Hydrosedimentological Processes Research Articles

Use of Statistical Tools and Artificial Intelligence for Hydrosedimentological Analysis in Experimental Units in Semiarid Brazil

Objective: To compare statistical and artificial intelligence tools to assist in the analysis and influence of hydrosedimentological variables on runoff generation and sediment yield in experimental units in the Brazilian semiarid region. Theoretical framework: The increasing frequency of extreme weather events in areas prone to natural disasters has led to economic, social, and loss of life due to the lack of urban planning, disorderly occupation in densely populated areas, intensive use of natural resources, and difficulty in maintaining and conserving natural ecosystems (CARMO & LIMA, 2020). Continuous monitoring of hydrosedimentological processes is necessary for natural resource planning and management. There is a need for expanding long-term environmental monitoring in Brazil, which is not simple and costly. In hard-to-reach areas, investment in tools and variable analysis for secure estimation of flow and erosion is necessary (AIRES, 2022). Method: Data obtained from the São João do Cariri Experimental Basin - SJCEB (13.6 km²) were used through conventional rain gauges and automatic data collection platforms from three micro-watersheds (0.16 – 1.63 ha) and two experimental plots (100 m²) located in the semiarid region of Paraíba, Northeast Brazil. For the analyses, 147 rain events that generated surface runoff in at least one of the five experimental units of the SJCEB were used, with data on area (m²), vegetation cover (0 no cover and 1 with vegetation cover over the entire area), average slope (%), daily precipitation (mm), period without rainfall (days), runoff depth (mm), and sediment yield (kg.ha⁻¹), from 2001 to 2008. The statistical and artificial intelligence tools used in the analyses were: simple linear regression, Pearson correlation, and Self-Organizing Maps (SOM). The models were implemented in the R programming language. Results and conclusion: In the statistical summary of the studied variables, precipitation ranged from 2.2 mm to 133.0 mm, with a mean of 21.1 mm. The period without rainfall ranged from 0 to 240 days, with an average of 16.2 days. Slope varied from 3.40% to 7.50%, with a mean of 5.7%. The average vegetation cover was 0.44, with a median of 0.60. The Pearson correlation analysis revealed that precipitation showed a positive and significant correlation with surface runoff and sediment yield variables, ranging from moderate to strong. Simple linear regression was unable to provide satisfactory answers to assist in the analysis of runoff and sediment yield, therefore considered inefficient for hydrosedimentological regionalization. The unsupervised SOM network, by presenting complex relationships among the studied variables, detected that sediment yield is particularly influenced by the interactions with precipitation, vegetation cover, and slope. Thus, it can be inferred that models based on unsupervised neural networks are attractive alternatives, as they were able to extract complex and nonlinear relationships among the analyzed variables. When there are complex cause-and-effect relationships between variables of hydrosedimentological processes, the use of nonlinear and multivariate models, such as artificial neural networks. Implications of the research: Manipulations in experimental areas have favored the measurement of flow and quantification of sediment yield. However, the high costs to maintain physical structures and the presence of specialized personnel for monitoring, processing, and dissemination of collected data are factors that may make it impractical to maintain and expand environmental monitoring networks in Brazil. In ungauged areas, the prediction of flow and erosion is necessary, and can be performed through mathematical, statistical, or artificial intelligence tools that facilitate the identification of priority areas for intervention. Manipulations in experimental areas have favored the measurement of runoff and sediment yield, but the high costs of maintaining the physical structures and the presence of specialized personnel to monitor, process and disseminate the collected data are factors that make maintenance and the expansion environmental monitoring networks in Brazil. In non-instrumented areas, forecasting and analysis of runoff and erosion becomes necessary, using mathematical, statistical or artificial intelligence tools that favor the identification of priority areas for intervention. Originality/value: Use of different statistical and artificial intelligence tools for hydrosedimentological regionalization in small experimental units in the Brazilian semiarid region, with emphasis on the application of unsupervised learning neural networks (SOM).

