Aplicação de ganhos

amplitude da intensidade de backscatter que vai se perdendo por espalhamento geométrico, absorção, refração. Esta etapa proporciona uma melhora na imagem, realçando as feições, uniformizando os tons de cores e ao mesmo tempo, dá o contraste ideal para diferenciar os padrões de fundo.

De acordo com Blondel (2009), o dado bruto é limitado, pois o sonograma apresenta artefatos e ruídos inerentes a aquisição, e necessita de processamento. Muitas vezes, a intensidade do backscatter apresenta valores mais elevados em relação aos ângulos de incidência normais especialmente para o fundo do mar liso em comparação com ângulos de feixe externo. Então, é perceptível a importância da aplicação de ganhos para compensar a atenuação do backscatter com o feixe externo. Dessa maneira, o efeito da variação lateral é removido normalizando as imagens do fundo marinho, favorecendo a elaboração do mosaico e a interpretação dos tipos de fundo.

Neste trabalho aplicou-se o ganho auto TVG (Time Varying Gain) com o índice 18 para ambos os lados, após tentativas de equalizar o dado com outros tipos de ganhos (exemplo: BAC e AGC) (Figura 3.10). O ganho TVG, é aplicado por uma função logaritmica e amplia o sinal refletido para melhorar a imagem e tem como principal função multiplicar as amostras de sonar por um valor de ganho que aumenta com o passar do tempo desde a transmissão de impulsos (Figura 3.11).

Figura 3.10 – Tela do SonarWiz 6 para mostrar o ganho aplicado TVG

3.5 Processamento dos dados de batimetria

Nesse trabalho, além dos dados sonográficos processados com o software SonarWiz

6, tem os dados batimétricos que foram processados com o software Hypack para obter assim

uma integração de dados e consequentemente, melhor interpretação das informações. A sequência das etapas no Hypack no módulo Hysweep 32 está na Figura 3.12.

Figura 3.12 – Sequência das etapas feitas no software Hypack.

3.5.1 Correção dos dados

A primeira etapa visa corrigir o dado bruto e após a escolha da linha a ser trabalhada, insere-se as informações de variação de maré e de velocidade do som e segue para a fase “Visualização dos Sensores” (Figura 3.13) na qual é feita uma análise dos sensores de movimento e de posição da embarcação, nesse momento é possível fazer uma primeira intervenção, caso observe-se elementos considerados incoerentes.

Figura 3.13 - Telas do módulo Hysweepdo Software Hypackmostrando, graficamente, as correções de maré, de velocidade do som; Informações dos sensores de movimento e de

posição.

A segunda etapa segue com a remoção dos dados espúrios (spykes) relacionados aos erros de localização e de medições de profundidade incoerentes (Figura 3.14). Ainda nesta etapa é realizado um ajuste angular da distribuição dos pontos das varreduras (patch test). Esta correção é fundamental para a qualidade de integração das linhas de varredura na formação do mosaico para gerar o mapa batimétrico e a modelagem digital 3D, que consiste a terceira etapa.

Figura 3.14- Em A, mostra em amarelo a parte que será retirada (ruidos) e em B, o dado já está editado.

Por fim, a ultima etapa antes de gerar o modelo digital de terreno (MDT) é gerar a matrix que salva o dado em *.XYZ e *.MTX e a partir disso, é possivel gerar o mapabatimétrico.

Figura 3.15- Dado na tela do hypack na fase da edição

3.6 Dados de sedimentos

A partir dos dados adquiridos por (Santos, 2018), foi obtido uma coleta de sedimentos em determinados pontos do estuário para correlacionar o tipo de sedimento com as feições de fundo encontradas e complementar o estudo da área.

A aquisição dos dados para coleta das amostras de fundo foi realizada no dia 04 de Março de 2016 através da embarcação Spirit of Noronha por (Santos, 2018) onde foram coletadas 31 amostras do fundo estuarino através do amostrador de fundo do tipo van veen (Figura 3.16) em 31 pontos previamente distribuídos ao longo do estuário (Figura 3.17).

