Effects of surfzone wave transformation on swash dynamics

Swash oscillations on two natural beaches were measured to show that the shape and magnitude of energy spectra can be largely dependent on processes occurring inside the surfzone. The observations took place on a steep, intermediate beach on the east coast (Tairua Beach), and a low-sloping, dissipative beach located on the west coast of New Zealand (Ngarunui Beach, Raglan), and aimed at improving the understanding of the effects of wave breaking, beach slope, and nonlinear wave interactions on the swash oscillations. These problems were addressed by analysing datasets obtained from field experiments undertaken at these two sites. A field experiment at Tairua Beach showed that swash oscillations were critically dependent on the stage of the tide which controlled the degree of wave energy dissipation over the sandbar crest. Under mild, near-constant offshore wave conditions, the presence of a sandbar and the tidally-controlled water depth over its crest determined whether most of the incoming waves broke before reaching the shoreline. This forced a change in the pattern of wave energy dissipation across the surfzone between low and high tide, which was reflected by changes to swash elevation (runup) on time-scales of a few hours. Significant runup height Rs, defined as 4 times the standard deviation of the waterline time series, varied by a factor of 2 between low tide, when most of the waves were breaking over the sandbar and high tide, when the waves were barely breaking. The increase in wave energy dissipation during low tide was associated with changes in swash maxima distribution, decrease in mean swash period and increasing energy at infragravity frequencies (0.004–0.05 Hz). Bispectral analysis suggested this infragravity modulation might be connected with the presence of secondary waves at the shoreline. Swash oscillations at Tairua were not homogeneous along the beach. Alongshore variability in Rs of up to 78% was observed and was mainly driven by changes in the sea-swell (0.05–0.4 Hz) band of the swash. This variability was predominantly controlled by alongshore changes in beach face slope, although alongshore patterning in wave breaking over the sandbar caused alongshore changes in wave dissipation and also resulted in alongshore swash variation in the sea-swell bandwidth. At infragravity frequencies, alongshore swash variability was not well associated either with changes in beach slope or wave breaking and was possibly linked to the presence of low-mode edge waves, observed from frequency-wavenumber spectra of the swash time series. A final experiment was conducted to understand the surfzone control on incident and infragravity runup on a gently-sloping beach. The observations showed that runup saturation at infragravity frequencies can occur under mild offshore energy conditions if the beach slope is sufficiently gentle. Infragravity saturation was observed for higher-frequency (> 0.025–0.035 Hz) infragravity waves, where typically less than 5% of the (linear) energy flux was reflected from the beach and where, similar to the sea-swell band, the swash energy was independent of offshore wave energy. The infragravity frequency range of saturation was determined by the tide, with saturation extending to lower frequencies at low tide when the local beach face slope over the concave-shaped profile was gentler. Runup was strongly dominated by infragravity frequencies, which accounted on average for 96% of the runup variance, and its energy levels were entirely consistent with strong infragravity wave dissipation observed in the surfzone, particularly when including the nonlinear contributions to the wave energy fluxes. Our observations show evidence of nonlinear interactions involving infragravity and high-frequency, harmonic waves, and suggest that these harmonics could play a role in the wave energy balance near the shoreline on low-sloping, dissipative beaches

The effects of tides on swash statistics on an intermediate beach

Swash hydrodynamics were investigated on an intermediate beach using runup data obtained from video images. Under mild, near-constant, offshore wave conditions, the presence of a sandbar and the tidally controlled water depth over its crest determined whether most of the incoming waves broke before reaching the shoreline. This forced a change in the pattern of wave energy dissipation across the surf zone between low and high tide, which was reflected by changes to swash on time scales of a few hours. Significant runup height (Rs, defined as 4 times the standard deviation of the waterline time series), was found to vary by a factor of 2 between low tide, when most of the waves were breaking over the sandbar (Rs/Hs ≈ 1.5, where Hs is the offshore significant wave height) and high tide, when the waves were barely breaking (Rs/Hs ≈ 2.7). The increase in wave energy dissipation during low tide was also associated with changes in swash maxima distribution, a decrease in mean swash period, and increasing energy at infragravity frequencies. Bispectral analysis suggested that this infragravity modulation might have been connected with the presence of secondary waves

Utilização de métodos diretos e vídeo-imagens ARGUS na caracterização morfodinâmica da zona de arrebentação da praia do Cassino, RS

