Analysis of a flood event that affected the Hispano-Andorran border the 1st August 2008 Laura Trapero¹, Natalia Gallego¹, Aina Margalef¹, Pere Esteban¹ Centre d’Estudis de la Neu i la Muntanya d’Andorra (CENMA), Sant Julià de Lòria (Andorra). Email: [email protected] INTRODUCTION During the afternoon of 1st August 2008 a heavy precipitation event took place in Sant Julià de Lòria (southern Andorra). In this episode the Runer River, that determines the Hispano-Andorran border, as well as nearby torrents, overflowed. The most affected infrastructure was the Hispano-Andorran border and road communications were interrupted. Fortunately, there were no casualties but the economic losses were around 300.000 €. The total precipitation amount registered on the border was 63,8 mm (period of accumulation: from 17:42 to 20:00 TU), with a 6-minute maximum of 15,2 mm (18:12 TU). Different factors contributed to worsen the effects: the stationary thunderstorm, the river basin orientation and the relief morphology. In this study we show the geomorphologic observations made in the basin after the event and also different meteorological aspects have been analysed using the Weather and Research Forecasting Model (WRF). Figure 2. (a) Localization of different important points: border between Spain and Andorra, the Runer River and automatic weather stations: 1- Borda Vidal (873 m), 2- Aixàs (1.566 m). GEOMORPHOLOGICAL IMPACTS AND DAMAGE (b) Orographic profile of the zone: in red, administrative limit; in blue, rivers and torrents; in OBSERVATIONS (Figure 1) purple, roads. (c) Discharge registered in the Valira River (3- La Seu d’Urgell gauge station). (a) (b) The basin area of the Runer River is about 7 km2 and contains numerous affluent torrents. The river is 5 kilometres long and drops 1153 m over its course (23.3% slope). At its lowest point it flows into the Gran Valira River. It is at this junction where the Hispano- 2 Andorran border, which was affected by the 1st August of 2008 1 0 5 km episode, is located. The blue line is the outline of the river, and the Riu Runer Discharge (m3/s) black one represents the approximate shape of the basin. (c) 45,00 3 40,66 m /s 40,00 35,00 Photo 1 and 2 show the aspect of the border two days after the rise 30,00 25,00 of the river. The Runer River usually runs the left of the facilities, but 20,00 15,00 during the rise, it overflowed to the right and left and deposited a 10,00 5,00 great amount of material on the road and in the border area. 0,00 01/08/2008 01/08/2008 02/08/2008 02/08/2008 03/08/2008 Photo 3 shows the effects that the rise had on the dog kennels 0:00 12:00 0:00 12:00 0:00 located very near the border. The kennels nearest to the river were METEOROLOGICAL OBSERVATIONS affected, as was much of the vegetation, including trees over 5 (1. Borda Vidal) meters high. 16 360 15,2 mm Photo 4 shows the effects probably provoked by the wind, caused by 16 64,4 mm 70 14 310 14 60 12 260 the wind channelling in the river valley, breaking off branches of 12 50 10 210 10 decimetric diameter from the highest parts of the trees. 40 8 160 8 P (mm) P 30 In photo 5 some of the geomorphologic effects that the flood 6 6 110 20 4 (m/s) vent Velocitat 4 60 Direcció vent (º) del provoked can be seen. Deposited decimetric and metric sized blocks 10 2 2 10 can be observed, as well as an important undermining of the river 0 0 Figure 1. In black is the area covered by the Runer River basin (7 km2). Photo 1 and 2 show the aspect of the border 0 -40 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 20:24 21:36 15:54 16:12 16:30 16:48 17:06 17:24 17:42 18:00 18:18 18:36 18:54 19:12 19:30 19:48 20:06 bank that led to some trees being undermined. These were dragged two days after the rise. Photo 3 shows the effects on the dog kennels next to the river. Photo 4 shows the effects of the TU TU by the current and accumulated in the low part of the river. wind, which broke branches of decimetric diameter. In photo 5 some of the geomorphologic effects that the flood Figure 3. Precipitation registered in Borda Figure 4. Temporal evolution of mean wind provoked can be seen (CENMA, 2008). Vidal weather station: dark blue shows the velocity and direction in Borda Vidal (deep red intensity and light blue the total accumulation line and dark blue dots, respectively), as well as between 15:30-20:10 UT 1st August 2008. the maximum wind velocities (red line) and their directions (light blue dots). WRF MODELISATION: TESTING CUMULUS OPTIONS AT 3KM RESOLUTION a b c Episode 20080801 SYNOPTIC DESCRIPTION Cold start: 20080801_00h 1 Chronological description of the synoptic atmosphere Scheme 1. Model configuration: defined domains, 00h Duration: 24h tested and fixed physical options that have been configuration: used in the model simulations. 