Beach Scarp Research Articles

Imprints of washover sediments by tropical storm Pabuk 2019 along the coast of the Gulf of Thailand; insight into coastal geomorphological response to storm surge

Tropical storm Pabuk (4th January 2019) struck southern Thailand, causing damage to the low-lying coastal area. The combination of waves, storm surge, and high tide resulted overwash and inundation of regions on the West coast of the Gulf of Thailand (GOT) side. After the event, we conducted a field survey from January 7–10 and 15–20, 2019 to investigate the effect of storm surges and the degree of damage from the site under the eye of storm affected area up to the north 500 km from the landfall sites. This paper investigates how variations in coastal geomorphological features respond to the storm surge generated by tropical storm Pabuk at a regional scale. Coastal damage was observed along the beach from the presence of the scour, beach scarp, knocked down trees, and destroyed buildings and infrastructure. The most damaged area was at Nakhon Si Thammarat (NST), where the tropical storm made landfall with maximum wind speeds of almost 100 km/h and a storm surge height of 5 m. At NST site, the washover deposits extended as far as 80 m from the coastline with a maximum inundation of 330 m. From this study, two types of washover deposits including perched fan and washover terrace were identified. The thickest washover sediment, 60 cm thick, was discovered in a relatively low-lying area adjacent to a small estuary. Mud rip-up clasts, planar stratification, cross stratification, foreset bedding, scouring at base, and sharp and erosional contact are characteristics of sedimentary structures found in storm sediments. Bedform surfaces, such as current ripples with indicated storm surge direction, were well-preserved at several sites. We suggest that the local geomorphological controlling factors, especially coastal elevation have played important roles in the difference of coastal geomorphological responses as well as the type of washover deposits.

Dynamics of beach scarp formation behind detached breakwaters

Detached (offshore) breakwaters are widely used as beach stabilization measures. However, there is potential for erosion at the openings between detached breakwaters, which may result in the formation of beach scarps. This study evaluated the formation of beach scarps around breakwaters using field surveys and numerical simulations. The evaluation was conducted using a combination of a recent advanced field survey method and a numerical modelling system combining GPV-MSM, SWAN, and XBeach. The unmanned aerial vehicle photogrammetry results revealed the height of the beach scarp generated behind the detached breakwaters to be approximately 3.0 m at Aoyama beach, Niigata coast, Japan, although sand deposition occurred below sea level towards the detached breakwaters. The numerical simulation using the results of the field survey showed that a combination of high waves and increased water level owing to radiation stress between the shoreline and breakwaters resulted in a steeper slope in the beach morphology. Second, avalanching occurred on the beach, leading to the formation of beach scarps. This study showed that detached breakwaters may lead to high water levels, resulting in the generation of beach scarps behind the breakwaters. This study implied that large beach scarps can be generated even when detached breakwaters are constructed.

Beach and Dune Subsurface Hydrodynamics and Their Influence on the Formation of Dune Scarps

AbstractErosive beach scarps influence beach vulnerability, yet their formation remains challenging to predict. In this study, a 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive event. Pressure and moisture sensors buried within the dune were used both to monitor the water table and to examine vertical pressure gradients in the upper 0.3 m of sand as the slope of the upper beach developed into a scarp. Concurrently, a line‐scan lidar tracked swash bores and monitored erosion and accretion patterns along a single cross‐shore transect throughout the experiment. As wave conditions intensified, a discontinuity in the slope of the dune formed; the discontinuity grew steeper and progressed landward at the same rate as the R2% runup extent until it was a fully formed scarp with a vertical face. Within the upper 0.15 m of the partially saturated sand, upward pore pressure gradients were detected during backwash, influencing the effective weight of sand and potentially contributing to beachface erosion. The magnitude and frequency of the upward pressure gradients increased with deeper swash depths and with frequency of wave interaction, and decreased with depth into the sand. A simple conceptual model for scarp formation is proposed that incorporates observations of upward‐directed pressure gradients from this study while providing a reference for future studies seeking to integrate additional swash zone sediment transport processes that may impact scarp development.

