Gerd Masselink

Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia

Abstract Seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastline of Perth, Western Australia, is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology. However, these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions. In summer, when northward sediment transport prevails due to sea breeze activity, beaches located south of coastal structures, headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle. These beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south. In contrast, beaches located north of obstacles will become narrower during summer and wider during winter. The usefulness of the dimensionless fall velocity Ω=H b /w s T (where Hb is the breaker height, ws is the sediment fall velocity and T is the wave period) as a predictor of presence/absence of bar morphology and beach type was investigated. It was found that Ω fluctuates around the threshold of bar formation (Ω≈1.5–2) over a variety of time scales (daily, weekly, and seasonally). These temporal variations in Ω in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Ω along the Perth coast.

BeachWin: Modelling groundwater effects on swash sediment transport and beach profile changes

Field and laboratory observations have shown that a relatively low beach groundwater table enhances beach accretion. These observations have led to the beach dewatering technique (artificially lowering the beach water table) for combating beach erosion. Here we present a process-based numerical model that simulates the interacting wave motion on the beach, coastal groundwater flow, swash sediment transport and beach profile changes. Results of model simulations demonstrate that the model replicates accretionary effects of a low beach water table on beach profile changes and has the potential to become a tool for assessing the effectiveness of beach dewatering systems.

Extreme wave activity during 2013/2014 winter and morphological impacts along the Atlantic coast of Europe

Studies of coastal vulnerability due to climate change tend to focus on the consequences of sea level rise, rather than the complex coastal responses resulting from changes to the extreme wave climate. Here we investigate the 2013/2014 winter wave conditions that severely impacted the Atlantic coast of Europe and demonstrate that this winter was the most energetic along most of the Atlantic coast of Europe since at least 1948. Along exposed open-coast sites, extensive beach and dune erosion occurred due to offshore sediment transport. More sheltered sites experienced less erosion and one of the sites even experienced accretion due to beach rotation induced by alongshore sediment transport. Storm wave conditions such as were encountered during the 2013/2014 winter have the potential to dramatically change the equilibrium state (beach gradient, coastal alignment, and nearshore bar position) of beaches along the Atlantic coast of Europe.

Rip current prediction: Development, validation, and evaluation of an operational tool

ABSTRACT Austin, M.J.; Scott, T.M.; Russell, P.E., and Masselink, G., 2013. Rip current prediction: development, validation, and evaluation of an operational tool. This contribution details the development, validation, and evaluation of an operational rip current prediction tool. Field measurements of rip current dynamics from a macrotidal beach in the southwest U.K. collected over 87 tidal cycles indicate that the rip currents are highly dynamic over a range of temporal and spatial scales. The morphology of the lower intertidal beach face provides the primary spatial control of the rip currents, whereas the variation in the pattern of wave dissipation due to the tidal translation of the surf zone at spring-neap and semidiurnal frequencies is the principle temporal control. The Lagrangian drift pattern associated with the rip currents displays three key behaviors: rotation, alongshore, and exit. Rotation and exit are observed under moderate conditions, whereas strong alongshore-directed currents prevail during energetic conditions. An operational regional wave model is used to force a two-dimensional horizontal (2D-H) nonstationary model for coupled wave propagation and flow to predict the rip current speed and behavior. The model is calibrated using measured Eulerian field data, and the resultant circulation patterns are validated against measured Lagrangian data. The model was run for a 2-month hindcast period, and the flow speed and behavioral output were combined to allocate a rip current hazard rating. The model performance was evaluated against beach lifeguard incident statistics; 64% of recorded incidents occurred under predicted high-risk conditions, and 36% occurred during medium-risk conditions. The rip hazard prediction model was subsequently run in forecast mode to provide an example of operational-type output.

Comprehensive Field Study of Swash-Zone Processes. II: Sheet Flow Sediment Concentrations during Quasi-Steady Backwash

AbstractSheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1-mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence, and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20–0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized...

Coastal cliff ground motions and response to extreme storm waves

Coastal cliff erosion from storm waves is observed worldwide, but the processes are notoriously difficult to measure during extreme storm wave conditions when most erosion normally occurs, limiting our understanding of cliff processes. Over January–February 2014, during the largest Atlantic storms in at least 60 years with deepwater significant wave heights of 6–8 m, cliff-top ground motions showed vertical ground displacements in excess of 50–100 µm; an order of magnitude larger than observations made previously. Repeat terrestrial laser scanner surveys over a 2 week period encompassing the extreme storms gave a cliff face volume loss of 2 orders of magnitude larger than the long-term erosion rate. The results imply that erosion of coastal cliffs exposed to extreme storm waves is highly episodic and that long-term rates of cliff erosion will depend on the frequency and severity of extreme storm wave impacts.

