Investigating sea‐state effects on flash flood hydrograph and inundation forecasting

Abstract

A common source of uncertainty in flood inundation forecasting is the hydrograph used.Given the role of sea-air-hydro-land chain processes on the water cycle, flood hydrographs in coastal areas can be indirectly affected by sea state. This study investigates sea-state effects on precipitation, discharge, and flood inundation forecasting implementing atmospheric, ocean wave, hydrological, and hydraulic-hydrodynamic coupled models. The Chemical Hydrological Atmospheric Ocean wave System (CHAOS) was used for coupled hydro-meteorological-wave simulations 'accounting' or 'not accounting' the impact of sea state on precipitation and, subsequently, on flood hydrograph. CHAOS includes the WRF-Hydro hydrological model and the WRF-ARW meteorological model two-way coupled with the WAM wave model through the OASIS3-MCT coupler. Subsequently, the 2D HEC-RAS hydraulic-hydrodynamic model was forced by the flood hydrographs and map the inundated areas. A flash flood event occurred on 15 November 2017 in Mandra, Attica, Greece, causing 24 fatalities, and damages was selected as case study. The calibration of models was performed exploiting historical flood records and previous studies. Human interventions such as hydraulic works and the urban areas were included in the hydraulic modelling geometry domain.The representation of the resistance caused by buildings was based on Unmanned Aerial System (UAS) data while the local elevation rise method was used in the urban-flood simulation. The flood extent results were assessed using the Critical Success Index (CSI), and CSI penalize. Integrating sea-state affected the forecast of precipitation and discharge peaks, causing up to +24% and from −8% to +36% differences, respectively, improving inundation forecast by 4.5% and flooding additional approximately 70 building blocks. The precipitation forcing time step was also highlighted as significant factor in such a small-scale flash flood. The integrated multidisciplinary methodological approach could be adopted in operational forecasting for civil protection applications facilitating the protection of socio-economic activities and human lives during similar future events.

Evaluating the Forecast Skill of a Hydrometeorological Modelling System in Greece

“…Respectively, WRF-Hydro is set up to daily produce 120-h streamflow and water level forecasts for two river basins in Greece, i.e., the Spercheios and Evrotas river basins. It is important to note that, the capability to provide inundation forecasting using the HEC-RAS model forced by WRF-Hydro results is a recent enhancement of the system based on knowledge obtained during previous flash flood studies in Volos city [24] and Mandra town [18]. Therefore, this study only presents the application of HEC-RAS in the flash flood event that occurred on 7 September 2016 at the Evrotas river overflowing a bridge at Skala town.…”

“…In more detail, the overland flow roughness scaling factor and the initial retention height scaling factor were set to 0.3 after extensive tests in the two rivers. Given the lack of continuous streamflow measurements in the two rivers, we exploited useful information derived from previous studies using the WRF-Hydro model in Greek catchments [17,18,22,24] as well as in catchments located at adjacent countries, such as Cyprus [42], Italy [43,44], and Turkey [45,46], to set the abovementioned calibration parameters so as to give reasonable results.…”

“…The free software HEC-RAS 6.0 version is capable of 2D unsteady flow simulation [47]. HEC-RAS 2D modelling and mapping had already been implemented successfully in several recent studies [18,24,[48][49][50][51][52][53][54]. Moreover, some studies tested the recent capabilities of the updated HEC-RAS [55] and the efficiency of the two-dimensional (2D) HEC-RAS model [56].…”

“…Greece also suffers from local flash floods causing severe damages, economic losses, and even fatalities [17]. Noteworthy flash flood events in Greece occurred at Mandra town on 15 November 2017 [17][18][19][20][21][22][23], at Volos city on 9 October 2006 [24], and at Rafina catchment on 11 December 2009 and 22 February 2013 [25]. The hydrometeorological forecasting and warning services in small-scale catchments demand a high resolution to capture the specific spatiotemporal characteristics of the flash flood events [26].…”

“…However, operational small-scale flash flood forecasting systems still require a considerable effort in research and development. Recently, flash flood modelling in smallscale basins of Greece has usually been based on high-resolution hydrological models forced by precipitation estimates using nowcasting approaches (e.g., [22]) regarding short-term (e.g., 1-3 h in advance) forecasts or by numerical weather prediction results (e.g., [17,18,24]) regarding forecasts exceeding even 1-2 days in advance.…”