensions (~100 × 1000 m) and conservative counts of 100 s of eels along camera transects of ~1000–2000 m length indicate 10,000 s individual eels may have been present. The absence of other known spawning locations indicates the Patience Seamount is a regional-scale spatial anchor for spawning. The aggregation was protected in a marine park in 2007 following a decades-long impact from bottom trawling, indicating that the population can be expected to stabilise and recover. Monitoring the aggregation’s status, and validating seasonal spawning, provide important opportunities to examine conservation-led recovery in the deep sea as part of Australia’s new national strategy of Monitoring, Evaluation, Reporting and Improvement (MERI) for conservation values within marine parks.

Effects of Temperature, Salinity, and Fluid Type on Acoustic Characteristics of Turbulent Flow Around Circular Cylinder

The effects of the temperature, salinity, and fluid type on the acoustic characteristics of turbulent flow around a circular cylinder were numerically investigated for the Reynolds numbers of 2.25 × 104, 4.5 × 104, and 9.0 × 104. Various hybrid methods—Reynolds-averaged Navier–Stokes (RANS) with the Ffowcs Williams and Hawkings (FWH) model, detached-eddy simulation (DES) with FWH, and large-eddy simulation with FWH—were used for the acoustic analyses, and their performances were evaluated by comparing the predicted results with the experimental data. The DES-FWH hybrid method was found to be suitable for the aero- and hydro-acoustic analysis. The hydro-acoustic measurements were performed in a silent circulation channel for the Reynolds number of 2.25 × 104. The results showed that the fluid temperature caused an increase in the overall sound pressure levels (OASPLs) and the maximum sound pressure levels (SPLT) for the air medium; however, it caused a decrease for the water medium. The salinity had smaller effects on the OASPL and SPLT compared to the temperature. Moreover, the main peak frequency increased with the air temperature but decreased with the water temperature, and it was nearly constant with the change in the salinity ratio. The SPLT and OASPL for the water medium were quite higher than those for the air medium.

Blue whiting Micromesistius poutassou diel feeding behaviour in the Irminger Sea

With warming ocean temperatures, the abundance of blue whiting Micromesistius poutassou is increasing in the waters around Greenland. However, in this region, knowledge about the species’ trophic role in the ecosystem is scarce. Consequently, we investigated the diet composition and diel feeding behaviour of blue whiting in the north-eastern part of the Irminger Sea in Greenland waters by analysing their stomach contents and the vertical position/movements of their prey from zooplankton samples and hydroacoustic measurements. We collected the data during a designated experimental survey in July 2016 with repeated sampling at the same location. Results from the stomachs of 624 blue whiting individuals ranging from 22-39 cm long (total length) showed that the highest food intake took place from noon until late evening, with minimum feeding occurring in the morning. The most essential prey groups consisted of euphausiids, copepods, amphipods and fish, in that respective order. Regarding copepod prey, blue whiting had a strong affinity for Calanus hyperboreus and Paraeuchaeta spp. and showed potential for local depletion of these large copepods. On the other hand, the more abundant but smaller C. finmarchicus was almost absent in the fish stomachs, in contrast to findings in other regions. This new understanding provides an early indication of some of the emerging trophodynamics in the Irminger Sea and similar subarctic zooplankton communities with increasing numbers of blue whiting. Our results confirm the importance of accounting for diel and size-specific differences in blue whiting feeding when studying various aspects of its food intake.

Statistical Characteristics of Flow Field through Open and Semi-Closed Bileaflet Mechanical Heart Valve

The formation of thrombi on the streamlined surface of the bileaflet mechanical heart valves is one of the main disadvantages of such valves. Thrombi block the valve leaflets and disrupt the cardiovascular system. Diagnosis of thrombosis of the bileaflet mechanical heart valves is relevant and requires the creation of effective diagnostic tools. Hydroacoustic registration of the heart noise is one of the methods for diagnosing the operation of a mechanical heart valve. The purpose of the research is to determine the statistical characteristics of the vortex and jet flow through the open and semi-closed bileaflet mechanical heart valve, to identify hydroacoustic differences and diagnostic signs to determine the operating conditions of the valve. Experimental studies were conducted in laboratory conditions on a model of the left atrium and left ventricle of the heart between which there was the bileaflet mechanical heart valve. Hydrodynamic noise was recorded by miniature pressure sensors, which were located downstream of the valve. The vortex and jet flow behind the prosthetic heart valve were non-linear, random processes and were analyzed using the methods of mathematical statistics and probability theory. The integral and spectral characteristics of the pressure field were obtained and the differences in the noise levels and their spectral components near the central and side jets for the open and semi-closed mitral valve were established. It was shown that hydroacoustic measurements could be an effective basis for developing diagnostic equipment for monitoring the bileaflet mechanical heart valve operation. Doi: 10.28991/SciMedJ-2020-0204-1 Full Text: PDF

Numerical investigation of geometrical parameters on the hydrodynamic noise characteristics of submerged bodies and comparisons with experiments