Figura3.16- Coleta de dados sedimentológicos. Em A: Embarcação Spirit of Noronha; Em B: Funcionamento da draga do tipo van veen (Compilados de Santos, 2018)

3.7 Dados de ADCP

Os ADCPs (Acoustic Doppler Current Profiler) são instrumentos hidroacústicos utilizados para medir a velocidade e a corrente da água em um determinado intervalo de profundidade. Segundo Gordon (1989), o funcionamento básico do ADCP, consiste em emitir pulsos acústicos ao longo de feixes estreitos em uma frequência conhecida. A diferença das frequências dos sons emitidos e refletidos é proporcional a velocidade relativa entre o barco e as partículas imersas na água (efeito Doppler) e portanto, a velocidade da corrente (Bensi, 2006). O instrumento trabalha em frequências variadas de 75, 150, 300,600, 1200 e 2400 kHz, dependendo do modelo (Thiago Filho et al., 1999). A frequência usada para esta coleta foi de 1200 kHz.

As medições foram coletadas nos dias 03, 04 e 10 de Novembro de 2017 próximas a desembocadura do estuário e à montante durante aproximadamente 10 horas, obtendo um total de 97 transectos. A partir dos dados adquiridos é possível ver dados de vazão, profundidade, direção das correntes, distância dos pontos.

A análise dos dados foi realizada no software WinRiver II, os transectos são analisados visualmente, seccionando os perfis longitudinais quando necessários.

Durante as medições no estuário, assim como nos trabalhos efetuados no mar, foi importante manter a velocidade do barco a mais baixa possível. O ideal seria uma velocidade próxima aquela da corrente medida, para obter mais precisão nos dados. No decorrer do trabalho no estuário do Rio Potengi (03,04 e 10 de novembro de 2017), a velocidade média do barco ao longo dos perfis foi mantida entre menor que 1 a 1,5 m/s, que se demonstrou uma velocidade suficientemente baixa para permitir a navegação no rumo estabelecido e obter uma boa qualidade dos dados registrados.

Capítulo 4

Submitted to: Pesquisas em Geociências - UFRGS

Morphology and bedform dynamics of a mesotidal tropical estuary, NE-Brazil

Flávia Valânea Souza Belchior¹ & Moab Praxedes Gomes²

1Post-Graduate Program of Geodynamics and Geophysics, Federal University of Rio Grande do Norte e-mails: flavia.valanea@hotmail.com; gomesmp@geologia.ufrn.br

Abstract

The Potengi River extends for over 170 km into the northeastern coast of Brazil, and its estuary cuts the metropolitan region of the Natal city. These tropical estuaries have the semi-diurnal mesotidal regime, with low sediment and river discharge. The estuarine channel has suffered periodic dredging and its margins with intense urbanization causing erosion and silting regions. We analyzed the channel morphology, the bedforms distribution, the influence of ebb and flood tidal currents on sediment transport, and the anthropic impacts. The data set consisted of interferometric bathymetry and sonography, sedimentological, and current data, collected in August 2017. The interferometric system (Edgetech 4600), operated in the frequency of 540 kHz, with a calibrated swath of 7 times depth. The sonar mosaic was processed in the SonarWiz through geometric editions and gains (TVG). Bathymetric data was filtered using Hypack and interpolated in ArcGIS. A total of 31 grabbed sediment samples were collected using a van ven grab sampler and statistically classified. ADCP profiles were collected during spring and neap tides. The estuary has an asymmetric main channel like a meander with depths varying from 3 m upstream to 20 m downstream. Bedforms occur along the estuary as 2D dunes, 3D dunes, small, medium and large dunes, flat bottom, rough bottom, anthropized regions. Flat and rugged flat bottoms are common on the banks of the channel with a depth ranging from 2 to 6 m. In addition, they are found in the central region surrounding 2D dunes. Both the 2D and 3D dunes are asymmetric dunes and vary according to the spacing between them: from 0.6 to 5 m (small size), 5 to 10 m (medium size) and from 10 to 100 m (large size). Both have depths ranging from 6 to 12 m. Although these dunes are well distributed throughout the channel, the 2D predominate over 3D bedforms.