Comprehensive Field Study of Swash-Zone Processes. I: Experimental Design with Examples of Hydrodynamic and Sediment Transport Measurements

AbstractA comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic m...

A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly

A pioneering and replicable method based on a 66-year numerical weather and wave hindcast is developed to optimize a climate index based on the sea level pressure (SLP) that best explains winter wave height variability along the coast of western Europe, from Portugal to UK (36–52 ∘ N). The resulting so-called Western Europe Pressure Anomaly (WEPA) is based on the sea level pressure gradient between the stations Valentia (Ireland) and Santa Cruz de Tenerife (Canary Islands). The WEPA positive phase reflects an intensified and southward shifted SLP difference between the Icelandic low and the Azores high, driving severe storms that funnel high-energy waves toward western Europe southward of 52 ∘ N. WEPA outscores by 25–150% the other leading atmospheric modes in explaining winter-averaged significant wave height, and even by a largest amount the winter-averaged extreme wave heights. WEPA is also the only index capturing the 2013/2014 extreme winter that caused widespread coastal erosion and flooding in western Europe.

Overwash experiment on a sandy barrier

ABSTRACT Matias, A., Masselink, G., Kroon, A., Blenkinsopp, C., and Turner, I.L., 2013. Overwash experiment on a sandy barrier This paper uses results obtained from the large-scale BARDEX II experiment undertaken in the Delta flume to investigate the morphological response of a prototype sandy barrier to wave and tidal forcing during overwash conditions. Since overwash processes are known to control short-term barrier dynamics and long-term barrier migration, the development of a robust quantitative method to define the critical conditions leading to barrier overwash is important both for scientific and practical management purposes. The Overwash Potential (OP), defined as the difference between the wave runup and the barrier elevation is used to define the overwash threshold condition, and to predict the morphological outcome of a particular overwash event. When OP is negative, wave runup is lower than the barrier crest and insignificant morphological changes are noticed at the barrier crest. When OP is positive, overwash occurs because predicted runup elevation is higher than the barrier crest. When OP is close to zero, overtop is expected with limited intrusion of water across the top of the barrier crest. To make effective use of OP it is necessary to identify a reliable runup predictor. Twelve runup equations were tested for this purpose, and the results were compared with the ones obtained using data from BARDEX experiment on a gravel barrier. A most reliable approach for the determination of OP for sandy barrier was similar to gravel barrier overwash experiments, with runup predictions provided by the equation of Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A.,Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53, 573–588]. This is striking, since different runup predictors would have been expected because beach slope, hydraulic conductivity, grain-size, amongst other factors, differ for both types of barriers. Nevertheless, the two main morphologic characteristics for the computation of OP are beach slope and the barrier crest elevation, both accounted for in the proposed equation. The use of OP values provides a practical means by which to identify potential coastal hazards associated with barrier overwash processes and is considered to have a range of practical coastal management applications.

BARDEX II: Bringing the beach to the laboratory - again!

ABSTRACT Masselink, G, Turner, I.L., Conley, D.C., Ruessink, B.G., Matias, A., Thompson, C., Castelle, B. and Wolters, G., 2013. BARDEX II: Bringing the beach to the laboratory – again! Proto-type laboratory experiments are particularly useful in coastal research when forcing parameters are modified in a way that is impossible to achieve in the field, and where installation and maintenance of instrumentation requires absence of waves. In 2008, the Barrier Dynamics Experiment (BARDEX) took place in the Delta Flume, the Netherlands. This project, funded by Hydralab III, focused on the effect of varying wave, sea level and beach groundwater conditions on a gravel beach (D50 = 10 mm). In 2012, a similar project was carried out, referred to as BARDEX II, this time funded by Hydralab IV and on a sandy beach (D50 = 0.42 mm). During the experiment, a 4.5-m high and 70-m wide sandy barrier was constructed in the flume with a lagoon situated to the landward. The barrier was instrumented with a very large number (> 200) of instruments and subjected to a range of wave conditions (Hs = 0.8 m; Tp = 4–12 s) and varying sea and lagoon water levels. Five distinct test series were executed over a 20-day period: series A focused on beach response due to accretionary/erosive wave conditions and a high/low lagoon water level; series B investigated the effect of a lower sea level on nearshore bar dynamics; series C simulated tidal effects; series D addressed the swash/overtopping/overwash threshold; and during series E the beach-barrier system was subjected to an extended period of energetic overwash conditions. This paper will describe the experimental design and the test programme during BARDEX II.