Abstract
Aim of this work is the development of an operational tool for pluvial flooding warning in an urban area based on off-line rainfall thresholds derived by coupling a rainfall–runoff modelling and a hydraulic routing. The critical conditions considered for issue flood warnings were not only based on the water stage, but also on the extension of the flooded area. Further, a risk assessment framework for quantifying the reliability of the rainfall thresholds has been included; rainfall thresholds used in pluvial flooding warning should be influenced by the uncertainties in the rainfall characteristics (i.e. rainfall duration, depth and storm pattern). This risk assessment framework incorporates the correlated multivariate Monte Carlo simulation method, an hydraulic model for the simulation of rainfall excess propagation over surface urban drainage structures, i.e. streets and pathways. Thresholds rainfall are defined using a number of inundation criteria, to analyze the change in the rainfall threshold due to various definitions of inundation. Starting from estimated water stages and flooded area from inundation simulation rainfall thresholds can be obtained according a specific inundation criterion, including, together, a critical water depth and a critical flooding area. Finally, the second phase concerns the imminence of a possible hydrological risk by comparing the time when cumulative rainfall and rainfall thresholds meet to each other. The developed procedure has been applied to the real case study of Mondello catchment in Palermo (Italy).
Highlights
Floods are natural phenomena which cannot be prevented; many studies show that the severity and frequency of floods have increased in recent years and underline the difficulty to separate the effects of natural climatic changes and human influences as land management practices, urbanisation, etc. [1,2,3,4,5,6]
The rapid transformation processes of urban areas induced, the increase of catchment imperviousness and the derived increase of surface runoff generated during rainfall events
The natural drainage network is, often, insufficient to convey such discharges and it is gradually substituted by artificial systems having the function to convey the runoff coming from urban areas towards the closest receiving water body
Summary
Floods are natural phenomena which cannot be prevented; many studies show that the severity and frequency of floods have increased in recent years and underline the difficulty to separate the effects of natural climatic changes and human influences as land management practices, urbanisation, etc. [1,2,3,4,5,6]. Flood warning systems are based on on-line hydrological and/or hydraulic models in order to provide forecasts of water stages or discharges at critical river sections [13,14,15]. This procedure is inappropriate for flash flood warning in urban areas or in catchments with a. In Europe, the Integrated Project FLOODSite [25], among others aims at assessing the advantage for using the rainfall threshold approach as an alternative to the traditional ones in the case of flash floods. Main uncertainty sources have been identified: observation uncertainty (rainfall, river discharge, current velocities«), flood frequency analysis, choice of hydraulic model (0-, 1-, 2-dimensional model), model parameterisation (i.e. hydraulic roughness), high resolution and high accuracy topographic data sets, representation of linear elements (i.e. road embankments)
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