Simulation Study of Scarp and Berm Formation Using a Suspended Sediment Transport Model

Vu, X.H. and Lee, J.L., 2021. Simulation study of scarp and berm formation using a suspended sediment transport model. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 504–508. Coconut Creek (Florida), ISSN 0749-0208. Beach berms and scarps are common morphological features that develop repetitively due to the cyclic incidence of mild and storm waves. However, beach scarp and berm formations, as well as their morphodynamics, have not been researched thoroughly. In particular, scarp generation is often observed at nourished beaches or under the incidence of oblique storm waves; scarps can pose severe risks to beach users and negatively impact local ecosystems. A numerical study was conducted using a suspended sediment transport model developed in a previous study to quantify the morphodynamic responses of beach berms and scarps to the foreshore region as well as offshore sand motion. In the present study, the destructive beach slope at the foreshore was controlled using Kim's empirical formula on the foreshore slope.

Morphodynamic Evolution of Post-Nourishment Beach Scarps in Low-Energy and Micro-Tidal Environment

Beach scarps are commonly associated with nourishment. Large and persistent beach scarps not only affect the performance of beach nourishment, but also are safety hazards to tourists. In this study, the morphological evolution of beach scarps was examined at a nourished beach in a low-energy and micro-tidal environment. Topographic surveys of nine beach profiles were carried out every 3–6 months after nourishment, lasting for nearly 4.5 years, combined with observed and simulated hydrodynamic data. The results showed that beach scarps were extensively developed after nourishment and migrated landward gradually. The formation of beach scarps was attributed to the higher designed berm, while the migration was possibly initiated by the subsequent higher total water level connected with the irregular tides. However, scarps were completely removed by the first post-nourishment severe storm and had been long absent ever since although two other energetic storms approached. This was different from the result of previous studies, which could be attributed to the much gentler upper beach slope. These results highlighted that the first post-nourishment storm played a key role in the evolution of beach scarps at low-energy and micro-tidal nourished beaches. This study also proposed two methods of determining berm elevation in beach nourishment according to China’s experiences, which would be helpful for other countries’ beach nourishment projects.

Beach scarp dynamics at nourished beaches

Beach scarps are nearly vertical seaward facing sandy cliffs within the cross-shore beach profile. These features are often associated with eroding (nourished) coastlines and can reach heights of O(2–3 m). An analysis of a six-year dataset of beach scarp presence at the nourished Sand Engine beach shows that the formation of beach scarps at the nourishment is linked to mildly erosive (summer storm) conditions, whereas destruction is often related to extremely erosive (winter storm) conditions. Additional experiments were carried out showing the formation, migration, and destruction of scarps from artificially constructed mounds with linear slopes. The field experiments show that steep initial slopes are more susceptible to beach scarp formation. Video observations at these experiments show that the scarp toe level, where the vertical slope meets the gently sloping beach, is related to the high wave runup events. The commonly used 2% exceedance wave runup estimates can therefore be used to predict the final elevation of the scarp toe. The strong connection of maximum runup elevation with the scarp toe elevation provides a direct relation between the final scarp height through nourishment platform height and hydrodynamic conditions. High platform nourishments will promote the formation of beach scarps and steep initial profiles increase the speed at which scarps form. This study suggests that by adjusting the design of beach nourishments, beach scarp formation and persistency can be limited by regulating the natural destruction of these features.

A three-dimensional laboratory investigation of beach morphology change during a storm event

This study provides laboratory observations of the nearshore hydrodynamics and detailed topographic surveys associated with the morphological response of a beach-dune system to oblique waves during a storm event. A physical model consisting of a 20 m long sandy steep beach constructed in a wave basin, and a wave paddle oriented obliquely to the beach shoreline is used to generate irregular waves at the offshore boundary. In the experiment, the wave height is varied over time in five segments to simulate the change in the energy from a passing storm and is varied alongshore to initiate an alongshore gradient in waves and wave-driven currents. To simulate a constant storm surge, a high mean water level relative to the dune is used, enabling waves to reach the dune toe. The results indicate the formation and evolution of a beach scarp. The retreat of the scarp initially corresponds to time-varying wave energy, but becomes controlled by the changing nearshore beach slope over time. The alongshore current magnitude is governed by the spatio-temporal variability in wave energy and is the main driver of sediment flux. The sand that is eroded from the dune initially deposits at the dune toe when nearshore circulation is weak. However, as the wave energy increases sediment is transported along the beach and is deposited, increasing the surf zone width. The experimental results indicate that small waves at high water levels relative to the dune can drive the formation of a beach scarp. In addition, different levels of incident wave energy drive different rates of sediment transport and morphological evolution with high alongshore variability throughout a storm event.

