Avaliação dinâmica do deságue de tubulações transversais de microdrenagem urbana nos canais artificiais de macrodrenagem: Estudo de caso — Canal do Rio Granjeiro, Crato

Abstract

Motivated by the events of a catastrophic flood that happened on January 28, 2011 at the city of Crato–CE, this work focuses on the collapsed section shortly downstream of the beginning of the Granjeiro River channel’s artificial concrete portion — which crosses the city centre and had the purpose of damping floods like that one. At that point of collapse, a transverse drainage gallery spews its flow, and that gallery’s output section ended up completely destroyed. Suspecting the action of excessive stream velocities and the very probable formation of turbulence at the joint, topographical and hydrological studies were made so as to characterize both structures, in order to be able to develop a hydraulic study that would manage to numerically analyse the problem at question. The topographical studies defined, through field surveying and the application of georeferencing software, not only the drainage gallery’s layout but also the drainage areas corresponding to the several focal points where flow rate was the main concern. The following hydrological studies were able to determine the runoff flow rates based on the area values previously delimited, as well as characterise the local hydrology during the flood event by analysing historical series of precipitation and by creating hydrographs based on the Natural Resources Conservation Service (NRCS, previously SCS) method. Finally, the hydraulic studies modelled the trapezoidal section of the artificial channel and the rectangular section of the gallery, determining the water blade height and flow velocity in both. Using those velocity values, a vectorial analysis of forces was elaborated with the intention of calculating the resultant force acting on the channel’s wall downstream of the gallery–channel junction. In addition to the vectorial analysis, the turbulence in the channel was also measured (in terms of Reynolds’ number) and the supercritical flow condition was confirmed (in terms of Froude’s coefficient), corroborating the initial assumptions and giving way to more complex studies to follow this one in the future.