Medida de processos hidrossedimentológicos no Nordeste brasileiro: aplicação ao Rio Jaguaribe (CE) e ao Núcleo de Desertificação de Gilbués (PI)

Abstract

The sedimentological processes are relevant for the Northeast of Brazil (NEB), whose main water sources are tens of thousands of dams, vulnerable to silting. Despite their relevance to environmental and water management in the region, these processes are rarely monitored. In addition, it is necessary to update measurement methods in order to improve data quality and spatially expand the monitored area. The objectives of this research was: (i) to propose and execute a technique for evaluating the suspended sediments concentration (SSC) in an intermittent semiarid river through remote sensing; (ii) propose and implement a technique for evaluating trap efficiency in a small reservoir based on data obtained on the hourly time scale; and (iii) to evaluate hydrosedimentological parameters of the Desertification Nucleus of Gilbués, Piauí, seeking correlation with the physical processes on the slopes and check dams. To achieve the first objective, the Upper Jaguaribe River, Ceará, was studied. Fifteen SSC indices based on the spectral bands of RapidEye satellite images were evaluated, as well as nine indices for other optical sensors. The measured SSC ranged from 67 to 230 mg.L−1. It was concluded that the images of the RapidEye satellite constellation identify moderate SSC, even for low flows, of the order of 2 m³.s−1. For objective (ii), an experimental area (15 ha) was selected in Gilbués, in which there are dozens of check dams. In an experimental check dam, rainfall, flow and sediment concentration were automatically and continuously measured, and silting was measured using a UAV. The trap efficiency (TE, measured from 2018 to 2019) was 86%. Three models failed to adequately simulate the experimental results: Churchill (TE = 99%), Maryland (94%) and Brune (75%); but the Camp (89%) and RETSED (81%) models were successful. The Camp model showed low sensitivity to hydrodynamics, while the RETSED model adequately represented not only the mean value, but also the hydrosedimentological dynamics on the hourly scale. Automatic techniques were successful in capturing accurate data, without which measurements would not have been possible. Regarding objective (iii), it is concluded that, in Gilbués, some parameters are similar to those of other areas in the NEB, such as the stable infiltration rate (20 mm.h-1) and bulk density (1.29 g.cm-3 of the soil and 1.21 g.cm-3 of the sediment). However, there is a notorious predominance of fines (silt and clay), both in the soil (95%) and in the sediment (97%). In addition, the delta of the check dams is composed of fine particles that, aggregated, are present in the gravel fraction; while in the bottom deposit, just a few meters away, the deposition pattern is noticeably different from the delta pattern, although there is no statistical difference in granulometry in the two cases.