Geotecnologia na determinação do assoreamento do açude Cachoeira dos Alves, Itaporanga-PB: um desenvolvimento metodológico.

Abstract

The accelerated growth in world population in conjunction with the problem related to environmental pollution has contributed to an unprecedented water shortage and unstableness of water resources. The need of space for living and for producing food led human beings to deforest large areas of native species and to use the land without the due technical orientation, creating large paved areas or even areas susceptible to erosion processes. As a consequence of those actions, it has been observed a decrease in infiltration and an increase in overland flow, which in turn removes and transports the soil nutrients and the sediments from the surface to the streams, rivers, and reservoirs. This sediment load contributes to the vanishing of some streams, the decrease in water quality, and to the reservoir sedimentation. Reservoir sedimentation is a natural and inevitable process. However, human impacts on the environment have accelerated this process. It is well known that sedimentation has caused troubles and irreparable damages to the water resources such as the increase in water level of rivers, and lost of the useful capacity of the reservoirs. Therefore, taking the river basin that contributes to the reservoir in account, methodologies to monitor, quantify precisely the level of sedimentation, to control, or even to prevent the velocity, in which reservoir sedimentation occurs, need to be developed. In this sense, the present work has the objective to fulfill some of these requirements through the analysis of reservoir sedimentation processes and its influential factors, considering as a study case a problem in curse at a reservoir in the West side of Paraíba State, Northeastern Brazil. The work consisted of field survey and the use of geoprossesing techniques to analyze the level of sedimentation of a reservoir named Cachoeira dos Alves, located at the municipality of Itaporanga City (Paraiba State). According to the original project, the storage capacity of the reservoir in 1984 was about 10,611,196 m3, the hydraulic basin of such dam was 2,085,136 m2, and the river basin area and the related perimeter was were 111.3 km2 and 72.28 km, respectively. Within this watershed a diversity of land use is developed with the predominance of cattle-rising. The reservoir is the main water supply source of Itaporanga City and part of its volume is also used for pisciculture and to supply some riparian communities. In all, the demand of water by those consumers is very high and a management of this resource should be well done to prevent water shortage. To quantify the amount of sedimentation from the dam construction up today, an automatic bathymetry associated to a geographical positioning system (GPS) was used to determine the underwater depth of the reservoir floor. Moreover, remote sensing techniques were used to identify area within the basin where spatial and temporal degradation of the soil surface and removal of vegetal cover is taking place. The comparison of the data from those techniques, and from the original project showed that, due to the sediment intake from the overland, from 1984 to 2006 the reservoir lost the capacity to store 1,775,833 m3 which correspond to 16% of its total capacity. This indicated that an advanced degradation of surface cover and a deforestation of the native vegetal cover are occuring around the reservoir. In addition the results showed that the use of geoprocessing to analyze those aspects and to support the management of water resources is a very promising tool due to the celerity in obtain data, reducing expenses and improving the accuracy in data collection.