Surface Elevation Measurements Research Articles

Detecting Ship Wakes for the Study of Coastal Processes

Rätsep, M.; Parnell, K.E., and Soomere, T., 2020. Detecting ship wakes for the study of coastal processes. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1258–1262. Coconut Creek (Florida), ISSN 0749-0208.Wakes from contemporary vessels may affect, and in some places dominate, coastal processes in the vicinity of major shipping lanes. The analysis of the properties and impact of wakes has generally been restricted to wakes that can be visually observed in raw data. In this work, spectral analysis of the time series of single-point measurements of water surface elevation from Tallinn Bay is used to highlight the structure of ship wakes using a Short Time Fourier Transform. This method makes it possible to determine the speed and distance of a vessel from the measurement site. Wakes are detected using an algorithm based on Gabor multipliers. The results are compared with vessel passages retrieved from the Automatic Identification System (AIS) data. The algorithm detects the majority of ship wakes that can be visually recognized in spectrograms and misses only those with low signal to noise ratio or those in close proximity to another vessel wake. The calculated speed and distance are consistent with the AIS data except for high-speed vessels sailing at ≥30 knots. The results indicate that by using these techniques the detection of vessel wakes from a single-point wave record is achievable under favorable weather conditions. The methods provide an option for mitigation of the impact of ship wakes in semi-enclosed water bodies.

Deep learning video analysis as measurement technique in physical models

In coastal engineering context, the use of video imagery is widespread. Especially in field settings along sandy coasts, many types of data have been derived from video imagery, often using non-learning analysis techniques. Recent advances in the field of computer vision and deep learning allow for the automation of image segmentation. In this paper, these techniques are combined with video imagery of physical model tests, resulting in an innovative non-intrusive measurement technique. This technique is validated for three different applications: the measurement of surface elevation, wave run-up and bed level development. In addition to demonstrating its potential as an alternative for existing measurement instruments, it is shown that the added detail in the spatial or temporal domain provided by the technique can lead to new insights. Examples of this are the detailed analysis on the variability of the run-up height over the width of the flume and the spatial distribution of run-up velocities over the slope.

ANN-Based Airflow Control for an Oscillating Water Column Using Surface Elevation Measurements.

Oscillating water column (OWC) plants face power generation limitations due to the stalling phenomenon. This behavior can be avoided by an airflow control strategy that can anticipate the incoming peak waves and reduce its airflow velocity within the turbine duct. In this sense, this work aims to use the power of artificial neural networks (ANN) to recognize the different incoming waves in order to distinguish the strong waves that provoke the stalling behavior and generate a suitable airflow speed reference for the airflow control scheme. The ANN is, therefore, trained using real surface elevation measurements of the waves. The ANN-based airflow control will control an air valve in the capture chamber to adjust the airflow speed as required. A comparative study has been carried out to compare the ANN-based airflow control to the uncontrolled OWC system in different sea conditions. Also, another study has been carried out using real measured wave input data and generated power of the NEREIDA wave power plant. Results show the effectiveness of the proposed ANN airflow control against the uncontrolled case ensuring power generation improvement.

Detailed force modelling of the secondary load cycle

Steep wave passage around vertical circular cylinders is associated with an additional force peak occurring after the main peak: the secondary load cycle. The secondary load cycle for a focused wave group typical of offshore wind turbine foundations at 33 m full-scale water depth is investigated in scale 1 : 50. Ensemble-averaged force, front face pressures and free surface elevation measurements are used as the basis for the investigation. A two-phase free-surface Reynolds-averaged Navier–Stokes solver is validated against generic cases of turbulent flow over a wall, wave-boundary layer flow for a Reynolds number, , of , two-dimensional (2-D) drag on a cylinder for and 2-D oscillatory flow for three combinations of and Keulegan–Carpenter number, , respectively. The solver is next applied to reproduce ensemble-averaged experimental results of the focused wave group and a good match for the inline force and free surface elevation is found along with a good match for the measured front face pressures. The numerical solution for the focused wave is next analysed in detail to explain the cause of the secondary load cycle. We find that the secondary load cycle is confined to an upper region ranging from just above the still water level to 1.5 cylinder diameters below. By a further break down of the pressure field into contributions from the individual terms of the vertical Navier–Stokes equation, we find that the local force peak in the secondary load cycle is mainly caused by suction effects around the still water level on the back side, contributed through the material time derivative of the vertical velocity, . The suction occurs due to the rapid decrease of water level below the generated water column at the back of the cylinder, which at this time has only just begun its downward motion. The first force local minimum in the secondary load cycle is aided by the hydrostatic pressure from the water column while the second local minimum of the secondary load cycle is aided by wash-down effects on the front side. Finally, the role of the observed vortices behind the cylinder is discussed and compared to reference computations with slip conditions. The results confirm findings from earlier slip boundary studies that the global force history through the secondary load cycle is not strongly affected by the boundary layer. The source of vortices behind the cylinder, observed in both sets of computations is discussed.

