Understanding satellite-derived bathymetry using Sentinel 2 imagery and spatial prediction models

ABSTRACT

Optical satellite data is an efficient and complementary method to hydrographic surveys for deriving bathymetry in shallow coastal waters. Empirical approaches (in particular, the models of Stumpf and Lyzenga) provide a practical methodology to derive bathymetric information from remote sensing. Recent studies, however, have focused on enhancing the performance of such empirical approaches by extending them via spatial information. In this study, the relationship between multibeam depth and Sentinel-2 image bands was analyzed in an optically complex environment using the spatial predictor of kriging with an external drift (KED), where its external drift component was estimated: a) by a ratio of log-transformed bands based on Stumpf’s model (KED_S) and b) by a log-linear transform based on Lyzenga’s model (KED_L). Through the calibration of KED models, the study objectives were: 1) to better understand the empirical relationship between Sentinel-2 multispectral satellite reflectance and depth, 2) to test the robustness of KED to derive bathymetry in a multitemporal series of Sentinel-2 images and multibeam data, and 3) to compare the performance of KED against the existing non-spatial models described by Stumpf et al. and Lyzenga. Results showed that KED could improve prediction accuracy with a decrease in RMSE of 89% and 88%, and an increase in R² of 27% and 14%, over the Stumpf and Lyzenga models, respectively. The decrease in RMSE provides a worthwhile improvement in accuracy, where results showed effective prediction of depth up to 6 m. However, the presence of higher concentrations of suspended materials, especially river plumes, can reduce this threshold to 4 m. As would be expected, prediction accuracy could be improved through the removal of outliers, which were mainly located in the channel of the river, areas influenced by the river plume, abrupt topography, but also very shallow areas close to the shoreline. These areas have been identified as conflictive zones where satellite-derived bathymetry can be compromised.     

Wave disturbance behind low-crested structures: Diffraction and overtopping effects

In recent years the hydraulic performance of low-crested structures has been widely studied both theoretically and through experimental analyses, but only a few studies have been focused on the combined diffraction-overtopping effects on wave transmission and induced wave currents.In this paper hydraulic model tests conducted at the 3D wave basin of Delft University of Technology (Cáceres et al., 2008) were used to obtain and discuss two simple methods for predicting the wave height at the lee of a single detached breakwater of finite length and the related current regime. For the first time diffraction effects are expressly accounted for. The agreement with experimental data is encouraging.The main objective of the paper is to aid engineers in the first stage of the design process, when using a mathematical model could be unnecessarily excessive.     

Magnetic anomalies in the Shikoku Basin: a new interpretation

Kaiko surveys over the Nankai Trough made available new magnetic and structural data for the northern Shikoku Basin. A survey of the oceanic lithosphere subducting below Southwest Japan along the central Nankai Trough revealed the existence of several north-south basement troughs. They are probably transform faults related to a north-south spreading system. We examine the possibility of a late phase of north-south spreading limited to the axial northernmost Shikoku Basin, active between 14 and 12 Ma. If this system was already active before that time, i.e. during the N55° opening of the southeastern basin, then a triple junction should be found between both areas.Based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin. From early east-west rifting to late north-south spreading, opening of the Shikoku Basin proceeded through multiple episodes of spreading. The analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space. Two successive counter-clockwise rotations of the spreading direction are postulated, at anomaly 6 (19 Ma) and at anomaly 5B (14 Ma), involving segmentation and rotation of the spreading ridge.     

Structuring factors and recent changes in subtidal macrozoobenthic communities of a coastal lagoon, Arcachon Bay (France)

Sediment stability in the shallow Venice Lagoon was investigated by means of numerical modelling. Results from a hydrodynamic model allowed for the determination of the wave climate and bottom effective parameters so that simulations with a Lagrangian model for suspended particulate matter could be performed. A spring–neap cycle in summer 1998 was chosen as integration period since data for calibration and verification were collected within the European project F-ECTS between summer 1998 and spring 1999. Deposition on shallow mud flats as well as short term erosion during a strong wind event were reproduced and mass balances for two areas computed. A relation of patterns of SPM in the water and in the sediment was found and can be ascribed to the displacement of material during storm events from shallow areas to the bottom of very small channels. Assuming about 10 to 14 storm events during the year comparable to the Bora event during the integration time, estimates for long-term trends of sediment loss on shallow flats by Day et al. [Day, J.W., Rybczyk, J., Scarton, F., Rismondo, A., Are, D., Cecconi, G., 1999. Soil accretionary dynamics, sea-level rise and the survival of wetlands in Venice Lagoon: a field and modelling approach. Estuarine, Coastal and Shelf Science 49, 607–628] are met by the simulation results. Evidently, long-term sediment evolution of the lagoon is therefore not dominated by the average (residual) processes that occur in the lagoon, but by the few peak events that happen randomly over the year.     

