Suzana Ilic

Coastal cliff geometry derived from structure-from-motion photogrammetry at Stara Baška, Krk Island, Croatia

The aim of this study was to examine the capability of structure-from-motion photogrammetry in defining the geometry of cliffs and undercuts in rocks of complex geomorphology. A case site was chosen along pocket beaches near the village of Stara Baška on the Adriatic Sea island of Krk, Gulf of Kvarner, Croatia, where cliff erosion of 5 m in breccias was identified by comparison of aerial photographs from 1960 and 2004. The 3D point cloud was derived from approx. 800 photos taken on 9 January 2014 by a single camera from various elevations and angles, and processed using the online software ReCap (Autodesk). Data acquisition was found to be quick and the method easy to implement. The difference between the georeferenced 3D cloud points and an RTK-GPS survey was 7 cm, i.e. within the limits of RTK-GPS precision. Quantifying the spatial variation in undercut geometries revealed that the deepest and largest (17 m3) undercut was in the south-eastern sector of the beach. Reconstructing the detailed geomorphology of this 3.8-m-deep undercut convincingly demonstrates the high efficiency of the method. Such assessments of spatiotemporal changes in undercut and overhang volumes can prove useful for evaluations of cliff erosion risk. Coupled with the low cost and relatively simple application, this is evidently an attractive technique for meaningful geotechnical and coastal engineering monitoring in the future on the island of Krk and, for that matter, also on other Adriatic islands and in similar settings worldwide.

Time development of scour around a cylinder in simulated tidal currents

A laboratory flume experiment was performed to investigate the time development of scour around a vertical cylinder acting as a scaled model of an offshore wind turbine monopile in tidal currents. The tidal current was simulated by resolving each half-cycle into three time steps, between which flow velocity and depth were varied. Flow direction was reversed between half-cycles, which were otherwise identical. Between them, the three time steps exhibited clear water, transitional, and live-bed conditions. The experiment was run over two full-simulated tidal cycles. The scour hole formed tended to a symmetrical shape after two half-cycles and was both shallower and slower developing than the scour hole in a unidirectional current test carried out in the same flume. This was due mainly to the variable rates of scour caused by the variable flow conditions within each half-cycle, and to a lesser extent to the infilling of the scour hole when the current direction reversed. The lower scour depth recorded in tidal conditions implies that the amount of scour protection required may be less than previous studies suggest.

An evaluation of directional analysis techniques for multidirectional, partially reflected waves Part 1: numerical investigations

Recent studies of advanced directional analysis techniques have mainly centred on incident wave fields. In the study of coastal structures, however, partially reflective wave fields are commonly present.In the near structure field, phase locked methods can be successfully applied. In the far field, non-phased locked methods are more appropriate. In this paper, the accuracy of two non-phased locked methods of directional analysis, the maximum likelihood method (MLM) and the Bayesian directional method (BDM) have been quantitatively evaluated using numerical simulations for the case of multidirectional waves with partial reflections. It is shown that the results are influenced by the ratio of distance from the reflector (L) to the length of the time series (S) used in the spectral analysis. Both methods are found to be capable of determining the incident and reflective wave fields when LIS > 0.5.The BDM provides, in most cases, more accurate estimates of incident significant wave height, average reflection coefficients and directional spreading.

EVALUATION OF BEACH MODELLING TECHNIQUES BEHIND DETACHED BREAKWATERS

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

The evolution of an equilibrium bay

This paper reports on laboratory morphological studies of a shore-parallel porous breakwater system. A physical model study of the Elmer breakwater scheme, West Sussex, was conducted in the UK Coastal Research Facility (UKCRF) at HR Wallingford as part of an EPSRC-funded composite model evaluation (LUPY project). Mobile bed experiments, using both sand and anthracite as model sediments are described and discussed. Subsequent analysis showed that the evolution of equilibrium bays and their final width depend not only on the breakwater length and gap width, but also on the properties of the chosen model sediment and the permeability of the structure. It was also found that the 3D morphological changes influence the hydrodynamics, which in turn influences the evolution of the equilibrium morphological features.