1. Introduction

The bottom morphology of an estuary reflects the sediment dynamics and the current direction (Kenyon, 1986; Ashley, 1990; Flemming, 2000). Bedforms are used as a proxy to study sediment transport in estuarine-coastal systems (Banard et al., 2013). The location, distribution, pattern, orientation, depth of bedforms occurrence might provide information of sediment pathways, grain-size distribution, and pattern of circulation. Many recent studies address on the distribution and morphology of the dunes to infer the sediment transport pattern in the seabed (Francken et al., 2004; Stow et al., 2013; Salvatierra et al., 2015; Giagante et al., 2008; Vecchi et al., 2008). The flood height, wave shape, and flood duration affect the formation and destruction of bedforms because these are first-order control over the river bed roughness and strength (Simons and Richardson, 1966; Van Rijn, 1984, 1993). Coherent dune-generated flow structures dictates a vertical change of momentum and sediment in the stream which affects energy dissipation within the river, and sediment budgets and bank erosion (Bennett and Best, 1995). Additionally, large bedforms represent a danger to navigation, aggravated by changes from anthropogenic activities, which alter the availability and distribution of sediments and disturb the natural balance of the system.

The Potengi River estuary is located on the eastern coast of the state of Rio Grande do Norte (RN) (Figure 1), receiving the fluvial tributaries of the Doce, Jundiaí and Potengi rivers, embedded on Tertio-Quaternary sedimentary rocks (Barreiras Formation) and sediments, surrounded by fixed and mobile dunes, alluviums, river terraces and mangrove vegetation. Holocene sediments, which fill the estuarine channel, are predominantly at muddy and sandy facies, ranging from sorted to well-sorted (Frazão, 2003; Santos, 2018). This estuary is approximately 800 m wide and has a maximum depth of 15 m in the main navigation channel (Silva et al., 2001). The Potengi Estuary inserted on a mesotidal regime, a hot and humid climate with sparse winters and wetter summers (Rocha & Vital, 2015).

The aim of this paper is to analyze the morphology of the estuarine channel of the Potengi River, to investigate the distribution of the bedforms and the morpho-sedimentary processes related to the dynamics of circulation and sedimentation.

Figure 1 – Study area map showing the Potengi river estuary crossing the Natal city, and the data set of interferometric survey area, the 10 bathymetric profile and ADCP profiles, and

sediment samples sites.

2. Methods

The present study used interferometric technique to collect data of bathymetry and from the reflected energy obtained sonography in August at 2017 and current profiles using an ADCP in November at 2017. In addition, grabbed sediment samples from previous work (Santos, 2018) were analyzed in comparison with the estuarine bedforms distribution. A medium-size boat, the Spirit of Noronha, was used in the acquisition with the interferometric system, and a small boat used (dugout) for the ADCP surveys. Bathymetric and sonographic surveys were carried through eight parallel lines, 8 km each, spaced approximately 80 m, longitudinally to the estuarine channel (Fig. 1).

The equipment used for the research was the EdgeTech 4600 interferometer, which provides acoustic backscatter and swath bathymetry based on acoustic wave phase differentiation. The data acquisition used the frequencyof 540 kHz with a calibrated swath of

7 times depth, a horizontal beam width of 0.5º in eight receive transducers one each side and one transmission element. These elements emit the electroacoustic signals and determine the response angle of the bed through the phase difference, provided by the distance between the receivers, which can vary between ¼ and 1 wavelength, converting this information into distance and consequently generateing the depth.

Positioning from DGPS and an electronic compass (heading) corrected the data during the surveys. The bathymetry data were processed using the Hypack software to remove spurious raw data and to enter the sound speed and tidal variation. Sound velocity profiles (SVP) were collected along the day and used to correct the sound speed of water column. Tidal charts from Brazilian Navy provided the water level corrections. Sonographic data were processed using SonarWiz 6 software for convert Slant Range to Ground Range, to split line length and range, to enhance the histogram and compensate lateral signal losses with Time Variable Gain (TVG) and thus getting the mosaic image (Blondel, 2009).