Climate change impact on streamflow and suspended sediment load in the flood-prone river basin

Abstract Understanding the relationship between climate change and changes in streamflow and suspended sediment load (SSL) at the regional scale is critical for future decisions on water resource management. Hence, this study assessed the projected climate change impact on streamflow and SSL based on CMIP6 projections in the flood-prone Mahanadi River Basin (MRB) using the SWAT model. Bias-corrected climate data from five global climate models under medium and extreme shared socioeconomic pathways (SSPs) were utilized as an input to the SWAT model for the baseline (1985–2014), near future (NF, 2020–2046), and mid future (MF, 2047–2073) periods. Rainfall is expected to increase by 2.86% (2.19%) and 13.35% (16.98%) in the NF and MF, respectively, under SSP245 (SSP585). Streamflow and SSL may increase by 6.38% (6.06%) and 4.43% (7.89%) in the NF and by 29.78% (36.90%) and 37% (46.43%) in the MF, respectively, under SSP245 (SSP585) scenario. Also, streamflow and SSL exhibited an increasing trend in most months as well as in most seasons. The findings may be helpful to decision-makers in identifying watershed management plans by offering more information on how climate change affects the hydro-sedimentological processes in a flood-prone river basin.

Impacts of land use and land cover changes on hydrological processes and sediment yield determined using the SWAT model

Land use and land cover (LULC) changes strongly affect local hydrology and sediment yields. The current study focused on a basin in the Brazilian Amazon and had the following three objectives: (1) to perform an effective diagnosis of flow and sediment yield, (2) to evaluate the impacts of LULC changes over the last 40 years on the hydro-sedimentological variables, and (3) to investigate the impacts of the possible trends or breaking points in the flow, surface runoff, and sediment yield series. The Soil and Water Assessment Tool (SWAT) model validation showed that the simulated results were consistent with the data measured in the dynamic reproduction of flow seasonality. Furthermore, changes in LULC altered surface runoff, sediment yield, and flow according to the Mann-Kendall and Pettitt non-parametric tests. It was also observed that the sub-basins in which pastureland is predominant are more susceptible to increased surface runoff and sediment yield. According to trend and homogeneity tests, these sub-basins had break points in the time series of these variables. On the other hand, in the sub-basins whose land cover is predominantly forest, the time series is homogeneous and trendless. The current study highlighted the impacts that 40 years of LULC changes in an Amazonian basin had on hydro-sedimentological variables, which, in turn, can play an important role in changing the hydrological cycle, consequently these changes can also impact diverse human activities in the region, such as agribusiness, livestock, energy production, food security, and public water supply. Thus, it is concluded that the SWAT model is a good estimator of hydro-sedimentological processes in Amazonian basins and can be used by decision makers in the management of water and environmental resources.

Investigation and Discussion on the Beach Morphodynamic Response Under Storm Events Based on A Three-Dimensional Numerical Model

A well-established 3D phase-averaged beach morphodynamic model was applied to investigate the morphodynamics of a typical artificial beach, and a series of discussions were made on the surfzone hydro-sedimentological processes under calm and storm events. Model results revealed that the nearshore wave-induced current presents a significant 3D structure under stormy waves, where the undertow and longshore currents exist simultaneously, forming a spirallike circulation system in the surfzone. Continuous longshore sediment transport would shorten the sediment supply in the cross-shore direction, subsequently suppress the formation of sandbars, showing that a typical recovery profile under calm waves does not necessarily develop, but with a competing process of onshore drift, undertow and longshore currents. Sediment transport rate during storms reaches several hundreds of times as those under calm waves, and two storm events contribute approximately 60% to the beach erosion. Sediment transport pattern under calm waves is mainly bed load, but as the fine sands underneath begin to expose, the contribution of suspended load becomes significant.

