Performance of remote sensing algorithms for shoreline mapping under different beach morphodynamic conditions

Morphodynamic variability of surf zones and beaches: A synthesis

The habitat harshness hypothesis (HHH) pos- tulates that in reflective beaches the harsh environment forces organisms to divert more energy towards main- tenance and they therefore have lower abundance, fecundity, growth and survival rates than in dissipative beaches. Recent investigations have tested this hypoth- esis through single comparisons of only two beaches, and thus the observed trends in population level vari- ables cannot be attributed incontestably to the beach state, but only to location. Here, abundance, reproduc- tion, recruitment, population structure and body size of the intertidal mole crab Emerita brasiliensis were com- pared between populations from eight microtidal ex- posed sandy beaches with contrasting morphodynamics, sampled bimonthly during 22 months throughout the 180 km Uruguayan Atlantic coast. Physical variables and compound indices of the beach state were used to categorize sandy beaches. The results of this bi-annual large-scale analysis were fully consistent with the pre- dictions of the HHH: abundance (total and population components), duration of the reproduction and recruit- ment seasons and the individual size of megalops and females of the mole crab E. brasiliensis decreased from dissipative to reflective beaches. This was reflected by linear or, mostly, nonlinear relationships between bio- logical and both physical variables and compound indices of beach state. In conclusion, this multi-beach sampling provides compelling evidence of a consistent response of demographic and life history traits of an intertidal beach species to morphodynamic characteristics.

<p indent="0mm">The special cape-shaped topography of a headland sandy bay is the reason its shoreline varies differently than that of a straight-line coast. The Narrabeen embayment is located on the southeastern coastline of Australia approximately <sc>20 km</sc> north of Sydney. The enclosed sandy beach is <sc>3.6 km</sc> in length and is composed of fine to medium quartz sand overlying sandstone bedrock, with a median sand grain size D₅₀ of <sc>0.3 mm.</sc> Due to the prevalence of moderate to high wave energy conditions and the exposure of the central and northern portions of the beach to the prevailing south-southeastern swell waves. The morphodynamic responses of the Narrabeen beach are highly variable and rapid alongshore. The beach state was predominantly influenced by episodic storm events throughout the year when erosion and accretion can occur. This study analyzed measurements of cross-shore profiles of the Narrabeen headland sandy bay from 1997-1998. With reference to an atypical huge-wave event, this paper describes in detail the beach erosion caused by huge-waves and the beach restoration caused by small waves after the huge-wave event. It also details the relationship between shoreline variation and wave parameters (wave height and wave angle). To discuss the regulations and causes of beach rotation, the study selected six huge wave events that occurred in 1990–2000, all of which were followed by over eight months of small waves, and analyzed the alongshore variation. The results show that the morphodynamic responses of the Narrabeen beach are highly variable and rapid alongshore. During the huge wave events, the beach was eroded and the shoreline retreated. In the aftermath, the small waves restored the beach. The difference in the shoreline variation due to small waves exhibits a rotational pattern. Wave height is related to sand volume above mean sea level and sand accumulation was rapid and high after the big nearshore wave events. The Southern Oscillation Index (SOI) is closely related to the variability in the shoreline. Time-lagged cross-correlations between monthly beach shoreline positions and the SOI were calculated to investigate the presence of stronger cross-correlations between the beach shoreline and the SOI; the central profile (profile 4) is the most sensitive to the feedback of SOI change. The small waves after a huge-wave event make beach rotation periodic. Analysis of directional wave data and the SOI suggests that beach change is driven by changes in wave height (power) and a subtle change in wave direction, both of which are positively related to the SOl. The change in the SOI induces changes in the wave angles, which leads to sediment transport. This is the most important factor in beach rotation. These results reconfirm huge-wave and small waves subaerial morphology, in particular, these results confirm that the wave height and angle, which are induced by the SOI after a huge-wave event, are key drivers of alongshore variation in headland sandy bay coastlines. Under small wave conditions, the northern and mid beach profiles have negative correlations with SOI index, while the southern beach profiles have positive correlations with SOI index. This lead to sediment erosion in the north while sediment deposition in the south. And the shoreline thereby shows a rotational pattern. With the variation of the SOI index, the sediment movement changes in each beach profile, and the shoreline's rotational pattern shows periodicity.