Salt marsh elevation and responses to future sea-level rise in the Knysna Estuary, South Africa

Sea-level rise is a significant threat to salt marshes. However, these habitats can respond by increasing their relative elevation, or by migrating into adjacent areas. Here we provide the first report on surface elevation change for salt marsh at the Knysna Estuary, South Africa, and predict the responses of these habitats to sea-level rise using the Sea-Level Affecting Marshes Model (SLAMM). Recent (2018) surface elevation measurements at lower intertidal salt marsh sites were compared with those from 2009–2011. The long-term (1960–2017) relative sea-level rise (RSLR) was estimated as 2.19 mm y−1 and only two of the seven sites assessed are keeping pace with this RSLR. The SLAMM, which was applied to a focus area near Thesen’s Island, predicted up to 40% loss of upper intertidal salt marsh by 2100, because development in the adjacent area prevents habitat migration. Currently, salt marsh habitats cover a total of 667 ha, and artificial structures are present along 20.7 km of the perimeter. Protecting areas for salt marsh migration should be a conservation priority, because this estuary supports the third largest area of intertidal salt marsh in the country. Monitoring the responses of coastal wetlands should be prioritised so that natural capital contributes towards future resilience.

Effects of decimetre-scale surface roughness on L-band brightness temperature of sea ice

Abstract. Sea ice thickness is an essential climate variable. Current L-Band sea ice thickness retrieval methods do not account for sea ice surface roughness that is hypothesised to be not relevant to the process. This study attempts to validate this hypothesis that has not been tested yet. To test this hypothesis, we created a physical model of sea ice roughness based on geometrical optics and merged it into the L-band emissivity model of sea ice that is similar to the one used in the operational sea ice thickness retrieval algorithm. The facet description of sea ice surface used in geometrical optics is derived from 2-D surface elevation measurements. Subsequently the new model was tested with TB measurements performed during the SMOSice 2014 field campaign. Our simulation results corroborate the hypothesis that sea ice surface roughness has a marginal impact on near-nadir TB (used in the current operational retrieval). We demonstrate that the probability distribution function of surface slopes can be approximated with a parametric function whose single parameter can be used to characterise the degree of roughness. Facet azimuth orientation is isotropic at scales greater than 4.3 km. The simulation results indicate that surface roughness is a minor factor in modelling the sea ice brightness temperature. The change in TB is most pronounced at incidence angles greater than 40∘ and can reach up to 8 K for vertical polarisation at 60∘. Therefore current and future L-band missions (SMOS, SMAP, CIMR, SMOS-HR) measuring at such angles can be affected. Comparison of the brightness temperature simulations with the SMOSice 2014 radiometer data does not yield definite results.

Experimental Study of the Statistical Properties of Directionally Spread Ocean Waves Measured by Buoys

AbstractWave-following buoys are used to provide measurements of free surface elevation across the oceans. The measurements they produce are widely used to derive wave-averaged parameters such as significant wave height and peak period, alongside wave-by-wave statistics such as crest height distributions. Particularly concerning the measurement of extreme wave crests, these measurements are often perceived to be less accurate. We directly assess this through a side-by-side laboratory comparison of measurements made using Eulerian wave gauges and model wave-following buoys for randomly generated directionally spread irregular waves representative of extreme conditions on deep water. This study builds on the recent work of McAllister and van den Bremer (2019, https://doi.org/10.1175/JPO-D-19-0170.1), in which buoy measurements of steep directionally spread focused waves groups were considered. Our experiments confirm that the motion of a wave-following buoy should not significantly affect the measured wave crest statistics or spectral parameters and that the discrepancies observed for in situ buoy data are most likely a result of filtering. This filtering occurs when accelerations that are measured by the sensors within a buoy are converted to displacements. We present an approximate means of correcting the resulting measured crest height distributions, which is shown to be effective using our experimental data.

