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Igor P. Holanda II ; Renisson N. Casado IV ; Arilmara A. Bandeira V. Fone: 81 E-mail: rochaigor hotmail. E-mail: fholanda infonet. E-mail: nepoaraujo hotmail. E-mail: arilmara ig.
The rainfall that occurs in the region is considered non-erosive, The evolution of erosion is explained by the increase in the outflow in February that peaked at 9, The rainfall regime interacts with the wind that blows predominantly from the eastern direction during the dry season and from the southeastern direction during the rainy season with maximum velocities of 5. Key words: river discharge, wind, rainfall, water resources. According to Casado et al.
Holanda et al. Coops et al. Similarly Holanda et al. Nanson et al. Christofoletti states that the main factors that contribute to the erosion, transport, and sedimentation processes in rivers are the current velocity; the physical characteristics of the sediments, specifically the size, density, and shape; the existence of accidents or obstacles in the bed; and the variations in river discharges, which, in turn, may also be related to climatic variations.
The current velocity is considered a key factor for the erosion development along streams, because, in general, its increase is proportional to the volume of the eroded soil Hooke, , especially when the bank comprises non-cohesive soils Rocha et al.
The river channel morphology is another important variable that reflects the adjustment of the river to the material excavation since it is a result of the action exerted by the flow on the material components of the bed and banks Christofoletti, The criteria established by Rosgen to classify rivers in terms of their morphological and hydraulic characteristics are channel form, velocity, and stream discharge; this clearly demonstrates the importance of these variables in the geomorphological processes of rivers.
The discharge of the main stream, especially in the lower courses, reflects the sum of the discharges of the upstream tributaries and is primarily influenced by the rainfall that occurs at the headwaters of the basin Tucci, ; further, it has a close relationship with the current velocity.
According to Takken et al. To investigate this specific river stretch, meteorological data were collected from the closest meteorological stations having the appropriate data related to the river stretch. The annual mean rainfall is Soils with this characteristic typically have extremely low values of cohesion and shear strength. The original vegetation of the area consists of Atlantic rain forest bioma and reef patches along the coastal region, but these ecosystems are seriously threatened by deforestation being carried out for the purpose of cattle ranching or for irrigated agriculture.
The river bank of the studied sections has as its main features a sharp retreat of the bank line, granulometric composition with a predominance of medium and fine sand fractions, high friability, and proximity to the thalweg of the river Figure 2.
Variables of erosion process. In each section, pins 1. The specification of the installed pins is in accordance with Fernandez , who adapted the pin method for studying erosive processes along tropical rivers. The pins were painted with red lead paint to protect against rust and then with a light color to disguise them on the riverbank in order to avoid anthropic interferences or pin removals.
The pins were identified and marked every 0. It was performed 34 data collections during the year varying from 7 to 15 days between them. The calculated average erosion rate was the result of the riverbank retreat sum divided by the sum of installed pins. In this way, the annual average erosion rate was the arithmetic mean of those measurements.
The data obtained from daily means were used in order to get the monthly means of each year and subsequently the monthly means for the analyzed period. The sectional area is not constant because of changes in the river channel, and hence, it became necessary to employ the historical records of cross-sectional profiles of the same station for the same period.
Meteorological variables. Among these variables, the annual mean rainfall and the velocity and direction of the prevailing winds are important. The annual mean rainfall recorded data ranged between However, if the influences on the hydraulic flow table are neglected, the rainfall that occurs in the lower course hardly contributes to the erosion process of its banks.
According to Pereira et al. Such increase promotes hydrological and geomorphological changes in the entire hydrosystem toward the river mouth, and in particular, accelerates the bank erosion along the lower stretch.
In December , a remarkable increase in the erosion rate was noticed Figure 4A ; this was caused by a river discharge peak of 2. This erosion event can be attributed to the water discharge from the hydroelectric dams since December is characterized by the onset of the rainy season in the upper and middle river courses; during this period, the reservoirs have elevated water storage capacities.
Increased water levels in the river channel appear to threat the deforested banks with very low cohesion soils as the erosion intensifies.
Further, notable erosion rates also occurred from May to July , characterized by a rainy season in Sergipe state but a dry season at the headwaters of the river. Since the water quota and water discharge are proportional, bank erosion increases due to the frequency of the water depth becoming less than the usual and promoting the undercutting of the bank at soil layers with considerably less resistance; this is in agreement with Holanda et al.
