Regional flood frequency analysis as a medium for flood mapping in a ungauged urban watershed

Abstract

Disorganized urbanization generates several negative impacts on the urban environment, such as the occurrence of intensified floods. Thus, an adequate urban planning as well as the understanding of the hydrological regime of a region are essential to (1) minimize human and economic losses caused by floods, and (2) guarantee an effective management of water resources. The main objective of this work is to evaluate simulated inundation boundaries for different design floods in Vermelho Creek Watershed in Lavras, Minas Gerais (MG), Brazil. In view of the lack of streamflow monitoring in the watershed, a study of regionalization of maximum streamflow was performed. A regional function of maximum streamflow was generated by the dimensionless curve method using data from six gauging stations in Lavras region, all located in the Grande River Basin. The gauging stations for the study were selected considering the analyses of stationarity and trends by Mann-Kendall test, and the homogeneity of the region by the heterogeneity (H) and discordancy (Di) measures. The applicability of the regional function to estimate design floods in ungauged watersheds was verified through (1) performance analysis, using the Camargo and Sentelhas coefficient and (2) uncertainty analysis through bootstrapping and Monte Carlo simulations. The maximum streamflow regional function developed in this work can be used to estimate design floods in ungauged basins in the region, e.g., Vermelho Creek. This method is a simple alternative for maximum streamflow estimates and can be used in flood risk management. To calculate the design floods in Vermelho Creek Watershed, two regional functions developed for the Grande River Basin were compared, also considering data-transfer technique to adjust the estimated values. Two monitored basins were selected for analyses: basin X was used in both regionalization studies, and basin Y had drainage area similar to the urban watershed of Lavras. The performances were evaluated using the percentage error and the Nash-Sutcliffe coefficient between the values estimated by the regional functions and calculated by the most adequate probability density function for each studied station. The regional function developed in this work proved to be more adequate for the estimation of design floods in the Vermelho Creek Watershed. Then, flood maps were generated for return periods of 5, 10, 50 and 100 years using the HEC-RAS software. The areas affected by the design floods were compared with the current land use and the Soil Use Directive Plan of the city of Lavras. Considering the current scenario, the results showed that the areas affected in by the simulated inundation boundaries would decrease by 75% if the municipality's use and occupation guidelines were followed correctly. Therefore, this work reinforces the importance of adequate urban planning and compliance with the proposed guidelines, in order to guarantee the safety of the population and the environment.