Coseismic coastal subsidence associated with unusually wide rupture of prehistoric earthquakes on the Kamchatka subduction zone: A record in buried erosional scarps and tsunami deposits

The prograding strand plain of Avachinsky Bay, Kamchatka, Russia, along the highly active Kamchatka subduction zone, exhibits geological evidence--buried erosional scarps--for coseismic subsidence only three times in the last four millennia, the last event about 1200 years ago. This same coast has a historical record (since A.D. 1737) of five subduction-zone earthquakes with large tsunami runup (>5 m), the last of which was the 1952 Mw 9 Kamchatka earthquake, and a geological record of more than 30 large tsunamis in the last 4000 years. This rarity of buried scarps relative to large earthquakes contrasts with the Cascadia strand plain in SW Washington State, where most or all large events are represented by buried scarps. A strong factor in the amplitude and sign of coseismic deformation is distance from the seaward edge of a subduction zone (the trench); the Avachinsky Bay coastline is 180–200 km from the trench, with ∼25° slab dip, requiring unusually wide ruptures to generate significant coseismic subsidence. This coastal zone is undergoing net subsidence approximately equivalent to the total of the three coseismic subsidence events, generating a sequence of beach ridges that increase in elevation seaward.Each of the three unusual (coseismic subsidence) events comprises a) an earthquake whose deformation field caused b) onshore coseismic subsidence, thus local sea-level rise and c) sufficient deformation offshore to produce a large tsunami; a,b,c followed by d) a period of coastal erosion and shoreline retreat, leaving e) an erosional beach scarp that was f) subsequently buried once progradation resumed. We identified, dated and correlated the scarps and tsunami deposits from these events with several field methods, including trenching, tephrostratigraphy and ground penetrating radar. The scarps were correlated over an alongshore distance of 50–70 km. The most recent event (event 1) occurred ∼800 cal AD (1100–1250 14С years BP), event 2–600 cal BC (2400–2450 14С years BP), and event 3–1700 cal BC (3300–3500 14С years BP). We developed methods for quantifying subsidence, coastal erosion and tsunami size for each of these events. All three retain evidence of ∼0.4–1.2 m of coseismic subsidence; coastal erosion in the case of event 1 averaged more than 100 m; all three “event” tsunamis were amongst the largest in the last 4000 years.

ИСПОЛЬЗОВАНИЕ МАТЕРИАЛОВ ДИСТАНЦИОННОГО ЗОНДИРОВАНИЯ ЗЕМЛИ В ЛИТОДИНАМИЧЕСКИХ ИССЛЕДОВАНИЯХ (НА ПРИМЕРЕ БЕРЕГОВОЙ ЗОНЫ ОХОТСКОГО МОРЯ)

. The features of coastal destruction on two key areas of the Sea of Okhotsk: Nabil-Lunsky and Nevelsky, are examined, using materials from remote sensing of the Earth. Section significantly different in natural conditions. The analysis of high-resolution satellite images of different times showed that in the Okhotsk sea (Nabil-Lunsky site) a significant factor affecting the dynamics of the coasts is the migration of cuspate bars and storm deformations. Morphologically, cuspate bars are large accumulative wave-like bodies with a period along the coast of about 350–500 m and an amplitude along the normal to the coast of 50–80 m. Cuspate bars s are known to slowly migrate in the direction of the coastal sediment flow. It was found that the average abrasion rate of the beach scarp according to the results of decoding different-time (1983–2010) satellite images and aerial photographs within the Nabil-Lunsky site was from 0.0 to 0.44 m/year. In the Nevel Strait, stability of the west coast is facilitated by wide watts and marches. The eastern coast (Sakhalin Island) in the area of Cape Ouangi is steep coast (slope is greater than 0.03), which favors the marginal erosion. Within the boundaries of the studied areas, the mean annual rate of abrasion was estimated. According to the results of deciphering the satellite images of the Cape Kamennyi area, it was found that the accumulative coasts from 1968 to 2006 were in relatively stable condition, and the abrasion-denudation shore at the seabed end of the cape was subjected to abrasion at speeds of 0.05–0.1 m/year