Large meandering bends with high width-to-depth ratios: Insights from hydro-sedimentological processes

Morphodynamics and hydro-sedimentological processes in meandering rivers are one of the most complex phenomena observed in alluvial channels. Because of the complex nature of the interaction between flow structure, morphology, sediment transport, and bank roughness, a better understanding of these morphological units is needed, particularly for low-gradient rivers characterized by large meandering bends with high width-to-depth ratios. The present research provides an accurate description of the interaction between suspended bed-sediment transport, flow structure, and bed morphology on three consecutive bends characterized by width-to-depth ratios higher than 50. The study focuses on a selected reach of the Colastiné River, which is a secondary channel of the Paraná River, Argentina. Acoustic measurement techniques with high spatial-time resolution were employed during two different events - a bankfull and a medium-flow stage event - to capture the three-dimensionality of the flow velocity, suspended bed-sediment transport, and variations in bed morphology. Although the core of maximum velocity shifts from bank to bank at the bend entrance, following the thalweg shifting, the suspended bed-sediment concentration remains in the center of the channel because of the strong influence of the secondary currents and the bed morphology. Two types of secondary flows are well-defined in the cross section: a unidirectional flow toward the outer bank generated by the topographic steering effect along the point bar, and the classical helical motion confined to the thalweg zone. Close to the outer bank, the core of maximum velocity shifts toward the center of the channel because of the presence of macro-roughness induced by banklines and downed trees, which in turn generate high turbulence patterns along the outer bank. The bed morphology shows an extended point bar occupying almost half of the channel width (in the cross section apex) and spreading downstream to the entrance of the following bend. The thalweg shows an abrupt change from bank to bank, producing high curvature at each bend entrance. The findings reported herein show a lack of correlation between the cores of maximum velocity and suspended bed sediment, exhibiting different behaviors than those observed in smaller alluvial channels with lower width-to-depth ratios.

Artificial Neural Network and Fuzzy Inference System Models for Forecasting Suspended Sediment and Turbidity in Basins at Different Scales

The monitoring of hydro-sedimentological processes is important for environmental control but depends on resources that are not always available. The estimation of sedimentological variables with mathematical models is often limited by the scarcity of data for a single basin. This research experiments with the simulation of suspended sediment concentration (SSC) and turbidity (T) using a regional model, with data from agricultural basins of different scales within the same hydrographic region, using hourly precipitation as one of the predictive variables, aggregated through the exponentially weighted moving average (EWMA) of past rainfall, in artificial neural network (ANN) and fuzzy inference system (FIS) models. The data monitoring was performed from January 2013 to February 2020 in four watersheds within the same region in southern Brazil, with areas ranging from 1.3 to 524.3 km2. For the turbidity estimation, the FIS model, which also made use of the discharge (Q) and area (A) of each basin as inputs, performed best, with a Nash-Sutcliffe efficiency (NS) of 0.860 for the verification samples. Several FIS and ANN models performed very well for SSC prediction (with NSs ranging from 0.950 to 0.977) due to the EWMA variable, including an FIS model that uses only this variable (NS 0.952). The results allow us to conclude that it is possible, with few data for the individual basin and a regional empirical model, to estimate SSC and turbidity, provided the aggregation of hourly precipitation by the EWMA, as long as the basins have similar physical and climatic characteristics.

Hydrosedimentological modeling with SWAT using multi-site calibration in nested basins with reservoirs

ABSTRACT Calibration and validation of hydrosedimentological models, usually performed at the outlet of a single basin, does not always correctly represent the hydrosedimentological processes in the different subdivisions of dammed river systems. The aim of this study was to evaluate simple calibration techniques (watershed outlet) and multi-site calibration (watershed outlet and internal reservoirs) with the Soil and Water Assessment Tool - SWAT model, using two nested basins in the southern region of Brazil. Three modeling procedures were analyzed, adjusting the hydrological and sedimentological parameters of the watershed and the reservoirs. It was found that (a) the simplest calibration does not correctly represent the processes in reservoirs; (b) the multi-site calibration provided a better simulation of the hydrosedimentological dynamics of the nested basins; and (c) parameterizations of the SWAT reservoir module have limitations in the context of the study area. The results showed that the multi-site calibration in watershed with reservoirs is more appropriate.

