Storm Wave Conditions Research Articles

Boulder transport by storms – Extreme-waves in the coastal zone of the Irish west coast

This study is concerned with large boulders located along exposed shorelines in higher latitudes, which have become dislocated onshore by winter storms and have moved against gravity. To identify the transportation processes that these boulders have undergone in detail, their direct investigation during storm wave conditions is necessary, or at least near time inspections after extraordinary wave events. As both methods are rare, a wide range of questions and contradictions with regards to the processes that have acted on these boulders, remains. Despite a lack of applicable methods as for fine sediments, the depositional environment and processes of boulder movement can be determined from geomorphologic evidence in the landscape itself. Examples are presented in this paper. To progress understanding of boulders in rocky coastal environments, qualitative and quantitative data are acquired during a near time inspection following extreme storms in winter 2013/14 with special focus on an extraordinary boulder site near Doolin at the entrance to Galway Bay (central west coast of Ireland). The comparison of these data to previously published research on coastal boulder movement results in agreements and discrepancies (e.g. on boulder forms and mode of transport, difference in wave and bore transport) which are discussed.

Ichnology of the Callovian-Oxfordian Rocks of the Katrol Hill Range, Kachchh, Western India

ABSTRACT On the basis of distinct lithologic features such as composition, grain size, bedding characteristics and sedimentary structures, six facies were identified in Callovian to Oxfordian rocks exposed southwest of Bhuj, Kachchh. They are interbedded calcareous shale-siltstone (ICSSF), limestone (LF), ferruginous sandstone (FerSF), felspathic sandstone (FelSF), grey shale (GSF) and oolitic limestone (OLF) facies. The rich and highly diversified trace fossils reveal a wide range of animal behaviours represented by dwelling, feeding, crawling and resting structures. Horizontal feeding structures are found abundantly in all lithofacies indicating low wave and current energy and deposition of poorly sorted muddy to sandy sediments. A few coarse layers containing Arenicolites, Ophiomorpha and Skolithos indicate the presence of opportunistic animals (due to their first appearance under harsh conditions) under -intermittently moderate wave and current energy or storm wave conditions (due to coarse grain size and dominance/presence of only vertical trace fossils) in the shoreface zone. Taenidium occurs mainly in the lower shoreface to transitional zone suggesting low to moderate energy conditions. Thalassinoides occurs in middle to lower shoreface settings under relatively low-energy conditions. Zoophycos represents offshore environment, where it occupies the deepest bioturbation levels. The characteristic lithofacies and assemblages of trace fossils in the rocks of the Chari/Jumara Formation indicate a depositional environment fluctuating from the upper shoreface to offshore zone.

Numerical modeling of the undertow structure and sandbar migration in the surfzone

A process-based 3D numerical model for surfzone hydrodynamics and beach evolution was established. Comparisons between the experimental data and model results proved that the model could effectively describe the hydrodynamics, sediment transport feature and sandbar migration process in the surfzone with satisfactory precision. A series of numerical simulations on the wave breaking and shoaling up to a barred beach were carried out based on the model system. Analyzed from the model results, the wave-induced current system in the surfzone consists of two major processes, which are the phase-averaged undertow caused by wave breaking and the net drift caused by both of the nonlinear wave motion and surface roller effect. When storm waves come to the barred beach, the strong offshore undertow along the beach suppresses the onshore net drift, making the initial sandbar migrate to the seaside. Under the condition of calm wave environment, both the undertow and net drift flow to the shoreline at the offshore side of the sandbar, and then push the initial sandbar to the shoreline. The consideration of surface roller has significant impact on the modeling results of the sandbar migration. As the roller transfer rate increases, the sandbar moves onshore especially under the storm wave condition.

Effect of beach slope and grain-stabilization on coastal sediment transport: An attempt to overcome the erosion overestimation by XBeach

The reproduction of seventeen laboratory tests for dune erosion by the surf-beat mode of the two-dimensional horizontal (2DH) model XBeach for storm wave conditions showed that the model overestimates the erosion and thus the deposition volumes. This overestimation becomes particularly significant for very large overtopping rates on coastal barriers, which are commonly induced by extreme storm surges. In this paper, the recent model improvements to overcome this problem are first summarised. Moreover, two physical reasons for these overestimations are identified: (i) the wave nonlinearity effect on sediment transport, which is described in XBeach by a calibration factor for the time-averaged flow depending on the wave skewness and asymmetry and (ii) the considerable excess of the actual shear stress required to initiate the sediment particle motion as compared to that predicted by the common Shields curve. To improve the prediction capability of XBeach in terms of erosion and overwash volumes, two new improvements, related to the two aforementioned reasons for the overestimation, are introduced and implemented in the model. The improved XBeach model is then tested for dune erosion, for barrier breaching as well as for a barrier island erosion and overwash under an extreme storm surge event. The results showed a very good prediction capability of the improved model. Particularly, the second model improvement opens the way toward further model improvements to account for spatially varying soil resistance, which is crucial for reliable prediction of a barrier breaching.

