Hydrological drivers on changes in Brazilian rivers conveyance capacity

Código

I-EBHE0131

Autores

Alondra Beatriz Alvarez Perez, Fernando Grison, KLEBER ISAAC SILVA DE SOUZA, Pedro Luiz Borges Chaffe

Tema

WG 1.08: Deep Explanation & Evaluation for Practices in Hydrological Changes (DEEPHY)

Resumo

The increase in flooding across the world has been associated with changes in stream-flow trends, with climate change and land use and occupation partly being identified as the main causes. However, we do not have a corresponding understanding of the change in channels cross-sections conveyance capacity, which is a fundamental feature in flow forecast. This gap is linked to the lack of systematic observations covering gradients of drainage areas and climate. The channels change cyclically between wet and dry seasons, leading to the hypothesis of stationarity in the shape of the sections. However, with flow being the main agent of change in the section, and with proven changes in the hydrological cycle and sediment cycle, it is important to question whether the assumption of stationarity in the shape of the channel is reasonable. In Brazil, the dramatic increase in flow regularization, sediment availability limited by large-scale damming and changes in land use are stress factors on water systems, and characterizing river sections is vital for risk management. floods. We questioned the stationarity in the channel's transport capacity, producing cross-section scour (aggradation) or fill (degradation), and we evaluated whether the trend of change in bed elevation (BE) is related to the change in flow. We estimated the trend in bed elevation and average daily flow over time for 508 stream gauging sites in Brazilian natural rivers, available on CAMELS-BR and the Brazilian National Water Agency. We calculated the trend of change using the Theil-Sen regression and its significance by Mann-Kendall test. For stations with time series trends in BE (p < 0.05), we derived annualized rates of aggradation/degradation. For those without significant trends, we calculated the interquartile range of BE IQR). Our results show that the transport capacity of the sections is changing; however, there are more stations with flow trends (89%) than in BE (69%). The flow presents regional patterns in the direction of change, indicating controls such as climate. In contrast, the sections change in a more spatially heterogeneous way, indicating local controls, dominating the silting trend (51%) in the northeast, southeast and extreme south regions, and degradation of the central-south region. At stations that have the same trend in flow and bed elevation, the correlation between them is strong, showing that the flow is capable of changing the shape of the section. In more arid regions, where average flow is decreasing, and sections are getting deeper, the change in magnitude increases with the degree of flow regularization and sediment availability. The BE variation range (IQR) in the sections has a trend over time and is inversely correlated with the degree of flow regularization. These unforeseen responses of bed elevation to climate, local characteristics and human interference may have consequences that are still poorly understood in flood management and have important implications for sediment transport.