Hydrosedimentological Modelling of a Small, Trained Tidal Inlet System, Currumbin Creek, Southeast Queensland, Australia

ABSTRACT Shaeri, S.; Strauss, D.; Etemad-Shahidi, A., and Tomlinson, R., 2018. Hydrosedimentological modelling of a small, trained tidal inlet system, Currumbin Creek, southeast Queensland, Australia. Small tidal inlets with an entrance width of less than 50 m or a cross-sectional area of less than 100 m2 have not been investigated as often as their larger counterparts. To discern the major processes responsible for entrance infilling in a small inlet system, the hydrosedimentological processes in the annually dredged Currumbin Creek (Australia) inlet system were investigated. Delft3D was applied and validated using local field measurements and regional data. A standalone regional hydrodynamic model and three nested standalone wave models provided detailed data for the offshore boundary of the more detailed creek entrance model, which was a coupled, local flow–wave–sediment transport model. The noncohesive sediment transport module was calibrated based on the volume of dredging and the net annual volume o...

Simulation of hydrosedimentological impacts caused by climate change in the Apucaraninha River watershed, southern Brazil

Abstract. Climate change can cause significant modifications in hydrosedimentological processes. Climate projections indicate the occurrence of extreme events, in terms of precipitation, droughts, floods and temperature. By increasing temperatures and altering precipitation regimes, climate change is expected to affect sediment dynamics. Predictions of the effects of climate change on streamflow and sediment yield vary widely, depending on the geographical location and climate scenarios used. Mathematical modelling can be used to simulate the hydrosedimentological processes in watersheds and enable the simulation of climate change effects on sediment yield. This paper aims to simulate the impacts of climate change hydrosedimentological dynamics in the Apucaraninha River watershed (504 km²), southern Brazil, considering the climate change scenarios A2 (pessimistic about the emissions of greenhouse gases) and B2 (optimistic about the emissions of greenhouse gases), developed by the IPCC. The Soil and Water Assessment Tool (SWAT) was used to evaluate the impacts of climate projections on the sediment yield in the Apucaraninha River watershed. The model was calibrated and validated using daily streamflow and sediment data from 1987 to 2012. The model presented satisfactory fit to the observed data allowing the reproduction of the current hydrological conditions of the watershed. Based on the satisfactory results in calibration and validation, the climate scenarios A2 and B2 were inserted to simulate streamflow and sediment conditions for the period 2071–2100. The results for both scenarios indicate that simulations of both climate scenarios resulted in changes in hydrosedimentological dynamics in the Apucaraninha River watershed, mainly in terms of decrease in average sediment yield due to the reduction in precipitation amount and increase in evapotranspiration. Our results also indicate that every 1% change in precipitation has resulted in 2.8% change in soil erosion and 1.6% change in runoff under scenario A2, and 2.3% change in erosion and 1.1% in runoff under scenarios B2, thus suggesting that climate change tends to affect sediment yield more than streamflow, although seasonally both could be impacted in similar ways.

Description of hydrological and erosion processes determined by applying the LISEM model in a rural catchment in southern Brazil

Hydrosedimentological modeling is a tool that can be used to understand better important processes occurring at the catchment scale, such as runoff and sediment yield. The aim of this study was to use the Limburg Soil Erosion Model (LISEM) to describe the runoff and sediment yield during rainfall–runoff events in a small rural catchment in southern Brazil. The study was conducted in the Lajeado Ferreira Creek catchment (drainage area of 1.19 km2) where intense land use has caused a negative impact on water resources. Thirteen rainfall–runoff events that occurred in 2010 and 2011, including high-magnitude events, were used to model hydrosedimentological processes. from the calibration and validation stages indicate that the model had a good performance when representing the hydrograph, including events with greater complexity. The use of a second soil layer in the model increased its efficiency, which is in accordance with the importance of subsurface flow in this catchment and its sensitivity to the physical properties of the soil, which are essential for controlling hydrosedimentological processes at the catchment scale. The simulation of sediment yield was overestimated by the model, constrained by the lack of sensitivity of the model to soil cohesion and the stability of soil aggregates. During the model calibration stage, these parameters had values different from those measured in the field. The LISEM model performed well in representing runoff for events of different magnitudes. The discretization of the physical–hydrologic properties in the soil profile enabled the evaluation of the effect of subsurface impediment layers on water infiltration and runoff. The simulation was less accurate for suspended sediment concentration than for runoff. This indicates the need for further studies to either identify other factors controlling erosion and sediment yield that have not been identified by the model, or identify if the representation of the physical parameters is inadequate, especially the values of soil cohesion and aggregate stability.