Coastal landforms of “Meso-Afro-American” and “Neo-American” landscapes in the periglacial South Atlantic ocean: With special reference to the clast orientation, morphology, and granulometry of continental and marine sediments

Coastal landforms of “Meso-Afro-American” and “Neo-American” landscapes in the periglacial South Atlantic ocean: With special reference to the clast orientation, morphology, and granulometry of continental and marine sediments

A first evaluation of the contribution of aeolian sand transport to lagoon island accretion in the Maldives

Aeolian sedimentation and dune development have not been reported from coral atolls at equatorial latitudes. This study presents high-frequency measurements of incident and near surface wind flow and aeolian sand transport on a lagoon sand cay (Maaodegalaa) in the Maldives. Sonic anemometers and Wenglor™ particle counters were operated at 1 Hz for 8 days during the Iruvai monsoon in February 2018. Sand traps were deployed to estimate sand flux and island topography and vegetation cover were surveyed using UAV (un-manned aerial vehicle) photogrammetry and a laser level (in 2017 and 2018). Flow over beach scarps is modelled using computational fluid dynamics. Maaodegalaa sand cay reaches just 0.9 m above the highest spring high tides. Nebkha, between 0.10 and 0.40 m high, are widespread and are associated with Scaevola taccada and Cyperus conglomeratus. Between 2017 and 2018 the eastern section of the sand cay accreted 0.3 m following Cyperus colonisation. Reptation and aeolian ripple development occurred during fieldwork when near-surface flows exceeded 6 ms−1. Saltation occurred at higher wind speeds (8 ms−1). The highest rates of sand transport occurred during north-east incident winds of 12 ms−1 (at 6 m), that were probably generated by surface-based density currents under cumulonimbus clouds. Spatially, higher rates of sand transport were recorded downwind of a beach scarp, probably forced by flow acceleration. We propose a conceptual model of lagoon island formation, with both over-wash and aeolian sedimentation contributing to island accretion. A period of aeolian sedimentation may be critical to the emergence of sand cays.

Yanliao Beach Nourishment Methods

Yanliao Beach, part of Taiwan’s Northeast Coast National Scenic Area, is a major sightseeing and recreation spot. In recent years, the number of tourists has increased, and maintaining the beach’s recreation function has become crucial. In 2007, Typhoon Krosa caused substantial beach erosion at Yanliao Beach; sand dunes collapsed, resulting in a beach scarp and endangering facilities toward the back of the beach. To protect the sand dunes and beach scarp of the post-typhoon topography, based on long-term Yanliao Beach topographic survey information and long-term tidal water level records, this study conducted an integrated artificial beach nourishment method that comprised replenishing the coastal beach and dunes. The planned areas for sand replenishment were those above the mean high water line. This beach nourishment measure protected the dunes from collapsing further and mitigated the risk of beach nourishment polluting the water in front of the beach. Topographic survey results revealed that the large amount of sand deposited in the intertidal zone resulted in the 0 m shoreline advancing toward the seaside. The width of the beach increased, indicating the success of this integrated beach nourishment method.

Mapping of coastal landforms and volumetric change analysis in the south west coast of Kanyakumari, South India using remote sensing and GIS techniques

