Stefan Aarninkhof

Variability of shore and shoreline evolution

Shore and shoreline evolution both due to natural and human-induced causes or factors can be variable over a wide range of different temporal and/or spatial scales. Our capability to understand and especially predict this variability is still limited. This can lead to misinterpretation of coastal change information, which hampers informed decision making and the subsequent design and implementation of (soft) engineering interventions. Collecting and describing example observations of shore and shoreline variability is one way to support and improve such human intervention. This paper describes causes and factors for the variability and the resulting possible evolutions of wave-dominated shores and shorelines, which are illustrated by a number of case studies. The new element of this work is that the variability is described in terms of a range of different time and space scales, which helps to structure such analysis. However, it is difficult to generalise the results for arbitrary situations, especially on decadal time scales. Scientific and engineering improvements require more quantitative insight into the physical mechanisms behind the free and forced shore behaviour responsible for the variability.

A video-based technique for mapping intertidal beach bathymetry

Abstract 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.

A New Alternative to Saving Our Beaches from Sea-Level Rise: The Sand Engine

ABSTRACT Stive, M.J.F.; de Schipper, M.A.; Luijendijk, A.P.; Aarninkhof, S.G.J.; van Gelder-Maas, C.; van Thiel de Vries, J.S.M.; de Vries, S.; Henriquez, M.; Marx, S., and Ranasinghe, R., 2013. A new alternative to saving our beaches from local sea-level rise: the sand engine. A boldly innovative soft engineering intervention, comprising an unprecedented 21.5 Mm3 sand nourishment known as the Sand Engine, has recently been implemented in the Netherlands. The Sand Engine nourishment is a pilot project to test the efficacy of local mega-nourishments as a counter measure for the anticipated enhanced coastal recession in the 21st century. The proposed concept, a single mega-nourishment, is expected to be more efficient, economical, and environmentally friendly in the long term than traditional beach and shoreface nourishments presently being used to negate coastal recession. Preliminary numerical model results indicate that this local nourishment will result in the widening of the beach along a 10 to 20 km stretch of the coastline and a beach area gain of 200 ha over a 20-year period. First observations show indeed a redistribution of the sand feeding the adjacent coasts, roughly 40% toward the south and 60% toward the north. While the jury is still out on this globally unique intervention, if proven successful, it may well become a global generic solution for combating sea-level-rise driven coastal recession on open coasts.

The Performance of Shoreline Detection Models Applied to Video Imagery

Abstract 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.

CZM Applications of Argus Coastal Imaging at the Gold Coast, Australia

Abstract A video-based ARGUS coastal imaging system is being used at the northern Gold Coast, Australia to monitor and quantify the regional-scale coastal response to sand nourishment and construction of the world-first Gold Coast artificial (surfing) reef. This automated monitoring system is used to obtain hourly daylight images from four cameras that combined provide continuous coverage of 4.5 km of the coast. Digital image processing techniques are then applied on a routine (weekly to monthly) basis to extract a range of CZM information from the growing image database. Analyses include: the mapping of changing shoreline position (and hence beach width); the measurement of three-dimensional inter-tidal morphology and resulting changes in subaerial sand volume; and the comparison of wave breaking frequency at the reef and adjacent nearshore bars, to quantify enhanced recreational surfing opportunities at the reef site. Based upon the results of image analysis, to date (January 2003) an additional 20–30 m of net beach width was achieved along the approximately 2 km of nourished coastline, relative to the adjacent unnourished beaches to the north and south. Due to a regional net accretionary trend during this same period, in January 2003 the nourished beach at Surfers Paradise was some 50–80 m wider than at the commencement of the video monitoring program in mid 1999.

Coastal Imaging Applications and Research in Australia

Abstract Remote sensing methods are increasingly being deployed to measure and investigate morphology and hydrodynamics in the littoral zone, across spatial scales ranging from centimetres to kilometres, and at time-scales ranging from seconds to years. In the past 5 years in Australia, the deployment of video-based coastal imaging systems has grown rapidly, and by 2004, some 32 cameras were operating at eight sites along the coasts of New South Wales and Queensland. Coastal imaging techniques are being applied to a range of coastline monitoring programs. Projects include large- and small-scale sand nourishment works, the construction of a nearshore artificial reef structure, and the ongoing management of sand bypassing operations. At the same time, the growing image databases are underpinning more fundamental coastal research. The focus of recent and current research includes rip current behaviour, climate impacts, nearshore bar dynamics, and the development of new image analysis methods to support future research.

Using a Shore-Based Video System to Hourly Monitor Storm Water Plumes (Adour River, Bay of Biscay)

Abstract Significant river discharges are usually associated with episodic rainstorms, leading to turbid storm water plumes clearly visible in the vicinity of river mouths. The composition of the suspended particulate matter transported by these plumes can alter the quality of coastal waters. The survey of marine coastal zones has thus become a major issue for water managers. For several decades, satellite imagery has allowed the daily monitoring of river plumes behavior over large distances. Nevertheless, it fails to provide data on their hourly evolution, which is important to operationally manage coastal waters in tourist areas. This study investigates the possibility of using the Argus video system to continuously survey the evolution of a storm water plume impact region. The system was installed in Anglet, at the Adour River mouth (southwest of France), in February 2005. An image-processing technique, based on pixel intensity clustering, is proposed to delineate river plume fronts along beaches from images collected with the Adour video station. Application of this technique, tested on storm water runoff plume events occurring under different climate conditions, shows promising results.

Response of a shoreline sand wave to beach nourishment

On Egmond beach, The Netherlands, the trough of a shoreline sand wave was filled by a beach nourishment. Monthly monitoring over a 4 year period revealed that the shoreline sand wave recovered in about half a year, exhibiting an amplitude that exceeded the prenourishment one. The more rapid response of the lower elevation contours relative to the higher elevated ones resulted temporarily in beach steepening in the trough area and flattening in the crest area. A first comparison to similar time-scale (months) variations in wave conditions revealed neither evidence that the shoreline sand wave amplitude would tend to flatten during high energetic conditions nor that it would tend to grow under highangle incident waves. The role of the slowly evolving nearshore morphology and its effects on the nearshore flow field at the monthly time-scale need further study.

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.

2DH-Quantification of Surf Zone Bathymetry from Video

Information on the actual state of the nearshore zone – in terms of topographic variability, surface waves, and circulation patterns is crucial in many naval and civilian applications. Spatial scales of interest generally span distances on the order of hundreds of meters to a few kilometers. Obtaining this information form in situ measurements or model predictions alone is not feasible. This paper presents results of the first efforts in assimilating high-resolution video observations of the surf zone with a 2DH morphological model to map 3D bathymetry in the surf zone. Two pilot applications demonstrate the model’s potential, returning a realistic rip channelled beach topography at Monterey (CA, USA) and preserving the characteristic bar-trough configuration at Egmond (NL) throughout the entire 9 month simulation period. Being fully embedded in the Delft3D modelling system, the assimilation model can be combined with any regular hydrodynamic or morphological model run. This sophisticated use of highresolution video observations in combination with a 2DH morphological model opens the door towards the nowcasting of nearshore bathymetric