Hydrosedimentology of nested subtropical watersheds with native and eucalyptus forests

Information on the effects of eucalyptus forests on hydrosedimentological processes is scarce, particularly at the catchment scale. Monitoring and mathematical modeling are efficient scientific tools used to address the lack of information for natural resource management and the representation and prediction of those processes. This study evaluates the effects of eucalyptus cultivation on hydrosedimentological processes in watersheds and to use the Limburg soil erosion model (LISEM) to represent and predict hydrological processes. The study was conducted in two forested watersheds: the main watershed (94.46 ha) and a nested sub-watershed (38.86 ha), both cultivated with eucalyptus and residual riparian native forest, located in southern Brazil. Hydrosedimentalogical monitoring was conducted from 16th February 2011 to 31st December 2012, and LISEM model calibrations were performed on the bases of six storms events. The sediment yield for 2011 was 41.6 Mg km−2 and 38.5 Mg km−2 for the watershed and sub-watershed, respectively. An extreme event in 2012 provided greater sediment yield for the sub-watershed (99.8 Mg km−2) than that for the watershed (51.7 Mg km−2). Rainfall events with a greater maximum intensity generated rapid discharge and suspended sediment concentration responses in the sub-watershed due to the smaller drainage area and steeper landscape. In the main watershed, the accumulation of flood waves occurred for most events, with less steep hydrographs, and a later occurrence of the discharge peak after that of the sub-watershed. The LISEM adequately reproduced the peak discharge and runoff for the calibrated events; however, the peak time and the shape of the hydrograph were not adequately represented. The hydrosedimentological patterns of the watershed and sub-watershed, both cultivated with eucalyptus, was characterized by sedimentographs preceding hydrographs during rainfall–runoff events where scale effects occur, with maximum discharge and specific sediment yield greater in the watershed than that in the sub-watershed. Empirical models based on hydrologic variables may be used for estimating the suspended sediment concentration and sediment yield. Therefore, LISEM may be used for the prediction of hydrological variables in these forested watersheds.

Análise de sensibilidade dos parâmetros do modelo SWAT e simulação dos processos hidrossedimentológicos em uma bacia no agreste nordestino

O processo erosivo constitui-se na principal causa de degradação dos solos e é acelerado por intervenções humanas nas bacias hidrográficas, resultando em prejuízos ao setor agrícola e ao meio ambiente. Para a previsão dos impactos de ações antrópicas ou de mudanças climáticas sobre os processos hidrossedimentológicos, os modelos distribuídos e de base física têm sido bastante eficazes. Neste estudo, aplicou-se o modelo SWAT à sub-bacia do Rio Japaratuba Mirim, em Sergipe, nas seções de medição das estações Fazenda Pão de Açúcar (PA) e Fazenda Cajueiro (CJ), com áreas de contribuição respectivamente de 137,3 e 277,8 km², visando à parametrização das duas bacias aninhadas por meio dos processos de calibração e de validação. Foram realizadas simulações do escoamento na bacia menor (PA), a partir dos parâmetros calibrados na bacia maior (CJ), e, reciprocamente, do escoamento na bacia maior (CJ). Em seguida, foram também realizadas simulações do escoamento e da produção de sedimentos para o período de dados de vazão disponíveis (1985 a 2000), para a seção CJ. Os resultados evidenciaram que o modelo calibrado simulou bem o escoamento superficial e conseguiu prever, de forma coerente, a produção de sedimentos com base nos 12 parâmetros mais sensíveis do modelo. A transposição dos parâmetros da bacia maior para a menor e, vice-versa, resultou em índices de Eficiência de Nash-Sutcliffe (NSE) e de Tendência Percentual (PBIAS), considerados satisfatórios para o primeiro caso e insatisfatório para o segundo.