Dissertação(mestrado) - Universidade Federal do Rio Grande, Programa de Pós-Graduação em Oceanografia Física, Química e Geológica, Instituto de Oceanografia, 2008.Bancos arenosos são feições morfológicas que comumente ocorrem dentro e próximas à zona de arrebentação das praias arenosas, locais onde a amostragem de dados de forma direta é difícil de ser realizada de forma contínua. Em função disto, métodos de sensoriamento remoto baseados em vídeo têm sido bastante utilizados para estudos neste tipo de ambiente. O presente trabalho tem por objetivos principais caracterizar a morfologia e a dinâmica dos bancos arenosos na Praia do Cassino, RS, utilizando métodos diretos (perfis praiais) e indiretos de amostragem (Sistema de vídeo-imagens Argus), bem como avaliar estimativas de posições dos bancos arenosos através destes tipos de imagens. Setenta e um perfis de praia foram obtidos diariamente durante um experimento de campo realizado na Praia do Cassino, RS, simultaneamente à aquisição de vídeo-imagens Argus e dados hidrodinâmicos medidos na zona de arrebentação. Foi observada baixa mobilidade da praia na porção emersa, e alta na porção submersa, sendo esta atribuída à mobilidade dos bancos arenosos na zona de arrebentação. Três bancos arenosos foram amostrados com os perfis de praia, e observados através de padrões de quebra das ondas através das imagens, sendo o primeiro do tipo inter-mareal e os outros dois submersos. O primeiro banco apresentou alta mobilidade em escalas temporais de horas, induzida por variações nas posições de espraiamento na praia, enquanto o segundo, com posição média de 99 m a partir da linha de praia, caracterizou-se por ser mais estável na escala de horas, porém apresentou alta mobilidade na escala temporal de dias. Uma seqüência de migração contínua do segundo banco em direção à costa foi observada durante um período de 13 dias, culminando com a união do primeiro e do segundo banco na região do alinhamento do perfil, e a formação de uma morfologia crescente, típica de estágios praiais intermediários. Foram encontrados indícios de que os padrões de dissipação de energia das ondas na zona de arrebentação estão bastante associados à morfologia do perfil. Imagens Argus do tipo Timex e Variance, as quais retratam padrões de longa exposição e de variabilidade na quebra das ondas, respectivamente, foram utilizadas para estimar as posições dos bancos arenosos. Foi observada boa correlação entre as posições medidas com os perfis e as estimadas com ambas as imagens. Entretanto, oscilações consideráveis em escalas de horas foram observadas, principalmente com as imagens do tipo Variance, induzidas em grande parte por variações em altura de onda e nível. Estas variações alteram o ponto de quebra das ondas com relação à posição dos bancos, e representam assim migração aparente, que deve ser eliminada. A remoção de parte desta variabilidade foi obtida através de modelos de regressão múltipla, os quais melhoraram as estimativas dos bancos, resultando em erros residuais predominantemente restritos a 10 m entre as posições medidas e as estimadas

Observations of alongshore variability of swash motions on an intermediate beach

Alongshore variability in swash motions - shoreline oscillations about the mean water level on the beach face - were investigated using video images and a high-resolution morphology survey on an intermediate beach. Under mild, swell-dominated offshore wave conditions, alongshore variation of up to 78$ in significant runup height . Rs (defined as 4 times the standard deviation of the swash time series) was observed. This variation was predominantly driven by energy at the incident (>0.05. Hz) frequencies (where most of the swash energy was observed), and, consistent with previous observations, was mainly controlled by changes in the slope of the beach face (measured at the mean swash location). However, alongshore patterning in wave breaking over the sandbar caused variation in the degree of wave dissipation along the beach and also resulted in alongshore changes to swash motions. Although alongshore changes in beach slope and wave breaking patterning over the bar were observed to be typically correlated, both were needed in a regression model to provide the best explaination of alongshore changes in . Rs. At infragravity frequencies (<0.05. Hz), alongshore variability was not well associated either with changes in beach slope or wave breaking patterning. Low-mode edge waves were observed in the swash measurements and their contribution to the total energy spectrum was greatest near the location where a shoal was observed, suggesting this shoal may play a role in forcing. The edge waves may have contributed to the swash variability observed at infragravity frequencies. However, in these reflective conditions, the infragravity band plays a secondary role in controlling alongshore variations to swash motions

Hazards and risks associated to coastal processes along the southern Brazilian coastline: a synthesis

