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

Métodos semiautomáticos para contagem de árvores em plantios de Pinus taeda L., derivados de dados LiDAR aerotransportado

Orientadora: Profa. Dra. Ana Paula Dalla CorteCoorientadores: Prof. Dr. Daniel Rodrigues dos Santos e Prof. Dr. Carlos Roberto SanquettaDissertação (mestrado) - Universidade Federal do Paraná, Setor de Ciências Agrárias, Programa de Pós-Graduação em Engenharia Florestal. Defesa : Curitiba, 30/07/2020Inclui referências: p. 84-91Área de concentração: Manejo FlorestalResumo: Atualmente demanda-se por metodologias que determinem com acurácia o número de árvores, em povoamentos de Pinus taeda L., com idades próximas a colheita, visando melhorar as estimativas florestais e reduzir custos relacionados à coleta de dados. Neste trabalho, investigou-se o potencial de três métodos de detecção automática de árvores, em povoamentos florestais de Pinus taeda L., com idades de 10 e 15 anos, por meio do uso de dados LiDAR (Light Detection and Ranging) aerotransportado de pulsos discretos e baixa densidade de pontos por m2 (densidade média, entre 3 e 5 pontos por m²). Para isso métodos automáticos de detecção podem ser aplicados para atender esta demanda. Os métodos de detecção automática estudados foram: filtro de máxima local; filtro de máximos globais; e segmentação por bacias hidrográficas. Os dados foram submetidos a algumas etapas de pré-processamento, como a remoção de outliers; a classificação da nuvem de pontos, empregando duas metodologias distintas, o filtro morfológico progressivo e o filtro simulador de tecidos; a normalização das nuvens de pontos e a modelagem digital, a combinação das etapas de pré-processamento resultaram em um conjunto de 15 tipos de dados, utilizados como entrada para os métodos de detecção. Para a verificação da qualidade da detecção de árvores obtida com os algoritmos, foi executado um censo florestal sendo contabilizados os fustes dos talhões em estudo. Os resultados são oriundos de 105 cenários, que consistem na combinação entre algoritmos, parâmetros de detecção e dados de entrada, sendo considerados aderentes aqueles que atingiram acurácia maior que 80%, quando comparados ao censo. Dentre os cenários analisados, nos talhões de 10 anos, apenas 2,9% se comportaram de modo aderente, atingindo uma acurácia média de 88,45%, por meio filtro de máxima local com janela de busca circular, com dimensões inversamente proporcionais à altura e com entrada de dados uma nuvem de pontos não normalizada. Já nos talhões de 15 anos de idade, 9,5% dos cenários foram considerados aderentes, com acurácia média de 93,63%, proporcionada pelo filtro de máxima local com janela de busca, fixa e circular, aplicada a uma nuvem de pontos normalizada e previamente classificada pelo filtro simulador de tecidos. Para a área de estudo e aplicação em ambas as idades, o filtro de máxima local, com janela fixa em formato circular apresentou maior acurácia nas identificações de árvores, 84,02% em média, quando aplicado a um modelo digital de elevação, construído pelo interpolador ponderado ao inverso da distância (idw). Visto todos os cenários empregados, conclui-se que atualmente não existe um algoritmo genérico para aplicações em povoamentos florestais de Pinus taeda L., com espaçamento regular de 2,5m e com idades de 10 a 15 anos, que apresente alto nível de acurácia. Palavras-chave: detecção automática de árvores, filtro de máxima local, filtro de máxima global, segmentação de bacias, censo florestal.Abstract: Currently, there is a demand for methodologies that accurately determine the number of trees in stands of Pinus taeda L., with ages close to harvest, in order to improve forestry estimates and reduce costs related to data collection. In this work, we investigated the potential of three methods of automatic detection of trees in forest stands of Pinus taeda L., aged 10 and 15 years, using LiDAR data (Light Detection and Ranging) airborne pulses discrete and low density of points per m2 (average density, between 3 and 5 points per m²). For this purpose, automatic detection methods can be applied to meet this demand. The automatic detection methods studied were: local maximum filter; global maximum filter; and segmentation by hydrographic basins. The data were submitted to some pre-processing steps, such as removing outliers; the classification of the point cloud, using two different methodologies, the progressive morphological filter and the tissue simulator filter; the normalization of point clouds and digital modeling, the combination of the pre-processing steps resulted in a set of 15 types of data, used as input for the detection methods. To verify the quality of the detection of trees obtained with the algorithms, a forest census was carried out, accounting for the shafts of the stands under study. The results come from 105 scenarios, which consist of the combination of algorithms, detection parameters and input data, being considered adherent those that reached accuracy greater than 80%, when compared to the census. Among the scenarios analyzed, in the 10-year plots, only 2.9% behaved in an adherent manner, reaching an average accuracy of 88.45%, using a maximum local filter with a circular search window, with dimensions inversely proportional to the height and with data entry a non-standardized point cloud. In the 15-year-old plots, 9.5% of the scenarios were considered adherent, with an average accuracy of 93.63%, provided by the local maximum filter with a fixed and circular search window, applied to a standardized point cloud. and previously classified by the tissue simulator filter. For the study and application area at both ages, the local maximum filter, with a fixed circular window showed greater accuracy in the identification of trees, 84.02% on average, when applied to a digital elevation model, built by weighted interpolator to the inverse of the distance (idw). Considering all the scenarios employed, it is concluded that currently there is no generic algorithm for applications in forest stands of Pinus taeda L., with regular spacing of 2.5m and aged 10 to 15 years, which presents a high level of accuracy. Keywords: automatic tree detection, local maximum filter, global maximum filter, basin segmentation, forest censu

