Can an early-warning system help minimize the impacts of coastal storms? A case study of the 2012 Halloween storm, northern Italy

• Published in: Natural hazards and earth system sciences 2016-01, Vol.16 (1), p.209-222
• Main Authors: Harley, M. D, Valentini, A, Armaroli, C, Perini, L, Calabrese, L, Ciavola, P
• Format: Article
• Language: English
• Subjects: Accuracy; Analysis; Beaches; Boundary conditions; Case studies; Coastal storms; Coastal zone management; Coasts; Early warning systems; Emergency communications systems; Emergency procedures; Erosion; Flooding; Floods; Halloween; Mathematical models; Morphology; Numerical models; Offshore; Parameters; Storm forecasting; Storms; Tidal waves; Warning systems; Water levels; Wave power; Weather forecasting; Weather hazards
• ISSN: 1684-9981; 1561-8633; 1684-9981
• DOI: 10.5194/nhess-16-209-2016

The Emilia-Romagna early-warning system (ER-EWS) is a state-of-the-art coastal forecasting system that comprises a series of numerical models (COSMO, ROMS, SWAN and XBeach) to obtain a daily 3-day forecast of coastal storm hazard at eight key sites along the Emilia-Romagna coastline (northern Italy). On the night of 31 October 2012, a major storm event occurred that resulted in elevated water levels (equivalent to a 1-in-20- to 1-in-50-year event) and widespread erosion and flooding. Since this storm happened just 1 month prior to the roll-out of the ER-EWS, the forecast performance related to this event is unknown. The aim of this study was to therefore reanalyse the ER-EWS as if it had been operating a day before the event and determine to what extent the forecasts may have helped reduce storm impacts. Three different reanalysis modes were undertaken: (1) a default forecast (DF) mode based on 3-day wave and water-level forecasts and default XBeach parameters; (2) a measured offshore (MO) forecast mode using wave and water-level measurements and default XBeach parameters; and (3) a calibrated XBeach (CX) mode using measured boundary conditions and an optimized parameter set obtained through an extensive calibration process. The results indicate that, while a "code-red" alert would have been issued for the DF mode, an underprediction of the extreme water levels of this event limited high-hazard forecasts to only two of the eight ER-EWS sites. Forecasts based on measured offshore conditions (the MO mode) more-accurately indicate high-hazard conditions for all eight sites. Further considerable improvements are observed using an optimized XBeach parameter set (the CX mode) compared to default parameters. A series of what-if scenarios at one of the sites show that artificial dunes, which are a common management strategy along this coastline, could have hypothetically been constructed as an emergency procedure to potentially reduce storm impacts.