Using post-flood surveys and geomorphologic mapping to evaluate hydrological and hydraulic models: The flash flood of the Girona River (Spain) in 2007

Abstract

This paper analyses the River Girona (Spain) flash flood, occurred the 12th of October 2007, combining hydrological and hydraulic modeling with geomorphologic mapping and post-flood survey information. This research aims to reproduce the flood event in order to understand and decipher the flood processes and dynamics on a system of *Manuscript Click here to download Manuscript: JoH_Girona_Flashflood.docx Click here to view linked References overlapped prograding alluvial fans. The hydrological model TETIS was used to characterize the shape and dimension of the October 2007 River Girona hydrograph. Subsequently, the flood event was reproduced using the free surface flow module of the model RiverFlow2D. The combination of hydrological and hydraulic models was validated using post-flood surveys defining maximum flooded area and flood depths. Then, simulations with different peak discharges were carried out to estimate the hydrogeomorphologic response of the Girona River floodplain, through the identification of the activation thresholds in different geomorphic elements. Results showed that the unit peak discharge of the October 2007 flood event was among the largest ever recorded in the area, according to the existing literature. Likewise, the hydraulic model showed a good performance (Fit A = 76 %, RMSE = 0.65 m and NSE = 0.6), despite the complexity of the case, an ephemeral and ungauged river. The model simulation revealed the existence of a geo-chronological activation pattern of palaeochannels and alluvial fans, which was altered by the presence of a tectonic depression and bridges construction.This multidisciplinary approach proved to be a useful strategy for understanding flash flood processes in ungauged catchments. It allowed understanding the mechanisms governing floods in alluvial fans systems and it represented a solid contribution for early warning plans and risk mitigation policies.

Evaluation of Hydrological and Hydraulic Models Applied in Typical Mediterranean Ungauged Watersheds Using Post-Flash-Flood Measurements

“…The dense urban structures influenced the DEM and the hydraulic simulation in the Stavros floodplain. CSI values over 65% are generally acceptable [13,56] and show that the accuracy of the flood simulation is very close to the real flood extent. The comparison between the simulated and observed flood depths at the 35 points of the Stavros floodplain showed an acceptable accuracy [13,56], where RMSE was estimated at 0.51 m.…”

“…flood simulation is very close to the real flood extent. The comparison between the simulated and observed flood depths at the 35 points of the Stavros floodplain showed an acceptable accuracy [13,56], where RMSE was estimated at 0.51 m. Regarding the Stavros floodplain, the simulated flood depth and flood extent were delineated very well compared to the respective observed values. It was not feasible to accurately depict the flood depth because of the dense building blocks.…”

“…The results revealed that the proposed methodology could be a very useful tool to researchers and policy makers for flood risk assessment of higher accuracy and effectiveness in ungauged Mediterranean watersheds.Hydrology 2020, 7, 12 2 of 24 settlements with the tolerance of the state have led to the gradual trespass of torrents, the reduction of streambed width, and in some cases to their complete disappearance. Under these circumstances, the hydraulic characteristics of streams and floodplains have been altered dramatically and have resulted in the intensification of the flood phenomena [10][11][12][13]. The protective role of forests from flash flood generation is questionable, and according to previous research studies is very limited, indicating that forests have negligible effects on the reduction of peak discharge during extreme rainfall events [10,12,14,15].Over the past years, considerable efforts have been made to estimate flood risk and simulate flood events, using hydrological and hydraulic models [16][17][18][19][20][21][22][23].…”

“…Hydrology 2020, 7, 12 2 of 24 settlements with the tolerance of the state have led to the gradual trespass of torrents, the reduction of streambed width, and in some cases to their complete disappearance. Under these circumstances, the hydraulic characteristics of streams and floodplains have been altered dramatically and have resulted in the intensification of the flood phenomena [10][11][12][13]. The protective role of forests from flash flood generation is questionable, and according to previous research studies is very limited, indicating that forests have negligible effects on the reduction of peak discharge during extreme rainfall events [10,12,14,15].…”

Low cost, multiscale and multi-sensor application for flooded area mapping

“…Very often, nadiral remote sensed platforms cannot define the level of water, and for this reason, field surveys and ground-based photos are still necessary. A possible solution is the use of models for the estimation of the water depth based on digital terrain model (DTM) or hydraulic models (Bates and De Roo, 2000;Segura-Beltrán et al, 2016), but ground truth validation is needed. Recent development of computer vision applications, such as structure from Motion (SfM) (Snavely, 2008), make this system a possible valid alternative for the creation of a 3-D dataset based on terrestrial or aerial image acquisition systems.…”