Modeling the development of wave-cut shore platforms (original) (raw)
Related papers
The Role of Bed Roughness in Wave Transformation Across Sloping Rock Shore Platforms
Journal of Geophysical Research: Earth Surface
We present for the first time observations and model simulations of wave transformation across sloping (Type A) rock shore platforms. Pressure measurements of the water surface elevation using up to 15 sensors across five rock platforms with contrasting roughness, gradient, and wave climate represent the most extensive collected, both in terms of the range of environmental conditions, and the temporal and spatial resolution. Platforms are shown to dissipate both incident and infragravity wave energy as skewness and asymmetry develop and, in line with previous studies, surf zone wave heights are saturated and strongly tidally modulated. Overall, the observed properties of the waves and formulations derived from sandy beaches do not highlight any systematic interplatform variation, in spite of significant differences in platform roughness, suggesting that friction can be neglected when studying short wave transformation. Optimization of a numerical wave transformation model shows that the wave breaker criterion falls between the range of values reported for flat sandy beaches and those of steep coral fore reefs. However, the optimized drag coefficient shows significant scatter for the roughest sites and an alternative empirical drag model, based on the platform roughness, does not improve model performance. Thus, model results indicate that the parameterization of frictional drag using the bottom roughness length-scale may be inappropriate for the roughest platforms. Based on these results, we examine the balance of wave breaking to frictional dissipation for rock platforms and find that friction is only significant for very rough, flat platforms during small wave conditions outside the surf zone.
Infragravity wave generation on shore platforms : bound long wave versus breakpoint 1 forcing 2 3
2020
Shore platforms are ubiquitous morphological features along rocky coastlines and display a spectrum of forms from gently-sloping to sub-horizontal with a low tide cliff. They generally front eroding coastal cliffs and play an important natural coastal protection role by dissipating wave energy, especially during energetic wave conditions. Sea-swell wave energy dissipates during wave breaking, but the transfer of incident wave energy to lower frequencies, resulting in infragravity waves, can enable significant amounts of wave energy to persist up to the shoreline. This residual wave motion at the shoreline can carry out geomorphic work, for example by directly impacting the cliff face, but also for removing cliff-toe debris. There are two main mechanisms for generating infragravity wave motiongroup bound long waves and breakpoint forcingand it is not known which of these mechanisms operate on shore platforms. Here we show, using field data collected at a sloping platform in England and a subhorizontal platform in New Zealand, and supported by numerical modelling, that the group bound long wave mechanism is most important on sloping platforms, whereas breakpoint forcing dominates on sub-horizontal platforms. Our results also suggest that the infragravity wave motion on the sloping platform is somewhat more energetic than that on the subhorizontal platform, implying that the latter type of platform may provide better protection to coastal cliffs. However, site-specific factors, especially platform elevation with respect to tidal level and platform gradient, play a key role in wave transformation processes on shore platforms and more field data and modelling efforts are required to enhance our understanding of these processes, especially collected under extreme wave conditions (Hs > 5 m).
Development of Shore Platforms along the NW Coast of Italy: The Role of Wind Waves
Journal of Coastal Research, 2017
This paper investigates whether waves are active morphologic agents capable of shaping the small shore platforms that characterize the rocky coast of NW Italy. Two study areas have been selected along this coastal tract: Calafuria (Livorno) and Lerici-Tellaro (La Spezia), located ca. 120 km apart, the first being shaped in sandstone and the second in dolomite bedrock. Propagation of waves in the nearshore has been simulated by numerical modeling. From wave model results, validated with data from an offshore wave meter buoy, it is inferred that waves break directly on the coast or very close to it at Calafuria, whereas for Lerici-Tellaro shores, waves mostly break up to 150 m seaward of the shore platform. This implies that the amount of energy delivered on the platform is much greater in the first case than in the second case. Given breaking depths and the height of breakers, maximum pressure and shear stress released at the breaking point were calculated for both areas. The results showed that wave forces released onto the shore platforms both at Calafuria and at Lerici-Tellaro never exceeded the compressive strength of the platform rocks, assessed using the Schmidt hammer test. It is concluded that, in the study area and with the present oceanographic conditions, wave forces are not directly capable of causing erosion on shore platforms.