Transects of acoustic doppler current profiles (ADCP), using an RDI instrument operating on 1200 kHz, were acquired during two days, crossing continuously the channel width during approximately 12 hours per day, of the spring tide (November 3rd and 4th), crossing the estuary near the estuarine mouth and in the middle estuary. The durations of ADCP surveys were limited due to the safety conditions of navigation. The data was analyzed using the WinRiver software.

3. Results

3.1 Estuary morphology and bedforms

The Potengi River estuary is meander-like, varying width from near the mouth with approximately 550 m to up to 300 m further upstream in the study area (Figure 1 and 2). The largest depth of the estuary is 15 m in the dredged area of the estuarine mouth. The south estuarine margin is the strait and very anthropic where the port town of Natal is inserted. The north estuarine margin has a gentle gradient, with sandbanks, mangroves and tidal channels. The main channel of the estuary develops the largest dunes, while smaller dunes, sand banks and the anthropized environments occur on the margins. The estuary can be divided into three zones (Figure 2): one on the upstream part of the estuary where shallow sand banks and narrow channels occur; the middle estuary, an extended intertidal margin associated with the mangrove on north and the main single channel of the estuary on south; the downstream part of the estuary and mouth bathymetry is a result of intense dredging and strongly anthropic

interference in the margins and mouth. These zones differentiate the distribution of bedforms and circulation.

Figure 2 - Bathymetric map of the study area showing the main channel and the margins of the Potengi Estuary.

Bathymetric profiles show channel morphology and the different dune dimensions in the estuary (Fig. 3). The channel has larger dunes in the mouth region and the dredged part, have a height of approximately 2m and are at a depth of 15m, the dunes are wide and asymmetrical. The central region of the channel presents smaller dunes compared to the mouth, at a depth ranging from 7 to 10 m and the margins show their differences. The north margin has sand banks and it is wider and and shallower than to the south margin that is urbanized, has greater depth and smaller dunes. The upstream region is the smallest part of the estuary, has a narrow channel width and the dunes are the smallest, at a depth of 3-4 m.

Figure 3 – Bathymetric profiles A-A’ to J-J’ crossing the channel width and perpendicular profiles 1 to 10 from estuarine mouth to the interior.

Based on the bathymetry, acoustic imagery, sediments and flows measurements of the estuary, a varied bottom configuration of 2D, 3D dunes, flat bottom, and rigid features, represents the interaction between current velocity and direction, bottom type and water depth. In general terms, the Potengi estuary is morphologically characterized by small to large dunes with wavelengths of 4–100 m, heights up to 3 m, and varying in sinuosity and superposition (Figure 4).

The flat bottom occurs commonly associated with the channel margins and only in restricted portions in the main channel (Figure 4A). Flat bottoms present fine sandy sediments, probably deposited at low energy flows or are associated with very muddy bottoms. Moreover, in some portions of the main channel, the bedforms are absent indicating variability of flows do not allowing the dune preservation (Figure 4A ). A rough flat bottom is found in the central part of the estuary at a depth of 3.5 m between the main channel and the intertidal bank (Figure 4K ). In the interior of the estuary, the flat bottoms occur at depths from 2.5 m to 8.5 m (Figure 4O). The North margin of the central region of the data at a depth of 2.6 m there is a variation in textures, as there are regions with a large predominance of smooth and rough texture. Finally, this bottom form is also found in upstream, on the south margin with a depth of 4 m.

The 2D and 3D dunes occur predominantly in the main channel varying sizes, shapes and depths. The dunes decrease size (height and length) upstream, confirming the predominance of marine influence on building bedforms. The biggest dunes at each section occur in the main channel, both at the mouth, in the central part and in the most upstream area. Most dunes recorded by the sonographic survey revealed relevant asymmetries indicating a tidal dominated environments, by flood and ebb tide currents (Figure 4 ),which might be the main control on sedimentation.

