Coping with Extreme Events: Effect of Different Reservoir Operation Strategies on Flood Inundation Maps

Elena Ridolfi,Chiara Biscarini,Piergiorgio Manciola,Silvia Di Francesco,Nicola Berni,Claudia Pandolfo

doi:10.3390/w11050982

Abstract

The need of addressing “residual flood risk” associated with structural protection measures, such as levee systems and flood-control reservoirs, has fostered actions aimed at increasing flood risk awareness. Structural measures have lowered risk perception by inducing a false sense of safety. As a result, these structures contribute to an underestimation of the “residual risk”. We analyze the effect of different reservoir operations, such as coping with drought versus coping with flood events, on flood inundation patterns. First, a hydrological model simulates different scenarios, which represent the dam regulation strategies. Each regulation strategy is the combination of an opening of the outlet gate and of the initial water level in the reservoir. Second, the corresponding outputs of the dam in terms of maximum discharge values are estimated. Then, in turn, each output of the dam is used as an upstream boundary condition of a hydraulic model used to simulate the flood propagation and the inundation processes in the river reach. The hydraulic model is thus used to determine the effect, in terms of inundated areas, of each dam regulation scenario. Finally, the ensemble of all flood inundation maps is built to define the areas more prone to be flooded. The test site is the Casanuova dam (Umbria, central Italy) which aims at: (i) mitigating floods occurring at the Chiascio River, one of the main tributaries of Tiber River, while (ii) providing water supply for irrigation. Because of these two competitive interests, the understanding of different scenarios generated by the dam operations offers an unique support to flood mitigation strategies. Results can lead to draw interesting remarks for a wide number of case studies.

Highlights

To alleviate droughts and floods, human societies have built structural devices such as levee systems and reservoirs, which can significantly affect the hydrological regime
Risk perception tends to decrease in areas protected by structural measures, as the reduced frequency of hydrological extremes is often associated with the fading awareness and preparedness of locals
Population living in flood prone areas may be not aware of the hydrological risk they face, as they feel protected by hydraulic structures such as dams and levee systems

Summary

Introduction

To alleviate droughts and floods, human societies have built structural devices such as levee systems and reservoirs, which can significantly affect the hydrological regime. Risk perception tends to decrease in areas protected by structural measures, as the reduced frequency of hydrological extremes is often associated with the fading awareness and preparedness of locals. In the literature, this phenomenon is known as the “safe-development paradox”, which was introduced for the very first time by White [2] with reference to floods (known as “levee effect”), and more recently shown with reference to drought, and termed “reservoir effect” by Di Baldassarre et al [3]. Population living in flood prone areas may be not aware of the hydrological risk they face, as they feel protected by hydraulic structures such as dams and levee systems

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