Intertidal Bathymetry Research Articles

Use of a Raspberry-Pi Video Camera for Coastal Flooding Vulnerability Assessment: The Case of Riccione (Italy)

Coastal monitoring is strategic for the correct assessment of nearshore morphodynamics, to verify the effects of anthropogenic interventions for the purpose of coastal protection and for the rapid assessment of flooding vulnerability due to severe events. Remote sensing and field surveys are among the main approaches that have been developed to meet these necessities. Key parameters in the assessment and prevision of coastal flooding extensions, beside meteomarine characteristics, are the topography and slope of beaches, which can be extremely dynamic. The use of continuous monitoring through orthorectified video images allows for the rapid detection of the intertidal bathymetry and flooding threshold during severe events. The aim of this work was to present a comparison of different monitoring strategies and methodologies that have been integrated into repeated surveys in order to evaluate the performance of a new camera system. We used a low-cost camera based on Raspberry Pi called VISTAE (Video monitoring Intelligent STAtion for Environmental applications) for long-term remote observations and GNSS-laser tools for field measurements. The case study was a coastal tract in Riccione, Italy (Northern Adriatic Sea), which is the seat of nourishment interventions and of different types of underwater protection structures to combat coastal erosion. We performed data acquisition and analysis of the emerged beach and of the swash zone in terms of the intertidal bathymetry and shoreline. The results show a generally good agreement between the field and remote measurements through image processing, with a small discrepancy of the order of ≈0.05 m in the vertical and ≈1.5 m in the horizontal in terms of the root mean square error (RMSE). These values are comparable with that of current video monitoring instruments, but the VISTAE has the advantages of its low-cost, programmability and automatized analyses. This result, together with the possibility of continuous monitoring during daylight hours, supports the advantages of a combined approach in coastal flooding vulnerability assessment through integrated and complementary techniques.

Intertidal bathymetry and foreshore slopes derived from satellite images for static coasts

The physical geography of the coastal areas of Bangladesh is highly diverse and dynamic. Despite its significance, getting reasonable estimates of the foreshore slopes is quite challenging for a large spatial scale along the coastline of Bangladesh. In this study, a method of estimating the slope from satellite-based shoreline positions in different times covering a tidal cycle is explained particularly for static coasts. The procedure is based on the shoreline elevations and their correlation with the temporal variation of the position of water within the intertidal zone. The methodology was tested against in-situ measurements with reasonably good agreement for a static coastal region. The proposed methodology does not demand any supplementary data other than satellite images and tidewater elevation for static coasts, and the result can produce intertidal bathymetry along with foreshore slopes without requiring any laborious and time-consuming field data collection. The technique can be used for developing intertidal models for monitoring the dynamics within intertidal regions, as well as morpho-dynamic modeling and also cartographic updates.

Intertidal Bathymetry Extraction with Multispectral Images: A Logistic Regression Approach

In this study, a methodology to estimate the intertidal bathymetry from multispectral remote sensing images is presented. The technique is based on the temporal variability of the water and the intertidal zone reflectance and their correlation with the tidal height. The water spectral behavior is characterized by high absorption at the infrared (IR) band or radiation with higher wavelengths. Due to tidal cycles, pixels on the intertidal zone have higher temporal variability on the near IR spectral reflectance. The variability of IR reflectivity in time is modeled through a sigmoid function of three parameters, where the inflection parameter corresponds to the pixel elevation. The methodology was tested at the Tagus river estuary in Lisbon, Portugal, and at the Bijagós archipelago, in the West African nation of Guinea-Bissau. Multispectral images from Sentinel-2 satellites were used, after atmospheric corrections from ACOLITE processor and the derived bathymetric model validated with in situ data. The presented method does not require additional depth data for calibration, and the output can generate intertidal digital elevation models at 10 m spatial resolution, without any manual editing by the operator. The results show a standard deviation of 0.34 m at the Tagus tidal zone, with −0.50 m bias, performing better than the Stumpf ratio transform algorithm, also applied to the test areas to derive intertidal bathymetry. This methodology can be used to update intertidal elevation models with clear benefits to monitoring of intertidal dynamics, morphodynamic modeling, and cartographic update.

Morphodynamic Feedback Loops Control Stable Fringing Flats

We apply a 2‐D horizontal process‐based model (Delft3D) to study the feedback mechanisms that control the long‐term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long‐term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long‐term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave‐dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact increases with increasing cross‐sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide‐induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.