On the physical interaction between ocean waves and coastal cliffs

Wave impacts have long been posited as the primary forcing mechanism of coastal cliff recession. Recent developments in the study of hydrodynamics at coastal structures such as seawalls and breakwaters have shown that wave pressures are stochastic in nature and have a broad range of first- and second-order controls. This understanding has yet to be translated to coastal cliffs, where it is still largely assumed that wave impact characteristics can be predicted by simple deterministic formulae. Hydraulic components in coastal models are limited by the lack of in-situ measurements of waves at the cliff toe due to the difficulties in deploying instrumentation in such energetic and inaccessible environments. To address this, I have approached the problem threefold. Monthly high-resolution terrestrial laser scanning (TLS) was undertaken over a year at multiple sites at Staithes, North Yorkshire, to evaluate the recession rate and detachment characteristics of the lower cliff section. Concurrently, wave gauges were deployed at the cliff toe of each site to monitor wave conditions. A novel method of measuring wave impacts was undertaken at one of the sites for nine low-to-low tidal cycles. New and established methods for processing this data were used. Analysis of the erosion dataset revealed distinct temporal patterns of erosion, with accelerated erosion rates during winter. Vertical variations in detachment volumes below 0.1 m3 related to the tidal elevation were also observed, suggesting a key marine influence. Detachment frequency and volume were found to be influenced by lithology type and joint density. Wave conditions over the study period were found to be depth-limited, yet some waves at the toe were found to be larger than those offshore due to shoaling. Wave breaking conditions were strongly influenced by platform morphology and tidal stage. Up to 9% of all waves were breaking on impact. Measurements of wave impacts revealed approximately 14% of wave exhibited high-magnitude impulsive pressures generated by breaking and broken waves. These were analysed probabilistically and found to be controlled primarily by the ratio between wave height and water depth. These data were used to develop a conceptual model of forcing at the cliff toe, including an evaluation of the ability of waves to remove material via enhanced pressure inside discontinuities and fragmentation of weathered material. These results have broad implications concerning the process geomorphology of rock coasts and the evaluation of wave forcing in coastal models

