Beach Profile Changes under Sea Level Rise in Laboratory Flume Experiments at Different Scale

Laboratory wave flume experiments have been used to provide improved understanding of beach profile evolution under different wave and water level conditions. However, the understanding of the processes involved in the evolution of beach profile under Sea Level Rise (SLR) toward equilibrium is unclear. Two similar, but distorted experiments were performed at large and medium scale in order to study the qualitative morphological changes involved in beach profile evolution under SLR. Both experiments showed similar beach profile evolution. The profile change predicted by the Profile Translation Model (PTM) and the Bruun Rule underestimated the observed reatreat in both experiments. The length of the active beach profile increased under SLR. For the large scale experiment, the reflection coefficient of the beach decreased while the vertical runup increased significantly. The beachface changed faster than the outer surf zone, making the beach more dissipative

Monitoring individual wave characteristics in the inner surf with a 2-Dimensional laser scanner (LiDAR)

This paper presents an investigation into the use of a 2-dimensional laser scanner (LiDAR) to obtain measurements of wave processes in the inner surf and swash zones of a microtidal beach (Rousty, Camargue, France). The bed is extracted at the wave-by-wave timescale using a variance threshold method on the time series. Individual wave properties were then retrieved from a local extrema analysis. Finally, individual and averaged wave celerities are obtained using a crest-tracking method and cross-correlation technique, respectively, and compared with common wave celerity predictors. Very good agreement was found between the individual wave properties and the wave spectrum analysis, showing the great potential of the scanner to be used in the surf and swash zone for studies of nearshore waves at the wave-by-wave timescale

Behaviour and performance of a dynamic cobble berm revetment during a spring tidal cycle in North Cove, Washington State, USA

In many places, sandy coastlines and their associated assets are at high risk of erosion and flooding, with this risk increasing under climate change and sea level rise. In this context, dynamic cobble berm revetments represent a potentially sustainable protection technique to armour sandy beaches, reduce wave runup and protect the hinterland against wave attack. However, the behaviour and performance of such structures is not well understood. The dynamic cobble berm revetment located in North Cove, WA, USA, was monitored over a spring tidal cycle in January 2019. A representative 60 m alongshore section was monitored over 10 days using 2D laser scanner (lidar) measurements, GPS ground elevation surveys, Radio Frequency Identification of individual cobbles and revetment thickness measurements. These data were used together to assess the dynamic behaviour and functionality of the revetment throughout the experiment. Over the course of the experiment, the surface elevation changed by up to ±0.5 m, and the revetment volume reduced by an average 0.67 m3/m. These changes were found to be caused by relatively large significant wave height and high water levels. The revetment demonstrated a dynamic stability and the capacity to quickly reshape under changing hydrodynamic conditions. The instrumented cobbles were transported along and cross-shore and accumulated at the toe of the revetment, but were never transported seaward of the toe. The revetment also managed to recover some of the lost volume under moderate wave conditions. The revetment behaviour was found to be influenced by variation in the cobble-sand matrix. The underlying sand dynamics – i.e., accumulation or removal of sand within the cobbles – were found to govern the overall volume changes and were important to the overall stability of the revetment. Seven possible transport regimes were identified, and a model of the internal sand dynamics was developed. During the spring tidal cycle measured here, the revetment protected the sand scarp immediately landward and prevented flooding of the hinterland, while armouring the underlying sand. Over time, renourishment will likely be required due to longshore sediment transport, and preliminary guidelines for this and other aspects of design are suggested.</p

Comparison of dynamic cobble berm revetments with differing gravel characteristics

Pressure on the coastline is escalating due to the impacts of climate change, this is leading to a rise in sea-levels and intensifying storminess. Consequently, many regions of the coast are at increased risk of erosion and flooding. Therefore coastal protection schemes will increase in cost and scale. In response there is a growing use of nature-based coastal protection which aim to be sustainable, effective and adaptable. An example of a nature-based solution is a dynamic cobble berm revetment: a berm constructed from cobble and other gravel sediments at the high tide wave runup limit. These structures limit wave excursion protecting the hinterland from inundation, stabilise the upper beach and adapt to changes in water level. Recent experiments and field applications have shown the suitability of these structures for coastal protection, however many of the processes and design considerations are poorly understood. This study directly compares two prototype scale laboratory experiments which tested dynamic cobble berm revetments constructed with approximately the same geometry but differing gravel characteristics; well-sorted rounded gravel (DynaRev1) and poorly-sorted angular gravel (DynaRev2). In both cases the structures were tested using identical wave forcing including incrementally increasing water level and erosive wave conditions. The results presented in this paper demonstrate that both designs responded to changing water level and wave conditions by approaching a dynamically stable state, where individual gravel is mobilised under wave action but the geometry remains approximately constant. Further, both structures acted to reduce swash excursions compared to a pure sand beach. However, their morphological behaviour is response to wave action varied considerably. Once overtopping of the designed crest occurred, the poorly-sorted revetment developed a peaked crest which grew in elevation as the water level or wave height increased, further limited overtopping. By comparison, the well-sorted revetment was characterised by a larger volume of submerged gravel and a lower elevation flat crest which responded less well to changes in conditions. This occurred due to two processes: (1) for the poorly-sorted case, gravel sorting processes moved small to medium gravel material (D50<70mm) to the crest and (2) the angular nature of the poorly-sorted gravel material promoted increased interlocking. Both of these processes led to a gravel matrix that is more resistant to wave action and gravitational effects. Both revetments experienced some sinking due to sand erosion beneath the front slope. The rate of sinking for the well-sorted case was larger and continued throughout due to the large pore spaces within the gravel matrix. For the poorly sorted revetment in DynaRev2, sand erosion ceased after approximately 28 h due to the development of a filter layer of small gravel at the sand-gravel interface reducing porosity at this location, hence a larger volume of sand was preserved beneath the structure. Both designs present a low-cost and effective solution for protecting sandy coastlines but from an engineering viewpoint it appears better to avoid well-sorted gravel material and greater gravel angularity has been seen to increase crest stability

