Using an X-band radar for an urban area early flood warning system

Floods represent a serious hazard in many areas of the world in terms of damage to properties and loss of life. For this reason, the European Floods Directive 2007/60/EC on the assessment and management of flood risk required the relevant bodies of the Member States to have adequate and efficient tools for assessing the flood risk of river basins and establishing flood risk management plans focused on prevention and protection of territory and people. The decision makers involved in flood defense often need some hydro-informatics tools to transform the flood forecast into relatively simple and clear messages that allow them to take prompt action to prevent and/or counteract the flood. From this perspective, the definition of flood-prone areas and critical rainfall intensity-duration thresholds can be a very useful tool to identify potentially critical areas, setting up early-warning systems or designing appropriate protection measures. Over the last decade, many studies have addressed the possibility of providing flood warnings at given river outlet based on the direct comparison of the quantitative precipitation forecast with critical rainfall threshold values. A rainfall threshold defines the amount of rainfall accumulated during a given time period over a basin that is enough to cause flooding at the outlet of the draining stream. Sometimes, traditional systems for the observation and measuring of rainfall, as a rain gauge network, may not be enough to make a good estimation of such thresholds, especially if the network is not dense enough. In this case, the availability of radar instruments can provide an important tool for the study, the characterization, and the operational usage of detailed precipitation information such as duration, intensity, and spatial distribution over a basin. Moreover, radars are able to provide short-term quantitative precipitation forecasts (nowcasts) that can potentially afford great benefits to flood warning and short-term forecasting in urban settings. The overall objective of this work is to derive rainfall thresholds for the hydraulic risk of the Oreto river basin, which is located in Sicily (southern Italy), using the hydrological model HEC-HMS and the hydraulic model HEC-RAS. Specifically, once defined the more flood-prone areas (critical points) of the Oreto basin by means of the HEC-RAS model, the HEC-HMS model has been used to estimate, for given duration and hydrological initial conditions, the amount of rainfall that causes the critical flow at the critical points. Estimations coming from an X-band weather radar will be used to evaluate the possible exceeding of the rainfall thresholds at critical points in order to assess the flood risk at those points. The X-band weather radar was installed and is managed by the hydrology laboratory of the Department of Engineering (DI) of the University of Palermo (Sicily, Italy). The radar is within the urban area of Palermo along with a rain gauge network of 18 tipping bucket gauges, a weight rain gauge, an optical disdrometer, and a weather station (Figure 1). The information provided by these devices can be combined in order to integrate different data and correct errors that traditionally affect the radar estimates. Observations from the radar and other sensors are embedded in an integrated early warning system able to provide warnings and decision support related to floods in the urban area of Palermo.