Incertezas no cálculo da declividade para a simulação chuva-vazão com abordagem distribuída NRCS-CN-HU

Abstract

In the last decades, the imperviousness has been observed in Brazil due to an accelerated and unplanned urbanization process. The overload of rainwater drainage systems is the most serious consequence of this and results in large environmental impacts and damages to society, especially in large cities. Currently, the hygienist practice of draining the rainwater as fast as possible prevails, which only transfers the problem downstream. Thus, to assist the management of urban drainage, sustainable practices began to gain emphasis, especially with mathematical models. Such models require, among other basic parameters, the concentration time of the catchment (Tc), which is the travel time of a drop of water from the farthest point of the catchment to its outfall. To determine Tc, several equations and methods can be used, most of them dependent on data concerning the physiographic characteristics of the catchment, such as its slope. In this sense, this work aimed to evaluate the influence of the slope calculation procedure on the rainfall-runoff simulation with the NRCS-CN-UH (Natural Resources Conservation Service - Curve Number - Unit Hydrograph) distributed approach. The catchment flow travel time was estimated by NRCS method and slopes were calculated using three different methods (individual slopes - DI, single slopes - DU and weighted averages - MP), which were evaluated and discussed. In addition, GIS (Geographic Information Systems) and geoprocessing tools were used, along with a distributed approach, which analyzes the value of the variables pixel by pixel. As a case study, the catchment of the Cuiá River, located in João Pessoa (PB), was chosen because it had already been target of previous studies. The results obtained were satisfactory and showed that the different calculation methods were coherent for the rainfall-runoff simulation and had little influence on the determination of the catchment resultant hydrograph, with a maximum peak flow variation of only 9.7%. The choice of the best methodology, therefore, will depend on the available data and the users’ experience and preferences. However, due to the more accurate representation of the reality, it is recommended to use the individual slopes method (DI).