Environmental signal shredding on sandy coastlines

How storm events contribute to long-term shoreline change over decades to centuries remains an open question in coastal research. Sand and gravel coasts exhibit remarkable resilience to event-driven disturbances, and, in settings where sea level is rising, shorelines retain almost no detailed information about their own past positions. Here, we use a high-frequency, multi-decadal observational record of shoreline position to demonstrate quantitative indications of morphodynamic turbulence – “signal shredding” – in a sandy beach system. We find that, much as in other dynamic sedimentary systems, processes of sediment transport that affect shoreline position at relatively short timescales may obscure or erase evidence of external forcing. This suggests that the physical effects of annual (or intra-annual) forcing events, including major storms, may convey less about the dynamics of long-term shoreline change – and vice versa – than coastal researchers might wish

Morphodynamical modelling of field-scale swash events

In the present work, data for three single swash events are selected from those available for an accretive tide that occurred at Le Truc Vert beach (France) during a measurement field campaign at that location. These events are primarily chosen because of the different final bed change they produced, namely variable accretion, seaward erosion/landward deposition and variable erosion along the swash zone. These data are compared to results obtained from a ‘state-of-the-art’ numerical fully-coupled 1D morphodynamical shallow water solver, driven by measurements made of those events in the lower swash/inner surf zone. It is found that the hydrodynamics is reasonably well represented, although the computed results exhibit reduced maximum inundations in comparison with the observed ones. The model reproduces the correct order of magnitude of the morphodynamic change after each event, and sometimes the pattern of erosion and deposition, but this change is generally underestimated. Sensitivity analyses are conducted with respect to more uncertain physical parameters and assumed initial conditions. They suggest that initial spatial distributions for velocity and pre-suspended sediment concentration play a key role in the quantitative and qualitative prediction of the bed change

Laboratory investigation of the Bruun Rule and beach response to sea level rise

Rising sea levels are expected to cause widespread coastal recession over the course of the next century. In this work, new insight into the response of sandy beaches to sea level rise is obtained through a series of comprehensive experiments using monochromatic and random waves in medium scale laboratory wave flumes. Beach profile development from initially planar profiles, and a 2/3 power law profile, exposed to wave conditions that formed barred or bermed profiles and subsequent profile evolution following rises in water level and the same wave conditions are presented. Experiments assess profile response to a step-change in water level as well as the influence of sediment deposition above the still water level (e.g. overwash). A continuity based profile translation model (PTM) is applied to both idealised and measured shoreface profiles, and is used to predict overwash and deposition volumes above the shoreline. Quantitative agreement with the Bruun Rule (and variants of it) is found for measured shoreline recession for both barred and bermed beach profiles. There is some variability between the profiles at equilibrium at the two different water levels. Under these idealised conditions, deviations between the original Bruun Rule, the modification by Rosati et al. (2013) and the PTM model predictions are of the order of 15% and all these model predictions are within ±30% of the observed shoreline recession. Measurements of the recession of individual contour responses, such as the shoreline, may be subject to local profile variability; therefore, a measure of the mean recession of the profile is also obtained by averaging the recession of discrete contours throughout the active profile. The mean recession only requires conservation of volume, not conservation of profile shape, to be consistent with the Bruun Rule concept, and is found to be in better agreement with all three model predictions than the recession measured at the shoreline

Measurements of morphodynamic and hydrodynamic overwash processes in a large-scale wave flume

Overwash is an important process that controls short-term barrier dynamics, as well as long-term barrier migration, but this process is difficult to study in the field due to its rare occurrence and the challenging conditions under which it occurs. This paper uses data collected during the BARDEX II experiment in the Delta Flume, the Netherlands, where a proto-type barrier was subjected to a range of wave and water level conditions. The objectives of this research are to: (1) compare the morphologic response to overwash on a gravel barrier (BARDEX 2008 experiment) with that on a sandy barrier (BARDEX II); (2) understand the influence of wave period on overwash characteristics and sediment transport; and (3) improve current knowledge of overwash hydrodynamics. The comparative analysis shows that barrier overwash can be affected by negative feedback that stabilises the barrier through barrier crest accretion on gravel barriers, and by submerged bar development on sandy barriers. An increase in the wave period induced a reduction in overwash frequency over the crest, but no significant relation was found between wave peak period and overwash discharge. Nevertheless, overall water discharge during an overwash episode significantly correlates with overall overwash sediment transport rate. Overwash flow depths during the experiment were relatively shallow and velocities were similar compared to those measured during previous studies and reported in the literature. Despite the controlled laboratory conditions, collection of reliable and accurate measurements of overwash velocities remains challenging. (C) 2015 Elsevier B.V. All rights reserved