Observed spectrum in the South China Sea during storms

The present work focuses on the measured wave spectrum in the South China Sea (scs) during storms. The measured spectra have been obtained from surface elevation measurements undertaken at 2 locations in scs over 7 years, both of which are related to intermediate relative water depths. Shell’s stringent ts-211 quality control procedure was applied to the raw data and from the data that passed this procedure, the five largest storms for each of the locations were isolated. Analysis of this data in conjunction with the corresponding wind data confirmed that for both of the locations, the largest ‘storms’ comprised of mainly swell. In keeping with this trend, a double-peaked spectrum consisting of a Gaussian swell component and a jonswap wind component provided the best fits to the measured spectra. Further analysis of the spectral parameters resulting from the fits for the individual wave records revealed that no particular trend was identifiable for the shape parameters. Due to this, for each location, a pragmatic fit that encompasses the average spectral shape parameters obtained from all the sea states has been suggested as a typical design spectrum for each location.

Post-20th century near-steady state of Batura Glacier: observational evidence of Karakoram Anomaly

Stable or marginal mass loss dominating in Karakoram has been reported widely through satellite and ground investigations. This work aimed to verify the variation in glacier mass by collecting ground-based data. By tracking profiles from the first survey by China–Pakistan Batura Glacier Investigation Group in 1974–1975, we revisited Batura Glacier and conducted an updated comparable measurement of the glacier surface elevation and ice thickness of this large valley glacier of Karakoram, in August 2017. Results of ground penetrating radar (GPR) measurement were used to improve the accuracy of an ice thickness distribution model (GlabTop2). The model calculation agreed reasonably with the measurement when the optimal basal shear stress (100 kPa for clean ice to 140 kPa for heavy debris cover) and shape factor (0.9) were used. We then used a glacier bed topographies map to calculate the ice flux. By subtracting the glacier surface topographies from the remote-sensing measurements, we observed a marginal thinning in Batura during 2000–2016, with a rate of variation in glacier surface elevation of −0.12 ± 0.27 m a−1. It indicated that the mass gain in the accumulation area nearly compensated the mass loss in the ablation area. In addition, both ground and satellite remote measurement reveal a steady rate of decrease in surface of the Batura tongue, implying an absence of significant variation during the past 40 years. Moreover, the mass conservation equation was applied to the Batura tongue, in combination with surface elevation variation and ice flux evolution. The tongue-averaged mass balance diminished by more than half from the 1970s to the 2010s. In summary, we inferred a near-steady state of Batura Glacier post 2000 based on the above-mentioned evidence of “Karakoram Anomaly”.

How will radar layover impact SWOT measurements of water surface elevation and slope, and estimates of river discharge?

How will radar layover impact SWOT measurements of water surface elevation and slope, and estimates of river discharge?

Evolution of rock cover, surface roughness, and flow velocity on stony soil under simulated rainfall

Erosion pavements occur commonly in many semiarid watersheds due to selective erosion. However, quantitative information regarding the dynamic feedback between soil erosion, surface morphology, and flow hydraulics as erosion pavement develops is limited. In order to quantify the spatiotemporal evolution of rock cover and surface roughness, and measure their effects on flow velocities as erosion pavement develops, a series of rainfall simulations were conducted on a 2 by 6.1 m soil plot under three slope treatments (5%, 12%, and 20%) with surface elevation and rock cover measurements. The total applied rainfall volume for each experimental replication ranging from 1,400 to 2,240 mm caused the development of erosion pavement. The results showed (1) rock cover increased from 15% to 90% as rainfall progressed, and the terminal rock cover was not slope gradient dependent; (2) random roughness was positively correlated with rock cover in the upper and middle plot sections, and increasing surface roughness and rock cover reduced the flow velocity following power functions; (3) surface roughness in the lower sections did not uniformly increase with increasing rock cover due to the formation of rills; (4) the terminal surface roughness values of full plots were 5.3 and 5.1 mm, 4.2 and 4.5 mm, and 2.9 and 3.2 mm for replications of 20%, 12%, and 5% slopes, respectively, indicating that steeper slopes produced greater surface roughness; (5) flow velocities measured at the end of experiments reached a relative constant value that was a function of unit flow rates alone for a given section; (6) hydraulic resistances were correlated with flow discharge, slope gradient, and rock cover, exhibiting no unique hydraulic coefficient for a given surface condition. These results improve our understanding of the evolution of semiarid hillslopes.