Submitted by gabriela rosa ([email protected]) on 2012-06-22T13:53:08Z No. of bitstreams: 1 PERIGOS E RISCOS ASSOCIADOS A PROCESSOS COSTEIROS NO.pdf: 4845492 bytes, checksum: 5f0c2862d8ef9ad7da045cbf1a899293 (MD5)Approved for entry into archive by Bruna Vieira([email protected]) on 2012-06-23T00:15:16Z (GMT) No. of bitstreams: 1 PERIGOS E RISCOS ASSOCIADOS A PROCESSOS COSTEIROS NO.pdf: 4845492 bytes, checksum: 5f0c2862d8ef9ad7da045cbf1a899293 (MD5)Made available in DSpace on 2012-06-23T00:15:16Z (GMT). No. of bitstreams: 1 PERIGOS E RISCOS ASSOCIADOS A PROCESSOS COSTEIROS NO.pdf: 4845492 bytes, checksum: 5f0c2862d8ef9ad7da045cbf1a899293 (MD5) Previous issue date: 2010Some processes related to hydrodynamics, geomorphology and sediment movement represent coastal hazards and risks along the littoral of Rio Grande do Sul (RS) state in southern Brazil. The hazards imply in coastal erosion, habitat loss and environmental change. The risks are related to serious accidents including fatalities to the coastal users. Shoreface morphology, storms and washouts induce erosion at specific locations of the coastline. Storms and washouts, together with beach morphodynamics and depositional processes both aeolian and hydrodynamic embody latent risks to the physical integrity of people that live in and use this environment. Wave refraction patterns from SE to SW cause erosion in two coastal sectors at the proximities of Conceição Lighthouse and Hermenegildo Beach respectively located at the central and southernmost portion of the RS coastline. In both areas, the high number of washouts amplifies the erosion destroying foredunes and the subaerial beach. In addition, the lowlands of the barrier adjacent to small pocket lagoons in the northern littoral are more susceptible to washout erosion. Both, the refraction patterns and the washouts can be associated with two storms tracks and extra-tropical cyclones which magnify erosion and inundation of the coastal zone. The combination of higher population during summer seasons and intermediate beaches with considerable amounts of medium sand make beaches from the northern littoral and extreme south more risky for bathers. Coastline orientation in relation to the predominant NE wind causes wind-blow sand to invade the coastal plain in the form of transgressive dunes causing several hazards mainly in the northern littoral between Tramandaí and Mostardas. Short-term effects associated with episodic events of mud deposition during heavy storms on Cassino Beach influence the morphodynamics behavior on the sectors affected by the mud deposits creating coastal risks relating to beach usage

Variação decadal dos perfis praiais do Farol da Conceição e Terminal

As praias da costa do estado do Rio Grande do Sul têm sido estudas desde o início da década de noventa. Dos aproximados 620 quilômetros de costa, duas praias merecem destaque: Farol da Conceição e Terminal Turístico. O objetivo do presente estudo é descrever variações apresentadas pelos perfis amostrados nessas localidades. Para calculo das taxas de retração e progradação da linha de costa foram utilizados os métodos de pontos finais e regressão linear. Através dos perfis estudados foi possível constatar um grande desenvolvimento ocorrido no campo de dunas frontais cuja causa acredita-se estar relacionada a ações de manejo. Taxas de retração de 1,5 e 3,7 m/ano foram encontradas para o Farol da Conceição bem como taxas de progradação de 3,2 a 4,7m/ano para o Terminal Turístico. Não houve uma grande diferença entre as taxas encontradas com os diferentes métodos utilizados

Perigos e riscos associados a processos costeiros no litoral do RS: uma síntese

Some processes related to hydrodynamics, geomorphology and sediment movement represent coastal hazards and risks along the littoral of Rio Grande do Sul (RS) state in southern Brazil. The hazards imply in coastal erosion, habitat loss and environmental change. The risks are related to serious accidents including fatalities to the coastal users. Shoreface morphology, storms and washouts induce erosion at specific locations of the coastline. Storms and washouts, together with beach morphodynamics and depositional processes both aeolian and hydrodynamic embody latent risks to the physical integrity of people that live in and use this environment. Wave refraction patterns from SE to SW cause erosion in two coastal sectors at the proximities of Conceição Lighthouse and Hermenegildo Beach respectively located at the central and southernmost portion of the RS coastline. In both areas, the high number of washouts amplifies the erosion destroying foredunes and the subaerial beach. In addition, the lowlands of the barrier adjacent to small pocket lagoons in the northern littoral are more susceptible to washout erosion. Both, the refraction patterns and the washouts can be associated with two storms tracks and extra-tropical cyclones which magnify erosion and inundation of the coastal zone. The combination of higher population during summer seasons and intermediate beaches with considerable amounts of medium sand make beaches from the northern littoral and extreme south more risky for bathers. Coastline orientation in relation to the predominant NE wind causes wind-blow sand to invade the coastal plain in the form of transgressive dunes causing several hazards mainly in the northern littoral between Tramandaí and Mostardas. Short-term effects associated with episodic events of mud deposition during heavy storms on Cassino Beach influence the morphodynamics behavior on the sectors affected by the mud deposits creating coastal risks relating to beach usage

Núcleos de Ensino da Unesp: artigos 2007

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq