Modelagem de cotas de cheias a partir de vazões derivadas de hidrogramas unitários instantâneos geomorfológicos.

Abstract

Given the lack of hydrological and meteorological monitoring in small and mediumsized watersheds and due to the current temporal discretization of the stream flow historical series freely available to the public, studies and development of techniques for estimating direct surface runoff (DSR) hydrographs have been increseangly conducted. Such studies have great relevance for the analysis of stream flows resulting from intense rainfall events. Among these techniques, the Instantaneous Unit Hydrograph (IUH) and the Geomorphological IUH (IUHGEO) theories deserve attention. Clark (CIUH) is a widely used IUH model and several geomorphological approaches have been developed for it by establishing relationships between CIUH’s parameters and watersheds’ physical characteristics. The modified version of the CIUH (ModClark) has been widely applied in order to verify its performance in relation to the discretization of the watersheds in modules. With regard to flood management in watersheds, the combination of hydrological and hydraulic models is of great importance, as it enables simulation of water levels and water surface profiles. In this context, the main objective of this study was to evaluate a tool for estimating water levels originated from extreme rainfall events, combining hydrological modeling – by means of the ModClark based on geomorphological parameters – and hydraulic modeling. The study area comprises the experimental Cadeia river watershed (CRW), an important sub-watershed of the Pelotas river watershed (PRW), which has active hydrological monitoring and is situated in the south of Rio Grande do Sul State. Different rainfall-runoff events were initially selected and the corresponding hyetograhs and hydrographs were then analyzed. The runoff separation and the DSR depth were determined for each event by the Inflection Method and the Numerical Integration, respectively, whereas, effective rainfall hyetographs were derived from the Curve Number (CN) method. The aforementioned dataset was used as input for hydrological modeling based on the ModClark applying the HEC-HMS software. With respect to the calibration of parameters, the Nelder Mead algorithm was taken into account. The peak stream flow values derived from hydrological modeling were used as input data in the HECRAS software for hydraulic modeling in order to compute water levels and water surface profiles in the streches under analysis of the Cadeia river. In addition, it was also necessary to obtain topographic and bathymetric data of cross sections that were surveyed along the main watercourse of the CRW, as well as their geometric characteristics and Manning’s roughness coefficient. Based on the results obtained, the main conclusions of this study were: a) the geomorphological approaches are not universal and need to be used with caution; b) the physiographic characteristics of the watershed under study had great influence on the geomorphological approaches; c) the geomorphological approaches exerted influence on the performance of the DSR hydrograph estimations; d) the choice of the methodology used in the hydrological modeling had a great impact on the results obtained in the hydraulic modeling; e) the input data used for hydraulic modeling had great impact on estimation of water level associated with floods.