A new video monitoring system in support of Coastal Zone Management at Apulia Region, Italy

New systems for video monitoring and surveillance of Apulian coasts (South-east of Italy) have been recently employed. The system design aims at obtaining a tool able to automatically perform image acquisition, image processing and dissemination of results on a web portal. In the present paper, the system implementation and video stations installation at two different sites are described. A new specific Shoreline Detection Model (SDM) has been developed, based on boundary extraction procedure from automatic segmented coastal area. In real time, processed images and shoreline maps are uploaded and make freely downloadable. Moreover, shoreline time variation analysis is available on user-selected transects in cross-shore direction. The SDM model validation is here presented at one location. Over a set of testing images, the pixel-based comparison between manually and automatically detected shorelines lead to a mean distance of about 1.17pixel. Then, the reconstruction of intertidal bathymetry from video is reported and compared with topographical (d-RTK) field survey, in order to assess the system efficacy and accuracy in coastal area monitoring, resulting in a mean RMS error overall the transects investigated equal to 0.025 m.

Intertidal Bathymetry Estimation Using Video Images on a Dissipative Beach

Sobral, F.N.C., Pereira, P.S., Cavalcanti, P.G., Guedes, R.M.C., Calliari, L.J., 2013. Intertidal Bathymetry Estimation Using Video Images on a Dissipative Beach.Located at southern of Brazil, Cassino beach is a multiple sandbar beach. A video technique to map intertidal bathymetry changes was applied using CMYK color model to find the waterline position, hydrodynamic data and runup 2% (R2) parameterizations. Linear regression between waterline video detected and elevation estimated with in situ measured maximum swash, showed a correlation coefficient of r2=0.82 and r2=0.91, respectively. Rmse values for those comparison was 10.14 m (horizontal detection) and 0.12 m (elevation). The empirical swash coefficient (Kosc) was 1.08 with a total error δz of rms=0.25 m and represents a waterline underestimated to compensate the overestimated horizontal determination. The principal error was found to be at horizontal position, which means that the R2 parameterization was efficient. The estimated bathymetry temporal evolution changes showed a similar morphologic evolution when compared with beach profile with a total rmse of 0.13 m.

Integrated monitoring of the hydro-morphodynamics of a beach protected by low crested detached breakwaters

The aim of this paper is to propose an integrated low cost system for monitoring the performance of beach defence works during storms. For this purpose, the site of Igea Marina, Northern Adriatic Sea, Italy, which is protected by low crested detached breakwaters, is selected. The monitoring system is composed by a video installation, for the identification of the shoreline position and of the intertidal bathymetry at high space and time resolution, and a 2DH numerical model, to reconstruct the hydrodynamics induced by coastal defenses. The accuracy of the monitoring system is verified against available measurements of waves and currents performed during a field campaign with acoustic Doppler profilers. Assimilation of data on the shoreline position in 2DH model operatively working is challenging in the improvement of the production of risk maps. These show the current intensities and flooded areas during forecasted storms. First promising results suggest the possibility of setting up an early warning tool.

The Performance of Shoreline Detection Models Applied to Video Imagery

Digital images of the intertidal region were used to map shorelines and the intertidal bathymetry along four geo-morphically and hydrodynamically distinct coastlines in the United States, United Kingdom, The Netherlands, and Australia. Mapping methods, each of which was originally designed to perform well at only one of the four sites, were applied to all four sites, and the results were compared to direct topographic surveys. It was determined that the rms errors of image-derived versus directly surveyed elevations depended on the prevailing hydrodynamic conditions as well as differences in each of the four different mapping methods. Before these differences were accounted for, rms errors ranged from 0.3 to 0.7 m. An empirical correction model that computed local estimates of setup, swash, and surf beat amplitudes reduced errors by about 50%, with residual rms errors ranging between 0.1 and 0.4 m. The model required tuning only one parameter that determined how each method was affected by swash at infragravity and incident wave frequencies. In environments where all methods successfully identify shorelines, the methods can be used somewhat interchangeably. The diversity of methods is advantageous in situations where one or more methods are likely to fail (e.g., lack of color imagery, high degree of alongshore variability). This remote sensing methodology can be applied to the shoreline and intertidal mapping problem across diverse nearshore environments.

