Geomorfologia da planície aluvial do alto rio Branco em Roraima : dinâmica e processos evolutivos

Abstract

This thesis aims to understand the geomorphological of the alluvial plain of the upper Branco River system, with its current dynamic, operative processes and their evolution as a system. The study included a survey of the basin characteristics and more detailed the upper Branco River channel along a length of approximately 70km, positioned between the confluence of the Uraricoera and Tacutu rivers (limit upstream) to the Serra Grande (limit downstream). The methodology included: geomorphological mapping of the alluvial plain; the lifting of the discharge and sediment load of the upper Branco River and its tributaries; measuring the velocity of flow and the discharge the study area; hydrological characterization of the basin; the morphological survey of channel; the collection and textural analysis of bed load; quantification of suspended sediment load; determining discharge of the solid in suspension; and the quantification of erosion on the canal. The surveys showed over the last decades, the prevalence of depositional processes in the upper Branco river system; between the period 1975 and 2013 the islands together showed an increase of 18% in your area, from 17,60km2 to 20,79km2; for the same period the number of lakes decreased by 11.71%; monitoring the banks of the channel indicated the predominance of erosion along the right bank and aggradation on the left bank of the Branco River, with great emphasis to the passage upstream of the mouth of the Cauamé River with superior erosion 1m/yr; the description of the deposits along the channels and islands showed three main types of profiles, the first type marked by succession of very complex facies, the second characterized by profiles with granodecreasing succession facies and the third group of Pliocene / Pleistocene origin of the Boa Vista Formation in the region of the Boa Vista city, not associated with the formation of the alluvial plain of the Branco River; on the hydrological behavior and the largest amplitude values of discharge and lower production flow by area were observed in the regions of higher concentration of rainfall; the variation of daily data showed amplitudes ranging between 17.29 and 74.69 times in Uraricoera River basin, with higher values towards the downstream; in the basin of Tacutu River the values were larger, with amplitudes between 57.33 and 5062.18 times, with the highest value obtained in Bonfim, in the middle Tacutu; monitoring of daily data of discharge over one year indicated that the Uraricoera River is the largest contributor to the Branco River, accounting for approximately 70% of the annual discharge of the system; the Tacutu River submitted a contribution was around 30%; as sediment production in suspension, the Tacutu River accounted for 48% of the production of the upper Branco River basin sediments, with average production of 41,53mg /L; the Uraricoera river contributed with 43% of the sediment discharge averaging 29,54mg /L; the Branco River (Boa Vista) had suspended load volumes ranging from 7 to 79,4mg/L, with an average 34,52mg/L, with the occurrence of the largest load values suspended at the beginning of the floods.