Beaches along the Rio Grande do Sul (RS) coast are fully exposed with the exception of the extreme northern end where a rocky headland composed of sandstone, basalt and volcano-clastic sequences provide a small degree of protection to coastal dynamics. The beaches generally consist of fine quartz sand with a low gradient swash zone usually containing low-relief beach cusps. Beaches range between intermediate and dissipative, with beach state controlled in part by grain-size variations through the contribution of shell fragments, gravel and coarse quartz sand. This wave-dominated coast is episodically exposed to storm waves, which favors the formation and maintenance of two to three shore parallel sand bars. Most of the beaches possess distinct hazard to bathers associated to their beach state and hazard rating. The longshore bar and trough (LBT) and rhythmic bar and beach (RBB) states are very common and the troughs are usually deeper than the height of the bathers. The combination of higher beach population during summer seasons and rip-dominated intermediate beaches result in the most hazardous beaches along the northern littoral and in the extreme south. Other beach hazards are related to the presence of washouts and flooding, as well as, to episodic deposition of fluid mud at the surfzone and beach as a result of anthropogenic influence. Approximately 80 % of the coastline is undeveloped and the main impacts are associated to car traffic and litter represented by solid waste and plastic beads. Foredunes along the littoral are affected by changes in coastal orientation in relation to the prevailing northeast winds and beach morphodynamics and range from well-developed to non-existent.

Simulating wave runup on an intermediate–reflective beach using a wave-resolving and a wave-averaged version of XBeach

depend on the accurate knowledge of the beach response to sea level regime on multi-time scale. For the long-term beach response, Bruun (1962) suggested that the equilibrium beach profile would move to new equilibrium profile in response to a rising sea level. In this concept called as Bruun rule, the upper part of the beach profile is eroded due to the sea level rise, resulting in the shoreline retreat. It is widely used for the future shoreline prediction. However, the Bruun rule predicts just only the final beach state with a constant wave impinging for an infinite period after sea level rise. On the other hand, simultaneous function of wave and sea level is more important on interannual to decadal-scale beach response. El niño in 2015 and 2016 increased wave energy and sea level, corresponding to large beach erosion across the US west coast (Barnard et al., 2017). Sea level influences the response sensitivity to the wave forcing as a subordinate factor on the morphological change. High water level anomalies made the beach more eroded even if the wave condition was equal. Beach morphology in the swash zone often changes on a 1-year cycle due to seasonal wave conditions. The effect of sea level on the annual cyclic beach morphology in swash zone is still unclear because long-term beach observation data required for the analysis are difficult to obtain. In this study, we investigated the simultaneous effects of the wave and sea level on annual cyclic beach morphology in the swash zone with spectrum analysis for 25-year Hasaki beach observation data.

This contribution presents the results of a study on the shoreline variability of a natural perched urban beach (Ammoudara, N. Crete, Greece). Shoreline variability was monitored in high spatio-temporal resolution using time series of coastal video images and a novel, fully automated 2-D shoreline detection algorithm. Ten-month video monitoring showed that cross-shore shoreline change was, in some areas, up to 8 m with adjacent sections of the shoreline showing contrasting patterns of beach loss or gain. Variability increased in spring/early summer and stabilized until the end of the summer when partial beach recovery commenced. Correlation of the patterns of beach change with wave forcing (as recorded at an offshore wave buoy) is not straightforward; the only discernible association was that particularly energetic waves from the northern sector can trigger changes in the patterns of shoreline variability and that increased variability might be sustained by increases in offshore wave steepness. It was also found that the fronting beachrock reef exerts significant geological control on beach hydrodynamics. Hydrodynamic modelling and observations during an energetic event showed that the reef can filter wave energy in a highly differential manner, depending on its local architecture. In some areas, the reef allows only low-energy waves to impinge on the shoreline, whereas elsewhere penetration of higher waves is facilitated by the low elevation and limited width of the reef or by the presence of an inlet. Wave/reef interaction can also generate complex circulation patterns, including rip currents that appeared to be also constrained by the reef architecture.

Uncertainty in runup predictions on natural beaches using XBeach nonhydrostatic

One of the most applied models for the characterization of sandy beaches is the Wright and Short (Morphodynamic variability of surf zones and beaches: A synthesis) Beach State Model (BSM) that empirically relates dissipative, intermediate and reflective beach types to wave conditions and sediment characteristics. Here we utilize the Delft3D-4 modelling system and create an idealized morphodynamics model to explain beach morphological changes under varying wave conditions. The model accounts for the 3D hydrodynamics and sediment transport processes and is capable of simulating both erosive upstate and accretive downstate sequences. Under an energetic condition, the profile develops into an alongshore-uniform dissipative two-bar profile. Under low wave-energy condition, the morphology develops downstate, leading to initiation and evolution of rip currents following onshore movement of the bar. The bar-formation process starts with sediment erosion at the point with maximum suspended load transport by undertow and sediment deposition on the offshore side of the breakpoint. Later, the sandbar develops as a result of sediment transport convergence of wave asymmetry and the undertow and moves offshore.