Roshanka Ranasinghe

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

Abstract Offshore reef systems consist of individual reefs, with spaces in between, which together constitute the reef matrix. This is the first comprehensive, large-scale study, of the influence of an offshore reef system on wave climate and wave transmission. The focus was on the Great Barrier Reef (GBR), Australia, utilizing a 16-yr record of wave height from seven satellite altimeters. Within the GBR matrix, the wave climate is not strongly dependent on reef matrix submergence. This suggests that after initial wave breaking at the seaward edge of the reef matrix, wave energy that penetrates the matrix has little depth modulation. There is no clear evidence to suggest that as reef matrix porosity (ratio of spaces between individual reefs to reef area) decreases, wave attenuation increases. This is because individual reefs cast a wave shadow much larger than the reef itself; thus, a matrix of isolated reefs is remarkably effective at attenuating wave energy. This weak dependence of transmitted wave energy on depth of reef submergence, and reef matrix porosity, is also evident in the lee of the GBR matrix. Here, wave conditions appear to be dependent largely on local wind speed, rather than wave conditions either seaward, or within the reef matrix. This is because the GBR matrix is a very effective wave absorber, irrespective of water depth and reef matrix porosity.

The State of the World’s Beaches

Coastal zones constitute one of the most heavily populated and developed land zones in the world. Despite the utility and economic benefits that coasts provide, there is no reliable global-scale assessment of historical shoreline change trends. Here, via the use of freely available optical satellite images captured since 1984, in conjunction with sophisticated image interrogation and analysis methods, we present a global-scale assessment of the occurrence of sandy beaches and rates of shoreline change therein. Applying pixel-based supervised classification, we found that 31% of the world’s ice-free shoreline are sandy. The application of an automated shoreline detection method to the sandy shorelines thus identified resulted in a global dataset of shoreline change rates for the 33 year period 1984–2016. Analysis of the satellite derived shoreline data indicates that 24% of the world’s sandy beaches are eroding at rates exceeding 0.5 m/yr, while 28% are accreting and 48% are stable. The majority of the sandy shorelines in marine protected areas are eroding, raising cause for serious concern.

Shallow water bathymetry mapping using Support Vector Machine (SVM) technique and multispectral imagery

ABSTRACT Satellite imagery along with image processing techniques prove to be efficient tools for bathymetry retrieval as they provide time and cost-effective alternatives to traditional methods of water depth estimation. In this article, a nonlinear machine learning technique of Support Vector Machine (SVM) is used to derive shallow water bathymetry data along Sint Maarten Island and Ameland Inlet, The Netherlands, by combining echo-sounding measurements and the reflectance of blue, green, or red bands of Landsat Enhanced Thematic Mapper Plus (Landsat 7 ETM+) and Landsat 8 Operational Land Imager (OLI) imagery with 30 m spatial resolution. In the analysis, 80% of data points of the echo-sounding measurements are used for training and the remaining 20% data points are used for testing. The model utilizes the radial basis kernel function (nonlinear) and the other training factors such as the smoothing parameter, penalty parameter C, and insensitivity zone ε are selected and tuned based on the learning (i.e. training) process. The overall errors during test phases for Sint Maarten Island (1–15 m) and Ameland Inlet (1.00–3.50 m) are 8.26% and 14.43%, respectively, reflecting that the model produces significant estimations for the shallow depths ranges, considered in this study. The results obtained are also compared statistically with those estimated from the widely used linear transform model and ratio transform model, which establish a linear relationship between the water depth and band reflectances. Based on the results, it is evident that SVM provides a comparable or better performance for shallow depth ranges and can be used effectively for deriving accurate and updated medium resolution bathymetric maps.

Modeling of Coastal Morphological Processes

Morphology is the study of forms, whether of animals, plants or words; coastal morphology is the study of coastal forms. Some of these forms are hard (like rock) and do not move much. Others, such as beaches, dunes, channels, sandbars, shoals, are made of mud, sand, or even gravel and move all the time. Understanding and predicting these motions are what coastal morphologists try to do.

Synthetic imagery for the automated detection of rip currents

ABSTRACT Pitman, S.J.; Gallop, S.L.; Haigh, I.D.; Mahmoodi, S.; Masselink, G., and Ranasinghe, R., 2016. Synthetic imagery for the automated detection of rip currents. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 912–916. Coconut Creek (Florida), ISSN 0749-0208. Rip currents are a major hazard on beaches worldwide. Although in-situ measurements of rips can be made in the field, it is generally safer and more cost effective to employ remote sensing methods, such as coastal video imaging systems. However, there is no universal, fully-automated method capable of detecting rips in imagery. In this paper we discuss the benefits of image manipulation, such as filtering, prior to rip detection attempts. Furthermore, we present a new approach to detect rip channels that utilizes synthetic imagery. The creation of a synthetic image involves the partitioning of the ‘parent’ image into key areas, such as sand bars, channels, shoreline and offshore. Then, pixels in each partition are replaced with the respective dominant color trends observed in the parent image. Using synthetic imagery increased the accuracy of rip detection from 81% to 92%. Synthetics reduce ‘noise’ inherent in surfzone imagery and is another step towards an automated approach for rip current detection.

Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand

ABSTRACT Gallop, S.L.; Bryan, K.R.; Pitman, S.J.; Ranasinghe, R., and Sandwell, D., 2016. Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 378–382. Coconut Creek (Florida), ISSN 0749-0208. The exchange of material between the surf zone and continental shelf can be driven by pulsations in rip current velocities. However, there is a poor understanding of the relationship of these pulsations to surf zone morphology and material exchange. Moreover, understanding of rip current dynamics has focused mainly on single-barred beaches in an intermediate state, and there have been few studies on high energy beaches. Therefore, this paper undertakes preliminary research on surf zone current velocity pulsations, on a high energy beach in New Zealand. This initial analysis presents results from two days of measurements using Acoustic Doppler Velocimeters and Lagrangian GPS drifters. Drifters revealed pulsations in current velocities on the order of ∼0.5–2 m s−1 throughout the surf zone, whether inside a rip current circulation cell or not. More infragravity wave energy was associated with constant pulsations in current velocity, and lower infragravity energy with pulsation bursts, lasting 5–10 minutes, interspersed with periods of relatively constant velocity lasting 15–25 minutes. However, higher wave conditions also reduced the exit rate from the surf zone.

Author Correction: The State of the World’s Beaches

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.