Rapid transition of sediment consolidation across an expanding mangrove fringe in the Firth of Thames New Zealand

Mangrove habitats are complex systems, which are subjected to both natural and human external forces such as tidal variations, sediment supply, deforestation, and climate change, which in many locations are causing mangroves to decline and even disappear. Global measurements of surface elevations have been conducted at many locations to understand if sedimentation rates in mangrove areas will keep pace with sea level rise. However, extending results to other areas requires a detailed understanding of subsidence processes in mangrove areas. Here, we provide a detailed geotechnical investigation (sediment cone resistance and friction, coefficient of consolidation, grain size, normalised tip resistance and friction ratio, etc.), critical for understanding surficial sediment dynamics and vertical sediment accretion rates, of the mangrove forest edge and its surrounding mud flat in the Firth of Thames, New Zealand. Eight in situ samples were collected and tested in oedometer experiments to evaluate the coefficient of consolidation. Furthermore, a kinematic penetrometer, NIMROD, was used to estimate the resistance forces of the mud flat and mangrove forest, from which soil properties were evaluated. Our results show that mangroves are able to change soil properties to enhance sediment resistance towards erosion. An increase of sediment strength correlated with an increase of mangrove tree density as well as a decrease in the coefficient of consolidation. Hence, the increase of tree density to a decrease of the coefficient of consolidation correlates as well. In this study, our results suggest that the Firth of Thames mangroves are able to accrete enough sediment to keep up with local sea level rises if sediment supplies remain similar in the future. The correlation of tree density to the coefficient of consolidation can be used to assess future mangrove-soil interaction and mud flat progression. Finally, the applicability of literature soil classifications for the kinematic penetrometer was realized for the first time and applied to mangrove and mud flat areas.

Soft computing applications in the field of industrial and environmental enterprises

Soft computing applications in the field of industrial and environmental enterprises

Accuracy analysis of multi‐base InSAR altimeter in ocean surface relative elevation measurement

A new sea-level elevation measurement system was introduced, which uses a distributed multi-base forward-scattering mechanism. After introducing the working principle of the system, this study analyses the main influencing factors of relative elevation measurement in detail. It is proved through quantitative expression that the interferometric phase error is the main error source affecting relative elevation measurement. In order to evaluate the system capability and prove that the multi-base InSAR (MbInSAR) can meet the application requirements of gravity field inversion through relative ocean surface elevation measurement, three aspects of decorrelation, geometric decorrelation, volumetric decorrelation, and SNR decorrelation, are analysed as the interferometric phase error sources.

How Does the Unique Space‐Time Sampling of the SWOT Mission Influence River Discharge Series Characteristics?

AbstractThe Surface Water and Ocean Topography (SWOT) satellite mission will, for the first time, provide simultaneous, high‐resolution measurements of water surface elevation and extent. Here we explore the applicability of SWOT's unique sampling to capture discharge frequency behavior throughout the Mississippi River Basin. Our findings suggest the mission may capture key variability in river discharge series. SWOT orbit specifications, US Geological Survey (USGS) discharge measurements, and potential uncertainty estimates are used to generate SWOT‐like river discharges. Frequency distributions and specific quantiles derived from synthetic SWOT discharge series are compared to those derived from daily USGS discharge series. Based on the Kolmogorov‐Smirnov test, SWOT temporal sampling has essentially no impact on derived frequency distributions. When including potential uncertainty, 78% of derived distributions are statistically identical. The combined effects of temporal sampling and discharge uncertainty mitigates the negative bias used for SWOT discharge uncertainty at larger discharge quantiles (i.e., ≥75% quantiles).

Greater Water Surface Variability Revealed by New Congo River Field Data: Implications for Satellite Altimetry Measurements of Large Rivers

AbstractLarge river hydrodynamics studies inform global and regional issues pertaining to biogeochemical cycling, ecology, water availability, and flood risk. Such studies rely increasingly on satellite measurements, but these are limited by resolution, coverage, and uncertainty and their inability to directly measure bathymetry or discharge. We obtain new in situ data covering 650 km of the Congo's main stem, including elusive bathymetry and discharge measurements that complement space‐borne data sets. Our key findings relate to our water surface elevation measurements, which show that spatial coverage of existing satellite altimetry for deriving river water surface profiles may be adequate through the globally important Cuvette Centrale but is not at its outlet where our field data reveal significant spatial variability in water surface slope. The findings have implications for altimetry‐based hydrodynamics studies of other large rivers, such as those that involve estimating discharge or modeling multichannel river hydraulics.

Estimation of Wave Crest Amplitudes Distribution and Freak Wave Occurrence in A Short Crested Mixed Sea

In this study we have for the first time proposed a novel transformed linear simulation method for the estimation of wave crest amplitudes distribution and freak wave occurrence in a short crested mixed sea with a bimodal 3D spectrum. For implementing the proposed transformed linear simulation method, a Hermite transformation model expressed in a monotonic cubic polynomial has been constructed so that the first four moments of the original true process match the corresponding moments of the transformed model. The proposed novel simulation method has been applied to forecast the freak wave occurrence in two short crested mixed sea states, one with a directional wave spectrum based on the measured surface elevation data at the coast of Yura, and the other one with a typical directional bimodal Torsethaugen wave spectrum. It is shown in the two cases that the proposed novel simulation method can offer more accurate forecasting results than those obtained from the traditional linear simulation method or by using Rayleigh distribution model. It is also demonstrated in this article that the proposed novel simulation method is more efficient than the nonlinear simulation method.