Produção de Sedimentos em Diferentes Escalas Espaço-Temporais de Duas Pequenas Bacias e um Lisímetro no Bioma Mata Atlântica

The present research aims to evaluate the hydrosedimentological processes of two small basins in the Vacacai Mirim River watershed - Rancho do Amaral (4.45km²) and Rincao do Soturno (11.98Km²) and in a volumetric lysimeter (1m³) - in the Atlantic Forest biome area in Santa Maria (RS, Brazil), by monitoring sediment production at different space and time scales. The variables selected for this study were monitored during the year of 2009. The precipitation was monitored with two electronic rain gauges, and the water levels established employing two electronic linigraphs. The measurements carried out comprised solid and liquid discharges at each measuring station installed. Samples of suspended sediments were collected using a conventional sampler and automatic point samplers. The sediment yield values found for the three sites monitored were: 1.7 .10⁻⁴mm(soil)/mm(effective rainfall) in the lysimeter; 2.6 .10⁻⁴mm/mm in Rancho do Amaral basin; and 3.0 .10⁻⁴mm/mm in Rincao do Soturno basin. According to these results, in each basin, human use and occupation of the area were the prevailing factor for sediment yield, besides effective rainfall

Análise da dinâmica hidrossedimentológica em uma bacia hidrográfica no sul do Brasil

O presente trabalho tem por objetivo analisar a dinâmica hidrossedimentológica em uma bacia hidrográfica no sul do Brasil, por meio da descarga líquida (Q) e da concentração de sedimento em suspensão (C SS), monitorados automaticamente por sensores. A Q e a C SS foram comparadas em 25 eventos pluviométricos, na bacia hidrográfica do Caeté, Alfredo Wagner/SC, entre setembro de 2006 e março de 2008, por meio das análises de regressão, correlação e do comportamento das variáveis ao longo do tempo (histerese). A C SS máxima não apresentou bons índices de correlação com a pluviosidade total e com a descarga líquida máxima, mas sim com a pluviosidade máxima (intensidade). Os índices de correlação entre vazão e C SS apresentaram coeficiente de Pearson (R) superior a 0,70 em 56% dos eventos. Contudo, verificou-se que, na ascensão do hidrograma, os índices de correlação são baixos, enquanto na recessão altos. A relação entre descarga líquida e concentração de sedimento em suspensão apresentou comportamento do tipo figura oito, com pico de concentração de sedimento em suspensão que antecede o da vazão, com a presença de mais de um pico por evento, representada por uma curva no sentido horário, seguida por outra curva no sentido anti-horário. Por fim, constatou-se que o monitoramento automático é de extrema importância para o entendimento da dinâmica dos processos hidrossedimentológicos, por possibilitar medições contínuas em intervalos de tempos pequenos (minutos), pois dados obtidos em intervalos de tempos grandes (dias) podem esconder variações e comportamentos significativos da dinâmica dos processos hidrossedimentológicos.

New tools for modelling water quality of hydrosystems: An application in the Seine River basin in the frame of the Water Framework Directive

The implementation of the European Water Framework Directive requires new tools for predicting the effect of expected measures taken in the watershed on water quality at the scale of large regional river systems. In the Seine basin, four models, developed in a research context, have been chained to each other to simulate water quality and biogeochemical functioning of the hydrosystem from headwater streams to the coastal marine area. All four models are based on a similar deterministic approach and share a common description of the biogeochemical processes, allowing them to exchange information. Each model differently represents the hydro-sedimentological processes, and uses different time and space resolution, in order to tackle with the specific problematic of each sub-system. This cascade of models has been used for testing a prospective scenario of water resources management at the horizon of 2015, established by Water Authorities of the Seine-Normandy district. The simulation predicts a general improvement of water quality concerning those variables linked to point sources of pollution (ammonium, oxygen, phosphate), even if, locally, this improvement can be insufficient for meeting the expected quality standards. The predicted improvement of the quality of the Seine River downstream from Paris and its estuary is large. However, the predicted very significant drop of phosphate contamination, although beneficial for limiting the problems of coastal marine eutrophication, does not lead to a significant control of phytoplankton development in the rivers upstream from Paris. The simulation also predicts a general increase in nitrate contamination mainly linked to diffuse sources from agricultural areas.