A video-based technique for mapping intertidal beach bathymetry

Measuring the location of the shoreline and monitoring foreshore changes through time are core tasks carried out by coastal engineers for a wide range of research, monitoring and design applications. With the advent of digital imaging technology, shore-based video systems provide continuous and automated data collection, encompassing a much greater range of time and spatial scales than were previously possible using field survey methods. A new video-based technique is presented that utilises full-colour image information, which overcomes problems associated with previous grey-scale methods, which work well at steep (reflective) sites, but are less successful at flatter (dissipative) sites. Identification of the shoreline feature is achieved by the automated clustering of sub-aqueous and sub-aerial pixels in ‘Hue–Saturation–Value’ (HSV) colour space, and applying an objective discriminator function to define their boundary (i.e., ‘shoreline’) within a time-series of consecutive geo-referenced images. The elevation corresponding to the detected shoreline features is calculated on the basis of concurrent tide and wave information, which is incorporated in a model that combines the effects of wave set-up and swash, at both incident and infragravity frequencies. Validation of the technique is achieved by comparison with DGPS survey results, to assess the accuracy of the detection and elevation methods both separately and together. The uncertainties associated with the two sub-components of the model tend to compensate for each other. The mean difference between image-based and surveyed shoreline elevations was less than 15 cm along 85% of the 2-km study region, which corresponded to an horizontal offset of 6 m. The application of the intertidal bathymetry mapping technique in support of CZM objectives is briefly illustrated at two sites in The Netherlands and Australia.

Morphodynamic Modelling of Intertidal Sediment Transport in Morecambe Bay

The construction of a morphodynamic model for the intertidal region of Morecambe Bay, England, and its calibration and validation using extensive sediment volume change data measured by remote sensing, is described. A properly validated sediment transport model of the bay would be of interest for coastal defence, management and economic purposes. The morphodynamic model consisted of a set of linked modules for predicting 2D depth-averaged tidal currents, waves, sediment transport rates and sediment budgets. The intertidal bathymetry was updated at regular intervals and the currents, waves and sediment transport patterns recomputed using the new bathymetry. Tidal input filtering using representative spring and neap tidal cycles was used to reduce computational requirements. Sediment volume changes during the period 1992–1997 were measured over the intertidal zone at a resolution of 240m using the waterline method employing remote sensing and hydrodynamic modelling. The total volume of sediment eroded was −102±16×106m3, and the total volume accreted was 107±15×106m3.Tidal asymmetry is shown to be the dominant agent of sediment transport in the intertidal zone, with waves being of secondary importance. A close correlation was found between the computed directions of tidal asymmetry and observed sediment transport paths. It was necessary to use the remotely sensed data for calibration of the sediment transport model as well as for validation. The model showed some limited capability for predicting gross changes in bathymetry, but there were substantial differences between the predicted and observed patterns of sediment movement. The possibility of improvement by a further incorporation of the spatially and temporally extensive field data into the modelling process is discussed.

Intertidal beach profile estimation using video images

In this paper, we present a technique suitable for measurement of intertidal bathymetry over a broad range of length scales (10 1 to 10 3 m) and time scales (days to decades). A series of time-averaged images of the swash zone are used to map contour lines of the beach surface. In each image, contours are identified using bands of maximum brightness associated with breaking waves at the shoreline. By mapping the location of these bands in a sequence of images collected over one tidal cycle, contour maps of the intertidal bathymetry are generated. We expect this technique to work best (smallest absolute error) under waves which are nearly reflective at the shoreline, but break enough to be observed visually. This is typical of a barred beach since the wave height at the shoreline is limited by wave breaking over the bar crest. The ability of the measurements made with this technique to resolve actual beach elevation variation depends on the ratio of the measurement error variance to the true beach elevation variance. Thus, large measurement errors may be compensated by either large tidal ranges or large temporal changes of the beach itself. In a comparison to bathymetry surveyed using a Differential Global Positioning System (DGPS) during the Duck94 experiment, in Duck, N.C., the image-based elevation estimates were well correlated with the actual bathymetry. The deviations (imagebased vs. DGPS measurements) may be partially attributed to effects scaled by wave height at the shoreline, wave-induced setup, and wave height saturation over the sand bar. In particular, setup was important during dissipative conditions. The rms deviation (vertical) between the DGPS and image-based bathymetry was reduced from 0.24 m to 0.06 m by correcting for the systematic deviations due to variations in setup and wave height saturation. Further improvement of the elevation estimates resulted from parameterizing the actual bathymetry with a simple plane beach surface, which reduced random (or unresolvable) measurement errors. This led to estimates of the beach slope that were accurate to within 10% of the actual slope and estimates of the cross-shore location of the mean sea level line accurate to about 0.50 m.