The coastal landforms along the south west coast of Kanyakumari have undergone remarkable change in terms of shape and disposition due to both natural and anthropogenic interference. An attempt is made here to map the coastal landforms along the coast using remote sensing and GIS techniques. Spatial data sources, such as, topographical map published by Survey of India, Landsat ETM+ (30m) image, IKONOS image (0.82m), SRTM and ASTER DEM datasets have been comprehensively analyzed for extracting coastal landforms. Change detection methods, such as, (i) topographical change detection, (ii) cross-shore profile analysis, (iii) Geomorphic Change Detection (GCD) using DEM of Difference (DoD) were adopted for assessment of volumetric changes of coastal landforms for the period between 2000 and 2011. The GCD analysis uses ASTER and SRTM DEM datasets by resampling them into common scale (pixel size) using pixel-by-pixel based Wavelet Transform and Pan-Sharpening techniques in ERDAS Imagine software. Volumetric changes of coastal landforms were validated with data derived from GPS-based field survey. Coastal landform units were mapped based on process of their evolution such as beach landforms including sandy beach, cusp, berm, scarp, beach terrace, upland, rockyshore, cliffs, wave-cut notches and wave-cut platforms; and the fluvial landforms. Comprising of alluvial plain, flood plains, and other shallow marshes in estuaries. The topographical change analysis reveals that the beach landforms have reduced their elevation ranging from 1 to 3m probably due to sediment removal or flattening. Analysis of cross-shore profiles for twelve locations indicate varying degrees of loss or gain of coastal landforms. For example, the K3-K3′ profile across the Kovalam coast has shown significant erosion (−0.26 to −0.76m) of the sandy beaches resulting in the formation of beach cusps and beach scarps within a distance of 300m from the shoreline. The volumetric change of sediment load estimated based on DoD model depict a loss of 241.69m3/km2 for 62.82km2 of the area and land gain of 6.96m3/km2 for 202.80km2 of the area during 2000–2011. However, an area of 26.38km2 unchanged by maintaining equilibrium in sediment budgeting along the coastal stretch. The study apart from providing insight into the decadal change of coastal settings also supplements a database on the vulnerability of the coast, which would help the coastal managers in future.

Assessment of runup predictions by empirical models on non-truncated beaches on the south-east Australian coast

This paper assesses the accuracy of 11 existing runup models against field data collected under moderate wave conditions from 11 non-truncated beaches in New South Wales and Queensland, Australia. Beach types spanned the full range of intermediate beach types from low tide terrace to longshore bar and trough. Model predictions for both the 2% runup exceedance (R2%) and maximum runup (Rmax) were highly variable between models, with predictions shown to vary by a factor of 1.5 for the same incident wave conditions. No single model provided the best predictions on all beaches in the dataset. Overall, model root mean square errors are of the order of 25% of the R2% value. Models for R2% derived from field data were shown to be more accurate for predicting runup in the field than those developed from laboratory data, which overestimate the field data significantly. The most accurate existing models for predicting R2% were those developed by Holman [12] and Vousdoukas et al. [40], with mean RMSE errors of 0.30m or 25%. A new model-of-models for R2% was developed from a best fit to the predictions from six existing field and one large scale laboratory R2% data-derived models. It uses the Hunt [17] scaling parameter tanβHoLo and incorporates a setup parameterisation. This model is shown to be as accurate as the Holman and Vousdoukas et al. models across all tidal stages. It also yielded the smallest maximum error across the dataset. The most accurate predictions for Rmax were given by Hunt [17] but this tended to under predict the observed maximum runup obtained for 15-min records. Mase's [22] model has larger errors but yielded more conservative estimates. Greater observed values of Rmax are expected with increased record length, leading to greater differences in predicted values. Given the large variation in predictions across all models, however, it is clear that predictions by uncalibrated runup models on a given beach may be prone to significant error and this should be considered when using such models for coastal management purposes. It should be noted that in extreme events, which are lacking in the dataset, runup may be truncated by beach scarps, cliffs, and dunes, or may overtop, and as a result, the probability density functions will have different tail shapes. The uncertainty already present in current models is likely to increase in such conditions.

Field experiments of beach scarp erosion during oblique wave, stormy conditions (Normandy, France)

