X-band radar coupled with a GR3H rain-flow model in mountainous areas for flood prediction

Intense and localised rain events are commonly observed in the Mediterranean area, suggesting the need of radar rainfall information. In the framework of RHYTMME (Hydro-meteorological risk in Mountainous and Mediterranean Area) project, a three X-band radar network will be installed on the Alpine French area. This particular area is very mountainous and is unsatisfyingly covered by the existing French radar network ARAMIS (Météo France) as well as by the rain-gauge network. The aim of this new radar network is then to complete the actual radar network in Alpine area.

Since June 2007, a first dual-polarized X-band radar (HYDRIX) has been installed in the Nice area, at the top of the Mt-Vial (1500 m ASL). The X-band radar was coupled with the rain profiling algorithm ZPHI®, which made it possible to correct the signal attenuation. This combination avoids the need of rain-gauge-based adjustment, thus allowing operating in areas where rain-gauge networks are missing.

This study aims at evaluating the capability of X-band radar to improve the estimation of flood, compared to rain-gauge network or to the actual radar network in the Alpine area. Comparison will be presented in terms of flows derived from rain gauges, the X-band radar and a neighbouring Météo France S-band radar through the distributed rain flow model at hourly timescale (GR3H). For this purpose, two particular catchments will be studied:

- The “Réal Collobrier” catchments (70 km² area), covered by a dense rain gauge network (17 stations) localized only at 4km from an S-band radar (MétéoFrance).

- The alpine “Roya” catchments (673 km2 area), localized in a very mountainous area, where only few rain gauges stations are available, 60 km far from the X-band radar, and 150 km far from the S-band one.