Badland dynamics in the Central Pyrenees: temporal and spatial patterns of weathering processes

AbstractThe genesis and development of badlands is controlled by bedrock lithology, topography and climate. The development of badlands in the Central Spanish Pyrenees is favoured by the presence of marls and a climate with a marked seasonality; the combination of these factors results in active hydrosedimentological processes. The aim of the present work was to study the dynamics of badlands in the Central Pyrenees, with a focus on the temporal and spatial patterns of the dominant weathering processes. The study area was a small mountain catchment, and we focussed on the continuous observation of a number of physical parameters that control regolith development and weathering processes. Samples collected from different depths at sites with different aspects enabled temporal comparisons of the factors involved in the development of surface regolith that can be considered as indicators of the weathering of the marls. These indicators were then related to environmental variables (moisture and temperature). Atterberg limits were established and related to moisture content to explain slope dynamics and related processes. Finally, analyses of carbonate content were performed in the laboratory. The results validate the employed methodology, supporting our description of weathering processes, the established chronological sequence and the relation of these processes to climatic and topographic parameters. The results obtained since January 2004 show that the chosen indicators are sensitive to seasonal dynamics, variations in temperature and water supply, and slope aspect. The development and dynamics of regolith on north‐facing slopes is more active than that on south‐facing slopes. Freeze–thaw is the main weathering process on north‐facing slopes, while south‐facing slopes are dominated by the development of crusts associated with wetting–drying processes. Washing effects and lower temperatures upon north‐facing slopes mean that the dissolution of carbonates is more pronounced in such areas compared with south‐facing slopes, especially within the upper layers of the regolith. Copyright © 2006 John Wiley & Sons, Ltd.

Modos de Transporte de Sedimentos Finos no Estuário do Rio Itajaí, SC

This paper presents an assessment about the modes of fine sediment transport in the Itajaí River estuary. ·The available information to approach a conceptual model is derived from three primary sources: (1) the river sediment delivery; (2) the distribution of salinity and particulate suspended mailer (MPS) along the estuary under influence of different river discharge conditions; and (3) near bed hydro-sedimentological processes. The river sediment delivery was achieved by daily monitoring the water and the suspended sediment discharge, since November 1998 at the Indaial lymnimetric station, which represents about 70 % of the drainage basin. The salinity and MPS distribution in the estuary were obtained through 47 weekly surveys along the estuary when vertical profiles of these variables were acquired every 1-1,5 km from the mouth to the upper limit of the salt intrusion. The surveys were carried out between November 1998 and November 1999, using an inflatable boat. The results allowed obtain the relationship between the salt wedge extension as function of the river discharge and the estuarine trapping efficiency. The near-bed hydro-sedimenlological processes were assessed through the deployment of an instrument tripod on the estuarine bed al the channel thalweg 4 km upstream from the mouth. The current speed, water level and MPS data were recorded hourly, 1 m above the bottom, from September to November 1999. Based on this data set, two main modes of fine sediment transport in the Itajaí River estuary were identified: a tidal driven and a fluvial driven mode. The tidal driven mode occurs when the river discharge is bellow 200 m3.s-1. Near bottom sediments arc in all erosion-deposition cycle following the semi-diurnal tides and the anomalistic cycle of spring and neap tides, with net sediment transport landwards. During such periods fine sediment is imported from the inner shelf. This mode occurs at time scales of weeks to months. The fluvial driven mode are fully developed when the river discharge exceeds 1,000 m3.s-1. accompanied by suspended sediment load as high as 10,000 ton.day-1. Fluvial advective process fully dominates the sediment transport resulting in zero trapping efficiency.