The effects of the geometrical parameters on the hydroacoustic characteristics of the flow over rectangular, square and circular cylinders are investigated by numerical analyses and experiments. The numerical simulations are carried out by using a hybrid method which combines RANS with FWH equation. In order to validate the numerical results, the hydroacoustic measurements are also performed for the circular cylinders. The circular cylinders with diameters of 9.5, 19.0, 38.0 and 65.0 mm and aspect ratios of 2.5, 5.0 and 10.0 are employed for the hydroacoustic measurements and analyses. The rectangular cylinders with side ratios of 0.3, 0.6, 1.8 and 3.0, and also square cylinder with the side ratio of 1.0 are considered in hydroacoustic analyses. The Reynolds numbers are in the range of 2.25 × 104 and 1.7 × 105. The hydroacoustic characteristics of the cylinders are obtained to be completely different due to the differences in the shear layer separation, reattachment mechanism and the intensity of disturbance. The shape of the noise spectrum significantly changes with the geometrical shapes of the cylinders. The spectrum becomes narrower by an increase in the side ratio. The main peak frequency reduces when the side ratio increases. The highest value of the maximum sound pressure level, SPLmax are observed for the square cylinder and the lowest value for the rectangular cylinder with the side ratio of 0.6. The peak spectrum becomes like a line spectrum as the cylinder diameter decreases. The main peak frequency decreases when the cylinder diameter increases but it is almost constant with the aspect ratio. At the constant Reynolds number, the broadband noise level and SPLmax decrease with an increase in the cylinder diameter and decrease in the aspect ratio. A good agreement between the numerical and experimental results are obtained.

Predicting Transverse Mixing Efficiency Downstream of a River Confluence

AbstractPredicting mixing processes, especially transverse mixing, downstream of river confluences, is necessary for assessing and modeling the fate of pollutants transported in river networks, but it is still challenging. Typically, there is a lack of transverse mixing solutions implemented in 1‐D hydrodynamical models widely used in river engineering applications. To investigate the mixing processes developing downstream of a medium‐sized river confluence, three high‐resolution in situ surveys are conducted at the Rhône‐Saône confluence in France, based on geolocated specific conductivity and hydroacoustic measurements. Contrasting mixing situations are observed depending on hydrological conditions. In some cases, the two flows mix slowly due to turbulent shear at their vertical interface. This can be modeled by an analytical solution of the advection‐diffusion equation. In other cases, the waters from one of the two tributaries move under the waters of the other tributary. The induced local circulation enhances transverse mixing but not vertical mixing and the flow remains stratified vertically, which may be missed when surface or satellite images are analyzed qualitatively. Stratification may be predicted by comparing the time scales for shear and density‐driven adjustment. Shear‐dominated transverse mixing of depth‐averaged concentrations can be predicted analytically and implemented in 1‐D hydrodynamical models. However, the initiation of apparently rapid transverse mixing due to density‐driven circulation remains to be better understood and quantified.

The bathymetry of coral reef area at Bonetambung Island (optical and hydroacoustical comparison to nautical chart)

Base-on time efficiency, cost, and wide coverage area, bathymetry mapping with a hydroacoustic measurement on a small island were avoided; only remote sensing surveys were conducted. Therefore, comparing the bathymetry map of Bonetambung Island, from Hydroacoustic data with that from Landsat 8 OLI images, is essential. Hydroacoustic measurements were carried out in September 2013, while Landsat 8 Satellite Imagery was recorded on April 25, 2018. After being corrected, Kriging (K) and Natural Neighbor (NN) interpolations methods were performed on both data. The 95% ANOVA test, conducted in a previous study, showed that the average depth for the two interpolation results did not differ significantly for the depth range of 0-5m. However, for the 0-30m depth range, this study shows that the accuracy of interpolation hydroacoustic data is much better with the range of RMSE = 0.8-1.2m, ME = 0.4-0.7m, and range of RMSE = 0.8-1.3, ME = 0.4-0.7 for each K and NN interpolation methods. On the other hand, only in the depth range of 0-5m, the accuracy of interpolation Landsat 8 OLI data shows a relatively good value with RMSE = 1.1m, ME = 0.8m, and RMSE = 1.1m and ME = 0.8m for the K and NN Methods respectively. The results of hydroacoustic data interpolation and Landsat 8 OLI were then visually compared with Dishidros Nautical chart number 124. It was concluded that although Landsat data can produce bathymetry maps with a wider coverage area, its accuracy is still unreliable compared to bathymetry maps from Hydroacoustic measurements.

Spatial and Temporal Changes of Tidal Inlet Using Object-Based Image Analysis of Multibeam Echosounder Measurements: A Case from the Lagoon of Venice, Italy

Scientific exploration of seabed substrata has significantly progressed in the last few years. Hydroacoustic methods of seafloor investigation, including multibeam echosounder measurements, allow us to map large areas of the seabed with unprecedented precision. Through time-series of hydroacoustic measurements, it was possible to determine areas with distinct characteristics in the inlets of the Lagoon of Venice, Italy. Their temporal variability was investigated. Monitoring the changes was particularly relevant, considering the presence at the channel inlets of mobile barriers of the Experimental Electromechanical Module (MoSE) project installed to protect the historical city of Venice from flooding. The detection of temporal and spatial changes was performed by comparing seafloor maps created using object-based image analysis and supervised classifiers. The analysis included extraction of 25 multibeam echosounder bathymetry and backscatter features. Their importance was estimated using an objective approach with two feature selection methods. Moreover, the study investigated how the accuracy of classification could be affected by the scale of object-based segmentation. The application of the classification method at the proper scale allowed us to observe habitat changes in the tidal inlet of the Venice Lagoon, showing that the sediment substrates located in the Chioggia inlet were subjected to very dynamic changes. In general, during the study period, the area was enriched in mixed and muddy sediments and was depleted in sandy deposits. This study presents a unique methodological approach to predictive seabed sediment composition mapping and change detection in a very shallow marine environment. A consistent, repeatable, logical site-specific workflow was designed, whose main assumptions could be applied to other seabed mapping case studies in both shallow and deep marine environments, all over the world.