1. During the morning, the surface pressure field showed a very weak gradient over the western Fixed options Figure 5. WRF 4-nested domain configuration (27, 9, 3 and 1 km). Land Surface Model: Mediterranean with presence of SE winds. This surface ·Noah LSM 1 pattern plays a very efficient role in the activation of WRF-ARW Radiation: 12h Tested cumulus options valley-mountain breezes (Figure 6a). 4-nested domains: ·Shortwave: Dudhia · Longwave: Rapid Radiative Transfer Model (RRTM) over 3 km domain: 2. After midday, a ridge developed over the Pyrenees, 27-9-3-1 km (Fig.5) Planetary Boundary Layer: · No parametrization which triggered a humid N flow (Atlantic origin) over the · Yonsei University (YSU scheme) Initial-boundary conditions: (experiment code: F1_CU1100) north face of the Pyrenees (Figure 6b). Surface Layer: Final analysis-GFS ( 1º x 1º) · Monin-Obukhov · New Grell scheme 3. Simultaneously, an upper air trough passed over Microphysics option: (experiment code: F1_CU1150) 2 Andorra generating unstable atmospheric conditions - WSM3 simple ice scheme: mixed phase processes (Hc=3)* 00h Cumulus option (over 27 and 9 km): (Figure 6c). Barcelona and Nimes radiosoundings -Kain-Fritsch confirmed the west flow at 700 hPa. (a) *(Hc= # hydrometeors categories) F1_CU1100 Figure 6. Domain 1 (27 km) graphic representation of different fields using the F1_CU1100 model configuration: (a) wind and MESOSCALE DESCRIPTION: KEY FACTORS (Figure 7) surface temperature; (b) wind and humidity at 500 hPa; (c) temperature and geopotential height at 500 hPa. 1. The north flow interacting with the Pyrenees originated a SE and SW 24-HOUR QUANTITAVE PRECIPITATION FORECASTED (QPF) component in the south face (Bessemoulin et. al., 1993). This developed a 27 KM 9 KM 3 KM 1 KM convergence zone over central Catalonia (north-south orange axe). with a (b) maximum confluence in southern Andorra (orange circle). AND 2. This flow coupled with the valley-mountain breezes triggered the convective activity, which was favoured by the simultaneous presence of cold air in the upper levels. F1_CU1100 (c) AND F1_CU1100 AND Figure 8. (a) Meteosat-9 image (MSG) 01/08/2008 18:00UT. RGB channel combination (HVIS 0.7 mm and MIR 3.9 mm). (b) CAPPI-1Km Radar reflectivity (dBZ) at 17:42 UT. (c) Cloud to ground lightning registered by the XDDE network. (These images are courtesy of the Figure 7. Mean 10m wind (m/s) at 18:00 UT 1st August 2008. Orange line Meteorological Service of Catalonia). F1_CU1150 24-HOUR QUANTITAVE PRECIPITATION shows the convergence axe and orange circles the areas were the model localize the thunderstorms. ESTIMATED (QPE) QPF F1_CU1100 versus QPF F1_CU1150 THE THUNTHERSTORM (Figure 8) Both experiments performed well in detecting the convective activity during the event and its The radar images show the presence of an intense convective cell over RAINGAUGE versus QPE time initiation, but underestimated the the Hispano-Andorran border, with reflectivity values close to 50 dBz intensity. (CAPPI-1km). The cell followed a quasi-stationary west-east trajectory - Borda Vidal weather station registered 63.8 - F1_CU1100: the convective activity was well over the entire course of the Runer River due to the W flow present mm during the event. detected. The model localizes 3 distinct cores over 2500 m (Trapero et. al., 2009). - CDV radar detected 3 convective cells over over Catalonia. But as can bee seen in the 3 The lightning detection network from the Meteorological Service of Catalonia and registered a maximum QPE km domain, its location is wrongly place to the Catalonia registered the local nature of this thunderstorm. It is worth greater than 20 mm over the affected area. This east, specially the one that affected the highlighting the presence of a secondary storm in central Catalonia subestimation is due to the distance from the Hispano-Andorran border. which had developed earlier in the afternoon. The presence of these radar (>110 km) and the high beam blockage - F1_CU1150: this experiment overestimated PRECIPITATION (mm) PRECIPITATION two thunderstorms confirms the important role of the convergence that affects this area (Trapero et. al., 2009). the area affected by the convective activity detected on the surface layer. (the 3 cores cannot be distinguished). Figure 9. 24 h-quantitative precipitation estimated in the CDV radar from the Meteorological Service of Catalonia (1/08/2008). CONCLUSIONS NEWSPAPERS This event represents a challenge in terms of meteorological and hydrological forecasting, especially in a small mountainous area like Andorra.