The impact of reach averaging Manning’s equation for an in-situ dataset of water surface elevation, width, and slope

The Surface Water and Ocean Topography Mission (SWOT) will generate global, spatially continuous maps of water surface elevation and extent for large inland water bodies when it launches in 2021. We present an analysis of water surface elevation, width, and bathymetry timeseries data from a medium-sized (average annual discharge 14 m3/s) river to explore Manning’s equation, an empirical open channel flow equation, in the context of SWOT discharge algorithms. While this equation is in theory inapplicable to natural channels due to the non-uniform and spatially heterogeneous nature of river systems, we explored approaches to adapt it to this context using reach-averaged variables. At twenty sites along a 6.5 km stretch of the Olentangy River in Ohio, USA, we collected automated and manual measurements of water surface elevation and river width, undertook a full bathymetric survey of the study area, and built a hydraulic model. The stretch of river was divided into five reaches, and hydraulic variables were reach-averaged. Using these variables, we used a modified form of Manning’s equation to compute a reach-averaged roughness coefficient. Reach-averaged roughness coefficients varied nonlinearly with discharge and were 2–10 times larger at low flow than at high flow in the in-situ data, ranging from 0.06 to 0.61 in one of the study reaches. These results were compared with the output of an unsteady flow simulation using a calibrated 1-D hydraulic model which was run with constant roughness coefficients at each cross section. When reach-averaged data was used, model-derived roughness coefficient also varied by more than an order of magnitude, with a range of 0.02–0.82 for one reach. For both in-situ and model-derived datasets, using a two-parameter roughness coefficient which scaled with a power law on either discharge or stage reduced discharge estimation error, with error for one reach dropping from 81% to 8% relative root-mean square error (rRMSE) in the in-situ data and 58% to 8% nRMSE in the modeled data. These results imply that spatial averaging of hydraulic variables leads to large variations in reach averaged Manning’s n, which we term the reach’s “effective resistance”, and suggest that this variability can be accounted for with a simple parameterization in estimates of discharge that use spatially averaged data.

Investigation of ultrasonic sensor type JSN-SRT04 performance as flood elevation detection

To measure water surface elevation, the measurement technique with ultrasonic technology has been recently being applied to various sectors, such as automotive, robotic and industrial requirement. This research aims to create water surface elevation measurement with JSN-SR04T ultrasonic sensor, which has 0.5 cm measurement resolution. Arduino-Uno microcontroller with a microprocessor-Atmega238 type was used as a measurement data processing system. The system test was conducted in a river where the system was placed under the bridge, and then the elevation water surface levels were captured by the sensor. After that, the results were displayed on the LCD screen. The test was executed in 37 testing scenarios of the condition of the river water surface. Then, the measurement results from the system were compared to the manual measurement. The results showed that the value of measurement error was 0.75%. This finding is in line with the datasheet which indicates that WSN-SR04T sensor can provide a maximum resolution of 0.5 cm.

Disentangling and Parameterizing Shallow Sources of Subsidence: Application to a Reclaimed Coastal Area, Flevoland, the Netherlands

AbstractThe natural surface area of many coastal and delta plains has been increased by land reclamation in response to growing populations. These reclaimed lands are often experiencing subsidence. The reclaimed South Flevopolder in the coastal plain of the Netherlands has experienced severe subsidence after its reclamation in 1968. The subsidence is caused by phreatic groundwater level lowering and the associated aeration of the former subaqueous shallow subsurface and increased effective stresses. In this study, surface elevation measurements that quantify the subsidence, conducted annually between 1968 and 1993, and in 2009 and 2012, have been used to constrain and estimate the parameters in models that describe subsidence. For the estimation an Ensemble Smoother with Multiple Data Assimilation was employed. For the forward models, we employed correlations for compression (primary consolidation and creep), oxidation, and shrinkage of coastal deposits. Shrinkage of the aerated clay and organic clay layers was found to be the main contributor to subsidence, and the measurements could be represented well. The quantification of the model parameters allows for better subsidence forecasts. The stochastic method that was employed further facilitates to define a quality measure for forecasts in terms of a covariance matrix or a confidence range.