A coupling of hydrologic and hydraulic models appropriate for the fast floods of the Gardon River basin (France)

Abstract

Abstract. Mediterranean catchments are regularly affected by fast and flash floods. Numerous hydrologic models have been developed, and allow modelling of these floods. However, these approaches often concern average-size basins of a few hundred km 2 . At larger scales (> 1000 km 2 ), coupling of hydrologic and hydraulic models appears to be an adapted solution. This study has as its first objective the evaluation of the performances of a coupling of models for flood hydrograph modelling. Secondly, the coupling results are compared with those of other modelling options. The aim of these comparisons is to clear up the following points. (1) Is a simplified routing model (lag and route) as efficient as a full hydraulic model for the modelling of hydrographs, in the intermediary downstream part of the stream? (2) Is adding lateral inflows necessary for all studied events? (3) What is the impact of the qualities of upstream hydrologic modelling feeding the coupling? The coupling combines the SCS-LR (Soil Conservation Service-lag-and-route) hydrologic model of the ATHYS platform and the MASCARET 1-D hydraulic model based on full Saint-Venant equations. It is applied to the Gardon River basin (2040 km 2 ) in the south of France. For the seven studied events, the results of the coupling are satisfactory, the calculated Nash indexes varying between 0.61 and 0.97. The comparisons with the other modelling options show the important role of the spatial distribution of rains during events: when rains are centered on the intermediary downstream part of the catchment, adding lateral inflows is necessary; when rains are more important in the upstream part, the quality of the hydrologic modelling upstream of the coupling has a strong impact. Furthermore, the used coupling of models seems well adapted for water rising and flooded area forecasting. The future developments of the tool will concentrate on this point.

Challenges, Opportunities, and Pitfalls for Global Coupled Hydrologic‐Hydraulic Modeling of Floods

“…Consequently, such a relatively simple coupling strategy might affect model results, but the impacts of dynamic model feedback were assumed negligible when compared to the other sources of nontrivial uncertainty (e.g., the resolution and accuracy of the topographic data). Moreover, the use of dynamic coupling for the modeling of floodplain inundation dynamics at the large scale would require extremely large computational resources that hamper both the feasibility and transferability of the methodology to other study areas (Laganier et al, 2014;Lerat, 2009). Conversely, external unidirectional coupling has been successfully applied in a number of previous analyses (Bravo et al, 2012;Lian et al, 2007;Mejia & Reed, 2011), and it was hence considered a sensible approach for the purposes of this study.…”

“…Instead of using observed rainfall records as model input, Nam et al (2014) used results from a numerical weather prediction model as input to the sequence of models for short-term flood inundation prediction. Laganier et al (2014), Nguyen et al (2016), and Mai and De Smedt (2017) showed that the coupled approach can adequately model flash floods and floodplain inundation. A coupled hydrologichydraulic modeling approach was recommended by Grimaldi et al (2013) for flood hazard modeling, by Felder et al (2017) for probable maximum flood risk estimation, and by Sindhu and Durga (2017) for flood damage mitigation.…”

Using a multi-hypothesis framework to improve the understanding of flow dynamics during flash floods

“…Douinot et al: Using a multi-hypothesis framework to improve the understanding of flash flood dynamics 1.2 Flash flood events: understanding flow processes Due to the challenges involved in forecasting flash floods, there has been considerable research done on the subject over the last 10 years. Examples include the HYDRATE (Hydrometeorological data resources and technologies for effective flash flood forecasting, 2006Gaume and Borga, 2013), which enabled the setting up of a comprehensive European database of flash flood flash events as well as the development of a reference methodology for the observation of post-flood events, the EXTRAFLO (EXTreme RAinfall andFLOod estimation, 2009-2013;Lang et al, 2014) to estimate extreme precipitation and floods for French catchments, the HYMEX project (HYdrological cycle in the Mediterranean EXperiment, 2010-2020; Drobinski et al, 2014) focusing on the meteorological cycle at the Mediterranean scale and particularly on the conditions that allow extreme events to develop, the FLASH project (Flooded Locations andSimulated Hydrographs, 2012-2017;Gourley et al, 2017) assessing the ability and the improvement of a flash flood forecasting framework in USA on the basis of real-time hydrological modelling with high-resolution forcing, or the FLOOD-SCALE project (Multi-scale hydrometeorological observation and modelling for flash floods understanding and simulation, 2012-2016Braud et al, 2014), based on a multiscale experimental approach to improve the observation of the hydrological processes that lead to flash floods.…”

A Dynamic Bidirectional Coupled Hydrologic-Hydrodynamic Model for Flood Prediction

“…The next step is to determine whether the discharge term is inflow or outflow fed back to the hydrodynamic model in the next time step. In this method, the current water level of the hydrologic model and hydrodynamic model have been used in velocity calculation at the mutual boundary, It does not consider present flow state (Bravo et al, 2012;Laganier et al, 2014). It is different from BCM using the lateral inflow conditions that velocity provided by hydrologic model will be added to the governing equations of hydrodynamic model directly, not considering present flow state.…”