How can an integrated model help understand a mysterious flooding event?

Following significant floods along one of the major rivers in the South West of France and its tributaries, a group of individuals organized themselves to understand what occurred on their territory during the rainy episode of May 10th and 11th, 2020, which was not exceptional. It is important to note that the two main left bank tributaries have a number of dams.

The objective of the study was to answer the following questions: (i) analyze the consistency of the observed hydraulic phenomena during the rainy episode of May 10th and 11th, 2020, and, if necessary, explain any peculiarities; (ii) evaluate the impact of utilizing the storage capacity of the dams for flood control; and (iii) evaluate the impact of maintaining the river beds.

A statistical analysis of available data for the May 10th and 11th, 2020 event highlights a peculiar feature. Specifically, the return period of the peak flow measured at two different locations was about 50 years, while the rainfall had a return period below 20 years. To explain this peculiar feature, an integrated hydrological and hydraulic model (PCSWMM) was developed and implemented to propose explanations.

Several simulations were run examining various scenarios. The most favorable outcomes in terms of peak flow and hydrograph shapes were achieved by incorporating the opening of a dam gate, which increased the downstream water volume by over 1.2 million cubic meters. The model accurately identified the pipe's diameter and the timing of its opening. This hypothesis is currently under discussion with the operator for confirmation or further clarification.

The calibrated model was then used to perform a sensitivity analysis, by considering (i) the suppression of the additional water supply highlighted previously; (ii) the fact that all the dams' reservoirs were empty before the rainy event; and (iii) a more regular and efficient maintenance of the river beds. The results clearly show the significant impact of the additional water compared to the other actions.

This study, including the role of the developed model, was essential in clarifying what happened in May 2020 and in providing direction on how to manage water resources to avoid future floods. Discussions are ongoing as there are many different uses, with high stakes, of the water on the watershed

 Click here to watch recorded presentation on YouTube.