A field-based experimental study of beach scarp morphodynamic evolution was conducted on the shoreface of a macrotidal sandy beach subject to storms combined with spring tide events (Luc-sur-Mer, France). Both video and in-situ measurements on an artificial berm are used to understand beach scarp evolution over one tide during stormy conditions. Image time stacks are used to analyze the swash action on the beach scarp and topographical data of the scarp are recorded with a terrestrial scanner laser to quantify the morphodynamic response of the beach scarp to wave action. This work provides a new and unique dataset about beach scarp changes and berm morphology in particular under rising tide and oblique wind-wave conditions. During one stormy event, the berm was completely destroyed. However, contrasting alongshore changes were measured during the erosive phase with different crest and foot scarp retreats and eroded volumes between the west and the east side of the berm. The beach in front of the scarp also shows a contrasting residual evolution, indicating an evident longshore sediment transport on the study area as a consequence of incident oblique wave conditions. A strong connection between beach evolution and beach scarp changes is clearly identified. The scarp erosion increases on the west side of the berm when the beach level is lowered and reduces when the beach surface rises on the east side. The beach slope and foreshore elevation as a result of a longshore sediment transport between east and west profiles, influence swash activity. Overall, water depth and swash activity became progressively different along the scarp during the experiment.Swash measurements indicate that the presence of the beach scarp strongly influences the swash motion. At high tide, the reflection of the uprush on the scarp front induces a collision between the reflected backwash and the following uprush dynamic. These collisions reduce and sometimes stop the motion of the following uprush, reducing the incoming swash excursion. Consequently, the scarp presence modifies the swash interaction that normally appears on a planar beach surface. With a beach scarp, the swash energy level is substantially attenuated and its spectrum is characterized by a large band. The number of uprush impacts on the scarp front calculated from video images reaches about 25 per 5min. In spite of the swash energy attenuation due to swash/swash interactions, these impacts provoke the berm destruction in about two hours. However, the onshore migration of the swash zone induced by the rising tide appears to be important to explain scarp destruction, compensating the attenuated swash activity due to backwash-uprush interactions.

Pleistocene sediments of Lake Baikal: Lithology and stratigraphic correlation

The Cenozoic sediments of Lake Baikal penetrated by boreholes and investigated by the manned submersible Pisces, as well as coeval deposits cropping out in beach scarps, recovered by mine workings, and drilled in the coastal zone were the object of this investigation. The main attention was paid to Pleistocene bottom sediments penetrated by Borehole BDP-99-2. The investigations included the detailed analysis of the lithology (grain-size composition, immersion mineralogy of light and heavy fractions, X-ray structural analysis of clayey fraction) and palynological assemblages to specify facies features of Cenozoic sediments, correlate all their known stratigraphic units constituting the sedimentary section of the lake with their analogs in the onshore part of the Baikal rift zone, and compile the composite Cenozoic section. The following features of these sediments are noted: (1) as a whole, Pleistocene sediments are characterized by the hydromica-smectite composition of their clayey fraction with an insignificant share of kaoline; (2) the heavy fraction is dominated by the terrigenous epidote-amphibole association poorly resistant to weathering; (3) Pleistocene sediments of the lake contain siderite, vivianite, pyrite, and goethite concretions and micrometeorites, in addition to well-known ferromanganese nodules; (4) the presence of relict palynomorphs in Pleistocene sediments of Baikal is determined by their erosion from Miocene and Pliocene cavernous clays cropping out on underwater slopes of the Posol’skaya Bank and subsequent reburial along with Pleistocene palynological assemblages.

Post-nourishment beach scarp morphodynamics

Ruiz de Alegria-Arzaburu, A., Marino-Tapia, I., Silva, R., Pedrozo-Acuna A., 2013. Post-nourishment beach scarp morphodynamics In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 576-581, ISSN 07490208. Large and persistent beach scarps can be safety hazards to beach users and result in serious social and economic implications. In this study the morphological evolution of beach scarps of large dimensions is examined on a nourished microtidal Caribbean Mexican beach. Beach profiles were measured three-to-four monthly along the beach after the nourishment in December 2009 and over 1.5 years. A beach scarp was defined as a feature with a slope larger than the critical angle of repose of 32° and a minimum height of 0.25 m. The top and bottom positions of the scarps were calculated from the minimum and maximum values of the second derivative of the measured beach profiles (slope gradient). The cross-shore morphological evolution of the scarps was related to wave runup (R2) and tides, and also to both with the contribution of the longshore energy flux (Pl). During calm conditions characterised by longshore uniform mean and maximum R2 of 0.73 and 0.83 m, and Pl=180KN/s, the scarps remained present along the beach. Energetic conditions with mean and maximum R2 of 0.83 and 1.2 m and Pl=400KN/s, increased the longshore rythmicity of the beach and induced significant cross-shore erosion (over 20 m) and the disappearance of ~50% of the scarps. The added contribution of the longshore energy flux, wave runup and tidal elevation explain 40% of the morphological evolution of beach scarps over the study period.