Reducing fluctuations in the sample variogram

In the analysis of regionalized data, irregular sampling patterns are often responsible for large deviations (fluctuations) between the theoretical and sample semi-variograms. This article proposes a new semi-variogram estimator that is unbiased irrespective of the actual multivariate distribution of the data (provided an assumption of stationarity) and has the minimal variance under a given multivariate distribution model. Such an estimator considerably reduces fluctuations in the sample semi-variogram when the data are strongly correlated and clustered in space, and proves to be robust to a misspecification of the multivariate distribution model. The traditional and proposed semi-variogram estimators are compared through an application to a pollution dataset.     

Urban climate and clues of heat island events in the metropolitan area of Rio de Janeiro

This paper aims to map the thermal field in the metropolitan region of Rio de Janeiro (MARJ) considering the atmospheric characteristics and the land use that contribute to understanding the urban heat island. Three thermal maps are defined through the use of Landsat5-TM satellite images for three winter events chosen for the decades of 1980, 1990, and 2000, respectively. The results reveal a concentration of warmer cores in urban central areas as well as some local warmer areas in suburban region. Sites with lower temperatures correspond to vegetated areas which are away from the central part of the MARJ, including points of suburban areas. This work emphasizes the importance of the combined analysis of surface temperature with land use and atmospheric conditions, depicting a distinct pattern of heat islands for tropical climate.     

A survey of ultraviolet-bright sources behind the halo of M31

We have performed a wide-area ultraviolet (UV) imaging survey using the GALaxy Evolution eXplorer to search for bright, point-like UV sources behind M31's extended halo. Our survey consisted of 46 pointings covering an effective area of ≈50 deg², in both the far-UV and near-UV channels. We combined these data with optical R -band observations acquired with the WIYN Mosaic-1 imager on the Kitt Peak National Observatory 0.9-m WIYN telescope. An analysis of the brightness and colours of sources matched between our photometric catalogues yielded ≈100 UV-bright quasar candidates. We have obtained discovery spectra for 76 of these targets with the Kast spectrometer on the Lick 3-m telescope and confirmed 30 active galactic nuclei and quasars, 29 galaxies at   z > 0.02  including several early-type systems, 16 Galactic stars (hot main-sequence stars) and one featureless source previously identified as a BL Lac object. Future UV spectroscopy of the brightest targets with the Cosmic Origins Spectrograph on the Hubble Space Telescope will enable a systematic search for diffuse gas in the extended halo of M31.     

Fighting for a future: an actor-oriented planning approach to landscape preservation in Galicia

Modification and channelization of streams and rivers have been conducted extensively throughout the world during the past century. Subsequently, much effort has been directed at re-creating the lost habitats and thereby improving living conditions for aquatic organisms. However, as restoration methods are plentiful, it is difficult to determine which one to use to get the anticipated result. The aim of this study was to compare two commonly used methods in small Danish streams to improve the physical condition: re-meandering and passive restoration through cease of maintenance. Our investigation included measurement of the physical conditions in 29 stream reaches covering four different groups: (1) re-meandered streams, (2) LDC streams (the least disturbed streams available), (3) passively restored streams (>10?years stop of maintenance) and (4) channelized and non-restored streams. The in-stream habitats were compared through analysis of the measured physical parameters and by applying a habitat model. We found that re-meandering is a more effective way of re-creating near-natural physical conditions in small streams compared to passive restoration. This is probably due to the limited energy in small streams which restricts re-shaping of the stream channel. However, based on habitat suitability modelling, the change to the physical condition did not translate into improved habitat suitability for young of the year brown trout highlighting the value of using several methods when evaluating restoration success.