Modelling the morphodynamics of sandy coastal systems under a changing climate

Coasts are vulnerable to changing environmental conditions and are likely to be affected by predicted sea level rise and wave climate variations over the next century. Predicting the likely response of coastal systems to these changes, including altering erosional patterns and flood potentials, is complex and involves understanding their morphodynamics and key sensitivities. Whilst numerical models can be powerful tools for the exploration and prediction of environmental behaviours, mesoscale coastal models tend towards one-line approaches that are unable to simulate the combined effects of wave action and sea level rise (e.g. COVE and CEM).In this thesis the development and application of a new two-dimensional numerical model is presented, designed to simulate coastal morphodynamics at the mesoscale to a higher resolution than afforded by existing models of its kind. The Coastline Evolution Model 2D (CEM2D) has been built upon the core principles of the one-line Coastline Evolution Model but with increased complexity in the domain structure and representation of sediment transport processes. CEM2D has shown to simulate fundamental cause-effect relationships in coastal environments and demonstrated its ability to evolve key shoreline shapes according wave-driven sediment transport processes (including cuspate headlands, sand waves and spits). The results of the modelling exercises reinforce the theory of high angle wave instability and compare well to the planform morphology of natural coastal environments worldwide, including the Carolina Capes (USA) and Spurn Point Spit (UK).Insightful dynamics are observed in the evolution of the simulated coastal environments when subject to sea level rise. Coastal landforms show a varying ability to keep pace with rates of rise at 1 m and 2 m per 100 years, representative of relatively extreme values of change over the coming century. The results suggest that there is a sensitive balance between the wave energy delivered to the shoreline, the balance of cross- and longshore sediment transport, the sediment budget and the level of geomorphic activity that occurs in the systems. These factors determine whether the shoreline erodes, landforms are submerged or progradation occurs and whether remnant morphologies can be preserved in the bathymetric profile. Whilst it is observed that sea level rise plays a significant role in coastal evolution, the wave climate conditions remain the principal agent in defining their planform morphology

On the Opportunities and Challenges of LIDAR Technology in Air-water Flows in Physical Hydraulic Modelling

Aerated free-surface flows commonly occur in high-velocity flows and hydraulic jumps. The aerated free-surface is characterized by intense fluctuations and air entrainment, and accurate measurements are challenging due to both the complexity of the aerated free-surface and limitations of classic instrumentation to single points. LIDAR technology has recently been introduced to measure time-varying free-surface motions in aerated flows with high spatial and temporal resolution. Despite recent LIDAR deployments, guidance on the applicability of LIDAR in a range of air-water flows in hydraulic structures engineering is missing. In this thesis, the LIDAR’s ability is investigated in typical air-water flow phenomena including classical hydraulic jumps, hydraulic jumps in stilling basins and high-velocity spillway flows in the laboratory, as well as at a low-head weir in the field. Advantages and challenges of LIDAR applications in hydraulic structures engineering are demonstrated and discussed. Laboratory experiments were conducted at the UNSW Water Research Laboratory. Comparison of the LIDAR against point-source measurements using phase-detection intrusive probes and acoustic displacement meters revealed close agreement in free-surface properties, such as mean free-surface elevations and free-surface fluctuations, etc., while the LIDAR provided additional benefits of high spatial resolution to create 3D surface maps of complex flows in stilling basins that can guide engineering design. Additionally, a robust method to align all free-surface properties using the measured hydraulic jump toe location was developed that can correct data scatter observed with traditional point measurement instrumentation. Experiments in a large spillway model confirmed LIDAR’s ability to remotely predict distributions of bubble count rate and void fraction in high-velocity air-water flows, which allows remote prediction of hydraulic design parameters such as energy dissipation rate and friction factor. In field applications, the LIDAR showed the ability to simultaneously monitor water depths and free-surface velocities in the subcritical flows upstream of a low-head weir, and the ability to record detailed free-surface features in aerated flows downstream of the weir and in river rapids. Overall LIDAR technology is demonstrated to be a reliable remote sensing tool that provides advanced understanding of complex air-water flows, and may be used to improve engineering design methods of hydraulic structures with promising prospects for field observations. Future research opportunities should explore LIDAR technology in aerated and non-aerated flows across large-scale hydraulic structures and field deployments