Implications of delta retreat on wave propagation and longshore sediment transport - Guadalfeo case study (southern Spain)

The influence of both natural and human-induced changes on the evolution of worldwide deltas has been widely addressed; however, the variations of the submerged morphology of the delta and their implications on coastal dynamics have received limited attention. This work details the spatial and temporal variability of the mouth of the Guadalfeo River (southern Spain) and focuses on the influence of submerged morphological changes, partly due to watershed regulation in 2004, on wave propagation and longshore sediment transport. Bathymetric measurements were carried out over a 15-year period (1999–2014), a wave propagation model was calibrated and applied, and an updated expression for longshore sediment transport was applied using downscaling techniques to obtain the complete littoral drift time series. The results show that the river damming led to coastline retreat and bed-level erosion up to 3m along a 1-km section around the river mouth, with maximum erosion rates in excess of 760m3/m. These subtidal morphological changes reduced wave refraction and led to higher breaking wave energy. Variations in wave climate during the study period have also played a role in influencing the coastline dynamics. Although the erosion around the river mouth has decreased since 2008, partly due to a sediment pulse in 2010, eastward longshore sediment transport rates under westerly storm wave conditions have significantly increased since then. This has led to the propagation of the sediment deficit towards the east of the mouth, endangering urban developments at this location. This paper provides insights into the shift from wave-river dominated deltas towards deltaic coasts increasingly controlled by wave directionality and longshore sediment transport, and represents an advance on the understanding of the dynamics of many worldwide deltas where the river sediment supply has decreased due to human activities.

Relevance of infragravity waves in a wave‐dominated inlet

AbstractThis study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave‐dominated inlet located on the western coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low‐frequency oscillations (i.e., 25–300 s) in water levels and current velocities. While these fluctuations were present over the ebb‐tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to 2 h after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m s−1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions.

Ancient high-energy storm boulder deposits on Ko Samui, Thailand, and their significance for identifying coastal hazard risk

Coastal geomorphic processes associated with high-energy storm events are difficult to estimate over recent geological history, though their frequency and magnitude are important to assess in order to understand present-day coastal vulnerability. Studying ancient coastal boulder deposits can shed light on the previous physical conditions necessary for their deposition. In this study, we estimated the physical processes required to move reef-derived coral boulders on the east coast of Ko Samui, a rapidly developing tourist island off eastern peninsular Thailand. The position and dimensions of 97 coral boulders (weight: mean 2.9t, max. 12.7t; transport distance: max. 125m) were measured at two sites and dated using uranium/thorium methods. Flow velocities of 2.3–8.6m/s were required to transport the measured boulders, with individuals deposited up to 4.7m above mean sea level. Age-dating suggests that events capable of the highest flow velocities occurred around AD 1600 and AD 1750. These were probably driven by tropical cyclones (typhoons). Boulder transport by events of similar magnitude has not been detected within the last 250years. The non-occurrence of similar events in living memory has implications for hazard perceptions at this important tourist destination. However, there is also evidence of substantial Holocene sea-level changes in the Gulf of Thailand, as observed at nearby Ko Phaluai. This potentially offers a challenge for the interpretation of older boulders dating from the mid-Holocene, as sea level may have been more than 2m higher than present. Thus, studies using coral boulders as a proxy for past storm-wave conditions must consider the broader sea-level history, and are probably best limited to the period post-2000BP in the Gulf of Thailand.

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

AbstractStudies 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.

Measuring Coastal Boulder Movement Under Waves Using Tri-Axial Accelerometers

ABSTRACT Stephenson, W.J. and Abazovic, A. 2016. Measuring Coastal Boulder Movement Under Waves Using Tri-Axial Accelerometers. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 607–611. Coconut Creek (Florida), ISSN 0749-0208. Boulder entrainment under tsunami or storm wave conditions has been a topic of interest among coastal geomorphologists for the last 25 years. Efforts have been directed to understanding the dynamic conditions under which boulder entrainment occurs and in particular the energy levels required to move boulders of specific sizes. Recent focus on storm waves has highlighted the complexity of the process and that more energetic tsunami waves are not always needed to move larger boulders. Consequently attempts have been made to develop better empirical equations to predict wave height or flow velocity and differentiate between stor...