An evaluation of directional analysis techniques for multidirectional, partially reflected waves Part 2: application to field data

Based on the findings of a numerical investigation, presented in the Part 1 companion paper, two methods of directional analysis, the maximum likelihood method (MLM) and the Bayesian directional method (BDM) are applied to over 80 field data sets. These cover a wide range of environmental conditions, for which multidirectional, partially reflective sea states exist. The results show that trends similar to those found using the numerical simulations are observed in the field estimates of relative predictions of incident significant wave height, average reflection coefficients, main directions and directional spreading. It is concluded that overall the BDM produces the more accurate results when applied to real sea waves.

Observations of shoreline changes on a pocket gravel beach: Baška beach, Croatia

The Baška beach, one of the largest gravel beaches in Croatia was monitored over 3 years. This paper presents analysis of the shoreline position derived from these measurements. The shoreline position and the beach width were calculated from beach profile measurements. Overall changes in shoreline positions were quite small except in areas affected by fresh-water input (stream and urban runoff) and near a groyne. Several storm events were covered during the observation period. In most of these cases, the shoreline retreat was registered as expected. This would be followed by a rapid beach recovery and resulting shoreline advancement. After NE (Bura) storm wind event the shoreline advancement was registered. However, the close inspection of the beach profiles showed that this registered shoreline advancement was due to accumulation of sediment at the toe of the beach. This material was eroded at the upper beach. This finding and the lack of beach rotation indicate that the cross-shore sediment processes dominate during the energetic events.

DUNE EROSION AND OVERWASH IN LARGE-SCALE FLUME EXPERIMENTS

The present paper gives a general overview of the large-scale physical model experiments performed at the LIM/UPC in Barcelona, within the EU-Hydralab III Integrated Infrastructure Initiative. The model tests have been carried out in a flume with a sandy dune exposed to a combination of water levels and wave conditions. Different regimes of wave attacks on the sandy beach/dune system have been investigated. In particular, the study provides a unique set of large-scale physical data concerning the wave-induced dune overwash. Measurements of hydrodynamics, sediment concentrations and beach-dune profile evolution have been carried out. Profile measurements have been used to calibrate and validate a numerical model (Kobayashi et al. 2007) to predict beach-dune profile modifications over the near-shore region. The numerical model is shown to be in agreement with the experimental data.

Prediction of beach morphological changes using a data-based approach

A data-based approach using linear transfer functions (TF) was adopted to predict the evolution of the nearshore beach profile volume at Duck, North Carolina, using different wave forcing variables. The best TF model relation was found with the squared monthly average direction resolved significant wave heights. This TF model explained 76% of the variance of the data and produced a very good fit of the long-term trend in beach volume. This suggests that the long-term behavior of the bulk morphology of the beach profile is strongly influenced by the monthly average wave conditions. Complimentary long-term patterns in behavior were also observed on comparing the beach morphology and wave data. The fit of this TF model was improved by including the inputs of past alongshore sediment exchanges between adjacent profiles. Here, the TF model reproduced 92% of the variance in the volume data and fitted the long-term trend as well as some short-term behavior. This model gave very good forecasts of beach volume over a 5 year period. Thus, the linear TF modeling approach shows strong potential for predicting beach morphological changes.

Data-Based Approaches to Analysing and Modelling Beach Morphological Changes at Duck, North Carolina

In this study, the ability of two data-based approaches in analysing and modelling beach morphology and wave data at Duck, North Carolina, USA, was investigated. Dynamic Harmonic Regression (DHR) was used to analyse bed elevation and wave height time series in order to identify the long-term trends and dominant periodic components characterising the morphological changes and wave climate at Duck. A linear Transfer Function (TF) modelling approach was used to relate bed elevation time series to nearshore wave height time series with the aim of investigating the suitability of such a simple model in predicting the longer-term morphological changes. The DHR analysis showed multi-scale variability of the beach profile and was in good agreement with observations made in previous studies. The linear TF model showed limited capability with only a small proportion of the data being described by the model.