MORPHODYNAMIC OF BEACH SCARPS ON A MACROTIDAL COAST DURING EXCEPTIONAL WATER LEVEL EVENTS (NORMANDY, FRANCE)

Although there are numerous studies on the morphology and physical processes affecting the dune's scarp and many conceptual models describing beach scarps on microtidal and mesotidal environments (Sunamura, 1985a ; Short,1999), really quantitative informations about the beach scarp formation is lacking, especially along tidal environments where the tide controls the level of wave attack. However, Sherman and Nordstrom(1985) give a qualitative description of beach scarp formations and evolution based on field observations but without data set. The coast of Calvados (Normandy, France) is a fine example of a macrotidal coast, where beach scarps have been often observed. Along macrotidal beaches, the formation of a berm is classically observed during fair weather conditions which contribute to stabilize the coastline. During stormy conditions, the low atmosphere pressure, short waves and onshore winds cause an increase of the water level, which during spring tides, increases the high tide water level time action. Consequently, the natural berm protecting the dune foot, or sometimes a seawall foot, is threatened by erosion and often destroyed. The goal of this study is to quantify the beach scarp destruction (foot and crest scarp retreat speed, eroded volume...) in relation with the hydraulic and morphologic local conditions.

ESTUDO MORFODINÂMICO DURANTE UMA MARÉ EQUINOCIAL DE SIZÍGIA EM UMA PRAIA DE MACROMARÉ DO LITORAL AMAZÔNICO (PRAIA DE AJURUTEUA-PA, BRASIL)

A zona costeira bragantina está susceptível a fortes processos hidrodinâmicos que continuamente remodelama morfologia praial local. Estas flutuações de energia são mais intensas durante as marés equinociais desizígia, registradas durante os meses de março/abril e setembro/outubro, na região. Para conhecer as variaçõesmorfodinâmicas durante um destes períodos, medidas de intensidade e direção de correntes, altura da ondana rebentação (Hb), período da onda (T), ângulo de rebentação (αb), declividade do perfil (m), surf scaling parameter(ε), coeficiente de rebentação (β) e variação da escarpa, foram realizadas, diariamente, entre os dias17 e 22 de março de 2003. As correntes foram medidas na zona de surf, através de um correntômetro SensordataSD30. As alturas, períodos e ângulo de rebentação foram medidos com o auxílio de régua, cronômetro ebússola, respectivamente. Os levantamentos topográficos foram realizados em dois perfis perpendiculares àlinha de costa, com auxílio de um nível, tripé e mira falante. As análises realizadas demonstraram que a área em estudo é, em sua maioria, morfodinâmicamente dissipativa, com ondas tipo deslizante principalmente, nosetor SE. Os processos erosivos na praia em estudo são conseqüência de causas naturais, intensificadas porações antrópicas (e.g. ocupação desordenada de edificações sobre campos de dunas e área de manguezal),sendo a energia hidrodinâmica gerada pela macromaré equinocial, um fator preponderante na modificação daconfiguração morfológica da praia de Ajuruteua, durante o período em estudo. Estas alterações refletiram emtransformações morfodinâmicas a curto período que corresponderam a alterações no volume sedimentar dosdiferentes subambientes praiais.

Laboratory investigation of beach scarp and dune recession due to notching and subsequent failure

Analytical models to calculate notch development and subsequent mass failure of dunes are presented. The notch evolution model is based on a transport equation for sediment from the dune and the sediment volume conservation equation, whereas the models of mass failure are derived using elementary engineering statics and soil mechanics. An empirical transport coefficient in the model describing the notch growth rate is found to be related to the hydrodynamic forcing at the dune normalized by geotechnical parameters describing the resistive strength of the dune. Two modes of mass failure are modeled whereby the overhang generated by the removal of material from the dune foot (notching) slides downward or topples over following the development of a tensile crack some distance shoreward of the maximum notch depth. The accuracy of the notch evolution and mass failure models are assessed by comparing calculated recession distances against measurements from a small-scale laboratory experiment.