Empirical wave run-up formula for wave, storm surge and berm width

An empirical model to predict wave run-up on beaches considering storm wave and surge conditions and berm widths (dry beach) has been derived through a synthetic data set generated from a one-dimensional Boussinesq wave model. The new run-up equation is expressed as a function of a new Iribarren number composed of three regions: the foreshore, the berm or dry beach width, and the dune. The dissipative effect of the berm is included as a reduction factor expressed as a function of the berm width normalized by the offshore wavelength. The equation is relatively simple but is shown to be applicable for a fairly wide variety of berm widths and storm wave conditions associated with extreme events such as hurricanes, and it is shown to be an improvement over existing empirical run-up models that do not consider the berm width explicitly. In addition, the new parameterization of the Iribarren number considering the three regions and the berm width reduction factor are shown to improve other empirical models.

The extreme 2013/2014 winter storms: hydrodynamic forcing and coastal response along the southwest coast of England

AbstractThe southwest coast of England was subjected to an unusually energetic sequence of Atlantic storms during the 2013/2014 winter, with the 8‐week period from mid‐December to mid‐February representing the most energetic period since at least 1953. A regional analysis of the hydrodynamic forcing and morphological response of these storms along the SW coast of England highlighted the importance of both storm‐ and site‐specific conditions.The key factor that controls the Atlantic storm wave conditions along the south coast of southwest England is the storm track. Energetic inshore wave conditions along this coast require a relatively southward storm track which enables offshore waves to propagate up the English Channel relatively unimpeded. The timing of the storm in relation to the tidal stage is also important, and coastal impacts along the macro‐tidal southwest coast of England are maximised when the peak storm waves coincide with spring high tide. The role of storm surge is limited and rarely exceeds 1 m.The geomorphic storm response along the southwest coast of England displayed considerable spatial variability; this is mainly attributed to the embayed nature of the coastline and the associated variability in coastal orientation. On west‐facing beaches typical of the north coast, the westerly Atlantic storm waves approached the coastline shore‐parallel, and the prevailing storm response was offshore sediment transport. Many of these north coast beaches experienced extensive beach and dune erosion, and some of the beaches were completely stripped of sediment, exposing a rocky shore platform. On the south coast, the westerly Atlantic storm waves refract and diffract to become southerly inshore storm waves and for the southeast‐facing beaches this results in large incident wave angles and strong eastward littoral drift. Many south coast beaches exhibited rotation, with the western part of the beaches eroding and the eastern part accreting. © 2015 The Authors.Earth Surface Processes and Landformspublished by John Wiley & Sons Ltd.

Coastal cliff ground motions and response to extreme storm waves

AbstractCoastal 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.

Wave-induced extreme water levels in the Puerto Morelos fringing reef lagoon

Abstract. Wave-induced extreme water levels in the Puerto Morelos fringing reef lagoon are investigated by means of a phase-resolving non-hydrostatic wave model (SWASH). This model solves the nonlinear shallow water equations including non-hydrostatic pressure. The one-dimensional version of the model is implemented in order to investigate wave transformation in fringing reefs. Firstly, the numerical model is validated with (i) laboratory experiments conducted on a physical model (Demirbilek et al., 2007)and (ii) field observations (Coronado et al., 2007). Numerical results show good agreement with both experimental and field data. The comparison against the physical model results, for energetic wave conditions, indicates that high- and low-frequency wave transformation is well reproduced. Moreover, extreme water-level conditions measured during the passage of Hurricane Ivan in Puerto Morelos are also estimated by the numerical tool. Subsequently, the model is implemented at different along-reef locations in Puerto Morelos. Extreme water levels, wave-induced setup, and infragravity wave energy are estimated inside the reef lagoon for different storm wave conditions (Hs >2 m). The numerical results revealed a strong correlation between the offshore sea-swell wave energy and the setup. In contrast, infragravity waves are shown to be the result of a more complex pattern which heavily relies on the reef geometry. Indeed, the southern end of the reef lagoon provides evidence of resonance excitation, suggesting that the reef barrier may act as either a natural flood protection morphological feature, or as an inundation hazard enhancer depending on the incident wave conditions.

The effect of climate change on extreme waves in front of the Dutch coast

Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000 years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (H s) and wave period (T m) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual T m maximum decreases significantly by 0.3 to 0.6 s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both H s and T m. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return H s and T m are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.

Prediction of Beach Changes Due to Selective Discharge of Medium Sand during Storm Wave Conditions

Prediction of Beach Changes Due to Selective Discharge of Medium Sand during Storm Wave Conditions

Prediction of Deposition of Fine Sand in Beach Nourishment Using Gravel and Shoreward Movement of Gravel During Storm

On the Akashi coast, beach nourishment using gravel with the grain size ranging between 3 and 15 mm was begun in November 2008. The beach changes were monitored using the photographs from several fixed points. It was found that fine sand was deposited over the gravel layer under the calm wave conditions, whereas the nourished gravel was transported onshore and deposited in front of the seawall under the storm wave conditions. Assuming that the equilibrium slope changes in response to the change in thickness of sand layer, a model for predicting the deposition of fine sand over the gravel layer in beach nourishment using gravel and the shoreward movement of gravel during storms was developed on the basis of the contour-line-change model.