Moreover, this event caused changes in the morphology of unprotected banks, leading to the formation of new terraces; this demonstrates the changes in the river discharges during the monitored period. However, the erosion rates were higher only in the months of March and April, probably due to the occupation of the floodplain by the floods that occurred in February; Parker et al. In addition, as the water level decreases, the friction provided by the water flow in contact with the unprotected bank allows higher removal of soil particles.
The erosion and depositional sites are active along the left and right banks of the river and controlled by the floods that occur cyclically; as a result, the proximity of the thalweg to the banks during the hydrodynamic cycle is also altered. Hence, in an attempt to achieve a new morphological balance, another erosion cycle begins with the verticalization of the riverbanks leading to significant erosion rates in the period of july to september, but at levels well below the previous period; nevertheless, river discharges in a regulated pattern are driven by the operation of the dams for power generation.
River water velocity depends on the discharge behavior and had a minimum and a maximum of and 1. The velocity curve for steep slopes shows that the water flow has a large kinetic energy that can be understood to represent the sheer power of the river flow. When transferred to the banks, this energy promotes the breaking of the bonding forces between the aggregation of the soil particles. Rosgen described a similar situation and identified the relationship between the flow velocity and its power to break the soil aggregation.
Wind velocity and direction. Wind velocity and direction are particularly influenced by the rainfall in the region. Slight changes in the winds behavior as a function of the period of the day are also observed. The daytime winds have a higher mean velocity than the nighttime winds due to fast heating of the land surface during the daytime, which causes fast changes in the pressure gradient due to changes in the air density.
However, the daytime wind velocity was higher than the nighttime one. Nevertheless, it is known that the velocity and wind direction associated with wind fetch and the friction with the water surface are the major factors influencing the formation of natural waves in any stream. Specifically in this river stretch, a specific form of joint action between waves and river current, occurs, and this interaction results in a bank morphology characterized by a concave curve wherein the highest retreat rates occur.
A similar situation was described by Williams , when in specific season the winds provide energy from the waves that hit the unprotected riverbanks. Our observations suggest that the natural waves are responsible for eliminating large volumes of the eroded soil, resulting in changes in the river geomorphological pattern, promotion of the deposition of sediments in its bed and formation of increasing zones as small islands and sandbars.
The erosive effects from the more intense flows of the thalweg are greater when it is closer to the bank; this is the zone of greatest turbulence having higher flow velocities and accumulation of greater energies that promote landslide.
An important consequence of this event is that it becomes possible to list the fast and progressive losses of soil from the riverbanks in Sergipe, the continuous formation of sandbars, and the increased water turbidity near the shore. From the geomorphological viewpoint, the effects of the investigated variables can be viewed as an adjustment to the channel morphology, which demonstrate the dynamics of the stream.
Further, this river stretch has a wide width and low depth, i. These results agree with those of Jiongxin who correlated the riverbed composition with the amount of energy employed by the channel flow. On the other hand, the rainfall upstream Itaparica dam seems to strongly influences this process.
There is an interaction between the occurrences of winds and rainfall in the study area prevailing winds of eastern origin blow during the dry period while winds of southeastern origin blow during the rainy season, suggesting that these waves have strong influence at riverbank erosion process. The wave fronts originate from the eastern direction are redirected toward the south or southwest when they hit against the right margin banks, where higher erosion rates have occurred.
Furthermore, the erosion process is more intensive in places where the thalweg is near the riverbank. Bhattacharyya, K. London: Springer, Brandt, S. Classification of geomorphological effects downstream of dams. Catena, v. Brighetti, G. Casado, A. B; Holanda, F. Brazilian Journal of Soil Science, v.
Christofoletti, A. Geomorfologia fluvial. Modelagem de sistemas ambientais. Coelho, A. Geomorfologia fluvial de rios impactados por barragens. Caminhos de Geografia, v. Coops, H. Interactions between waves, bank erosion and emergent vegetation: An experimental wave tank. Aquatic Botany, v. Ellis J. Assessing the impact of an organic restoration structure on boat wake energy.
Journal of Coastal Research, v. Fernandez, O.
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Christofoletti, Antônio 1936-