8. Low-Crested and Submerged Breakwaters in Presence of Broken Waves
2003
Functional design of low crested breakwaters requires an accurate prediction of wave transmission and set up in the protected areas. Nevertheless, commonly used formulae do not appear to be reliable enough, especially for structures located in shallow waters. The paper describes results from large-scale model tests conducted on rubble mound breakwaters exposed to breaking waves. Tests were carried out at
Infragravity wave generation on shore platforms: bound long wave versus breakpoint forcing
Geomorphology
Shore platforms are ubiquitous morphological features along rocky coastlines and display a spectrum of forms from gently-sloping to sub-horizontal with a low tide cliff. They generally front eroding coastal cliffs and play an important natural coastal protection role by dissipating wave energy, especially during energetic wave conditions. Sea-swell wave energy dissipates during wave breaking, but the transfer of incident wave energy to lower frequencies, resulting in infragravity waves, can enable significant amounts of wave energy to persist up to the shoreline. This residual wave motion at the shoreline can carry out geomorphic work, for example by directly impacting the cliff face, but also for removing cliff-toe debris. There are two main mechanisms for generating infragravity wave motion-group bound long waves and breakpoint forcing-and it is not known which of these mechanisms operate on shore platforms. Here we show, using field data collected at a sloping platform in England and a sub-horizontal platform in New Zealand, and supported by numerical modelling, that the group bound long wave mechanism is most important on sloping platforms, whereas breakpoint forcing dominates on sub-horizontal platforms. Our results also suggest that the infragravity wave motion on the sloping platform is somewhat more energetic than that on the sub-horizontal platform, implying that the latter type of platform may provide better protection to coastal cliffs. However, site-specific factors, especially platform elevation with respect to tidal level and platform gradient, play a key role in wave transformation processes on shore platforms and more field data and modelling efforts are required to enhance our understanding of these processes, especially collected under extreme wave conditions (H s > 5 m).
Modelling the development of dynamic equilibrium on shore platforms
Marine Geology, 2020
A numerical model was used to determine whether the landward migration of shore platforms at the low tidal level, due to downwearing and possibly other mechanisms, can match the recession caused by wave erosion at the high tidal level. The hybrid model calculated the time required to undercut a cliff face and to remove the debris, and the amount of recession accomplished at the low tidal level during that time. These iterations were repeated over the equivalent of a 40,000 year period. The rates of high tidal erosion calculated during model runs were representative of cliff recession rates recorded in the field, and downwearing rates, specified for each run, were based on data recorded on shore platforms with micro-erosion meters. Almost all runs demonstrated that platform gradients decline asymptotically to a state of dynamic equilibrium that is strongly related to tidal range and rock resistance, although its relationship to wave height varies according to attenuation rates in the surf zone. Periodic changes in rock resistance and continuous changes in cliff height can introduce perturbations that delay equilibrium or, depending on their severity, prevent its occurrence. The conclusion that platforms trend towards dynamic, as opposed to static, equilibrium is contrary to other models which have considered only the effect of wave erosion. The results of this work have important implications for coastal modelling and the estimation of platform lowering for cosmogenic dating.
2016
Two-dimensional laboratory experiments were conducted by changing the random wave conditions and structure configurations to develop a formula to predict run-up level on the tetrapod armoured rubble mound structure. The incident waves in the experiments included non-breaking, breaking, and broken wave conditions at the toe of the structure. In this study, a steep front slope (1:1.5) was set up to suggest the wave run-up formula while most of the previous studies focused on the milder sloped structures. The experimental results were compared to the previous research by van de Meer and Stam (1992). The results showed that the relative run-up height (the ratio of the run-up height to the significant wave height at the toe) converged as the incident wave steepness increased. On the other hand, the relative run-up height was highly dependent of the relative wave height (the ratio of the significant wave height to the water depth at the toe) as the incident wave steepness decreased. Then,...
Wave Transformation Across a Rock Platform, Belinho, Portugal
2009
Much is known about wave attenuation across sandy nearshore environments or coral reef platforms. Limited field work has aimed to measure and assess attenuation across rock platforms. We designed and implemented a field experiment at Belinho, Portugal, to address this need. Field work was conducted in June, 2006, over a sequence of five tidal cycles. We deployed a shore-normal array of seven KPSI pressure transducers, installed 0.15 m above the bed and spaced between 10 and 15 m apart, to measure surface water levels. When all instruments were submerged by the rising tide, they were sampled at 20Hz.. The instrument array spanned the inter-tidal platform composed of schist. The surface, about 70 m wide, is irregular and cut by many shore perpendicular channels. To assess the attenuation and transformation of waves across this surface as a result of shoaling and breaking, we derived spectral estimates of wave height and period through each high tide series. Wave heights at the outermost PT ranged between about 0. 4 to 0.9 m. At the innermost PT, corresponding values were 0.5 to 1.14 m. Wave periods averaged about 10 s through the study. Our results showed that a breaking criterion of 0.42 + c fit our data.