Field and laboratory measurements of mean overtopping discharges and spatial distributions at vertical seawalls

Within the CLASH project, wave overtopping at the vertical seawall at Samphire Hoe was measured by HR Wallingford (HRW), and compared laboratory tests in 2 & 3 dimensions carried out at the University of Edinburgh and HRW. At Samphire Hoe, overtopping volumes were captured in three volumetric tanks capable of measuring wave-by-wave and total overtopping volumes. The three tanks were placed progressively farther back from the seawall edge so that the spatial distribution of the overtopping discharges could be determined. The field measurement equipment was successfully deployed on three occasions, and measured overtopping discharges ranged from that barely considered to be hazardous to the public to over q = 3.0 l/s/m. The 2d testing at Edinburgh was modelled at a scale of 1:40, and the 3d model at HRW was modelled at 1:20. For both sets of laboratory tests, a range of conditions, representative of the storm wave conditions and water levels, was reproduced in addition to a set of parametric conditions. The storm conditions allowed a direct comparison between the field and laboratory measurements, and the parametric conditions were used to test the generic overtopping behaviour of the structure. For both sets of laboratory tests, mean overtopping discharges and the spatial distribution were measured separately. Analysis of the distribution data relates the proportion of the discharge that has landed as a function of ( x / L o); where x is the distance behind the crest, and L o is the offshore wavelength. Analysis of the field, 2d & 3d laboratory data, and empirical prediction methods have not identified any scale effects for overtopping discharges at vertical and near-vertical seawalls.

Modelling the processes of cliff-top erosion and deposition under extreme storm waves

At exposed sites on the deep water coasts of the British Isles, cliff-top storm deposits (CTSDs) occur on cliffs at a variety of elevations above sea level and may occasionally reach up to 50 m above sea level. Time-series field mapping of CTSDs has demonstrated their formation over the historical period and their continued modification during major past storms. This paper seeks to clarify the morphogenetic context of CTSDs, model the wave conditions and forces encountered at the cliff-face and cliff-top platform and propose mechanisms to link wave processes to cliff-face quarrying and landwards cliff-top transport of quarried blocks to deposition zones at the rear of the cliff-top platform. We report on wave-tank experiments using scaled cliff and wave conditions from a cliff in the Shetland Islands to focus on three situations: incident waves lower than the cliff edge height; at the same height as the cliff edge height; and higher than the cliff edge height. The modelling suggests that steep waves of 10 m and above impacting on a 15 m high cliff will result in impact pressures sufficient to promote crack propagation, block detachment and lifting of large blocks. Large, but not necessarily steep, waves of the same height as the cliff edge produce sufficient impact pressures and water flow over the cliff edge and platform to entrain blocks, transport and deposit them on the cliff-top. Where cliff-top height is below wave crest elevation “green water” bore flow occurs, sufficient to force rotation or lifting of blocks out of cliff-top and cliff-top platform ‘sockets’. High flow velocities rapidly accelerate and transport blocks inland until the flow attenuation results in deposition of blocks at the limit of run-up. The model results provide a good explanatory framework to account for the quarrying of the upper part of the cliff-face and cliff-top platform under storm wave conditions and provide an insight into the exceptional velocities experienced over the cliff-top platform under bore flow conditions. The modelling results show that extreme storm waves are capable of quarrying, transporting and depositing large blocks at altitude and significant distances inland and so present serious questions about the use of such deposits as diagnostic of palaeo-tsunami.

Safety Traffic Control for a Coastal Bridge Constructed on a Reef under Storm Wave Conditions

Safety traffic control measures for a temporary bridge, which was constructed with a permeable floor over a road (National Route 229) on a coastal reef, were studied through a hydraulic model experiment and field observation. When the offshore wave height exceeded 4 meters, a large wave force was acted to passing vehicles. It was thus revealed that road closure or other measures would be necessary before the offshore wave height reached 5 meters to ensure the safety of vehicles. It was also found that the wave height could be estimated from the wave pressure measured with wave pressure gauges installed on a bridge pier, and that it would be possible to make full use of such wave pressure data for traffic control on the bridge.

消波ブロック積み傾斜提を対象としたマウンド被覆ブロックの耐波安定性について

Armor block stability for the mound of sloping breakwaters was examined by 2-D model tests. Damage patterns of armor blocks at the mound berm were reproduced by wave force tests. By the action of breaking wave, armor blocks at the offshore side of mound were uplifted by the vertical wave force, and overturned by. the wave induced flow. Based on the stability test results, the stability number of armor blocks was formulated as the function of relative depth, relative mound height and sea bottom slope. Furthermore, the relationship between the damage level and the number of wave action was shown for storm wave conditions exceeding the design wave height.