At the mouth, asymmetrical straight-crested dunes, mainly small 2D dunes,occur at depths of 10 m,with wavelength of 2.0m to 2.5m, and a lateral extension about 60 m (Figure 4b). In central estuarine, 2D dunes occur in the main channel at a depths around 8 m with wavelength of 80 m (Figure 4c ). And in the most upstream area, the 2D dunes are at a depth of 6 m, the spacing between the dunes is 4 m.

The spacing between dune crests nearly constant along the channel. This is observed in mid-size 2D dunes (Figure 4C, P) spread along the main channel, which may have lateral extension up to 100 m. The dunes near the mouth are in 12 m water depth, with crests spaced 8 m apart. The central area has dunes at depths of 8 m with spacingin between of 5 and 7 m. In the interior estuary, the spacing is 8 to 9 m with a depth of 5.0 m (Fig. 4C). An example of

this 2D dunes is at depth of 7 m, with a height of 1.5 m, a lateral extension of 61 m, and a wavelength of 20 m between them (Fig. 4D).

The 3D dunes are sinuous dunes with a smaller occurrence in the estuary without depth preference. The figure 4E shows small 3D dunes near the mouth with depths of 12 to 13m with lateral extension of 57 m and wavelength of 5 m.Mid-size 3D dunes present spacing between crests from 5m to 10m (Fig.4F and 2M). The largest 3D dunes may reach a wavelength of 25 ma lateral extension of 105 m at water depth below 10 m, with superposition by smaller dunes.

Rigid features, rocky, wrecks, bridge pillars, are mainly present on the south channel margins, with a depth ranging from 4 m to 9 m and varying in size (Fig. 4H, I and J).

Figure 4 - Sonographic image with background features. In A is a flat bottom, in B, C and D are small, medium and large 2D dunes. In E, F and G are medium, small and large 3D dunes, respectively. In I, J, K are rigid features. In L are medium sized 2d dunes, in M are medium sized 3D dunes, in N are small 2d dunes, in O are rigid features and in P are medium sized 2d dunes.

3.2 Estuary sedimentology

The Potengi River estuary has a predominance of sandy sediments, this area has sediments classified as very fine sand, fine sand, medium sand and coarse sand (Table 1). The estuarine margins are plenty of medium sand with the development of small to medium-size dunes. The coarse sand is isolated in patches at the mouth, in the central region and in the upstream, associated with small dunes and sand bodies. Very fine sand is in flat bottoms and the fine sand predominates on the shores but might be associated with both dunes and flat bottoms. The amount of carbonate (CaCO3) present in the Potengi River estuary ranges from 0.60% to 88%. The mouth region has a percentage below 5%, the central region and upstream the values range from <5% to 15% and in isolated patches can reach 35%. The value of 88% is inside the Jaguaribe tidal channel.

Table 1- Sedimentary facies and their composition from the average of the total values

3.3 Estuarine Circulation

The current profiles were acquired near the mouth of the estuary on the spring tide of November 3rd (Figure 5) show the current flow at ebb phase (profile A), the speed is quite varied, the north margin is shallower and its speed ranges from 0 to 0.6 m/s, the south margin has a higher speed, reaching 1 m/s and the flow (Q) reaches 1948m³ / s. The profiles B and C show the highest speed in the main channel (0.4 to 0.9 m/s) while at the margins the current speed varies between 0 to 0.3 m / s. Profile D has the highest flow (-2450 m³/s) reached at 14:30 h, the speed varies from 0.6 to 0.9 m/s on the north margin and main channel, however the speed is lower on the margin south (0.0 to 0.3 m/s). The profile E still remains at tide with a flow of -2056 m³/s, the current speed decreases for the south and north margin, the channel has a speed between 0.6 and 0.9 m/s. Finally, the F profile shows the current flow starting at low tide, with a flow rate of 1649 m³/s, the margins are faster than the channel and vary from